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    Supported in part by the United States Department of Energy under Grant DE-FG06-90ER40537 This report was prepared as an account of work sponsored in part by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately-owned rights.


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    This has been a year of change at the Nuclear Physics Laboratory. We are pleased to have added several new faculty members to our group. R.G. Hamish Robertson and John Wilkerson have joined the Department of Physics as Professors. Peter Doe and Steve Elliott have joined the Research faculty as Professor and Assistant Professor, respectively. The primary interest of these new faculty is solar neutrino physics. They are presently playing a major role in the design and construction of detectors for the Sudbury Solar Neutrino Observatory (SNO). John Lestone has also recently joined the Research faculty as Assistant Professor. His primary interest is in nuclear reactions, particularly fission. The Nuclear Physics Laboratory of the University of Washington has for over 40 years supported a broad program of experimental physics research. The current program includes "in-house" research using the local tandem Van de Graaff and superconducting linac accelerators and non-accelerator research in double beta decay and gravitation as well as user-mode research at large accelerator and reactor facilities around the world. We are also now actively using or developing solar neutrino observatories in Canada and Russia. In June 1994 we completed an upgrade of the tandem accelerator. We now have a pelletron charging system, new resistors, a terminal control computer system and spiral inclined field beam tubes in the tandem. The position, energy and pulse transit time stability of the beam accelerated by this completed system as well as the transmission of the accelerator for heavy ions is significantly improved. Some highlights of our research activities during the past year are given below. Motivated by nuclear structure and astrophysical considerations, we have continued our studies of the distribution of Gamow-Teller strength. We focused on the high-energy-release beta decays of 37Ca and 36Ca, and find in both cases considerably more integrated Gamow-Teller strength than predicted by the shell model with effective operators. The GT matrix elements for 37Ca decay can be compared to the isospin-mirror GT matrix elements inferred from 37Cl(p,n) studies; to the extent that isospin symmetry is valid this comparison tests the procedure for extracting GT matrix elements from hadronic charge-exchange reactions. We find that the charge-exchange values can vary by up to a factor of two compared to the beta-decay values. Both of the discrepancies mentioned here provide challenges to nuclear theory. We have considered the gravitational lensing properties of a plausible model for naturally-occurring wormholes, and find that they should be detectable by MACHO searches currently in progress. We find that the expected dipole anisotropy in the distribution of gamma ray bursts is not large enough to determine if they are cosmic in origin. The Russian-American Gallium experiment (SAGE) continues to give a flux well below the Standard Solar Model. A calibration with 51Cr is in progress. The University of Washington group continues to have major involvement in several aspects of the SNO project. The research and development effort for the acrylic vessel which holds the 1000 tons of heavy water has been completed. We are involved in the development of the data acquisition system for the readout of the SNO detector. In collaboration with groups at LASL and LBL we will provide an independent detector array for recording the neutral current signal of the SNO detector. In our studies of sub-barrier fusion reactions we have measured the distributions of barriers for a prolate and an oblate target nucleus bombarded with 40Ca. The barrier distributions demonstrate important contributions


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    from the hexadecapole as well as from the sign of the quadrupole moment. A study of entrance channel effects on the energy spectra of light charged particles has revealed significant effects. The spectral shape for 12C + 144Sm is hotter than for 64Ni + 96Zr when the bombarding energies are chosen to match the excitation energy in the compound nucleus. Recent results from the APEX collaboration on the positron spectra for uranium projectiles do not reveal peak structures and call into question previous GSI results. The AMS group, with partial support from NSF, has continued its development and refinement of techniques for the isolation of essentially pure pollen fractions from lake sediment and peat samples for AMS radiocarbon (14C) dating. These techniques were successfully applied, for the first time, to a low-organic- carbon-content core taken from an Arctic lake. An apparent age-depth reversal in earlier "bulk carbon" results obtained by others for this core was rectified. The ultra-relativistic heavy ion group participated in a very successful initial run of CERN experiment NA49, which used two large time-projection chambers to track charged reaction products from the new CERN 33 TeV lead beam on a fixed lead target. The University of Washington group played a major role in developing the tracking software used with the main time-projection chamber of the experiment. They also developed new general purpose slow control software used in the experiment. We have devised an approximate procedure for analyzing Bose-Einstein correlation data without background generation which may be useful for single-event physics at CERN and RHIC. We have made progress in analyzing expected "wiggles" in multi-particle Bose-Einstein correlation data arising from non-Gaussian source shapes. As always, we welcome applications from outsiders for the use of our facilities. As a convenient reference for potential users, the table on the following page lists the vital statistics of our accelerators. For further information, please write or telephone: Professor Derek W. Storm, Director Nuclear Physics Laboratory University of Washington Box 354290 Seattle, WA 98195 (206) 543-4085 e-mail: storm@npl.washington.edu We close this introduction with a reminder that the articles in this report describe work in progress and are not to be regarded as publications or to be quoted without permission of the authors. In each article, the names of the investigators have been listed alphabetically, with the primary author underlined, to whom inquiries should be addressed. Robert Vandenbosch Editor


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    Barbara Fulton Assistant Editor


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    1. NUCLEAR PHYSICS 1.1 Beta-delayed alpha-particle emission of 16 N and the E1 S-factor of the 12 C(α , γ )16 O reaction E.G. Adelberger, P. Chan, L. De Braeckeleer, P.V. Magnus, D.M. Markoff, D.W. Storm, H.E. Swanson, K.B. Swartz, D. Wright and Z. Zhao The 12 C(α , γ )16 O cross section at astrophysical energies is an incoherent sum of the electric dipole (E1) and electric quadrupole (E2) components. The E1 component is dominated by a broad level at 9.6 MeV and a subthreshold level at 7.12 MeV (alpha particle threshold 7.16 MeV) and the interference between them. The E2 component is dominated by a subthreshold level at 6.92 MeV, and direct capture, and the interference between these two terms. The beta-delayed alpha spectrum of 16 N provides a unique way to determine the alpha-particle width of the 7.12 MeV level, which is crucial to our understanding of the E1 cross section at low energies. We have measured the alpha-particle spectrum using the rotating arm apparatus built for the mass-8 beta-alpha correlation measurements. The deuteron beam (30 µA at 3.5 MeV) from the FN tandem was incident on a rotating target which consisted of Ti15 N on a Ni backing. When the beam was on, the recoiling 16 N nuclei were collected by a carbon catcher foil of 10 or 20 µg / cm 2 . When the beam was off, the catcher was transferred to the counting area. There are two silicon surface barrier detectors in the counting area. The energies of the alpha-particles and the carbon nuclei were detected simultaneously. The slow timing between the two particles was recorded at the same time to determine the random coincidences between two counters. The lifetime of the decay, measured by clocking each event relative to the arm rotation, is found consistent with the known lifetime of the 16 N nuclei. In addition, the absolute efficiency to measure carbon ions and detector response to low energy carbon ions were also studied using the 12 C( p, p)12 C reaction. Our results are consistent with previous measurements of TRIUMF1 and Yale.2 The data were analyzed in an R-matrix formalism using a 3-level approximation. In Fig. 1.1-1, we show an R-matrix fit to our data. Possible l = 3 contribution is included in the fit. Fig. 1.1-1. Alpha energy spectrum. 1L. Buchmann et al., Phys. Rev. Lett. 70, 726 (1993). 2Z. Zhao et al., Phys. Rev. Lett. 70, 2066 (1993). 1


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    1.2 Detector tests for a measurement of the 12 C(α , γ )16 O cross section at low energies L. De Braeckeleer and Z. Zhao The main uncertainties in the E1 component of the 12 C(α, γ )16 O cross section were the uncertainty in the alpha-particle width of the subthreshold 7.12 MeV level and also the uncertainty in the sign of the interference between 7.12 MeV level and 9.6 MeV level. The beta-delayed alpha spectrum of 16 N provides a unique way to determine the alpha-particle width of the 7.12 MeV level, as discussed in the previous report,1,2,3 but provides no new information on the sign of the interference in the 12 C(α, γ )16 O reaction. The E2 component is obtained by measuring the angular distribution of the gamma rays, and the extrapolation to astrophysical energies is also largely uncertain. Although the combined four sets of previous cross section measurements4,5,6,7 favors constructive interference in the E1 12 C(α, γ )16 O reaction, examination of individual data sets reveals inconsistency among them especially regarding the sign of the interference in the E1 channel. Since the alpha-particle width of the subthreshold level is determined by the 16 N data, the constructive and destructive interference in the E1 channel of the 12 C(α, γ )16 O reaction should differ by a factor of two at energy Ec.m. = 1.2 MeV. A 10% measurement at this energy should allow us to distinguish between two interference schemes. We plan to do measurements in two steps; step one is a two-detector assembly at 90° to determine the E1 component (mainly the sign); step two is a measurement of the angular distribution of the gamma rays to determine the E2 component. Recently, technological advancements have made available a high intensity pulsed ion beam and large volume germanium counters. We plan to surround the Ge counter with a NaI(Tl) annulus and plastic scintillator. The Ge counter will be running in escape mode (requiring coincidence with 511 keV gamma rays in the NaI(Tl) counter). Therefore the Ge counters provide good energy resolution, and escape gamma rays in the NaI(Tl) provide excellent timing relative to the pulsed beam. We expect a significant background reduction relative to previous measurements. Using two Ge counters (8 cm long and 8 cm in diameter), the total efficiency of this detector system is 1 × 10 − 3 . For a 120 µg / cm 2 target, the expected count rate is 4.5/day at Ec.m. = 1.2 MeV. The E2 contamination in the 90° detector arrangement is less than 10% for the entire accessible energy range. We expect the beam independent background to be negligible. We are currently testing such a concept using a smaller Ge and BGO counters. We also studied the beam independent background in such a system. We plan to study the beam related background in the near future. 1L. Buchmann et al., Phys. Rev. Lett 70, 726 (1993). 2Z. Zhao et al., Phys. Rev. Lett. 70, 2066 (1993). 3Z Zhao et al., to be published. 4P. Dyer and C.A. Barnes, Nucl. Phys. A233, 495 (1974). 5A. Redder et al., Nucl. Phys. A462, 385 (1987). 6R.M. Kremer et al., Phys. Rev. Lett. 60, 1475 (1988). 7J.M.L. Ouellet et al., Phys. Rev. Lett. 69, 1475 (1992). 2


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    1.3 On the dipole distribution of gamma ray bursts J.G. Cramer and P.B. Cramer* Gamma ray bursts (GRB) constitute an outstanding puzzle of contemporary astrophysics. In recent satellite experiments GRB have been observed to occur in a time scale of milliseconds to seconds at the rate of a few per day from directions uncorrelated with the galactic plane and to deliver the remarkably large integrated energy of about 6 MeV per cm 2 of detector area. No radio, optical, or X-ray counterparts of GRB have been observed, lending ambiguity to their origin and distance scale. The leading scenarios for GRB source locations are: (1) GRB are at galactic halo distances, perhaps produced by unusual neutron stars or quiet supernovas, or (2) GRB are at cosmological distances, perhaps produced by merging neutron stars. If GRB are galactic and are emitted in an angular cone with a half-angle θ (expressed as a fraction of π ), the energy release of each GRB is 10 51 θ 2 ergs (or, with θ =1, roughly the mass of Mars' largest moon Phobos converted entirely into gamma ray energy). On the other hand, if GRB are cosmological, each GRB has an energy release of 10 51 θ 2 ergs (with θ =1, this is roughly a Jupiter-mass converted completely into gamma rays in a few seconds). In either case, both the generation and the transport of this quantity of energy constitute very formidable and unsolved theoretical problems. We here consider the possibility of using the motion-induced dipole moment of the GRB distribution as an indication of the GRB source. COBE data shows that the Earth is moving through the microwave −3 background radiation with a velocity of β CB = 1.23 x 10 in the direction l = 264.4º ± 0.3º and b II = 48.4º II ± 0.5º in galactic coordinates. On the other hand, the Sun moves with respect to the galactic center with a − 4 velocity of β gal = 7.92 x 10 in the direction l =91.1º ± 0.4º and b II =0º in galactic coordinates. These II vectors thus point in nearly opposite directions, making an angle of about 130º with each other. The solid angle of a moving source is modified by its proper motion, an effect well known in nuclear and high energy physics. This solid angle modification will induce a dipole component in the GRB angular distribution of magnitude 2 β with its peak pointing in the direction of β , i.e. the GRB events will be distributed on the sky with a probability given by P(θ , φ ) = 1 + 2 β cos θ . Consider an observed sample of N observations of GRB, with the ith event detected at a polar angle of θ i with respect to the source velocity direction of interest and with an observational efficiency ε i . The estimated boost is < β obs > = N N ∑ ∑ ω cosθ i/ i =1 ω i , where ω i = 1/ ε i . Assuming ω i =1 and treating the dipole component as small, the i =1 i values of (ω i cosθ i ) will fall on a uniform flat distribution of values between -1 and +1. The mean of such a distribution is zero and its standard deviation is σ = 1/ 3 . Therefore, for N observations, the observed standard deviation of < β obs> will be σ obs = 1/ 3N . We conclude that a determination of the dipole distribution at the 1σ level for the COBE velocity would require 220,000 GRB observations and for the galactic velocity vector would require 532,000 GRB observations. Unfortunately, the most recent catalog of GRB events contains only 1121 events. Thus, because the proper motion of the solar system is relatively small and the number of detected GRB is still quite small, it is not feasible to use the expected dipole distribution as an indication of GRB origin. * Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany. 3


