avatar University of Washington Inc Services

Pages

  • Page 1

    ANNUAL REPORT Center for Experimental Nuclear Physics and Astrophysics University of Washington May, 2004 Sponsored in part by the United States Department of Energy under Grant #DE-FG02- 97ER41020/A000.


  • Page 2

    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, make any warranty, expressed or implied or assumes any legal liability or responsibility for accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe on privately-owned rights. Cover photos, clockwise from upper right: 1. A view into the SNO acrylic vessel showing the Remotely Operated Vehicle (ROV), which is used to install the Neutral Current Detectors (NCDs), some of which also appear in the picture. See Sec. 2.10. (Photo by John Amsbaugh) 2. Minesh Bacrania mounting targets in the 24” scattering chamber. See Sec. 3.3. (Photo by Derek Storm) 3. A torsion pendumum used for measuring gravity gradients. See Sec. 1.7. (Photo by Ki-Young Choi) 4. Joe Formaggio and Laura Stonehill operating the ROV while installing NCDs. See Sec. 2.10. (Photo by Jaret Heise)


  • Page 3

    UW CENPA Annual Report 2003-2004 May 2004 i INTRODUCTION CENPA pursues a broad program of research in nuclear physics, astrophysics and related fields. Research activities are conducted locally and at remote sites. The current program includes “in-house” research on nuclear collisions and fundamental interactions using the local tandem Van de Graaff, as well as local and remote non-accelerator research on fundamental interactions and user-mode research on relativistic heavy ions at large accelerator facilities in the U.S. and Europe. Our 7 Be(p,γ)8 B measurements have been completed and published. Three separate cross section measurements with 3 different (radioactive) targets are in excellent agreement, and determine the astrophysical S-factor to an experimental precision of ± 3% (± 4% including theoretical extrapolation uncertainty). We have begun work on the 3 He(α, γ)7 Be reaction, which is also very important in solar neutrino production. The Gravity group has become a member of the LISA gravity wave detector project. The end masses of the LISA interferometer must be kept inertial at very delicate levels. An ultra- sensitive torsion balance instrument has been built to characterize small forces that may act on the end masses. An initial broad survey of correlations and fluctuations in RHIC Au-Au and p-p colli- sions has been completed, revealing substantial evidence for a dissipative colored medium in central Au-Au collisions and several surprises relative to theoretical predictions. Among the findings: 1) p-p momentum distributions are precisely separated into soft and hard com- ponents, corresponding to string and minijet fragmentation respectively, which provide an essential reference for Au-Au collisions, 2) minijets in central Au-Au collisions are found to be strongly elongated along the collision axis and narrowed in azimuth, this trend being true for both angular correlations and transverse momentum correlations, and 3) the geometry of hadronization in central Au-Au collisions is demonstrated to be two-dimensional, in contrast to the previously-observed one-dimensional string fragmentation geometry in p-p. The second phase of operation of the Sudbury Neutrino Observatory, in which salt was added to the heavy water to enhance sensitivity to the neutral-current interaction of solar neutrinos, was completed in September, 2003. An analysis of 254 live days of these data was released at the Topics in Astroparticle and Underground Physics international conference in Seattle. The new results have ruled out maximal mixing for solar neutrinos at more than 5 standard deviations, and disfavor the higher-mass “LMA-II” solution to the solar neutrino problem at greater than 99 % confidence level. The salt has been removed from the SNO detector and 40 strings of 3 He counters have been installed to give SNO an event-by-event capability for discriminating neutral-current events from charged-current. Initial performance of the array is very satisfactory. An integrated data-acquisition system has been commissioned to make possible the si- multaneous collection of data from the photomultiplier array and the NCD array in SNO. The system integrates a new object-oriented realtime control and acquisition system, ORCA, with the existing SHaRC system, both developed at UW. Design and construction of a grid to suppress backgrounds in the prespectrometer for the


  • Page 4

    ii KATRIN tritium beta decay experiment is nearing completion. The prespectrometer, the first module of this major new neutrino mass experiment, will be commissioned in 2004. The emiT experiment collected over 350 million coincidences during its recently concluded data collection run at the NG-6 beamline at the NIST Center for Neutron Research (NCNR) in Gaithersburg, MD. Analysis of these data is underway, and should yield a statistical sensitivity to the time-violating coefficient “D” of 2 × 10−4 , which would exceed emiT’s original design goal by about a factor of 1.5. The source of Ultra-cold neutrons at LANL has been built, and the first production run successfully produced UCNs. We have started mounting the apparatus for measuring the beta-asymmetry from polarized-neutron decay and we expect to start taking data in October of 2004. Using the Tandem, we have taken data that should yield the mass of the lowest T=2 state in 32 S to within ∆m/m ≈ 10−8 . We are currently analyzing the data and calculating systematic uncertainties. In collaboration with the INT, we sponsored the Eighth International Workshop on Topics in Astrophysics and Underground Physics during September. This major conference is held every two years, and sponsoring such conferences is part of CENPA’s mandate. Initial exploration into the establishment of a Joint Institute for Advanced Detector Tech- nology between the University of Washington and Pacific Northwest Laboratories received strong endorsement by the administrations of both institutions. A more detailed proposal is being pursued. As always, we encourage outside applications for the use of our facilities. As a conve- nient reference for potential users, the table on the following page lists the capabilities of our accelerators. For further information, please contact Prof. Derek W. Storm, Executive Director, CENPA, Box 354290, University of Washington, Seattle, WA 98195; (206) 543- 4080, or storm@npl.washington.edu. Further information is also available on our web page: http://www.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 are listed alphabetically, with the primary author underlined, to whom inquiries should be addressed. Derek Storm, Editor Barbara Fulton, Assistant Editor


  • Page 5

    TANDEM VAN DE GRAAFF ACCELERATOR A High Voltage Engineering Corporation Model FN purchased in 1966 with NSF funds, operation funded primarily by the U.S. Department of Energy. See W. G. Weitkamp and F. H. Schmidt, “The University of Washington Three Stage Van de Graaff Accelerator,” Nucl. Instrum. Meth. 122, 65 (1974). Recently adapted to an (optional) terminal ion source and a non-inclined tube #3, which enables the accelerator to produce high intensity beams of helium and hydrogen isotopes at energies from 100 keV to 5.5 MeV. Some Available Energy Analyzed Beams Ion Max. Current Max. Energy Ion Source (particle µA) (MeV) 1H or 2 H 50 18 DEIS or 860 3 He or 4 He 2 27 Double Charge-Exchange Source 3 He or 4 He 30 7.5 Tandem Terminal Source 6 Li or 7 Li 1 36 860 11 B 5 54 860 12 C or 13 C 10 63 860 ∗14 N 1 63 DEIS or 860 16 O or 18 O 10 72 DEIS or 860 F 10 72 DEIS or 860 ∗ Ca 0.5 99 860 Ni 0.2 99 860 I 0.001 108 860 *Negative ion is the hydride, dihydride, or trihydride. Additional ion species available including the following: Mg, Al, Si, P, S, Cl, Fe, Cu, Ge, Se, Br and Ag. Less common isotopes are generated from enriched material. In addition, we are now producing a separated beam of 15-MeV 8 B at 6 particles/second. BOOSTER ACCELERATOR See “Status of and Operating Experience with the University of Washington Superconducting Booster Linac,” D. W. Storm et al., Nucl. Instrum. Meth. A 287, 247 (1990). The Booster is presently in a “mothballed” state.