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    1.4 Natural wormholes as gravitational lenses J.G. Cramer, R.W. Forward,* M.S. Morris,† M. Visser,‡ G. Benford£ and G.A. Landis§ Visser has suggested a wormhole configuration, a flat-space wormhole that is framed by a variant of the cosmic string solutions of Einstein's equations with a negative string tension of -1/4G and therefore a negative mass density. The inflationary phase of the early universe might produce closed negative-mass string loops framing stable Visser wormholes or expand microscopic wormholes in the Planck-scale spacetime foam to macroscopic dimensions, thereby creating stable natural wormholes. When a massive object passes through a wormhole, from back-reaction the entrance mouth should gain mass and the exit mouth lose mass. If the mass flow occurs in the early universe from a high to a low density region, the exit wormhole mouth could acquire a stellar-scale negative mass. We have christened such objects "gravitationally negative anomalous compact halo objects'' (GNACHOs). We have considered the gravitational lensing of such objects as a way of detecting them. We find that the lensing of a negative mass is not analogous to a diverging lens. In certain circumstances, it can produce more light enhancement than does the lensing of an equivalent positive mass. The intensity modulation of a background star that occurs when the GNACHO lensing mass crosses near the source-detector line of sight shows light enhancement profiles that are characteristically bounded by two caustics, each of which provides a very sharp increase in light intensity. Between these caustics is an umbra region with no transmitted light at all. This light enhancement profile is qualitatively different from that of a positive lensing mass of the same magnitude and geometry. In particular, the negative mass light curves is much sharper, showing stronger but briefer light enhancements, and a precipitous drop to zero intensity in the central region. Our calculations show that objects of negative gravitational mass, if they exist, can provide a very distinctive light enhancement profile. Since three groups are presently conducting searches for the gravitational lensing of more normal positive mass objects, we have suggested that these searches be broadened so that the signatures of the objects discussed above are not overlooked by over-specific data selection criteria and software cuts. While the analysis of this brief report is phrased in terms of wormholes, the observational test proposed is more generally a search for compact negative mass objects of any origin. The question of whether quantum field theory is consistent with negative mass in a spacetime that is asymptotically flat and semiclassical near infinity is as yet undecided. Observation of GNACHOs would give an experimental and definitive answer to this question. We recommend that MACHO search data be analyzed for evidence of GNACHOs. This work has been accepted for publication in Physical Review D, and is available as electronic preprint astro-ph/9409051 on World Wide Web at http://babbage.sissa.it. *Forward Unlimited, P.O. Box 2783, Malibu, CA 90265. †Department of Physics and Astronomy, Butler University, Indianapolis, IN 46208. ‡Physics Department, Washington University, St.Louis, MO 63130-4899. £Physics Department, University of California at Levine, CA 92717-4575 §NASA Lewis Research Center, Mail Code 302-1, Cleveland OH 44135-3191. 4


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    1.5 36 Ca b - decay and the Isobaric Multiplet Mass Equation E.G. Adelberger and A. Garcia* This work was a byproduct of our study of 37 Ca decay at ISOLDE, which is presented elsewhere in this report. The 36 Ca activity was produced in a fluorinated Ti target, and data were taken with both A = 36 (for 36 Ca ) and A = 55 (for 36 Ca 19 F ) beams. The A = 36 beam was dominated by 36 K; the A = 55 beam was extremely pure but had such a low intensity (because the fluorine leak in the target had expired) that we detected only the intense proton group corresponding to the superallowed decay of 36 Ca . The delayed proton energy scale was calibrated using well-known proton groups from 37 Ca and 36 K decays. We found that the superallowed 36 Ca group had a lab energy of E p - 2550.2 ± 2.2 keV, which corresponds to a mass excess of -13135.6 ± 2.4 keV for the lowest T = 2 level in 36 K. When combined with the well-known masses of the 36 S, 36 Cl , and 36 K members of the isospin quintet and the less well-known mass of 36 Ca , our result provides one of the most precise tools of the isobaric multiplet mass equation (IMME) M ( A1T1T3 ) = a( A1T ) + b( A1T )T3 + c( A1T )T32 . The results of fitting these data to the IMME (plus extensions containing T33 and T34 terms) are shown in Table 1.5-1. Table 1.5-1. Coefficients of multiplet mass equation for the lowest quintet in A = 36. ______________________________________________________________________________ a (keV) b (keV) c (keV) d (keV) e (keV) c2 /n P( c 2 , n )a ______________________________________________________________________________ -19378.4±1.0 -6043.8±1.3 200.5±0.7 1.5 0.22 -19377.6±1.5 -6044.5±1.6 199.1±1.9 0.8±1.0 2.4 0.12 -19.3771±1.5 -6043.6±1.3 197.6±2.5 0.6±0.5 1.6 0.21 -19377.1±1.5 -6039.2±3.8 195.4±3.1 -4.2±3.4 2.7±1.8 ______________________________________________________________________________ aProbability of getting a c 2 as large as that in the previous column. *Dept. of Physics, University of Notre Dame, South Bend, IN. 5


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    1.6 Gamow-Teller strength in 36 Ca b + decay E.G. Adelberger, B.A. Brown,* Z. Janas,† H. Keller,† K. Krumbholz,‡ V. Kunze,‡ P. Magnus F. Meissner,‡ A. Piechaczek,† M. Pfützner,£ E. Roeckl,† K. Rykaczewski,£ W.-D. Schmidt-Ott,‡ W. Trinder† and M. Weber† We have used the FRS projectile fragment separator at GSI to study the decay of 36 Ca . Beta delayed g 's and protons were observed using the same detector setup we used for the 37 Ca decay study discussed in the following report.. A secondary 36 Ca beam with an intensity of 0.25 atoms/s was produced by a primary beam of 300 MeV/u 40 Ca impinging on a 1g/cm2 9 Be target. A total of 2.8 x 104 36 Ca atoms were implanted during the experiment. The 36 Ca lifetime, 102 ± 2 ms, was extracted from the time distribution of proton events with energies above 2.5 MeV, mainly originating from the superallowed transition. We observed strong transitions to g emitting states in 36 K at 1112.8(4) and 1619.0(2) keV, and to proton decaying 36 K states at 3370(41), 4287(39), 4451(33), 4687(37), 5947(47) and 6798(71) keV. Fig. 1.6-1 compares our measured B(GT) values to shell model predictions using the USD interaction. We find a situation strikingly similar to that observed in 37 Ca decay1 where the theory with "quenched" GT operators gives a good account of the strengths to transition at low energy ( E x <~ 3.5 MeV), but predicts much too little strength at higher energies. In fact the B(GT) integrated strength up to our experimental cutoff (see Fig. 1.6-1) agrees much better with that predicted by the unquenched theory. There are now three examples of high energy-release b decays ( 37 Ca , 36 Ca , and 33 Ar ) where the shell model fails, in similar ways, to predict the distribution of GT strength. This systematic shortcoming poses an important problem for nuclear structure calculations. This work has been published in Physics Letters B. WWW: Select "Fig 1.6" from Table of Contents page Fig. 1.6-1. The heavy lines show the ±1σ error band of the summed B(GT) strength in 36 Ca decay as a function of final state Ex . Curves b) and c) are shell model calculations using free and renormalized GT operators, respectively. * Michigan State University, East Lansing, MI. †GSI, Darmstadt, Germany. ‡II. Phys. Inst., Univ. Göttingen, Germany. £ Inst. of Exp. Physics, Univ. of Warsaw, Poland. 1E.G. Adelberger, et al., Phys. Rev. Lett. 67, 3658 (1992). 6


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    1.7 Study of 37 Ca b decay at GSI and the 37 Cl cross section for 8 B neutrinos E. G. Adelberger, Z. Janas,* H. Keller,* K. Krumbholz,† V. Kunze,† P. Magnus, F. Meissner,† A. Piechaczek,* M. Pfützner,‡ E. Roeckl,* K. Rykaczewski,‡ W.-D. Schmidt-Ott,† W. Trinder,* and M. Weber* The observation of significant discrepancies between mirror B(GT) values extracted from 37 Ca b decay and 37 Cl( p, n) studies (mentioned elsewhere in this report) motivated this measurement that provided absolute intensities of the previously observed1 b -delayed proton groups and, for the first time, detected the beta-delayed gamma rays following 37 Ca decay. In addition we obtained a precise measurement of the 37 Ca halflife. The FRS projectile fragment separator at GSI produced a secondary 37 Ca beam of about 30 atoms/s, from the bombardment of a 1g / cm 2 Be target with a 300 MeV/u beam of 40 Ca . A total of 2.6 million 37 Ca ions were stopped in the central element of a 3 counter Si telescope that was surrounded by 2 large-volume Ge detectors. Beta delayed protons were detected in the central Si counter, and delayed gammas were detected by the Ge detectors in coincidence with a b pulse in the outer two Si counters. The 37 Ca halflife was obtained from the time distribution of the intense proton group corresponding to the superallowed decay. Our value of 181(1) ms is two standard deviations larger than the previously accepted result. A high-quality spectrum of g -rays following 37 Ca decay (see Fig. 1.7-1) showed peaks at 1370.9(2), 2750.4(2) and 3239.3(2) keV with branching ratios of 2.1(1)%, 2.8(1)%, and 4.8(2)% respectively. Combining these results with the delayed proton intensities from ref. 1 we find that the 3239 keV state of 37 K has Gg / Gp = 22(2 ) . This is a surprisingly large value for a level that lies nearly 1.4 MeV above proton threshold, and explains most of the discrepancy noted earlier regarding the mirror B(GT)'s for this level. We have used our results to recompute the cross section for 8 Be neutrinos on a 37 Cl detector. Our result 1.09(3) ¥ 10 -42 cm 2 is consistent with Bahcall's standard value of 1.06(10 ) ¥ 10 -42 cm 2 . WWW: Select "Fig 1.7" from Table of Contents page. Fig. 1.7-1. g -ray spectrum from the decay of 37 Ca . The dominant lines are labeled by the residual nucleus in which the transitions occur. Weaker lines are from weaker transitions in 36 Ar or are single or double escape peaks. This work has been published in Physics Letters B. *GSI, Darmstadt, Germany. †II. Phys. Inst., Univ. Göttingen, Germany. ‡ Inst. of Exp. Physics, Univ. of Warsaw, Poland. 1A. Garcia, et al., Phys. Rev. Lett. 67, 3654 (1991). 7


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    1.8 Study of 37 Ca decay at ISOLDE E. G. Adelberger, A. Garcia,* P.V. Magnus, H.E. Swanson, D.P. Wells,† F.E. Wietfeldt,‡ O. Tengbald,£ and the ISOLDE Collaboration Comparison of the isospin analog B(GT) values from our b -delayed proton work on 37 Ca 1 and a new, high-resolution study2 of 37 Cl( p, n) showed some large discrepancies. The discrepancies at low Ex could arise if g -decay competed successfully with proton decay for a few of the low-lying final states in 37 K.3 We studied b -delayed g emission in 37 Ca at ISOLDE and found unexpectedly significant g -ray branches from 2 of the unbound daughter states in 37 K. Our 37 Ca source was produced using a 37 CaF beam from the ISOLDE general-purpose on-line isotope separator at the CERN PS/booster. The fluorinated A = 56 beam had very little radioactive contamination. In particular, it had virtually no 37 K activity that in an A = 37 beam is so intense that it would have been impossible to do our experiment. The fluorination process was very efficient; the A = 56 beam had about 30 37 CaF ions per second, 50% of the 37 Ca intensity observed in the A = 37 beam. The 37 CaF beam was focused onto a three-element particle telescope that was surrounded by an annular eight-segment NaI detector that covered DW / 4p ª 0.9 . We observed b -delayed g branches of (1.5±0.4)%, (3.6±0.8)%, and (4.4 ±0.6)% to 37 K levels at 1.37, 2.75, and 3.24 MeV. These results are consistent with Ge detector data taken at GSI (see Section 1.7 of this report). Combining this b -delayed g work with our earlier b -delayed proton results1 we find the B(GT) values listed in Table 1.8-1. It is apparent that although the larger anomalies have essentially been eliminated, problems remain at the factor of 2 level. This work was recently published in Physical Review C.4 Table 1.8-1. Analog B(GT) values obtained from 37 Ca b decay and 37 Cl( p, n ) . Ex ( 37 K) B(GT ; b + ) a Ex ( 37 Ar) B(GT ; p, n) b 0.00 ( 4.8 ± 0.1) ¥ 10 -2 0.00 ( 4.8 ± 0.1) ¥ 10 -2 1.37 (9.2 ± 2.5) ¥ 10 -3 1.41 (1.4 ± 0.1) ¥ 10 -2 2.75 (1.2 ± 0.1) ¥ 10 -1 2.80 ( 7.0 ± 0.2 ) ¥ 10 -2 3.24 (8.2 ± 1.6) ¥ 10 -2 3.17 (1.28 ± 0.04) ¥ 10 -1 3.62 ( 7.5 ± 0.4) ¥ 10 -2 3.60 (5.8 ± 0.3) ¥ 10 -2 3.84 (9.4 ± 0.5) ¥ 10 -2 3.94 (1.9 ± 0.1) ¥ 10 -2 4.19 (2.0 ± 0.7) ¥ 10 -3 4.41 ( 4.3 ± 0.2) ¥ 10 -2 4.50 (6.0 ± 0.3) ¥ 10 -2 4.57 (8.5 ± 0.2) ¥ 10 -2 aFrom this work. bFrom the E p ª 100 MeV 37 Cl( p, n) data of Ref. 2. Data are normalized so that the ground-state transition has the correct value determined by the 37 Ar lifetime. *Department of Physics, University of Notre Dame, South Bend, IN. †Department of Health, Washington State, Olympia, WA. ‡Lawrence Berkeley Laboratory, Berkeley, CA. £ CERN, Switzerland. 1A. Garcia, et al., Phys. Rev. Lett. 67, 3654 (1991). 2D.P. Wells, private communication regarding experiment at Indiana University Cyclotron Facility. 3C.D. Goodman, et al., Phys. Rev. Lett. 69, 2446 (1992). 4A. Garcia, et al., Phys. Rev. C 51, R439 (1995). 8


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    2. NEUTRINO PHYSICS 2.1 Solar neutrino research Q.R. Ahmad, J. Beck, P.J. Doe, S.R. Elliott, J.V. Germani, A.W.P. Poon, T.D. Steiger, R.G.H. Robertson and J.F. Wilkerson Understanding the properties of neutrinos ranks as one of the major unresolved issues in nuclear and particle physics. In particular, we do not know if neutrinos have mass or not. Neutrinos with mass would provide conclusive evidence of new physics beyond the minimal Standard Model of elementary particles and fields while possibly also comprising a significant portion of the dark matter thought to abound in the universe. Increasingly, the "solar neutrino problem", in which all four of the existing experiments observe far fewer neutrinos from the Sun than predicted by solar model calculations, appears to point to neutrino oscillations. Neutrino oscillations convert one "flavor" of neutrino into another, and can only occur if neutrinos have mass. Our experimental program to resolve this question includes a collaboration with the Russian Academy of Sciences, Los Alamos National Laboratory, and the University of Pennsylvania on a 71 Ga radiochemical measurement (SAGE) of the pp neutrino flux and participation in the Sudbury Neutrino Observatory (SNO), a joint Canadian, US, UK effort to measure the spectral distribution and flavor composition of the flux of the higher-energy, 8 B neutrinos from the Sun. 2.2 The Sudbury Neutrino Observatory Q.R. Ahmad, J. Beck, P.J. Doe, S.R. Elliott, J.V. Germani, A.W.P. Poon, T.D. Steiger, R.G.H. Robertson and J.F. Wilkerson The Sudbury Neutrino Observatory will be the first solar neutrino detector capable of registering and distinguishing both the flux of electron neutrinos and the total flux of all left-handed neutrinos from the Sun. As such, it can make an unambiguous statement that neutrino oscillations are occurring, if the total flux is found to be larger than the ν e flux. The conclusions will be essentially independent of solar models. The sensitive medium of the SNO laboratory is 1000 tonnes of ultra-pure D 2 O . The charged-current ( interaction of electron neutrinos on deuterium produces a fast electron that emits Cerenkov radiation in the water. An array of 9500 photomultipliers records the amount of light and, therefore, the energy of the electron. Our group is involved in developing the data acquisition system for the readout of the SNO detector, directing the research and development effort for the acrylic vessel that separates the heavy water from the light water shield, coordinating the SNO detector turn-on and commissioning efforts, and providing an independent detector array for recording the neutral-current signal of the SNO detector. Neutral-current interactions of all flavors of neutrino disintegrate the deuteron into a proton and a neutron, and the rate of these interactions can be determined by measuring the rate of neutron production. 9


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    2.3 Development of a compact 20 MeV gamma-ray source for energy calibration at the Sudbury Neutrino Observatory M.C. Browne,* N.P. Kherani,† A.W.P. Poon, R.G.H. Robertson and C.E. Waltham‡ A compact 20 MeV gamma-ray source is being developed for energy calibration at the Sudbury Neutrino Observatory (SNO). The gamma-rays are produced through the radiative capture reaction 3H(p,γ)4He. There are several constraints in designing the source. It has to be compact and portable, as it will be lowered to the center of the SNO detector through the 57" diameter deck of the acrylic vessel. The neutron production rate has to be low to ensure SNO's neutral current sensitivity to supernova neutrinos during the scheduled high energy γ calibration. Even though the Q-value of 3H(p,n)3He is -0.763 MeV, which corresponds to a reaction threshold of 1.02 MeV, impurities in the beam and the target will give rise to undesired neutrons through the 2H(t,n)4He, 3H(d,n)4He, and 3H(t,nn)4He reactions. So the discharge hydrogen gas and the target tritium must be of very high purity. The solid tritium target must also have a high thermal stability to minimize tritium gas mixing into the discharge gas. Fig. 2.3-1 shows the design of the source. It can be divided into two sections: a cold cathode Penning ion source and a target chamber. The anode E2 is biased at +2 kV, while the cathodes E1, and E3 are grounded. Pyrex®-stainless steel couplings (C) are used to isolate high voltages. Permanent magnet rings M provide a ~1 kG axial magnetic field causing the electrons to spiral around the field lines, generating more ions. A SAES® getter pump is attached to E1. The getter pump serves as the dispenser of hydrogen gas. Ions (H+, H2+ and H3+) are accelerated through a -15 to -20 kV bias towards the target mounted at the end of the beam line. In this scheme, the construction of complicated accelerating and focusing electrodes is avoided, thus keeping the length of the source to a minimum. In fact, the length of the source is only 16". The target selected for this source is a solid scandium tritide (Sc3H2), primarily because of its good thermal stability. It will be prepared at the Tritium Laboratory of Ontario Hydro Technologies in Toronto, Canada. It is clear from the drawing that the source is a sealed one, minimizing the hazard of tritium contamination. The electro-optics of the ion source were first computer simulated and subsequently tested experimentally. The beam current has been measured using two independent methods: by measuring temperature change of a copper target and by integrating the beam profile measured using a Faraday cup. In the thermal measurement, the temperature of the copper target was monitored by a type T thermocouple. The beam power was later calibrated by an electric heater embedded in the target. We designed and constructed a special Faraday cup for measuring the beam profile. The cup was biased such that the secondary electron effects were minimized. Fig. 2.3-2 shows the results of the beam current measurements. We also designed and constructed a mass spectrometer to measure the mass composition of the ion beam. Ions of different masses are separated by a magnetic field perpendicular to the ion beam propagation. Our experiment showed that protons compose (63±15)% of the beam, the rest being H2+ and H3+ molecular ions. We are now building a target evaporation chamber, similar to that used by Kherani et al.,1 to test fully the target fabrication procedure of Sc3H2 before using the tritium facility at Ontario Hydro Technologies. *Department of Physics, North Carolina State University, Raleigh, NC. †OntarioHydro Technologies, 800 Kipling Avenue, Toronto, Ontario, Canada M8Z 5S4. ‡Department of Physics, University of British Columbia, Vancouver, B.C., Canada V6T 1Z1. 1N.P. Kherani and W.T. Shmayda, Fusion Tech., 21, 334 (1992); N.P. Kherani, Ph.D. thesis, University of Toronto, 1994 (submitted to University Microfilm Inc., Ann Arbor, Michigan). 10


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    M C UHV VALVE E1 E2 E3 Fig. 2.3-1. Design of the SNO 3H(p,γ)4He 20 MeV gamma-ray source. 160 140 Proton Source 120 Beam Current (µA) 100 80 60 40 (1.87±0.05)mTorr (1.40±0.05)mTorr 20 (0.82±0.12)mTorr (0.47±0.12)mTorr 0 5 10 15 20 Accelerating Voltage (kV) Fig. 2.3-2. Measured beam current of the ion source as a function of the beam accelerating voltage. 11


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    2.4 A neutral current detector array for Sudbury Neutrino Observatory P.J. Doe, S.R. Elliott, J.V. Germani, A.W.P. Poon, T.D. Steiger, R.G.H. Robertson and J.F. Wilkerson Neutrons can be detected with high efficiency in 3 He -filled proportional counters without interfering significantly with the Cerenkov light from charged-current processes. We plan to install in SNO an array of such counters, with a total length of 800 meters, and constructed of highly purified materials. Our rationale follows: 3 He proportional counters offer fundamental advantages. Neutral-current and charged-current events are recorded separately and distinguished event by event. The effective CC rates are doubled and the NC rates quadrupled in comparison with the dissolved-salt method. For the basic dissolved- salt method, the rate of neutron capture is obtained by subtraction of data without salt, after correcting for capture in deuterium. Recently, however, it has been suggested that gamma and electron events can be distinguished to some degree on-line. The secular variation in the NC rate due to the Earth's orbital eccentricity would become observable at the 95% confidence level. Time variations in the neutrino flux could be followed simultaneously in the NC and CC channels on the time scale of milliseconds to years. All signals and backgrounds can be determined at the same time, and there is no need to compare and subtract data taken at different times and under different conditions. The method is fully compatible with the dissolved-salt approach, allowing, by two different techniques, a valuable systematic check of important physics results. The duty factor for full-efficiency NC detection rises from 50 to 100%, and the possibility of missing NC data from a supernova or other interesting event is correspondingly diminished. Even if salt is present in the water, the ( conversion of NC events to Cerenkov light makes inferences about which events are CC and which NC indirect. Event-by-event NC detection offers the prospect of determining a ν µ or ν τ mass (especially in the cosmologically interesting range of 20-100 eV) if a supernova should occur. We are collaborating with scientists at Los Alamos National Laboratory and Lawrence Berkeley Laboratory to design, fabricate, and deploy this array of 3 He proportional counters. The key issue in building such an array is to select materials that will minimize the amount of radioactive U and Th chain elements. A special chemical vapor deposition (CVD) process to produce ultra-pure nickel has been found that meets these requirements. The research and design phase is nearly complete and preparations are underway for full scale construction of the array. 12


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    2.5 SNO neutral-current detector position readout J.V. Germani, S. Konsek,* R.G.H. Robertson, A. Myers and T. Van Wechel Development is underway to permit modest, ~1 meter resolution, position readout of the 3 He filled proportional counter detector strings that will be used as neutral-current detectors in the SNO experiment. Position readout along the length of the proportional counters is desirable for several reasons. The identification of backgrounds internal to the counters, as well as localized concentrations of Th or U in either the counters or the acrylic vessel, will be easier to identify with position information. Furthermore, the ability to construct spherical radial distributions of neutron events will improve our ability to identify, and correct for, backgrounds from Th and U uniformly distributed in the acrylic vessel. Single-ended readout of the detectors is highly desirable in order to minimize the amount of material in the heavy water and to keep the mechanical design and string deployment as simple as possible. The method presently under development relies on leaving the remote end of the counter unterminated. The pulse that propagates toward the remote end is reflected back with the same sign and adds with the pulse that heads directly to the preamp. The maximum delay time between the leading edge of the direct and reflected pulses is approximately 100 ns for an 11 m string of counters. A delay line of approximately 25 ns will be used at the remote end to define the minimum time delay for pulses originating close to that end. Thus, the time delays will range from 50 to 150 ns. Since the widths of current pulses range from 20 ns to 4 ms, the position signal in the combined pulse shape will vary from a double-peaked shape to a kink in the leading edge. We have set up a facility at NPL for studying position readout techniques and pulse propagation in the detector strings. A two-meter proportional counter is used to approximate one segment of a neutral-current string. This counter is coupled to six meters of transmission line, which simulates other counters in the string. The low-noise current preamplifier is a custom Robertson design that is built by the laboratory's electronics staff. Pulse shapes are digitized with a digital oscilloscope with sample rates as high as 1 GS/s, and are then analyzed off-line for pulse reflections. In addition, the affect of log amplification is being studied, as a means of begin able to cope with the large dynamic range required to digitize current pulses. Initial results show that position resolution of 1 meter is possible. This is consistent with the requirements of the neutral-current array, since the neutrons liberated in neutral-current interactions travel one meter on average before being absorbed by a proportional counter. Further measurements are planned with the present apparatus. Several digital signal processing techniques are being studied for the extraction of position information, and improvements in preamplifiers and log amplifiers are ongoing. *Department of Physics, University of Washington, Seattle, WA. 13


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    2.6 SNO DAQ: Development of a 100 channel noise test system Q.R. Ahmad, J. Beck, S.R. Elliott and J.F. Wilkerson The SNO data acquisition (DAQ) system will acquire data on an event-by-event basis from the 9500 PMTs that comprise the SNO detector. An integral part of this system is the Front End Card (FEC). It will provide dead timeless sub-nanosecond time and charge (Q&T) measurement for photomultiplier pulses in the range of 1 - 1000 photoelectrons. The FEC will handle 32 PMT signals per 9U x 280 or 340 mm card. The electronics is implemented using three full custom integrated circuits as well as commercial ADCs, memory and logic. The DAQ interface is VME compatible. In close collaboration with the University of Pennsylvania SNO group we are finishing up tests of the prototype FECs (pFECs) which consist of 8 instead of the final 32 channels per board. Results from these tests are providing valuable information as we finalize the design of the Front End Cards. The mass production of custom chips and boards will start before the end of 1995. We are developing a 100 channel PMT noise test data acquisition system using the pFEC boards and a single VME crate that will be used to perform noise test of the PMTs and cables as they are installed in the upper hemisphere of our underground detector in Sudbury. We are developing the software to perform this task under the Object Oriented Progamming (OOP) environment of Macintosh's Think C compiler using a realtime data acquisition OOP code developed by McGirt and Wilkerson at Los Alamos. This task can serve as a quick and simple DAQ/Electronics diagnostic. The task will first initialize all channels and then calculate the noise rate as the voltage is ramped to the PMT's operating voltage. During the course of the test it will provide the user with a real-time histogram of noise rate per channel. Drastic deviation from the norm would be indicative of tube or cable problems. The noise data will also be written to disk so that one can later look at the time history of the noise for each individual PMT. Furthermore, we are developing the necessary software to look at the Charge and Time (Q&T) information for each channel which we can compare to the acceptance testing Q&T database provided by Queen's University. 14