  • Page 6


  • Page 7

    UW CENPA Annual Report 2003-2004 May 2004 v Contents INTRODUCTION i 1 Fundamental Symmetries and Weak Interactions 1 Weak Interactions 1 1.1 Parity non-conserving neutron spin rotation in liquid helium . . . . . . . . . . 1 1.2 Progress on measuring the β-asymmetry in ultra-cold neutron decay . . . . . 3 1.3 The completed second run of the emiT experiment . . . . . . . . . . . . . . . 4 1.4 Search for a permanent electric dipole moment of 199 Hg . . . . . . . . . . . . 5 Torsion Balance Experiments 6 1.5 Sub-mm test of Newton’s inverse-square law . . . . . . . . . . . . . . . . . . . 6 1.6 Spin pendulum update . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.7 A new equivalence-principle test . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.8 Small force measurements for LISA . . . . . . . . . . . . . . . . . . . . . . . 9 1.9 The development of a torsion-pendulum based axion search . . . . . . . . . . 10 2 Neutrino Research 11 SNO 11 2.1 The Sudbury Neutrino Observatory . . . . . . . . . . . . . . . . . . . . . . . . 11 2.2 16 N production by muons in SNO . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 Muon-induced neutrons at the Sudbury Neutrino Observatory — refined and extended results plus new Monte Carlo capabilities . . . . . . . 14 2.4 Atmospheric-neutrino-induced muons at the Sudbury Neutrino Observatory . 15 2.5 NUANCE: atmospheric-neutrino simulation in SNO . . . . . . . . . . . . . . 17 2.6 Estimation of the background to an ν e analysis of SNO data . . . . . . . . . 18 2.7 Neutral-current results from the salt phase of SNO . . . . . . . . . . . . . . . 19


  • Page 8

    vi 2.8 The day-night measurement in the salt phase of SNO . . . . . . . . . . . . . . 20 SNO NCDs 21 2.9 NCD backgrounds and data cleaning . . . . . . . . . . . . . . . . . . . . . . . 21 2.10 Deployment of an array of Neutral Current Detectors (NCDs) in SNO . . . . 22 2.11 NCD analysis tools and electronics calibrations . . . . . . . . . . . . . . . . . 23 2.12 PMT calibrations in SNO during the NCD phase . . . . . . . . . . . . . . . . 24 2.13 Upgrade to the NCD electronics for SNO . . . . . . . . . . . . . . . . . . . . 25 KATRIN 26 2.14 The KATRIN neutrino-mass experiment . . . . . . . . . . . . . . . . . . . . . 26 2.15 KATRIN electron gun testing of detector dead layers . . . . . . . . . . . . . . 27 2.16 KATRIN pre-spectrometer electrode . . . . . . . . . . . . . . . . . . . . . . . 28 Majorana 29 2.17 CENPA contributions to the Majorana experiment . . . . . . . . . . . . . . . 29 2.18 A Joint Institute for Advanced Detector Technology . . . . . . . . . . . . . . 31 3 Nuclear Astrophysics 32 3.1 Precise measurement of the 7 Be(p,γ)8 B S-factor . . . . . . . . . . . . . . . . 32 3.2 3 He(α, γ)7 Be γ-ray background analysis . . . . . . . . . . . . . . . . . . . . . 33 3.3 Search for the 8 B(2+ ) → 8 Be(0+ ) ground state transition . . . . . . . . . . . 35 4 Nuclear Structure 36 4.1 β − ν correlation in A=8 and neutrino spectrum from 8 B . . . . . . . . . . . 36 4.2 Response function of Si detectors for α particles . . . . . . . . . . . . . . . . 37 4.3 A Monte Carlo simulation of 35 Cl(n, γ) lines . . . . . . . . . . . . . . . . . . . 38 4.4 A stringent test of the Isobaric Multiplet Mass Equation using 31 P(p, γ) . . . 39 5 Relativistic Heavy Ions 40


  • Page 9

    UW CENPA Annual Report 2003-2004 May 2004 vii 5.1 Summary of event-structure analysis . . . . . . . . . . . . . . . . . . . . . . . 40 5.2 Soft and hard components of inclusive pt distributions from RHIC p-p collisions √ at s = 200 GeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 5.3 Minimum-bias hard component (minijets) for inclusive pt distributions from √ Pythia-V6.131 p-p collisions at s = 200 and 630 GeV . . . . . . . . . . . . . 42 5.4 Soft and semi-hard components of transverse-momentum-dependent two-particle √ correlations from p-p collisions at sN N = 200 GeV . . . . . . . . . . . . . . 43 5.5 Minijet structure of two-particle axial-momentum correlations in p-p collisions √ at s = 200 GeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.6 Fluctuations, correlations and inverse problems . . . . . . . . . . . . . . . . . 45 5.7 Charge-dependent n and p⊥ correlations in Pythia and Hijing events . . . . . 46 5.8 Charge-independent n and p⊥ correlations in Pythia and Hijing events . . . . 47 5.9 n-p⊥ covariances in Pythia and Hijing Monte Carlo events . . . . . . . . . . . 48 5.10 Minijets as velocity structures: hpt i fluctuation scaling and inversion to joint √ pt autocorrelations from Au-Au collisions at s = 200 GeV . . . . . . . . . . 49 √ 5.11 Energy dependence of hpt i fluctuations from sN N = 10 to 200 GeV . . . . . 50 5.12 Minijet dissipation and transverse-momentum correlations in Au-Au collisions √ at sN N = 130 GeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 5.13 Minijet deformation on axial-momentum space and charge-independent num- √ ber autocorrelations from Au-Au collisions at sN N = 130 GeV . . . . . . . 52 5.14 Hadronization geometry and charge-dependent number autocorrelations on √ axial-momentum space in Au-Au collisions at sN N = 130 GeV . . . . . . . 53 5.15 Overview of HBT physics at STAR . . . . . . . . . . . . . . . . . . . . . . . 54 5.16 Pion opacity in RHIC collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5.17 Pion phase space density from STAR data . . . . . . . . . . . . . . . . . . . . 56 5.18 Pion entropy in RHIC collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.19 Bichsel functions for particle tracks in STAR TPC . . . . . . . . . . . . . . . 60 6 Electronics, Computing, and Detector Infrastructure 61 6.1 PC-based data-acquisition developments at CENPA . . . . . . . . . . . . . . 61


  • Page 10

    viii 6.2 Nanopore DNA sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 6.3 Electronic equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 6.4 NSAC - a data-acquisition and control system for the parity non-conserving neutron spin-rotation experiment . . . . . . . . . . . . . . . . . . . . . . . . 64 6.5 EötWash data-acquisition development . . . . . . . . . . . . . . . . . . . . . 65 6.6 Development of the ORCA DAQ system . . . . . . . . . . . . . . . . . . . . . 66 6.7 Laboratory computer systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 7 Accelerator and Ion Sources 68 7.1 Deck and ion sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.2 Van de Graaff accelerator operations and development . . . . . . . . . . . . . 69 7.3 Progress toward UCN detectors with the 860 SpIS . . . . . . . . . . . . . . . 70 7.4 Removal of large pieces of the old cyclotron . . . . . . . . . . . . . . . . . . . 71 8 The Career Development Organization: year four 72 9 CENPA Personnel 73 9.1 Faculty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 9.2 Postdoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . 73 9.3 Predoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . . 74 9.4 Research Experience for Undergraduates participants . . . . . . . . . . . . . . 74 9.5 Professional staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 9.6 Technical staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 9.7 Administrative staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 9.8 Part Time Staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 10 Publications 77 10.1 Degrees Granted, Academic Year, 2003-2004 . . . . . . . . . . . . . . . . . . . 87