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    2.7 The SNO PMT test facility Q.R. Ahmad, J. Beck, A. Goldberg, K.P. Issacson, J.Knox, A.W.P. Poon, W.G. Weitkamp and J.F. Wilkerson The SNO Data Acquisition group has been developing a PMT test facility that will utilize 96-120 SNO PMTs (including reflectors) to perform the following series of tests (in order of priority): Test and debug the SNO electronics and data acquisition (DAQ) systems with a reasonable number of PMTs. Develop and test calibration sources. Determine calibration constants, attenuation coefficients, and possibly even total efficiency of PMTs and reflectors. The initial tests of the electronics and DAQ systems can be done in air, but a series of more sophisticated tests to check out calibration constants and more realistically simulate the SNO detector must be done in water. The second and third items also require that the PMTs be immersed in water. The original plan for a PMT test facility was to share space in the neutral current detector test pool at Los Alamos. However, when the SNO data acquisition development effort moved to the University of Washington, it quickly became clear that the water shield room of the old 60-inch cyclotron could be made into a suitable test facility. The room is 14 feet wide by 20 feet long and can be filled to a depth of 14 feet. It was originally designed to shield the cyclotron scattering chamber from neutrons coming directly from the cyclotron. The room is accessible through a 20 in high x 28 in wide port located 56 in above the floor. The room has several advantages as a test facility: it is not used now (the cyclotron has been turned off since 1986), it has held water in the past, it is easier to install detectors inside provided they fit through the port, and it can be overhauled to make a suitable test volume with relatively modest amount of work. The adjacent experimental cave also offers a reasonable laboratory space to set up the SNO electronics and data acquisition systems. The first question was whether gamma ray background levels might not be too high for the tests in question. A 5" diameter x 4" NaI detector was set up to measure the radioactive background inside the water room. It was found that the backgrounds in the water room are similar to the ambient backgrounds in the adjacent laboratory and are dominated by low energy 40 K decays. At this point the dominant background to the test array is expected to be from cosmic rays. The next problem to be addressed was conversion of the water room into a suitable SNO PMT test facility. An aluminum duct which traversed the room was removed. Access holes near the ceiling for cables running to the test detectors have been cut and plugs for all the ports in the walls have been fabricated. A contractor will complete resealing the room in the near future. We have been working with collaborators at Lawrence Berkeley Laboratory to design the optimized PMT support structure that will fit within the room. The current design has a spherical geometry and can accommodate from 96-120 PMTs. Installation of the support structure should commence after initial wet tests of the water room are completed. 15


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    2.8 Acrylic vessel activities 1994-1995 P. Doe The principal activity on the acrylic vessel over the past year has been the fabrication of the individual components from which the vessel is constructed. The spherical shell of the acrylic vessel is constructed by bonding together 120 panels in the underground laboratory. These panels are first thermoformed from flat sheet into spherical shapes, machined to exact dimensions and then annealed to relieve machining stress. Careful monitoring is required throughout this fabrication process to ensure that the components are radioactively and mechanically acceptable. 80% of the panels required to form the sphere have been completed. To ensure that the machined panels were of the correct dimensions, the whole upper hemisphere of the vessel was dry assembled at the fabricator's facility in Colorado. This also allowed testing of the final assembly jigs, fixtures and survey technique and refining of assembly procedures. After dry assembly the individual panels are being cleaned, packed and shipped to the SNO site. The five cylindrical castings that comprise the vessel chimney have also been fabricated, machined and shipped to the SNO site. A second "qualification wall", measuring 20' long by 15' high and consisting of 8 spherically formed panels has been successfully completed by the fabricator, thus completing the demonstration of the bonding techniques required in the vessel assembly and for the attachment anchor points for the neutral current detectors. The acquisition of the 10 Kevlar@ suspension ropes has begun following successful completion of the selection, R&D and radioassay program established and overseen by LANL. The coming year will be a challenging one. Installation of the vessel chimney begins in early June '95 followed by the upper hemisphere in August '95. The suspension ropes will be required in October '95. Once the upper hemisphere is suspended, the lower hemisphere along with the neutral current detector anchors will be installed. The vessel is expected to be complete in May 1996. Ways of improving the schedule are being investigated. 16


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    2.9 The Russian-American Gallium Experiment (SAGE) S.R. Elliott and J.F. Wilkerson The Russian-American Gallium Experiment (SAGE) is a radiochemical solar neutrino flux measurement based on the inverse beta decay reaction 71 Ga(υ e , e − ) 71 Ge . The threshold for this reaction is 233 keV which permits sensitivity to the p-p neutrinos which dominate the solar neutrino flux. The target for the reaction is in the form of 55 tonnes of liquid gallium metal stored deep underground at the Baksan Neutrino Observatory in the Caucasus Mountains in Russia. About once a month, the neutrino-produced Ge is extracted from the Ga. 71 Ge is unstable with respect to electron capture (t1/2 = 11.43 days) and, therefore, the amount of extracted Ge can be determined from its activity as measured in small proportional counters. The experiment has measured the solar neutrino flux in 31 extractions between January 1990 and October 1993 with the result; 69±10 (statistical) +5/-7 (systematic) SNU. Additional extractions through the end of 1994 are awaiting analysis. The collaboration is currently using a 500-kCi 51 Cr neutrino source to test the experimental operation. 7 The energy of these neutrinos is similar to the solar Be neutrinos and the source thus makes an ideal check on the experimental procedure. The extractions for the Cr experiment took place in January and February of 1995 and the counting of the samples will continue through the summer. The University of Washington plays a major role in the statistical analysis of the data and in the determination of systematic uncertainties. We are very active in the planning and analysis of the Cr experiment data. 17


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    3.0 NUCLEUS-NUCLEUS REACTIONS 3.1 Distributions of fusion barriers extracted from cross section measurements on the systems 40 192 Ca + Os, 194 Pt J.D. Bierman, P. Chan, J.F. Liang, M.P. Kelly, A.A. Sonzogni and R. Vandenbosch As described in last year's report,1 the goal of this project is to experimentally determine the fusion cross sections of the two systems to high precision spanning the entire barrier region. These data may then be twice differentiated to yield the distribution of barriers. These distributions should differ as a result of the difference in permanent quadrupole deformations of the targets. This will be a valuable test of the sensitivity of the barrier distribution or "fingerprint" method of determining the relative importance of different coupling channels. We have measured fission fragment angular distributions at energies both well above and within the barrier region. The differences in these distributions were very small. More importantly, the center of mass angle where the angular distribution intersects a properly normalized isotropic distribution is constant over the entire energy region. As a result, rather than measure the entire distribution at each energy, we simply measure the differential cross section at this angle for each energy and then convert this measurement to a total fusion cross section as if the distribution were isotropic. This allows us to acquire data of sufficiently high statistics spanning the entire barrier region in fairly small energy steps in a reasonable amount of beam time. To this point, fusion cross sections have been measured for the 40 Ca + 192 Os system in roughly 1.25 MeV energy steps covering the entire barrier region. The measured cross sections range from 400 mb down to 0.1 mb. Measurements were also made at energies well above the barrier for this system. Analysis of these data is nearly complete. The product of energy and cross section has been differentiated to extract the distribution of barriers for the osmium system which is shown in Fig. 3.1-1. This "fingerprint" is similar to that expected for a spherical projectile and a target with a prolate quadrupole and small negative hexadecapole deformation. Data have also recently been taken for the 40 Ca + 194 Pt system in roughly 1.25 MeV energy steps spanning its barrier region. The analysis of these data is currently being performed. Fig. 3.1-1. Fusion barrier distribution extracted from experimentally measured cross sections for 40 Ca + 192 Os . 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 10. 18


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    40 46,48,50 3.2 Sub-barrier fusion for Ca + Ti J.D. Bierman, P. Chan, J.F. Liang, A.A. Sonzogni and R. Vandenbosch Several mechanisms have been used to explain the observed enhancement in the sub-barrier fusion cross section, among them nuclear deformation, coupling to inelastic channels and transfer reactions. The 46, 48,50 Ti isotopes exhibit a decrease in β 2 with increasing mass number and simultaneously the neutron transfer Q-values become more positive, which implies that collective motion should counteract transfer reactions as responsible for the enhancement. A measurement of the enhancement for the three isotopes should indicate which process is stronger. The projectile was the doubly magic 40 Ca to minimize projectile structure effects. Details of the experiment can be found in a previous report.1 Briefly, evaporation residue angular distributions were taken at a selected set of energies and integrated over angles. From these complete angular distributions we calibrated the relation between the differential cross section at 5 degrees and the integrated one. For a far larger set of energies, the differential cross section at 5 degrees was measured and the integrated one was then obtained. Results for 46 Ti are presented in Fig. 3.2-1. The full line is from a coupled channel calculation, which includes inelastic and transfer couplings. For the inelastic part we considered the 2+ and 3+ states for both projectile and target with strengths taken from the literature. The pickup of 1 and 2 neutrons was considered for the transfer part and their coupling strengths were adjusted to match the data. The dashed line is from an uncoupled calculation. A preliminary analysis suggests that for these systems, neutron transfer reactions are playing the most important role in the sub-barrier enhancement of the fusion cross section. A future experiment is being planned, where we will extend and improve the data already taken. Fig. 3.2-1. Fusion excitation function for 40 Ca + 46 Ti . 1Nuclear Physics Annual Report, University of Washington (1994) p. 11. 19


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    3.3 Properties of light charged particles produced by 25 A MeV 16O on 159 65Tb, 181 73 Ta, 197 Au and 79 14N nat 181 35 A MeV on 62 Sm, Ta 72 D. Bowman,* G. Cren,* R. DeSouza,† J. Dinius,* A. Elmaanni,‡ D. Fox,† C. K. Gelbke,* W. Hsi,* C. Hyde-Wright,§ W. Jiang, W. G. Lynch,* T. Moore,† G. Peaslee,* D. Prindle, C. Schwarz,* A. A. Sonzogni, M. B. Tsang,* R. Vandenbosch and C. Williams* In last year's Annual Report we reported on our analysis1 of an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University using the miniball array.2 In this experiment we measured light charged particles (LCPs) in coincidence with fission fragments (FF tag) or evaporation residues (ER tag). The ER tagged events were from more central collisions than the FF tagged events and by varying the target mass we change the average impact parameter over a large range while changing the total fusion cross section only slightly.3 In last years report we stated that we observed a substantial variation in the LCP multiplicity depending on the tag. For every target the observed LCP multiplicity is higher for the ER tag than it is for the FF tag. We show the characteristics of these "extra" LCPs by plotting in Fig. 3.3-1 the number of observed protons as a function of energy for 10,000 ER and 10,000 FF tagged events from the 25 A-MeV 16 O on 181 73Ta . At high energies the spectra are essentially identical. For lower, evaporative energies, there is a clear excess of protons. This excess is clearly observed for protons and alphas from all targets and beam energies used in this experiment with the exception of the 35 A-MeV 14 N beam on the nat 62 Sm target which has a very low ER cross section. The excess is also observed but not as pronounced for deuterium and tritium emission. It seems clear that the fission process must occur before the compound nucleus reaches the end of the particle emission de-excitation chain. We should be able to use the number of LCPs emitted after the time at which fission occurs to estimate how quickly fission must occur. We plan to pursue this through the use of statistical model calculations. 3 10 350 Protons/MeV ER-FF Protons/MeV 300 250 2 10 200 150 100 10 50 0 1 -50 20 40 60 20 40 60 E (MeV) E (MeV) Fig. 3.3-1. Number of protons as a function of energy for 25 A-MeV 16 O on 181 73Ta data. The solid line is ER tagged data and the dashed line is FF tagged data. In the panel on the right we show the difference of the histograms. *National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI. †Indiana University Cyclotron Facility, Indiana University, Bloomington IN. ‡Present Address: Battelle Memorial Institute, Columbus, OH. §Department of Physics, Old Dominion University, Norfolk VA. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 14. 2R. DeSouza et al., Nucl. Instrum. Methods A, 295, 109 (1990). 3D. Prindle et al., Phys. Rev. C 48, 192 (1993). 20


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    3.4 Entrance channel dependence of light particle emissions in the 156 Er compound nucleus decay J.D. Bierman, P. Chan, M.P. Kelly, J.F. Liang, A.A. Sonzogni and R. Vandenbosch The decay of a compound nucleus can be described by a statistical theory where the decay processes are assumed to be independent of the formation channels.1 Measurements on the decay of the 156 Er compound nucleus populated by 12 C + 144 Sm and 64 Ni + 92 Zr entrance channels gave mixed results as compared to the statistical model.2 The 12 C + 144 Sm data were in good agreement with statistical model calculations. However, the statistical model overestimated the neutron multiplicity for the 64 92 Ni + Zr reaction over a wide range of excitation energies.3 A program of studying the light-charged particles emitted from the 156 Er compound nucleus was initiated recently. The 156 Er compound nucleus was populated by 12 C + 144 Sm and 60 Ni + 96 Zr reactions at an excitation energy of 113 MeV where the fission barrier falls below the neutron binding energy and the maximum angular momentum, in the spin distribution, leading to evaporation residues is reached. The experiment was carried out in the Nuclear Physics Lab of the University of Washington. The angular distributions of light-charged particles were measured in coincidence with the evaporation residues by CsI scintillators coupled to PIN diodes. The evaporation residues were separated from the incident beam by a pair of electrostatic deflector plates and identified by energy vs. time-of-flight using the linac rf signal as stop. The evaporated proton and α particle energy spectra are shown in Fig. 3.4-1 for the two reactions studied. The spectral shape of the C+Sm system is harder than that of the Ni+Zr system. It suggests that the protons and α particles were emitted from a hotter source for the mass asymmetric system. The result of statistical model calculations using the code PACE4 are shown by solid curves. The level density parameter used in the calculations was A/10 for both systems. Good agreement between the data and the calculations can be seen for the 12 C induced reaction. For the Ni+Zr reaction, the calculation predicts a less steep slope than the measurement. More analyses comparing the entrance channel dependence of the 156 Er compound nucleus decay is underway. Fig. 3.4-1. Energy spectra of protons and α particles emitted from the decay of the 156 Er compound nucleus. 1N. Bohr, Nature (London) 137, 344 (1936). 2B. Fornal et al., Phys. Rev. C 42, 1472 (1990). 3R.V.F. Janssens et al., Phys. Lett. B 181, 16 (1986). 4A. Gavron, Phys. Rev. C 20, 230 (1980). 21


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    3.5 Bremsstrahlung and the GDR in 16,18 O + 92,100 Mo reactions J.D. Bierman, M.P. Kelly, J.F. Liang and K.A. Snover Recent measurements on C+Mo1 and C+Sn2 systems in the region 5-11 MeV/u bombarding energy have suggested a large target isotope effect on the production of high energy γ −rays (E γ ≥ 30 MeV). These γ −rays were presumed to be bremsstrahlung primarily from first chance neutron-proton collisions. Another measurement,3 on the C+Sn system at 10 MeV/u bombarding energy finds, in conflict with the earlier measurements, no evidence for an enhanced bremsstrahlung yield when using the heavier target isotope. More recently, the Stony Brook group has presented results backing up earlier findings on the same system together with an explanation of the isotopic effect.4 In this report, we present progress in the search for a similar effect in O+Mo systems. An important part of our procedure is the use of angular distributions to better define the relative contributions of bremsstrahlung and statistical decay. We have measured the γ −rays produced in the four possible reactions of 16,18 O + 92,100 Mo using the Seattle 10" x 15" NaI spectrometer. The incident energy for both 16,18 O projectiles is 9.4 MeV/u. Gamma ray yields were measured at five angles between 40° and 140° in the lab frame relative to the beam axis. These γ −rays cross sections were transformed to the compound nucleus center-of-mass and fit to a linear combination of the first three Legendre polynomials. The coefficient of the first polynomial, A 0 ( E γ ) , is a measure of the γ −ray cross section, σ ( Eγ ) . The second coefficient, a1( E γ ) , is a measure of the forward- backward anisotropy relative to the compound nucleus center-of-mass. 18 92 Fig. 3.5-1. shows the extracted A 0 ( E γ ) and a1( E γ ) for the inclusive O + Mo data. In the left panel, a bremsstrahlung component (solid line) has been determined by manual iteration of the strength and slope parameters. The dashed line in the right panel shows the a1 that results from this bremsstrahlung component alone. The bremsstrahlung component will always have a positive a1 (for A tgt > A proj ) since the nucleon-nucleon center-of-mass moves at half the beam velocity while the compound nucleus center-of- mass moves more slowly. In the region where GDR emission competes significantly with bremsstrahlung (Eγ ≤ 30 MeV ) the A 0 bremsstrahlung component of course falls below the data points. Since GDR emission has no forward-backward anisotropy in the compound nucleus center-of-mass, its effect is to dilute the a1 . Hence the data points in the right panel fall below the dashed curve. The bremsstrahlung a1 diluted by the contribution of statistical γ −rays (solid line, right panel) follows the data reasonably well. Observed nonzero values of a1 below approximately 16 MeV are due to some other reaction mechanism. The use of a multiplicity gate on the gamma spectrum has been shown to greatly reduce the nonzero a1 at low E γ , but has little effect on the region E γ ≥ 20 MeV other than to worsen the statistical errors.5 Efforts are underway using the CASCADE statistical model code to do a simultaneous fit of statistical and bremsstrahlung components to both the A 0 ( E γ ) and the a1( E γ ) spectra. 1C.A. Gossett et al., Phys. Rev. C 42, R1800 (1990). 2Vojtech et al., Phys. Rev. C 40, R2441 (1989). 3R. Pfaff et al., Z. Phys. A 347, 67-70 (1993). 4N. Gan et al., Phys. Rev. C 49, 298-303 (1994). 5Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 8. 22


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    Fig. 3.5-1. The points are the inclusive data for 18 O + 92 Mo at 9.4 MeV/u bombarding energy and the curves are the calculated bremsstrahlung component. The dashed line in the right panel is the a1 that results from the bremsstrahlung component alone, as shown by the solid line in the left panel. The solid line in the right panel is the bremsstrahlung a1 diluted by the statistical γ −rays contribution. 22


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    3.6 APEX update T.A. Trainor and the APEX collaboration During the past year the full APEX detector system has accumulated several million positrons resulting from several-pnA beams of ≈6 MeV/u uranium projectiles incident on several targets. Under no conditions have narrow peaks in positron-electron coincidence or positron singles energy spectra been observed (except for the 206Pb results described below). This overall result now constitutes a major disagreement with the results of several collaborations at the GSI in Darmstadt, Germany. APEX was designed to investigate in a kinematically complete way the production mechanism for reported anomalous electron pairs in several very-heavy-ion collision systems. The initial null results from APEX reported last year prompted an extensive study by Monte Carlo techniques of the detailed acceptance of APEX, especially as compared to the GSI experiments. This study has revealed no fundamental reason for a failure to observe anomalous pairs from an acceptance standpoint. More recently two collision systems have been used both to calibrate the APEX ability to detect such pairs and to examine the 'best case' GSI result. For the former the collision system 206Pb- 206Pb was used to produce pairs from internal conversion of a 3- → 2+ transition. These pairs have been seen as a narrow peak in the coincidence spectrum. The detailed peak shape is now being compared with Monte Carlo studies to further elucidate the APEX acceptance. In an attempt to examine the best-case GSI result (narrow peaks in positron singles spectra where the question of acceptance is much simpler) data were acquired for the uranium-thorium system. A total of 37,000 positrons was detected, 200 times that for the original GSI (EPOS) result and 20 times that for a more recent EPOS result. After a preliminary analysis no peak structures have been observed outside statistical fluctuations. In light of these results the reality of the electron-pair phenomenon as reported by GSI is called into question. Further analysis of the APEX data and data acquisition are planned for the coming year. 24


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    4.0 FUNDAMENTAL SYMMETRIES AND WEAK INTERACTIONS 4.1 CVC and SCC in the mass-8 isotriplet L. De Braeckeleer, E.G. Adelberger, K.A. Snover, D.W. Storm and D. Wright The conserved vector current (CVC) hypothesis predicts the existence of a small weak magnetism correction to the β -decays of 8Li and 8B related to the isovector M1 and E2 decay strengths of the isospin analogue state in 8Be. By measuring the difference in α − β angular correlations in the decays of these two nuclei,1 we may extract the quantity b/AC + dII/Ac, where b is determined by the M1 decay amplitude, c the GT decay amplitude, the first term is the CVC-predicted weak magnetism correction, and the second term represents second class current (SCC) contributions. In previous experiments in this laboratory,2 we have measured the GT strength of 8Li and 8B decay, as well as the isovector M1 strength of the decay of the isospin analogue doublet in 8Be, to excited 8Be. We have combined these measurements to produce a CVC prediction for the weak magnetism term, and compared this prediction to the α − β correlation data of both Tribble and Garvey3 and McKeown, Garvey, and Gagliardi.4 The CVC prediction and the two data sets are shown in Fig. 4.1-1. If we assume CVC to be correct, we may allow for the possibility of an SCC contribution by fitting the difference between our prediction and the data. We obtain dII/Ac=0.0+0.3+0.3 from Tribble and Garvey's data, and dII/Ac=-0.5+0.2+0.3 from the data of McKeown, Garvey, and Gagliardi. The first error bar quoted represents the uncertainty in the α − β correlation data, the second the uncertainty of our CVC prediction. If, on the other hand, we assume the absence of SCC, we may test the validity of CVC by comparing our prediction directly to the data. If we call the multiplicative difference between our prediction and the data κ , we obtain κ =1.00+0.04+0.05 for Tribble and Garvey's data, and κ = 0.93 +0.03+0.05 for the data for McKeown, Garvey, and Gagliardi. We thus conclude that present experimental data are consistent with both CVC and the absence of SCC.5 Uncertainties in the measured isovector gamma decay strength give a significant contribution to the uncertainty in the CVC prediction of the weak magnetism term. We are therefore planning improved measurements of both the total strength and its distribution to various excitation energies of 8Be. To this end, we have carried out a careful investigation of the sources of background in the 4He(α , γ ) experiment used to measure the gamma decay strength. Although difficulties in obtaining a correct normalization make it unattractive for a measurement of the total strength, the use of a long (~40cm) gas cell target which allows the center of the cell to be viewed by a detector while its Kapton windows are shielded by lead, eliminates an important source of γ -ray background and is therefore preferable for a measurement of the decay strength distribution. The remaining background may be attributed to γ -rays and neutrons produced when beam particles scattered by the windows or the target gas interact with the walls of the cell. By subtracting the γ -ray spectrum observed with the cell empty from the spectrum observed with a He-filled cell, we can compensate for the background produced by beam scattered from the cell windows, but not for background produced by beam scattered from the target gas. Crude estimates of the magnitudes of each of these backgrounds are consistent with experimentally observed spectra before and after subtraction. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 19. 2Nuclear Physics Laboratory Annual Report, University of Washington (1994) pp. 20-21. 3R.E. Tribble and G.T. Garvey, Phys. Rev. C 12, 967 (1975). 4R.D. McKeown, G.T. Garvey and C.A. Gagliardi, Phys. Rev. C 22, 738 (1980). 5L. De Braeckeleer et al., Phys Rev C, in press. 25


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    The remaining background after subtraction is unfortunately still comparable to the signal for γ -ray energies below ~8 MeV. To reduce this background, we have designed a new gas cell with a geometry chosen so that no α particle singly scattered by the target gas hits the cell wall in a region visible to our detector with an energy of more than ~20 MeV. We plan to line these portions of the gas cell wall with Tantalum metal, whose Coulomb barrier will greatly inhibit background-producing interactions. The new cell will be used in a precision measurement of the gamma decay strength distribution. Fig. 4.1-1. The panels compare the +1σ CVC prediction (assuming no SCC), shown as a solid curve, to the data points of Tribble and Garvey (upper panel) and McKeown, Garvey, and Gagliardi (lower panel). 26


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    4.2 Measurement of the β − α angular correlation in mass-8 E.G. Adelberger, J.F. Amsbaugh, P. Chan, L. De Braeckeleer, P.V. Magnus, D.M. Markoff, D.W. Storm, H.E. Swanson, K.B. Swartz, D. Wright and Z. Zhao Among the measurements needed to test CVC and/or search for second class current in the Mass-8 isotriplet, the measurements of the β − α angular correlations in 8 Li and 8 B are the most difficult to achieve with high accuracy. Both the weak magnetism and the weak electricity induced currents are proportional to the momentum transferred in the decay and therefore contribute to the observables at the percent level. The accuracy of these measurements determines the ultimate sensitivity of the symmetry tests. In particular, the β energy dependence of the a 2 term in the β − α angular correlation is presently quite uncertain. The two previous experiments1,2 support a large quadratic term, a fact not compatible with CVC and our recent measurement of the E2/M1 ratio, (see Section 4.1). Our apparatus has been designed with particular attention to the response function of the beta counters (delta E, active veto and stabilization). The data accumulated in August (1 week) and November 93 (3 weeks) have been analyzed. The β − α angular correlation measured in 8 Li is fitted by 1 + a1 cos(θ ) + a 2 cos 2 (θ ); the a1 and a 2 coefficients are shown in Figs. 4.2-1 and 4.2-2. The kinematical term a1 shows a little deviation from the expected value at high energy. We are currently investigating the origin of this systematic effect. The a 2 coefficient does not have a significant quadratic energy dependence. Two additional β counters have been built and set up at 0 and 180 degrees. This has required a modification of both the alpha counters and the hardware/software of the acquisition and stabilization system. We are now in the process of testing this new apparatus. We will also spend a couple of weeks adding to the existing statistics of the 8 Li experiment. Finally, the 8 B run is awaiting the completion of the high intensity 3 He terminal ion source. Fig. 4.2-1. The kinematical coefficient a1 measured Fig. 4.2-2. The a 2 coefficient measured in the 8 Li in the 8 Li decay. decay. 1Tribble et al., Phys. Rev. C 12, 967 (1975). 2McKeown et al., Phys. Rev. C 22, 738 (1980). 27