  • Page 11

    UW CENPA Annual Report 2003-2004 May 2004 1 1 Fundamental Symmetries and Weak Interactions Weak Interactions 1.1 Parity non-conserving neutron spin rotation in liquid helium E. G. Adelberger, A. Garcı́a, B. R. Heckel and H. E. Swanson We have undertaken a project to measure the PNC neutron spin rotation in a 50-cm long liquid-helium target to a precision of 10−7 rad and to explore the possibility of measuring the PNC interaction between bare neutrons and protons. This project has been supported by the NSF in addition to the support received from the DOE through CENPA. The collaboration for this project includes E. Adelberger, A. Garcı́a, B. Heckel and E. Swanson from CENPA, M. Snow and C. Bass from Indiana University, and D. Markoff from TUNL. Despite a considerable body of data of PNC measurements in nuclei, it has not been possible to extract a consistent picture of the weak force between nucleons. Uncertainties about the nuclear wave functions make it difficult to compare experiment to theory unam- biguously. The weak isovector coupling constant, fπ , is of particular interest because it is strongly influenced by the neutral current Z 0 exchange, yet it has proven to be very difficult to measure. We have proposed to measure fπ in PNC n − α scattering, a few-body nucleon system that may be calculated with relative confidence. The PNC rotation of the spin vector of a neutron beam as it traverses a liquid helium target, φpnc , provides a measurable observable: φpnc (n − α) ∝ +fπ + .33h0ρ + .23h0ω − .11h1ρ − .23h1ω − .02h01 ρ where the coupling constants are the standard DDH amplitudes.1 If fπ vanishes, then PNC p-α measurements, the isospin conjugate system to n-alpha, predict a value for φpnc of ap- proximately 5 × 10−7 rad in a 50-cm long liquid-helium target, five times larger than the experimental limit we propose to achieve. Cold neutrons, having de Broglie wavelengths longer than 0.4 nm, propagate through material targets as neutron waves. The target averaged interaction with the neutrons creates a neutron index of refraction. PNC neutron spin rotation arises from a term G0~σ · p~ in the index of refraction that is generated by the weak interaction (where ~σ is the Pauli spin and p~ is the momentum of the neutron, and G0 is proportional to the universal weak coupling constant.) The spins in a neutron beam whose polarization vector is transverse to p~ will experience a torque about p~, resulting in a total rotation angle of: φpnc = −4πN LRe(G0 ) independent of neutron momentum, where N is the number density of target nuclei and L the target length. 1 V. F. Dmitriev, V. V. Flambaum, O. P. Sushkov and V. B. Telitsin, Phys. Lett. 125, 1 (1983).


  • Page 12

    2 The experimental challenge is to distinguish the small PNC rotations from much larger neutron spin rotations due to residual magnetic fields. We have constructed a neutron spin polarimeter that includes a cryostat surrounding a liquid-helium target region and have per- formed the first measurement of the PNC neutron spin rotation in a liquid-helium target. The first data runs with the cryogenic polarimeter, taken at the NIST reactor, achieved a result of φpnc = (3.8 ± 6.5) × 10−7 rad, limited by count-rate shot noise.2 To achieve an additional factor of five in experimental sensitivity, we are in the process of rebuilding the polarimeter. There are three essential improvements. The first is to rebuild the cryostat so that superfluid helium can be used as a target. Superfluid helium has a density 20% higher than normal liquid helium; it will scatter 30% less neutrons out of the beam; and it provides better thermal conductivity to reduce systematic errors. The second improvement is to employ a long wavelength neutron filter in the beam to remove the longest wavelength neutrons. This is expected to increase the detected beam polarization by 50%. Finally, the beam collimation will be opened to allow a larger count rate. In the past year, we have purchased the hardware required to rebuild the cryostat, our shops have machined new liquid-helium target chambers to be used in the cryostat, and our collaborators at Indiana University are in the process of assembling the rebuilt cryostat. A collaboration meeting was held at NIST in April 2004 and plans were made to test the cryostat at Indiana University in the summer of 2004 in preparation for data collection to begin at the NIST reactor at the end of 2004. In 18 weeks of beam time, we should have the statistics to measure φpnc at the level of 10−7 rad. 2 D. Markoff, Ph.D. thesis, University of Washington, (1997).


  • Page 13

    UW CENPA Annual Report 2003-2004 May 2004 3 1.2 Progress on measuring the β-asymmetry in ultra-cold neutron decay A. Garcı́a, S. A. Hoedl, A. L. Sallaska and S. K. L. Sjue Ultra-high precision studies of neutron decay present a unique opportunity to improve the present uncertainty in Vud , test the unitarity of the CKM matrix, and probe for physics beyond the standard model. We are members of the UCNA collaboration whose first goal is to measure the angular correlation between the electron momentum and the neutron spin five times better than the current uncertainty. Such a measurement, when combined with the presently known value of the neutron and muon lifetimes, will permit a determination of Vud at the same level of precision as 0+ → 0+ decays without nuclear corrections. The principal advantage of this experimental effort is the use of Ultra-Cold Neutrons (UCN). Since UCN reflect at all angles of incidence from most materials, they do not activate the experiment itself (in contrast to a cold neutron beam) and can be efficiently transported to a well-shielded low radiation environment. In addition, their low energy (E ≤ 2 × 10−7 eV) permits a simple polarization scheme: passage through a 7-T magnet is sufficient to reject 100% of the wrong polarization state. Note that polarization has been the limiting systematic error in previous measurements of A. Our collaboration has built a new super-thermal solid deuterium source of UCN, which has recently been commissioned, and which we expect to achieve a world record density. Our principal contribution over the past year has been the design and construction of UCN detectors and absorbers. Detectors monitor the UCN density in the fiducial volume, while absorbers confine UCN inside the fiducial volume. The detectors consist of a thin foil which converts UCN into energetic (∼ 2 − 3 MeV) ions which can be readily detected by a silicon surface barrier detector. The principal challenge was finding materials with small UCN reflectivity which could be readily manufactured at CENPA. We developed two types of converter foils. In one type, we evaporated 300µg/cm2 of natural isotopic abundance LiF onto 2000Å thick nickel foils. In this foil, the neutrons will be captured according to n+6 Li→ α+3 H, with σ = 4.7 × 105 Barns. The nickel serves as a strong substrate, which also has the added benefit of reflecting UCN and effectively doubling the path length of neutrons in the absorbing medium. Naturally occurring LiF has a low UCN potential (54.4 neV) and is easy to evaporate. In the other type, we implanted 10 B (see Section 7.3) into a 2000Å thick layer of V coated onto a 100Å adhesion layer of chrome in turn evaporated onto a 2000Å thick nickel foil. Neutrons are captured according to the reaction, n+10 B→7 Li+α, with σ = 1.8 × 106 Barns. V also has a low UCN potential (-7.2 neV) and can be e-beam evaporated. We have also built two types of UCN absorbers. The challenge was finding material with a low UCN reflectivity which does not increase the radiation backgrounds. We have constructed absorbers consisting of TPX, which is a 3-methylpentene-1 based polyolefin with a nearly zero UCN potential, and LiF coated on stainless steel. TPX has a lower UCN potential; however, because n+H→D+γ, it may produce higher backgrounds. We anticipate that both the absorbers and the detectors will be tested in calender year 2004.


  • Page 14

    4 1.3 The completed second run of the emiT experiment H. P. Mumm, A. Garcı́a, M. F. Wehrenberg and J. F. Wilkerson The emiT experiment is a search for time-reversal (T) invariance violation in the beta decay of free neutrons. Current observations of CP(T) violation in the Kaon and B-meson systems can be accommodated within the standard model of particle physics. However, baryogenisis, as well as attempts to develop unified theories, indicate that additional sources are required. The standard model predicts T-violating observables in beta decay to be extremely small (Second order in the weak coupling constant) and hence are beyond the reach of modern experiments.1 However, potentially measurable T-violating effects are predicted to occur in some non-standard models such as those with left-right symmetry, exotic fermions, or lepto- quarks.2 Thus a precision search for T-violation in neutron beta decay provides an excellent test of physics beyond the Standard Model. The emiT experiment in sensitive to the T-odd P-even triple correlation between the neutron spin and the momenta of the neutrino and electron, D ~σn · P~e × P~ν , in the neutron beta-decay distribution. The coefficient of this correlation, D, is measured by detecting decay electrons in coincidence with recoil protons from a polarized beam of cold (2.7 meV) neutrons. Four electron detectors (plastic scintillators) and four proton detectors (large-area diode arrays) are arranged in an alternating octagonal array concentric with the neutron beam. Following extensive upgrades, the emiT detector was moved from the University of Wash- ington to NIST in June 2002. A number of set-up tasks were carried out from June until the start of production data taking in October of that year. Data was taken until December 2003. In all about two thousand data runs of up to four hours apiece were collected. These runs were of three types: coincidence runs used for extraction of the D coefficient, calibration runs, and Asymmetric beam Transverse Polarization (ATP) runs for understanding the dominant systematic effect in the experiment. In addition to a raw data stream, singles histograms were saved for the fast TDCs, QDCs, Shaper spectra, and proton energy versus delay time (summed over the entire detector). In total, approximately 350 million coincidence events were collected, involving approximately 78 Gb of data and resulting in an expected sensitivity of D ≈ 2 × 10−4 Considerable effort has been put into understanding systematic uncertainties to at least the 10−5 level. Monte Carlo calculations suggest that the dominate systematic effect would be a combination of a beam asymmetry and a polarization misalignment. To fully understand this possibility, a series of ‘ATP runs’ were carried out. In these runs the polarization was intentionally misaligned in a variety of directions. This allows a probe of the ATP effect as well as verifying the scaling used to extract the experimental uncertainty. An initial assessment indicates DAT P < 3 × 10−5 . Monte Carlo calculations are also being used to understand the effects of backscattering and proton-recoil effects. These and all other systematic effects appear to be below 1 × 10−4 . A final result is expected by the end of 2004. 1 M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973). 2 P. Herczeg, Progress in Nuclear Physics, W.-Y. P. Hwang, ed., Elsevier Sciences Publishing Co. Inc. (1991) p. 171.


  • Page 15

    UW CENPA Annual Report 2003-2004 May 2004 5 199 1.4 Search for a permanent electric dipole moment of Hg W. C. Griffith, M. D. Swallows,∗ M. V. Romalis† and E. N. Fortson∗ An experimental effort is underway to improve the limit on, or possibly measure, a permanent electric dipole moment (EDM) of 199 Hg. The measurement of a non-zero EDM would likely reveal a new source of CP -violation, and would represent possible experimental evidence for supersymmetry. Our previous measurement1 obtained a limit of |d(199 Hg)| < 2.1×10−28 e cm by comparing the 199 Hg spin-precession frequencies in a stack of two Hg-vapor cells in a common magnetic field and oppositely directed electric fields. We are currently working on a version of the experiment using a stack of four vapor cells, where the two additional cells have zero electric field applied to them, and are used as magnetometers to improve statistical sensitivity and our understanding of systematic effects. As of one year ago, we were attempting to begin dedicated data taking toward the new measurement. However, multiple instances of EDM-like false signals over the next several months led us to shift all of our efforts to trying to understand and eliminate the source of this systematic effect. In our experiment, the signature of an EDM would be a shift in the 199 Hg spin-precession frequency correlated with the electric field direction, and the most dangerous systematic effects are caused by high voltage correlated changes in the magnetic field observed by the vapor cells. The false signals we have generally observed would correspond to a local magnetic field change of 20 pG, or an EDM of 2 × 10−27 e cm, an order of magnitude larger than our previous limit. Besides the large size of the false signals, we are certain that they are not evidence of an actual EDM because the frequency shifts do not appear as symmetrically opposite shifts in the 2 cells with opposite electric fields, and they often occur in the magnetometer cells. In the course of our investigation we have ruled out several leading suspects. The measured steady-state electrical currents are much too small to cause the effect through direct magnetic field generation, even if all of the current maliciously flowed in a complete loop around a vapor cell. It is still a possibility that short current bursts due to high voltage sparks might lead to magnetic orientation of ferromagnetic contaminants, though. Cell movement due to an electric field induced force (e.g. piezoelectric) in the existing magnetic field gradient would lead to an EDM-like effect, but when the magnetic field gradient was increased to about 5 times its normal size there was no increase in the signal. This makes cell motion an unlikely culprit, although motion of a nearby magnetized material is still a possibility. The fact that the occurrence of false signals has been much more frequent with the 4-cell setup compared to the previous 2-cell measurement led to the suspicion that one of the changes implemented in the apparatus has led to an increased vulnerability to a particular systematic effect. This was tested by resurrecting the former 2-cell setup, and the resulting data has been much more susceptible to false signals than it was during the original dataset, indicating that the false signals are not caused by a specific apparatus difference. The origin of the false signals is still unresolved, and understanding this systematic effect remains the focus of our efforts. ∗ Physics Department, University of Washington, Seattle, WA 98195. † Department of Physics, Princeton University, Princeton, NJ 08544. 1 M. V. Romalis, W. C. Griffith, J. P. Jacobs, and E. N. Fortson, Phys. Rev. Lett. 86, 2505 (2001).