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    4.3 Completion of an apparatus to measure the PNC spin rotation of cold neutrons in a liquid helium target E.G. Adelberger, B.R. Heckel, D.M. Markoff, S.D. Penn and H.E. Swanson The apparatus to measure the parity non-conserving (PNC) spin-rotation of transversely polarized neutrons through a liquid helium target is nearly complete. This apparatus is designed to measure a neutron spin-rotation predicted to lie between 0 and 5 × 10 −r7 radians1,2 in 46 cm of helium with an error of 5 × 10 −8 radians. This observable, in conjunction with A L ( p + α ),3 will determine the PNC isovector pion-exchange amplitude of the NN interaction, Fπ . This amplitude is sensitive to the neutral weak current contribution. Current experimental results provide upper limits for Fπ that are smaller, by a factor of three, than the early theoretical calculations.4 Recent QCD sum-rule calculations5,6 predict a smaller value for Fπ that is consistent with existing data, and an expected rotation of 2 × 10 −7 radians in our targets. Our experiment is motivated by the need for a more precise measurement of Fπ . The two coaxial µ-metal shields have been constructed and tested. With the cryostat in place, we measured axial magnetic fields of 15-20 µ G in the center region, rising to approximately 40 µ G on the ends. This is comfortably below the maximum tolerable field of 100 µ G, set by the requirement that spin-rotations arising from diamagnetic effects of helium will be within our desired errors. The cryostat insert housing the liquid helium targets, the π-coil, and the pump and valve system that fill and empty the target chambers has been built. The feedthrough system which will drive the pump and valve will be completed shortly. The neutron detector, a segmented 3 He ionization chamber, is currently under construction. It is designed so that neutrons of different velocity ranges can be detected separately. The PNC effect is velocity independent, while spin-rotations from magnetic fields are velocity dependent. Velocity separated detection will allow us to monitor the integrated magnetic fields along the neutron path. In addition, the detector signal plates are divided into four quadrants that will allow us to monitor false signals from geometric asymmetries. The central π-coil has been constructed, wound, and assembled. Preliminary tests show the leakage fields to be on the order of 10 −4 G near the coil, falling off to 10 −5 G 5 cm away. The qualitative field shape was consistent with the sum of the coil symmetry (comparing favorably with computer predictions) and the winding asymmetry. The input and output coil forms have been constructed and are currently being wound with 1 mm wire. With µ-metal pieces connecting the main coil with the return coils, we expect field homogeneities of a part in 10 4 . The computer program to control the experiment and the data acquisition is currently being written and tested. Our scheduled beam time at the NIST (National Institute of Standards and Technology) reactor facility in Maryland will begin approximately two months after reactor start-up following the scheduled down-time for facility improvements. We expect to take data in the fall of 1995. 1Y. Avishai, Phys. Lett. 112B, 311 (1982). 2V.F. Dmitriev et al., Phys. Lett. 125, 1 (1983). 3J. Lang et al., Phys. Rev. C 34, 1545 (1986). 4E.G. Adelberger and W.C. Haxton, Ann. Rev. Nucl. Part. Sci. 35, 501 (1985). 5E.M. Henley, private communications. 6G. Feldman et al., Phys. Rev. C 43, 863 (1991). 28


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    4.4 Measurement of Newton's constant G E.G. Adelberger, J.H. Gundlach, B.R. Heckel and H.E. Swanson Newton's constant G is one of the most fundamental yet least precisely known constants of nature (G = (6.6726 ± 0.0008) × 10 −11m 3kg −1s −2 ) 1 and recently even this value has also been brought into question. The PTB (the German Bureau of Standards) obtained a value 0.6% (~40 standard deviations!) higher2 than the accepted value, a group from New Zealand reported a value which is 0.1% lower (~7 standard deviations)3 while another German group4 measured a value that is in accordance with the CODATA value. In addition a Russian group claims to observe a temporal variation of G at the 0.7% level.5 We are pursuing a novel and elegant technique that we believe may ultimately make a determination of −5 G at the 10 -level possible. We plan to use the new rotating torsion balance (described in Section 4.5) currently being developed for testing the universality of free fall. We will use the l,m = 2,2 gravitational coupling from two masses (30- 70kg) on opposite sides of the pendulum to induce a torque on the torsion pendulum. The turntable rotation rate will be servoed so that the pendulum does not move with respect to the turntable i.e. so that the torsion fiber never twists. This essentially transfers the angular acceleration of the pendulum to the turntable. The angular acceleration will be measured using a high-quality shaft encoder attached to the turntable. We plan to use a flat vertical pendulum; the expression for the q 22 -moment of a two-dimensional pendulum contains the same integral over the mass distribution as does the moment of inertia. This largely avoids the important experimental problem of knowing the dimensions and density uniformities of the small pendulum. To eliminate spurious torques caused by masses in the laboratory we will rotate the attractor masses on a second turntable at a different and possibly opposite rotation rate. We made computer simulations to study this feedback scheme. From these we expect that a 10 −5 acceleration measurement would ultimately be possible. We already implemented the rotation feedback in our existing rotating balance and were able to verify the computer simulations. The dominant noise source we encountered was due to gravitational effects from pedestrian and vehicular traffic in the vicinity of the apparatus. The site of the new rotating balance is in a relatively isolated spot on the campus and should reduce this problem. Furthermore the rotation rate of the attractor masses can be chosen to give a fairly high signal frequency which will reduce gravitational 1/f-noise. 1CODATA (Committee on Data for Science and Technology) 1986, based on "1986 Adjustments of the Fundamental Physical Constants" By E.R. Cohen and B.N. Taylor, Rev. Mod. Phys. 50, 1121 (1987). 2W. Michaelis et al., presented at the Conference on Precision Electromagnetic Measurement, Boulder, CO, 27 June - 1 July, 1994. 3M. Fitzgerald and T. Armstrong, presented at the Conference on Precision Electromagnetic Measurement, Boulder, CO, 27 June - 1 July, 1994. 4H. Meyer, presented at the Seventh Marcel Grossmann Meeting on General Relativity, Stanford, July, 1994. 5V.P. Izmailov et al., Measurement Techniques 36, no. 10 (1993). 29


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    4.5 Construction of a new rotating torsion balance instrument E.G. Adelberger, J.H. Gundlach, B.R. Heckel, S. Penn, Y. Su and H.E. Swanson We are preparing a new, more sensitive rotating torsion balance apparatus to search for violations of the Equivalence Principle over length scales ranging from 1 m to infinity. The dominant limitations of our previous results arose from: • Brownian motion of the torsion pendulum due to gas damping in the residual vacuum of ≈0.1 Torr. • coherent imperfections in the turntable such as fluctuations in the rotation rate or vertical "rumble''. • residual gravity-gradient couplings. • daily variations in the tilt of the laboratory floor that required corrections to the data. • vertical seismic motion was suspected (but never proved) to contribute to our fluctuating errors. Our new instrument is designed to minimize the first four of these problems. 7 • an ion-pump will be used to evacuate the chamber to < 10 − torr. • the torsion balance will be rotated on a high-quality air-bearing turntable on which a state- of-the-art angle encoder and an eddy current motor are mounted directly. The turntable system is being manufactured by a commercial firm, and is scheduled for delivery in late spring. • the tilt-sensitive parts of the apparatus will hang from a 2-axis gimbal constructed using flexures to avoid sticking and hysterisis. • the pendulum will be more symmetric; it will have 8 testbodies that mate reproduceably in conical seats. • the gravity gradient compensation will be improved by placing the compensators farther from the balance. We will develop the apparatus in two phases. First we will upgrade our existing Eöt-Wash balance for high vacuum operation, and hang it from a gimbal mechanism attached to the new turntable. This will allow us to thoroughly test the turntable and identify the necessary improvements for the second phase. We have built a massive concrete platform 5 m above the floor of the old cyclotron cave on which the turntable will be mounted and below which the apparatus will hang. The pendulum will be 2.5m above the floor to reduce the ambient Q 21 gravitational gradient. This initial setup should yield a factor of 5 more precise test of the Equivalence Principle. The second phase will have a gimbaled structure inside the vacuum vessel to support the tilt-sensitive components and a much longer torsion fiber. We will explore using a liquid-nitrogen cooled jacket to reduce thermal noise and fiber relaxation noise. With this second phase we hope to probe the Equivalence Principle with a substantially improved sensitivity. 30


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    4.6 Progress with the rotating-source torsion balance experiment E.G. Adelberger, J.H. Gundlach, M.G. Harris, B.R. Heckel, G.L. Smith and H.E. Swanson In last year's Annual Report,1 we reported preliminary results that were apparently limited by systematic errors from gravity gradients arising from imperfections in the 3 ton Uranium source. The largest systematic error arose from the l, m=3,1 coupling. • the stray Q 31 gradient of the source (due to machining imperfections) could not be measured precisely (and therefore minimized) because the Q 51 gradient of the earlier source did not vanish by design. • the coupling to the stray q 31 moment of the pendulum (due to small test body misplacements) led to variations larger than the statistical error. We therefore modified our 3 ton Uranium source mass to have vanishing Q 31and Q 51 moments. This was accomplished by introducing two horizontal gaps above and below the midplane of the Uranium brick stacks. We used flat aluminum bearing plates as spacers; these allow us to rotate the central stack by 180° into a configuration which maximizes the Q31 moment. Using this source configuration to measure the stray q 31 moment of the pendulum and special q 31 test bodies to measure the stray Q31 gradient of the source in its normal configuration, we were able to demonstrate that most of the offset (torques that are independent of the test body configuration on the pendulum tray) is due to the Q31 q 31 -coupling. We have tuned the source and the pendulum so that Q 21 q 21 offset torques are small, and we verified that the corrections due to Q 41 q 41 coupling are negligible. In addition we have built a set of new Pb and Cu precision test bodies that seat more reproduceably in the pendulum tray. We now operate our instrument at high vacuum and take data with the source mass rotating faster ( ≈ 1rev/ 20 min) than before and we achieve a statistical error of 5nrad day . We have made several tests for non-gravitational systematic sources of error, and established that magnetism, thermal variations, and apparatus tilt now lead to insignificant uncertainties. To reduce our thermal sensitivity we installed a glass tube which surrounds the fiber. The Ag coated tube increases the pendulum time response to temperature changes to approx. 10h. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 28. 31


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    4.7 New tests of the universality of free fall E.G. Adelberger, M. G. Harris, B. R. Heckel, G. Smith and Y. Su Our tests of the Universality of Free Fall (UFF) have reached the practical limits of the current version of the Eöt-Wash torsion balance. We studied differential accelerations of Be-Cu and Be-Al test-body parts in the fields of Earth, the Sun, or Galaxy, and in the direction of the cosmic microwave dipole. We also compared the acceleration towards the Sun and our galactic center of Cu and single-crystal Si in an Al shell (this pair of bodies approximates the elemental compositions of Earth's core and the Moon or Earth's crust, respectively). In terms of the classic UFF parameter h , our Earth-source results are h( Be, Cu) = ( -1.9 ± 2.5) ¥ 10 -12 and h(Be,Al) = ( -0.2 ± 2.8) ¥ 10 -12 where all errors are 1s . Thus our limit on UFF violation for Be and a composite Al/Cu body is h = (-11 . ± 1.9) ¥ 10 -12 . Our solar-source results are Da(Be,Cu) = ( -3.0 ± 3.6) ¥ 10 -12 cm / s 2 , Da(Be,Al) = ( +2.4 ± 5.8) ¥ 10 -12 cm / s2 , and Da(Si / Al,Cu) = ( +3.0 ± 4.0) ¥ 10 -12 cm / s2 . This latter result, when added to the lunar laser-ranging results that senses both composition-dependent forces and gravitational binding-energy anomalies, yields a nearly model-independent test of the UFF for gravitational binding energy at the 1% level. A fivefold tighter limit follows if composition-dependent interactions are restricted to vector forces. Our galactic-source results test the UFF for ordinary matter attracted toward dark matter, yielding h DM (Be,Cu) = ( -1.3 ± 0.9) ¥ 10 -3 , h DM ( Be,Al) = ( +1.8 ± 1.4) ¥ 10 -3 , and DM -3 h (Si / Al,Cu) = ( +0.7 ± 1.0) ¥ 10 . This provides laboratory confirmation of the usual assumption that gravity is the dominant long-range interaction between dark and luminous matter. We also tested Weber's claim that solar neutrinos scatter coherently from single crystals with cross sections ~ 10 23 times larger than the generally accepted value and rule out the existence of such cross sections. These results have recently appeared in print.1 We are now designing a new instrument with improved sensitivity (discussed elsewhere in this report). 1Y. Su et al., Phys. Rev. D 50, 3614 (1994). 32


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    4.8 Search for γ rays following the ββ decay of 100 Mo to the first excited 0 + state of 100 Ru L. De Braeckeleer, M. Felton* and A. Poon A natural attempt to reduce the background of a very low counting rate measurement is to set up a coincidence experiment. It has been known for a long time that the ultimate sensitivity of a 0 + → 2 + ββ 0 − ν decay search could exceed the one of 0 + → 0 + because the photon deexcitation provides an additional signature that can be used to reject the background. Moreover, the physics of a 0 + → 2 + 0ν ββ decay has its own particular features. The neutrinoless 0 + → 2 + transition is extremely interesting for particle physics since its observation would imply both a finite value of the neutrino mass and the existence of a right handed current. It is also interesting for nuclear physics because it involves the ∆ ↔ nucleon transition, a process strictly forbidden for the 0 + → 0 + case. However, the pessimistic estimates of the rates for 0 + → 2 + transitions due to both phase space suppression and small nuclear matrix elements have limited the enthusiasm of experimentalists for the search of this process. Is it possible to use the experimental advantage of the extra signature of the γ −rays of the nuclear deexcitation and to avoid the theoretical disadvantage of the small matrix elements governing the rate of 0 + → 2 + transitions? In the long wavelength approximation, the double beta decay operators can connect an initial 0 + state with 0 + , 1+ , 2 + states in the daughter nucleus. As a general rule, a decay to the 1+ is kinematically forbidden due to its higher excitation energies. In some cases, the decay to the first excited 0 + state is allowed. A favorable case is 100 Mo , with a Q value of 2 MeV to the + 100 first excited 0 state in Ru. Recently, two groups have attempted the observation of the ββ decay of 100 Mo to the first excited 0 + state of 100 Ru. Assuming similar matrix elements as the ones governing the transition to the ground state, one expects a partial half-life of 10 21 years. Presently the two groups report conflicting results: (8.1+−21..54 ) × 10 20 years1 and a null result at the level of 2 × 10 21 years.2 Previous experiments have focused on a very low background, special materials and underground laboratory. We are investigating a different approach: the detection in coincidence, of the 2 γ rays following the ββ decay of 100 Mo to the first excited 0 + state of 100 Ru. As a preliminary test, we are using 2 medium size (55%) BGO suppressed Germanium detectors (side mounted). We are measuring the coincidence background between these 2 counters. The BGO's are used to veto the cosmic ray background as well as the natural radioactivity. The apparatus is covered by a 4" thick layer of OFHC copper and a 4" thick layer of lead. The background level at the energies of 540 and 590 keV is (0.02 ± 0.01) count per (1 keV)2 per year. We are now measuring the coincidence background with a small sample of molybdenum (35 g.) to find out how the apparatus responds to the radioactivity inserted in the sample itself. A Monte Carlo calculation of the efficiency of an apparatus equipped with 2 large detectors (170%) is under development. *Department of Physics, University of Washington, Seattle, WA 98195. 1A.S. Barabash et al., Nucl. Phys. B, S28A, 236 (1992). 2D. Blum et al., Phys. Lett. B 274, 506 (1992). 33