  • Page 16

    6 Torsion Balance Experiments 1.5 Sub-mm test of Newton’s inverse-square law E. G. Adelberger, T. S. Cook, J. H. Gundlach, B. R. Heckel, D. J. Kapner and H. E. Swanson Our main systematic effects are associated with the rotation frequency of our motor that drives the short-range attractor. These effects are predominantly mechanical or magnetic. Normally, our gravitational signal of interest is at such a high harmonic of the rotation fre- quency that any effect from the motor is highly attenuated. By increasing the isolation between our vacuum vessel and the motor, adding flexible couplings for a potential misalign- ment of the motor, and stiffening our optical mounts, we have reduced the spurious signals at the motor frequency by greater than a factor of four. We also added additional layers of magnetic shielding around our motor. These reductions are helpful in determining our absolute gravitational calibration, as that signal frequency is at the third harmonic of the motor (as opposed to the short-range signal, which is at the 21st.) This calibration is typically a measurement of a long-range, easily- calculated gravitational torque on the pendulum from the interaction of small pendulum spheres and larger spheres on a turntable. We redesigned the pendulum frame to have much lower multipole moments (up to a factor of 400 lower for m=4) that could couple spuriously to our calibration turntable. The new frame is shown in Fig. 1.5-1. This frame has three spheres permanently mounted at a large distance from the short-range plate, to provide a continuous gravitational calibration, important for accounting for detector non-linearities. To increase our sensitivity, we need to reduce the vertical separation between our pen- dulum and attractor disks. This separation is comprised of the thickness of our electrostatic shield (10µm) the separation between the attractor and the shield (previously 17µm) and the separation between the pendulum and the shield (previously 70µm.) We have recently reduced the latter to at least 40µm with the construction of a new shield and diligent dust- removal. We are set to take data with our new improvements in the next month. Figure 1.5-1. The new short-range frame and 21-fold symmetric disk.


  • Page 17

    UW CENPA Annual Report 2003-2004 May 2004 7 1.6 Spin pendulum update B. R. Heckel, T. S. Cook, E. G. Adelberger and H. E. Swanson The next-generation spin pendulum has been assembled and is currently mounted in the original rotating Eöt-Wash torsion balance for the purpose of searching for new forces that couple to spin. The spin pendulum is a torsion balance that utilizes the magnetic properties of Alnico and SmCo to produce a spin-polarized test body.1 By accurately measuring the affinity of the test body for a prefered direction in space (or lack thereof), the pendulum puts limits on possible simultaneous Lorentz and CPT symmetry violation as described by Colladay and Kostelecky.2 Over the past year, a number of design improvements have been implemented to increase the overall sensitivity to such a symmetry violation. Smaller - Lighter: The diameter of the pendulum has been reduced while maintaining the same active mass as previous pendulums. In the past, pendulums were built with an aluminum harness to place rectangular magnets into an octagonal ring. For the new pendu- lum, the magnets are precision machined into trapezoidal wedges to form the octagonal ring. The maximum diameter of a ring has been reduced from 1.67” to just 1.04”. This reduced geometry requires less inert mass to support the ring structures, lowers the torsion moment of the pendulum and, due to the decreased mass, allows the inclusion of a magnetic shield on the pendulum while still using a 30-µm diameter suspension fiber. Improved Optics: The angular position of the pendulum is monitored with an auto- collimator that reflects a laser off a mirror attached to the pendulum. The spin pendulum experiments run in the old Eöt-Wash apparatus which is designed for a single reflection off the pendulum mirror. However, one can double the angular resolution by double reflection: reflecting the light to an auxiliary mirror and then back to the pendulum mirror before return- ing it to the auto-collimator. We have refitted the old Eöt-Wash apparatus to accommodate a double reflection system. As may be expected, the attenuation of all magnetic “leakage” fields from the pendulum is essential to performing a precise measurement. A magnetic coupling between the pendulum and the earth’s field or any ambient laboratory fields could overwhelm a potential signal or even produce a false signal. To ascertain the strength of our leakage fields accurately, we have constructed a stepper-motor driven turntable and data acquisition system to read data from a 3-axis hall probe as the pendulum is rotated. With this system we have been able to measure the fields at any given radius from the pendulum to less than 50 µG. Data with the new spin pendulum have been taken for six months. There is no evidence for magnetic coupling of the pendulum to the environment and the new angular reflection system is working well. An unknown systematic signal has been observed that appears to be associated with the torsion fiber pre-hangar and magnetic damper at the top of the torsion fiber. Measurements are being made to understand and eliminate this error before undertaking a search for new spin-coupled forces. 1 M. G. Harris, Ph.D. thesis, University of Washington, 1998. 2 D. Colladay and V. A. Kostelecky, Phys. Rev. D 55, 6760 (1997); ibid. 58, 116002 (1998).


  • Page 18

    8 1.7 A new equivalence-principle test E. G. Adelberger, K. Choi, J. H. Gundlach, B. R. Heckel, S. Schlamminger and H. E. Swanson We are testing the weak equivalence principle using a rotating torsion balance. A composition dipole, consisting of titanium, beryllium, or aluminum test bodies, is suspended from a torsion fiber inside a vacuum chamber. The vacuum chamber is hanging from a constantly rotating turntable. A violation of the equivalence principle would result in a periodic differential acceleration of the two materials directed towards a large variety of sources. We will be able to test the equivalence principle for Yukawa ranges from 1 m to ∞. In particular we can test for differential accelerations between the two different materials toward the Sun and the center of our Galaxy. By combining our measurement toward the Sun with lunar laser-ranging results, we will set new limit for the Strong Equivalence Principle. Since about 25% of the acceleration towards the center of the Galaxy is caused by dark matter, the measurement toward the center of our Galaxy allows us to test for the Equivalence Principle for the galactic dark matter. During the past year we built a new gradiometer pendulum (Fig. 1.7-1) to measure the strength of the Q21 , Q31 and Q41 gravity gradient fields at the location of the pendulum. The gradiometer pendulum consists of four equally spaced aluminum disks, which have chamfered holes to seat titanium balls. By changing the position of 16 titanium balls, we can create large q21 , q31 and q41 moments of this pendulum. In all three formations the moment of interest is maximized, while the other moments vanish by design. Thus we can measure each of the Q21 , Q31 and Q41 ambient gravity-gradient fields independently. We compensated the fields with Q21 , Q31 gravity-gradient compensators masses so that their strength1 was about 3% and 4% the uncompensated strength, respectively. Small tuning screws on the EP-pendulum were adjusted to reduce the q21 and q31 moments. The residual coupling due to gravity gradients is calculated to be less than 1 nrad. Other systematic effects, which include tilt, magnetics, thermal and etc. are highly reduced.2 Our current differential acceleration sensi- tivity is 1.3×10−14 m/s2 per day measurement which corresponds to an angle of 2 nrad. We plan to start a long data run in the near fu- ture with the goal of achieving a sensitivity of 1.0 × 10−15 m/s2 for accelerations toward the Sun and the center of our Galaxy. Figure 1.7-1. Gradiometer pendulum. Shown is the q31 -configuration for the measurement of the Q31 gravity gradient. 1 The fields change slowly due to rainfall about 2 % seasonally. 2 CENPA Annual Report, University of Washington (2003) p. 13.


  • Page 19

    UW CENPA Annual Report 2003-2004 May 2004 9 1.8 Small force measurements for LISA E. G. Adelberger, R. C. Carr, T. Cook, J. H. Gundlach, B. R. Heckel, M. J. Nickerson, B. R. Osting, S. Schlamminger and H. E. Swanson The scientific goal of the Laser Interferometer Space Antenna, LISA,1 a joint project of the ESA and NASA, is to observe gravitational waves in the frequency range of 10−4 to 10−1 Hz. The interferometer is formed by three identical spacecrafts 5, 000, 000 km apart at the corners of an equilateral triangle. Each spacecraft carries two freely floating cubical masses (gravitational reference sensors), which define the ends of the interferometer arms. The spacecraft is servoed to maintain a constant distance of 2 to 4 mm between the sensor housing and the sensor. Major concerns of the mission are small spurious forces acting on the small masses, caused by e.g. patch effects, which limit the performance of LISA by introducing additional noise at low frequencies. In order to investigate these effects we have built a sensitive torsion balance. The torsion pendulum consists of a gold coated glass plate (38.1 mm × 38.1 mm × 1 mm) and two gold coated aluminum cylinders mounted perpendicular to the plate at top and bottom of the glass. These cylinders compensate the q22 moment of the plate and therefore minimize the sensitivity to moving masses in the lab. Adjacent to the pendulum, a gold coated glass plate can be moved near to the pendulum plate. This setup allows us to test for forces that may arise between the LISA housing and the proof mass. We have developed a new capacitive method to measure the position of the pendulum. To avoid large deflection of the pendulum from the equilibrium position, only small voltages can be applied between the grounded plate and the pendulum. The capacitor formed by the pendulum and the plate is part of a low-pass filter. The phase shift of a small (2 mV) AC signal caused by this filter is detected with a lock-in amplifier and measured for various plate positions. In the course of the last year we have im- proved the torque noise of the apparatus by 1e-14 various measures. We have added a magnetic z) shield to reduce coupling to magnetic fields. H 1e-15 torque (Nm/√ In order to minimize vibrational couplings, the thermal insulating house was mechanically iso- lated from the concrete foundation. To in- 1e-16 crease the torque sensitivity of the pendulum, we have employed a thinner torsion fiber (di- 1e-17 ameter 12.7 µm). Finally, we have improved 0.001 0.01 the magnetic damper, a device used to damp frequency (Hz) out the swing motion of the pendulum and ex- cess vertical vibrations of the pendulum. As Figure 1.8-1. Measured torque noise (dotted line) and thermal limit (solid line) of the tor- a result of these improvements, the measured sion pendulum at 11 mm separation between noise of the torsion pendulum is at the thermal the attractor plate and the pendulum. limit, see Fig. 1.8-1. 1 http://lisa.jpl.nasa.gov/.


  • Page 20

    10 1.9 The development of a torsion-pendulum based axion search E. G. Adelberger, B. R. Heckel, S. A. Hoedl, F. V. Marcoline and H. E. Swanson We have designed a torsion-pendulum search for the axion which offers an improvement of 1018 over the most recent measurement1 for an axion mass of ∼ 200 µeV. The axion is the result of the hypothesized Peccei-Quinn symmetry and is a favored cold dark matter candidate.2 The mass is constrained by the known flat geometry of the universe to be heavier than 1 µeV (λa < 20 cm), and is constrained by the neutrino flux from SN1987A to be lighter than 1000 µeV (λa > 0.02 cm). Note that microwave cavity searches probe for light axions (1.2 µeV < ma < 12.4 µeV). A torsion-pendulum based search is possible because the axion mediates a macroscopic pseudo-scalar potential between polarized and unpolarized fermions. The axion pendulum (see Fig. 1.9-1) will consist of four 4 × 4 cm planes of 250 µm thick germanium. In each quadrant defined by the germanium planes will be placed a quarter piece of a toroidal electro- magnet. The pole faces of these magnets will be positioned 100 µm away from the germanium planes, and will have a maximum field strength of 20 kG. When energized, the oriented electron spins in the ferromagnetic core provide the source of polarized fermions. The nucleons in the germanium provide the source of unpolarized fermions. By reversing the magnetization orientation at a fixed frequency, the pendulum will feel an axion-mediated torque at that frequency. After 100 days of measurement time, we anticipate a limit on the axion coupling as a function of the axion Compton wavelength as presented in Fig. 1.9-1. An unexplained positive signal for an axion can be rigorously identified as a systematic error. The axion torque scales as e−h/λa , where h is the distance between a pole face and the germanium. Magnetic systematic errors (i.e. due to fixed electron spins in the germanium) will have a much slower fall off with h. Axion Sensitivity 1e-15 Proposed Axion Experiment Ni et al. Youdin et al. 1e-20 1e-25 (gs gp)/(−h c) 1e-30 1e-35 ΘQCD = 10-9 1e-40 ΘQCD = 10-14 1e-45 0.1 1 10 Range (cm) Figure 1.9-1. The axion pendulum and our expected sensitivity to the axion electron-nucleon coupling as a function of the axion Compton wavelength compared with recent experimental searches and the expected coupling for different values of ΘQCD . 1 W. T. Ni et. al. Phys. Rev. Lett. 82, 2439 (1999). 2 L. J. Rosenberg and K. A. van Bibber, Phys. Rep. 325, 1 (2000).


  • Page 21

    UW CENPA Annual Report 2003-2004 May 2004 11 2 Neutrino Research SNO 2.1 The Sudbury Neutrino Observatory J. F. Amsbaugh, T. H. Burritt, G. A. Cox, P. J. Doe, C. A. Duba, J. A. Formaggio, G. C. Harper, M. A. Howe, K. K. S. Miknaitis, S. McGee, A. W. Myers, N. S. Oblath, J. L. Orrell,∗ K. Rielage, R. G. H. Robertson, M. W. E. Smith and L. C. Stonehill The Sudbury Neutrino Observatory (SNO) is a heavy water C̆erenkov detector that was designed to study the flux, spectrum, and oscillation characteristics of 8 B neutrinos from the Sun. Previous solar-neutrino experiments, able to detect primarily electron neutrinos, have observed a deficit in the flux of solar neutrinos relative to the predictions of solar models. This discrepancy between the observed flux of electron neutrinos and the expected flux is known as the Solar-Neutrino Problem. The use of heavy water as a target medium in SNO allows sensitivity not only to electron neutrinos, but also to muon and tau neutrinos, through the neutral current (NC) reaction of neutrinos on deuterium. The three primary reactions through which we detect solar neutrinos are νx + 2H → νx + p + n Neutral Current (NC) (1) 2 νe + H → e− +p+p Charged Current (CC) (2) − νx + e → νx + e− Elastic Scattering (ES) (3) where x denotes any of the three active neutrino flavors. A comparison of SNO’s measured CC and NC rates has shown that solar electron neutrinos are oscillating into muon and tau neutrinos prior to reaching our detector. SNO’s results over the past two years have thus provided strong evidence for the oscillation of solar neutrinos and solved the long-standing puzzle of the missing solar neutrinos. A wide range of activities has taken place in the past year for the Sudbury Neutrino Observatory. At the Summer TAUP’03 Conference, SNO announced first results from data taken during the salt phase of the experiment. The 254-day data set provided a more sensitive measurement of the neutral-current-neutrino flux from the sun. Results were consistent with SNO’s previous analyses and helped restrict the solar mixing parameters solely to the Large Mixing Angle solution (see Fig. 2.1-1). In October 2003, the salt was extracted from the D2 O, paving the way for the third and final phase of the experiment, known as the neutral- current phase. An array of discrete neutral-current detectors (NCDs) is installed to provide an independent means of detecting the NC neutrons. The array deployment commenced and was successfully executed this past winter. It is anticipated that the data from this NCD phase, along with data from the previous phases, will significantly enhance our understanding of the properties of neutrinos. ∗ Presently at Pacific Northwest National Laboratory, Richland, WA 99352.