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    4.9 Test of time reversal symmetry: The emiT experiment S.R. Elliott, R.G.H. Robertson, T.D. Steiger, D.I. Will and J.F. Wilkerson The fact that CP (combined charge conjugation and parity symmetry) conservation is violated in kaon decays was discovered over three decades ago.1 Despite the considerable efforts of numerous researchers, however, this phenomenon remains to be adequately explained. The observation of CP symmetry violation combined with the CPT theorem implies that T (time reversal) symmetry must also be violated. Thus, tests of T symmetry provide a probe of the thirty-year-old CP puzzle which complements direct CP symmetry tests. Scientists at the Nuclear Physics Laboratory along with colleagues from Los Alamos National Laboratory, the National Institute of Standards and Technology (NIST), Notre Dame University, the University of California at Berkeley/Lawrence Berkeley National Laboratory, and the University of Michigan, have formed the emiT Collaboration to carry out the most precise test of T symmetry ever performed using neutron decay. The complete neutron beta-decay distribution may be written: dP(p e , pν ) p •p p p p × pν dΩ e dΩν dEe ( = G( Ee ) 1 + a e ν + Aσ n • e + Bσ n • ν + Dσ n • e Ee Eν Ee Eν Ee Eν ) where p e and Ee are the momentum and energy of the electron, pν and Eν represent the emitted neutrino, σ n is the spin direction of the neutron, and G( Ee ) includes phase-space factors and the Fermi function. In this equation, the term proportional to the triple correlation σ n • ( p e × pν ) is the only term which is odd under time reversal. Thus, the goal of the emiT experiment is to measure or place limits on the coefficient, D, which describes the strength of time reversal violation. The experiment will be performed by observing in-flight decay of low-energy (<10 meV) neutrons from the Cold Neutron Research Facility at NIST in Gaithersburg, MD. Using energy and momentum conservation, the unobservable variables describing the neutrino may be replaced by measurable variables describing the proton. Thus, D may be written in terms of σ n • ( p e × p p ) , and this quantity may be measured by detecting both the electron and the proton, and monitoring the angular correlation between their momenta as the neutron polarization is flipped. The emiT detector consists of four plastic scintillator paddles for electron detection and four arrays of large-area PIN diodes to detect the protons. These eight detector segments are arranged in an alternating octagonal array about the neutron beam so that each segment of one type lies at an angle of 135º relative to two segments of the other type. This geometry takes advantage of the fact that the electron-proton angular distribution is strongly peaked due to the disparate masses of the decay products. The emiT experiment is currently concluding the design phase and entering the construction phase. Assembly on the floor of the NIST reactor will begin before the end of 1995. The primary responsibility of the UW team is the production of the proton detector segments including read-out electronics, detector support frame, and associated vacuum systems. The feasibility of the proposed proton detection scheme was decisively proven during a test run at the NIST reactor in 1992.2 The proton detector segments and the front-end electronics are currently under construction at the Nuclear Physics Laboratory. 1J.H. Christenson et al., Phys. Rev. Lett. 13, 138 (1964). 2E.G. Wasserman, Ph.D. thesis, Time Reversal Invariance in Polarized Neutron Decay, Harvard University, 1994. 34


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    5. ACCELERATOR MASS SPECTROMETRY (AMS) 5.1 Paleoclimate studies using AMS radiocarbon ( 14 C) dating of pollen from lake sediments and peat deposits T.A. Brown,* G.W. Farwell and P.M. Grootes† We have continued our development and refinement of techniques for the isolation and AMS 14 C dating of essentially pure pollen fractions from lake sediment and peat samples under partial funding through an NSF grant under the Paleoclimate from Arctic Lakes and Estuaries (PALE) Initiative of the ARCSS Program (Grant No. ATM91-23963). We have applied these techniques to a number of cores taken from lakes and bogs in Washington, British Columbia, and Alaska. Some of our results were summarized in last year's Annual Report.1 A comprehensive description of the work and a discussion of the results are given in the Ph.D. dissertation of Thomas A. Brown.2 A paper describing the principal features of our 14 C AMS system and its performance (absolute accuracy, ± 0.5% or ± 40 years) has now been published.3 Radiocarbon dating of organic material from lake sediments and peat deposits has been used for decades in paleoclimate studies. During the past several years, AMS measurements have increasingly supplanted the traditional b -counting method of 14 C dating since they offer greatly increased sensitivity (sub- milligram samples can be dated) and, in many instances, greater accuracy; they are also very much faster. However, the typical "bulk carbon" sample preparation techniques used for both AMS and b -counting leave unanswered the question of just what is being dated, a grave disadvantage in palynological and other paleoclimate studies. In contrast, the extraction procedures2 that have been developed here typically produce purified pollen samples which can be clearly identified under the microscope; thus, an unambiguous proxy climate indicator --pollen-- is dated, and nothing else. Additionally, our results to date2,4 show that the extraction and dating of pollen fractions eliminates "hard water effects" as sources of dating errors (sometimes thousands of years) and demonstrate that significant age differences can exist between pollen and macrofossils at the same level in a sediment core. Recent results from the application of our pollen extraction/AMS dating procedures to a low-organic- content Arctic lake sediment core2 were useful in rectifying an apparent age-depth reversal in the results obtained from "bulk carbon" samples from the core. They also demonstrated a clear need for careful screening of prepared samples, through microscopic examination, for suitability in obtaining valid and consistent radiocarbon dates for palynological and other paleoclimate studies. With the recent departures of Thomas A. Brown and Pieter M. Grootes it has become unfeasible to continue the program of radiocarbon AMS measurements here in the Nuclear Physics Laboratory. The NSF- PALE paleoclimate work will continue, however, as a collaborative effort, with the AMS measurements to be carried out at the Lawrence Livermore National Laboratory, Livermore, California. *Now at Center for Accelerator Mass Spectrometry, L-397, Lawrence Livermore National Laboratory, Livermore, CA, USA 94551. †Now at C-14 Leibnitz Labor, Leibnitzstrasse 19, Christian Albrechts Universität, 24118 Kiel, Germany. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) pp. 30-31. 2Thomas A. Brown, Ph.D. Dissertation (Geophysics), University of Washington (1994). 3T.A. Brown, G.W. Farwell, and P.M. Grootes, Proceedings of the 6th International Conference on Accelerator Mass Spectrometry (1993), Nucl. Instrum. Methods B 92, 16 (1994). 4T.A. Brown, G.W. Farwell, and P.M. Grootes, presented at the 15th International Radiocarbon Conference, August 15-19, 1994, University of Glasgow, Scotland. 35


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    6. ATOMIC AND MOLECULAR CLUSTERS 6.1 Stopping powers of atoms and atomic clusters J.F. Liang, R. Vandenbosch and W.G. Weitkamp We have continued our effort to determine vicinage effects in the stopping power of small carbon clusters. We are particularly interested in the energy region where with decreasing energy electronic stopping gives way to nuclear stopping. In this energy region the differences in stopping are small as the nuclear and electronic vicinage effects tend to cancel each other. Last year we reported our results for the stopping of single carbon atom anions.1 We have extended these measurements to C −2 and to a lesser extent C3− anions. The measurements consist of comparing the energy loss of C − with C −2 at the same bombarding energy per carbon. This means the degraded carbon ions exiting from the stopping foil have close to the same energy, with any difference reflecting vicinage effects. Our first series of measurements were made with a surface barrier detector with a thin Au window. We first determined that the vicinage effects were independent of the stopping foil thickness in the range explored, 5-25 µg / cm 2 . This is to be expected as the clusters will break up and the atoms separate soon after they enter the stopping foil. In view of the independence of the vicinage effect on foil thickness it is more appropriate to express the effect as the difference in stopping power for clusters as compared to single atoms, rather than the ratio of the stopping powers. We have found that C −2 clusters lose energy faster than C − ions at the highest energy studied, 165 keV per carbon. As the energy decreases the difference disappears. Since the vicinage effects are very small we have decided to explore an alternate detection technique based on energy analysis of the degraded ions in an electrostatic deflector. We have reoriented a 90 degree bend deflector originally built to deflect the polarized ion source beam into the injection line of the tandem accelerator. This deflector transmits more than 70% of undegraded beam to a Faraday cup. Our measurements to date with this deflector have concentrated on measurements with low energy beams and a 5 µg / cm 2 stopping foil, and have confirmed that vicinage effects are small in the 30-50 keV per carbon energy range. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 59. 36


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    6.2 Size distributions for RbCn clusters R. Vandenbosch and D.I. Will It has been known for some time that low-mass carbon clusters exhibit an odd-even intensity pattern largely independent of the method of production of the clusters. The yield of negative ions, C −n , is larger by typically a factor of 2 to 3 for even mass clusters through n about 8 or 10. This favoring of even-n clusters is attributed to the larger electron affinities of carbon chains with an even number of carbons, as can be understood from the occupancy of delocalized pi electron orbitals.2 Middleton3 first reported a much stronger odd-even dependence in the yield of CsC −n clusters, for which the enhancement of even clusters is typically two orders of magnitude. No explanation of this striking effect has been offered. We decided to see if it occurs for mixed clusters with other alkali metals. We have measured the intensity of RbC −n clusters from the sputtering of graphite with Rb+ ions. The results are given in Fig. 6.2-1. As was observed for CsCn clusters, the enhancement of even-n clusters is more than two orders of magnitude. In an attempt to understand this enormous enhancement, we have initiated ab initio quantum chemical calculations of the electron affinities of Cn and RbCn clusters. We are using the Gaussian 92 program4 with the LANL2DZ basis functions. These basis functions are capable of reproducing the odd-even alternation in electron affinities of linear Cn chains. Our preliminary results for RbCn indicate an odd-even variation of similar magnitude to that for Cn. The absolute values of the electron affinities however are much less, with the odd-n RbCn clusters (with the exception of RbC) not having stable anions (not having positive electron affinities). Although the calculations may not be accurate enough to conclusively determine whether the odd-n clusters have negative affinities, the results of the calculations strongly suggest that the origin of the large enhancement is the much smaller absolute values of the electron affinities of RbCn as compared to Cn. In the course of these calculations we have also explored the relative energies of different geometrical configurations for the RbCn clusters. We find that linear clusters with the Rb on one end are appreciably more stable than linear clusters with the Rb in the middle, or than for small bent clusters. Fig. 6.2-1. Intensity distributions for RbCn clusters produced by Rb sputtering of graphite. The small yields for n=3,5 and 7 are upper limits. 2R.Middleton, Nucl. Instrum. Methods B 58, 161 (1977). 3R.Vandenbosch et al., Nucl Instrum Methods B 88, 116 (1994). 4Gaussian 92, Revision G.1, M.J. Frisch et al., Gaussian, Inc., Pittsburgh, PA, 1992. 37


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    6.3 High energy fragmentation of C60 R. Vandenbosch The unusual stability and high symmetry of C 60 results in a great deal of interest in the mechanisms by which this molecule dissociates when sufficiently excited. Two rather different mechanisms have been put forward to account for the approximately exponential fall off in yield as the heavy fragment mass decreases from the mass of the parent molecule. One is the successive removal of a number of C 2 molecules. The other is a decreasing probability for emitting fragments of increasing mass in a binary process. We report here a semi-quantitative examination of the latter mechanism. It was motivated by a recent report of the C +n yield distribution, where both the light (n < 20) and heavy (n > 40) fragments were reported.5 A remarkable feature of the data, shown in Fig. 6.3-1, is the near-symmetry of the size distribution. This is exhibited by reflecting the yield of light fragments with n ≥ 19 and plotting them as the closed symbols at 60-n. This symmetry is very suggestive of a binary fragmentation mechanism, C 60 → C n + C60 − n . We have developed a binary fragmentation model based upon the assumption of unimolecular dissociation from a statistically equilibrated system. One ingredient in such a model is the activation energy for a particular fragmentation channel leading to a C 60− n and a C n primary product pair. DeMuro, Jelski, and George6 have considered the general problem of removing carbon chains under a constraint to leave the resultant heavy fragment as close as possible to the original buckyball. They find that loss of 4, 6, 8 . . . atoms can occur via an "unzipping" process, yielding low-energy structures down to 44 atoms. The activation energy for a particular fragmentation channel should be approximately proportional to the number of bonds broken in this unzipping process. We have used the tabulated results of deMuro et al. for the number of bonds broken when chains of different length are extracted from C 60 to estimate the relative yields of different fragmentation channels. A fit to the data yields the full curve shown in Fig. 6.3-1. This mechanism also leads to a natural explanation of the dominance of even-n for the heavy fragments. Odd-n primary heavy fragments cannot be produced by this unzipping process. The observed yields of odd-n fragments for the lighter fragments may be the consequence of C1 and C3 evaporation from excited chains. The chains produced by unzipping C 60 will have considerable strain energy in addition to their share of the residual excitation energy. Fig. 6.3-1. The open squares represent the singly positive charged yields of LeBrun et al. for 625 MeV bombardment of C 60 vapor. The open circles are the reflection of the heavy yields assuming binary fragmentation. The full curve is a binary fragmentation fit to the heavy fragment yields. 5T. LeBrun et al., Phys. Rev. Lett. 72, 3965 (1994). 6R.L. deMuro et al., J. Phys. Chem. 96, 10603 (1992). 38