  • Page 22

    12 The University of Washington has played a major role in the experiment. Primary ac- tivities include analysis of the data from the salt phase of the experiment, coordination and installation of the neutral current detectors and development of the data acquisition system for the main SNO detector. Figure 2.1-1. Global neutrino oscillation contours. (a) Solar global: D2 O day and night spectra, salt CC, NC, ES fluxes, SK, Cl, Ga. The best-fit point is ∆m2 = 6.5 × 10−5 , tan 2 θ = 0.40, fB = 1.04, with χ2 /d.o.f. = 70.2/81. (b) Solar global + KamLAND. The best-fit point is ∆m2 = 7.1 × 10−5 , tan 2 θ = 0.41, fB = 1.02. In both (a) and (b) the 8 B flux is free and the hep flux is fixed.


  • Page 23

    UW CENPA Annual Report 2003-2004 May 2004 13 16 2.2 N production by muons in SNO N. S. Oblath, J. L. Orrell∗ and R. G. H. Robertson Any 16 N that is produced in SNO would be a significant background to the charged current (CC) neutrino signal. 16 N decays via a 6.13-MeV γ and an electron with an energy of up to 4 MeV, or by a 10 MeV electron. The resulting spectrum resembles the CC spectrum. 16 N is produced primarily by muons through spallation or capture on 16 O, or by fast-neutron interactions with 16 O, where the fast neutrons come from cosmic ray interactions in the water. An efficient method for identifying muons has been developed by John Orrell for his work on solar antineutrinos. We used his list of muons and searched for evidence of the decay of 16 N in a time window after each muon. The minimum edge of the time window was chosen to exclude the high multiplicity of free neutrons that are created by muons. The neutron capture time during the salt phase is 4 ms. Therefore, after 1 second all of the neutrons will have been captured. The time window extended to 50 seconds after each muon. The half-life of 16 N is 7.13 s, so the time window extends to 7 times the half-life, and should include almost any 16 N which is produced by a muon. Two other cuts were used to reduce backgrounds. The radial fiducial volume cut was chosen to be the same as that used for SNO’s published results. We also made a cut on the minimum number of PMT hits. This reduced the low-energy background, but sacrificed approximately 16% of any 16 N events. The analysis was performed on SNO’s 400-day salt-phase data by fitting an exponential with the appropriate decay constant, plus a flat background, to the data in the post-muon time window. The fit is shown in Fig. 2.2-1. The actual value for the initial activity is negative: N0 = −0.57 ± 1.34 s−1 . This is consistent with zero 16 N produced by muons in SNO. Analyses at other SNO institutions of D2 O-phase data have shown that a small Figure 2.2-1. The fit of the post-muon events. The fit includes an exponential decay plus a flat background. The results of the fit are consistent with zero: N0 = −0.57 ± 1.34 s−1 . amount of 16 N may have been produced by several high-energy muons. We hope to perform an analysis of the D2 O-Phase data to check those results in the near future. ∗ Presently at Pacific Northwest National Laboratory, Richland, WA 99352.


  • Page 24

    14 2.3 Muon-induced neutrons at the Sudbury Neutrino Observatory — refined and extended results plus new Monte Carlo capabilities J. A. Formaggio and J. L. Orrell∗ In a previous Annual Report1 we presented an analysis of the events following within one- half second of the passage of a cosmic-ray muon through the Sudbury Neutrino Observatory (SNO) detector. These events are predominately spallation neutrons. Of particular interest2 is the number of neutrons produced by a single muon, succinctly, the neutron multiplicity. Fig. 2.3-1 presents the results of a refinement of the method presented in the previously mentioned Annual Report. Data from both the D2 O and salt phases of the SNO experiment D2O Selecting Muons + Muon Followers RAW 5945 435 Salt Selecting Muons & Muon Followers hMulti_hi_0 Entries19021 Integral 1.402e+04 hMulti_hi_1 19021 Entries Integral 853 RAW 4 Muon cuts: Simple ID Muon cuts: Simple ID 10 Number Number Follower cuts: Instrumental, N hit >25 Follower cuts: Instrumental, N hit >20 Muon cuts: Simple ID Muon cuts: Simple ID Follower cuts: Instrumental, N hit >20, 3 Follower cuts: Instrumental, N hit >25, 10 3 Energy & volume Energy & volume 10 Muon cuts: Simple ID, Fit, Decay Ring Test Muon cuts: Simple ID, Fit, Decay Ring Test Follower cuts: Instrumental, N hit >20, Follower cuts: Instrumental, N hit >25, Energy & volume 2 Energy & volume 2 10 10 10 10 1 1 1 10 100 1 10 100 Multiplicity Multiplicity Figure 2.3-1. The multiplicity of spallation neutrons following a muon for different selection criterion on the muons and muon followers. In each plot, the lowest curve is predominately below a multiplicity of 10 while the two higher curves extend above a multiplicity of 100. have now been analyzed. The results clearly show muons with neutron multiplicities over 100. However, the decay ring test1 shows high- and low-multiplicity cases can be partitioned by looking for high-energy C̆erenkov events following within 11 microseconds after a muon. In conjuction with the analysis shown above, work has proceeded so as to properly sim- ulate photoneutron production from high-energy muons. Photoneutron production involves the exchange of a virtual photon as the muon passes through matter. Theoretically, it is possible to calculate the muon photonuclear cross-section by use of the Equivalent Pho- ton Approximation. This technique, originally proposed by Fermi3 and later developed by C. F. Weizsäcker4 and E. J. Williams,5 relates the real photonuclear cross-section to the virtual cross-section. Other processes that are responsible for neutron production, such as secondary interactions, are also included. Comparisons of the experimental data against the- oretical predictions are underway and should provide useful information to next-generation deep-underground experiments. ∗ Presently at Pacific Northwest National Laboratory, Richland, WA 99352. 1 CENPA Annual Report, University of Washington (2003) p. 20. 2 Y.-F. Wang et al., Phys. Rev. D 64, 013012 (2001). 3 E. Fermi, Physik Z. 29, 315 (1924). 4 C. F. Weizsäcker Z. Phys. 88, 612 (1934). 5 E. J. Williams et al., Kgl. Dan. Vidensk. Selsk. Mat. Fys. Medd.XIII, 4 (1935).