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    7.0 ULTRA-RELATIVISTIC HEAVY ION COLLISIONS 7.1 URHI Group Overview S.J. Bailey, H. Bichsel, P. Chan, J.G. Cramer, P.B. Cramer,* G.C. Harper, M.A. Howe, G. Odyniec,† D.J. Prindle, J.G. Reid, R.J. Seymour, T.A. Trainor, P. Venable and J. Zhu The major achievement for our group in the past year has been the highly successful first NA49 160 GeV/u lead beam run at CERN this past November and December. In preparation for this run the UW group has accepted a leading role in the production of Main TPC tracking software (Sections 7.5 and 7.6) and slow- controls system coordination software (Sections 7.4 and 9.4). The CERN and heavy ion communities have recognized that NA49 has carried out a major achievement by bringing into operation a highly complex experimental system with almost perfect functioning during its first running period. This experiment is considered to be the flagship experiment for the CERN heavy ion program through the year 2000. The full NA49 system will consist of two 1.5T vertex magnets, with one vertex TPC installed in each magnet gap, and two large main TPCs on either side of the beam and 10 m downstream from the main target. Of these the second vertex TPC and the right Main TPC have been installed for this first lead beam run. The full TPC complement will be installed in time for the 1995 run next fall. The Main TPCs are each 3.5 m square by 1.2 m high in active volume. Each TPC has about 64,000 electronics channels with 512 ADC samples per channel per event. This produces a data rate for one main TPC of about 3-4 Mb/s. The total data volume after five days of NA49 operation was about 1.5 Tbyte. Successful Main TPC tracking was accomplished minutes after the first data were recorded. Track and charge distributions were displayed with a UW-produced display package (Section 7.7) and were used to check out and optimize the functioning of the TPC on line and to analyze the performance of the tracking software offline. The UW group also played a lead role in preliminary analysis of Main TPC data (Section 7.3) in preparation for the Quark Matter 95 meeting at Monterey, CA in January. Charged-particle momentum spectra, collision system temperature distributions and net-charge momentum distributions were extracted during the week following the end of the run. These UW efforts, in combination with other VTPC and MTPC analysis efforts by NA49 collaborating institutions such as MPI-Munich, Lawrence Berkeley Laboratory, and IKF-Frankfurt provided a good preliminary description of this new lead-lead collision system only weeks after the first availability of beam. In addition to our NA49 activities we have continued an active role in the STAR collaboration, which will mount a solenoidal detector at the RHIC accelerator in time for turn on in 1999. Some of these activities included development of STAR trigger algorithms based on correlation measures (Sections 7.11, 7.12 and 7.13), development of a servo-controlled TPC high voltage control system, and simulations in support of the STAR silicon vertex tracker (Section 7.8). In addition to collaboration-specific activities we continue to pursue a strong interest in development of HBT or Bose-Einstein correlation determinations of collision system space-time geometry (Sections 7.9 and 7.10). *Max-Planck Institut für Physik, Föhringer Ring 6, D-80805 München, Germany. † Lawrence Berkeley Laboratory, Berkeley, CA. 39


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    7.2 NA49 Pb run first results S.J. Bailey, H. Bichsel, P. Chan, J.G. Cramer, P.B. Cramer,* G.C. Harper, M.A. Howe, G. Odyniec,† D.J. Prindle, J.G. Reid, R.J. Seymour, T.A. Trainor, P. Venable and J. Zhu This past November and December NA49 carried out at CERN the first data acquisition with 33 TeV lead projectiles incident on a lead target. Operational detector elements included an in-magnetic-field vertex TPC, an out-of-field main TPC, a ring calorimeter covering the pseudorapidity region 2.1 < η < 3.4, and a veto calorimeter covering the very forward pseudorapidity region which accepts spectator projectile particles, i.e., those which have not participated in the collision. A minimum-bias correlation plot between E veto and E T for the veto and ring (transverse energy) calorimeters respectively shows an expected anticorrelation of these observables extending from E veto = 33 TeV at E T = 0 (no collision) to E veto = 6 TeV at E T = 0.5 TeV (central collision). Position of an event along this correlation is determined mainly by the particular collision geometry (impact parameter) for the event. The observed minimum E veto energy is equivalent to about 38 noninteracting projectile particles (spectators) incident on this calorimeter, whereas simple model calculations predict about 13 spectator particles or 2 TeV. Therefore, about 4 TeV equivalent energy in produced particles must fall within the veto calorimeter acceptance. Using these calorimeter data the degree of stopping of projectile particles while passing through the target can be compared for these new Pb-Pb results with that for S-Au obtained by NA35. The degree of stopping is found to be similar, for a corresponding mean number of interactions of projectile particles, indicating that collective effects in the stopping process do not seem to dominate. More detailed analysis of calorimeter data indicate an energy density in the collision system of about 3 GeV/fm 3 in a volume (from HBT measurements) roughly 3.5 times larger than that for 200 GeV/c sulfur- sulfur. Preliminary analysis of vertex and main TPC data indicate that the negative charged particle (mainly pion) distribution in pion rapidity has a peak value at midrapidity of about 230 per rapidity unit. This distribution is significantly broader than would be expected for particle emission isotropic in the CM. A preliminary study of Bose-Einstein correlations of negative particles indicate that the source size is about 7.4 fm, as compared to a source size of about 4.7 fm for sulfur beam. These preliminary results indicate the achievement of energy densities and produced particle multiplicities consistent with predicted conditions for color deconfinement. The similarity of the degree of stopping for sulfur and lead on heavy targets indicates the absence of rescattering or other collective effect. *Max-Planck Institut für Physik, Föhringer Ring 6, D-80805 München, Germany. †Lawrence Berkeley Laboratory, Berkeley, CA. 40


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    7.3 NA49 Pb run preliminary spectrum analysis P.Chan and T.A. Trainor When the 160 GeV/u lead beam became available at the CERN SPS last fall, we carried out a preliminary spectrum analysis for experiment NA49's main TPC in order to provide an initial look at the data. This analysis also served as a check on the status of the hardware and analysis software. The NA49 right main TPC is located 11 m downstream from the target and double magnet system with a lateral displacement of 2 m to the right of the beam. Straight tracks are deduced from correlation of ionization deposited by charged particles traversing the TPC active volume. For each reconstructed track, the corresponding particle's physical parameters, such as transverse momentum ( p t ) , rapidity assuming pion and proton mass (Yπ , Yproton ), and transverse mass (m t = p 2t + mπ2 ) are determined. Due to the condition of the software environment in these early stages, only 100 events of negative particles (h − ) and 70 events of positive particles (h + ) were used for this analysis. On average, there are 320 reconstructed tracks per event for h − and 390 for h + . The only correction applied to the raw spectrum was the TPC geometrical acceptance. Two acceptance correction factors were determined, one for p t and one for m t spectra. The p t acceptance correction was determined by comparing the simulated output of GEANT with flat phase space input. On the other hand, a VENUS-generated phase space distribution was used to determined the acceptance correction factor for the m t distribution. Due to the lack of statistical power over certain regions, the acceptance correction was most reliable for 4 <Yπ < 5. For a pion mass hypothesis, p t acceptance corrected data are then used to produce event averaged p t < dN/dp t > and rapidity <dN/dYπ > distributions for h − and h + . The preliminary result shows a mean p t of 370 and 440 MeV/c for h − and h + respectively. The m t acceptance correction factor is used to obtain the m t distribution 1/m t <dN/dm t > which is fitted with an exponential dependence on m t . The fitted inverse slope parameter (or temperature) is 190 and 250 MeV for h − and h + spectra respectively. A "net baryon'' distribution can be deduced from the charge excess between the h + and h − spectra. The preliminary analysis shows a mean p t of 580 MeV/c for this distribution. The rapidity distribution peaks near mid rapidity. This set of results from the preliminary data analysis provides us with a first substantive glance at the data obtained from a new generation of ultrarelativistic heavy ion experiments. It also makes clear the challenge to our software analysis system in analyzing the bulk of data to be generated by NA49 in coming years. 41


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    7.4 Experience using SControl in CERN experiment NA49 J.G. Cramer, P.B. Cramer* and M.A. Howe SControl was designed (see Section 9.4) for CERN Experiment NA49 and was used extensively in the initial Autumn 1994 run of NA49. The program operated on a dedicated HP-712 workstation in the NA49 counting room. In this application its function as a "control'' system for actively changing experimental parameters was minimal. The principal tasks of SControl were: (a) to collect experiment-status information from 6 satellite processors, (b) to maintain an archive of this information by updating an archive file, (c) to maintain a data structure (BOS bank) that was read at irregular intervals by the data acquisition system, (d) to provide user-controlled displays of experiment parameters of interest, and (e) to generate and manage alarms. The experiment was represented by pages organized in a hierarchical structure, with a "home'' page at the top level showing an orthographic representation of the whole experiment. On this diagram the major subsystems were outlined in white, and each of these outlined regions provided a hyper-link to the top-level page of the subsystem. The subsystem top-level pages typically showed a block diagram of the subsystem, with blocks outlined in red providing hyper-links to the appropriate sub-sub-systems. This pattern of diagrams with hyper-links was repeated at one or more levels until a page was reached that was designed to monitor several related functions of a particular subsystem. For example, a page displayed as a strip chart the measured drift velocity of gas used in a TPC, the pressure and temperature at which the measurement was made, a "normalized'' drift velocity that had been corrected for variations in temperature, pressure, and electric field, and a scatter plot correlation of measured drift velocity with measured pressure. An alarm system was created which generated appropriate levels of alarms when (a) certain temperatures went out of range and (b) when one of the "heartbeat'' signals provided by the satellite processors failed to recur at the expected interval. Archives were updated every 10 minutes and organized into daily files, each initialized at midnight. About 200 Mb of archived parameter files were accumulated during the NA49 run. Measured gas temperatures, pressures, and other data from these files have already proved very valuable in providing comprehensive time-dependent estimates of the drift velocities in the NA49 TPCs. Experience during the 1994 run of NA49 provided a number of lessons that bear on the application of SControl to future experiments and NA49 runs: (1) It is important to establish and enforce a naming convention for the slow control parameters archived. Names supplied by satellites were used to identify parameters in the archives. Parameters with time-dependent names proved troublesome. (2) Whenever an alarm link is created, a message describing the action to be taken when the alarm occurs should also be provided. No alarm is "obvious''. (3) In the 1994 NA49 run, all slow control information was written each day to a master archive file. Because of the large volume of array information (up to 12 Mb/day) stored in the archive file by the two time-of-flight systems, this arrangement proved unwieldy. In future runs, when there will be more detectors on line, we will maintain separate archive file structures for several subsystems. (4) A better way of reviewing archived information is needed. We are considering a facility for converting slow control archives to a PAW ntuple for analysis. (5) One of the great advantages of SControl is the ease with which the user interface can be modified and expanded while running. However, this can lead to confusion and duplication unless the application developer exercises restraint. *Max-Planck Institut für Physik, Föhringer Ring 6, D-80805 München, Germany. 42


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    7.5 NA49 main TPC tracking software S.J. Bailey, P. Chan, D.J. Prindle, S. Schönfelder,* T.A. Trainor, P. Venable and X. Zhu The original NA49 tracking software was an extension of NA35's TRAC program, modified to match the geometry and tracking needs of NA49. All steps of the analysis from reading the raw data through the momentum calculation were incorporated into a single large stand-alone program (TNT). To allow a more modular design and interactive access to the data, the NA49 Server Environment (NASE) was created at IKF, Frankfurt, for use by all NA49 analysis programs. This server provided a central memory manager that separate client programs could access, thus sharing data. After a client had finished, its output data was stored in the central server where users could access the data to immediately view the results from a PAW-like environment. The stand-alone tracking software was modified and split up into several separate clients for NASE. This collection of clients, STIRN, became the basis for NA49 MTPC tracking. STIRN includes modules for reading in init files with setup parameters, raw-data reading and space-point finding, tracking, dE/dx calculation, and momentum determination. It also comes with a collection of utility clients for matching simulated tracks to reconstructed tracks, calculating two-track resolution, and other programs for debugging and testing the primary clients. Although improvements continue to be made to STIRN, the clients were ready and tested on simulated data before the first data acquisition run took place at CERN this last Fall. Thus, we were able to view reconstructed tracks literally within minutes of the initial raw data acquisition. The modular nature of STIRN allowed numerous improvements and suggestions to be rapidly implemented during the run. Although useful, NASE suffers from being slow, using memory inefficiently, and instability. Because of these problems STIRN was recently converted to run under DSPACK, an alternate server system. Because DSPACK uses shared memory instead of copying data through Unix sockets, the performance of STIRN under DSPACK is significantly enhanced. DSPACK is also a more stable system than NASE and initial results have been promising. Until both systems have been fully tested, STIRN is being simultaneously developed for both servers with the choice of which server to use being made at compile time. This allows both systems to be tested and compared while maintaining the same source code for the core of the analysis to ensure accountability between tests of the different systems. *Max-Planck Institute für Physik (MPI), Föhringer Ring 6, D-80805 München, Germany. 43

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