  • Page 25

    UW CENPA Annual Report 2003-2004 May 2004 15 2.4 Atmospheric-neutrino-induced muons at the Sudbury Neutrino Ob- servatory J. A. Formaggio, N. S. Oblath and J. L. Orrell∗ High energy muons and neutrinos are produced constantly by the interaction of primary cosmic rays with nuclei of the Earth’s upper atmosphere. These primary interactions produce mesons (e.g. π, K and short-lived charmed D mesons) which decay into neutrinos and muons. 14 p+ N → π + (K + ) + X (1) + + + + π (K , D ) → µ νµ (2) + + µ → e νe ν̄µ (3) At the depth of 2092 meters, where the Sudbury Neutrino Observatory (SNO) is located, only muons and neutrinos are able to penetrate. SNO is currently in a unique position amongst world experiments located underground. At the depth of over 6-km water equivalent, it is the deepest underground laboratory currently in operation. The vertical flux intensity at this depth is 4.1 × 10−10 µ/cm2 ·s·sr, which corresponds to a rate of about 3 muons/hour. SNO can make two important measurements with respect to muons present in the de- tector. First, SNO is sensitive to the downward-muon rate coming from primary cosmic ray interactions. The muon rate is expected to fall rapidly as a function of depth: h0 α h Iµ (h) = Aν ( ) exp (− ) (4) h h0 where h is the depth of rock, and h0 is the scale height. By measuring the muon flux as a function of zenith angle, it is possible to test predictions regarding the energy dependence of the muon flux at the surface, as well as the transport mechanism of extremely high-energy muons. Second, SNO’s unprecedented depth allows for an unprecedented measurement of at- mospheric neutrinos (via the detection of neutrino-induced muons) at inclinations as large as cos (θZenith ) ' 0.4. These atmospheric-muon neutrinos can interact with the rock around SNO. They produce penetrating muons that travel up to 10 km w.e. The predicted rate of neutrino-induced muons is about 120 muons/year and, as such, it is mainly a measurement limited by statistics. This rate is small compared with the downward muon rate, but SNO’s angular resolution is sufficient to make a clean separation at an angle of cos (θZenith ) ≤ 0.4. This feature is very important; all comparable experiments are at a much shallower depth than SNO and, thus, cannot distinguish neutrino-induced muons above the horizon. SNO’s unique niche allows it to make important model-independent checks of atmospheric neutrino oscillations. ∗ Presently at Pacific Northwest National Laboratory, Richland, WA 99352.


  • Page 26

    16 SNO is progressing in completing measurements of both the downward and upward cosmic-ray muons. The neutral-current detectors do not add to the measurement of such muons, but the additional running time greatly improves the measurement, especially for neutrino-induced muons. A total dataset of 1000 days (an additional 300 live days from the NCD phase) would mean a total data set of 360 neutrino-induced muons, or a ∼ 5% mea- surement (statistics only). SNO is currently exploring a number of ways to also improve the systematic errors associated with muon identification. SNO is also sensitive to atmospheric-neutrino interactions that are contained within the fiducial volume of the detector itself and to measure the neutron flux induced from cosmic rays. SNO’s sensitivity to these reactions is currently being explored.


  • Page 27

    UW CENPA Annual Report 2003-2004 May 2004 17 2.5 NUANCE: atmospheric-neutrino simulation in SNO J. A. Formaggio, N. S. Oblath and J. L. Orrell∗ NUANCE is an atmospheric-neutrino simulator. The code was written by David Casper, and it is used as a stand-alone program with links to the CERN libraries. NUANCE simulates six categories of scattering interactions: quasi-elastic, resonance, coherent, diffractive, deep- inelastic and elastic. Also, it simulates neutral-current and charged-current interactions, as well as neutrino oscillations and matter effects. In the end, kinematic information is provided for each vertex. NUANCE takes as its inputs information about the neutrino flux and information about the detector geometry. The flux inputs can be in the form of an arbitrary beam, or a histogrammed flux. We have used a flux which is specific for SNO, as determined in 1996 by the Bartol group.1 The low-energy flux (< 10 MeV) is particular to Sudbury due to geomagnetic effects, while the high-energy flux is a general flux. The geometric input is a simple description of SNO built up with concentric spherical shells of specified materials. At the center is a 6-m radius sphere of heavy water. Surrounding that is a 5-cm thick shell of acrylic, and then a 2.5-m thick shell of light water. NUANCE is now being used as the atmospheric-neutrino generator for SNO Monte Carlos. The output of NUANCE is a file which contains kinematic information about the vertices that result from the atmospheric-neutrino interactions. This file is then converted into a format which can be read by the Monte Carlo program, SNOMAN. By running the NUANCE output through SNOMAN, we can study the effects of atmospheric neutrinos in SNO. We have studied the systematics of the NUANCE simulation based on the uncertainties of eight parameters: the axial mass in quasi-elastic scattering, Pauli suppression in oxygen, un- certainties in the resonance channels, neutrino oscillation parameters (θ12 ,θ23 , ∆m212 , ∆m223 ), and the uncertainty in the total atmospheric neutrino flux. We found that for neutral-current interactions there is a 28% uncertainty, and for charged-current interactions there is a 30% uncertainty. Table 2.5-1 shows a summary of the systematics. Systematic CC % Error NC % Error +1.49 θ12 −5.91 N/A ∆m212 +1.77 N/A θ23 +4.37 N/A +2.11 ∆m223 −1.31 N/A +9.23 +9.22 Axial Mass −6.44 −9.39 +14.42 +19.35 Pauli Suppression −15.37 −19.18 Resonance Uncertainty ±6.78 ±7.31 Total Flux ±20.00 ±20.00 +27.71 +30.24 Total −27.58 −30.22 Table 2.5-1. Listing of NUANCE systematics. ∗ Presently at Pacific Northwest National Laboratory, Richland, WA 99352. 1 V. Agrawal, T. K. Gaisser, P. Lipari and T. Stanev, Phys. Rev. D 53, 1314 (1996).


  • Page 28

    18 2.6 Estimation of the background to an ν e analysis of SNO data J. L. Orrell∗ Work done at CENPA is, in large part, responsible for the estimation of the background to an electron-antineutrino (ν e ) analysis of the pure heavy-water phase of the Sudbury Neutrino Observatory (SNO) experiment. The electron-antineutrino analysis of SNO is a low-signal, low-background analysis employing extensions to the well known Feldman & Cousins1 method of confidence level determination for signals and limits on signals. A reported result using a Feldman & Cousins style confidence level determination requires an estimation of the back- ground, hence the importance of this work. The electron-antineutrino analysis of SNO data is a continuing effort2 to measure or limit the conversion of 8 B solar electron neutrinos (νe ) into electron antineutrinos (ν e ), comparable to recent results3,4 from other subterranean neutrino detectors. This conversion (νe → ν e ) is a possibility for massive, Majorana-type neutrinos via an interaction between a neutrino mag- netic moment and solar magnetic fields.5 A measured signal would provide new insight into the properties of massive neutrinos, while a limit on the conversion mechanism implies limits on the combination of the magnitude of the neutrino magnetic moment and the maximum magnitude of solar magnetic fields. Table 2.6-1 presents a preliminary estimation of the background to a search for a solar electron-antineutrino signal. Note that electron antineutrinos are detected by coincidences of events produced by any of the three product particles in the reaction ν e + d → e+ + n + n. The estimated number of coincidences reported is for the pure heavy-water phase of the ν e background Non-ν e background Source Coincidences Process Coincidences Atmospheric ≤ 0.072 Atmospheric ν 1.46 Reactor < 0.019 238 U: spontaneous fission < 0.79 Diffuse supernova ≤ 0.005 Accidental coincidences 0.13 Table 2.6-1. Estimated number of background coincidences in a search for a solar electron antineutrino signal. Smaller, non-ν e induced backgrounds are not shown. experiment composed of data recorded between 2 November 1999 and 28 May 2001. Separate Annual Reports provide further details of the reactor6 and atmospheric neutrino (See Section 2.5) calculations. The SNO group at CENPA has played an important role in bringing the electron antineutrino analysis closer to publication. ∗ Presently at Pacific Northwest National Laboratory Richland, WA 99352. 1 G. J. Feldman and R. D. Cousins, Phys. Rev. D 57, 3873 (1998). 2 CENPA Annual Report, University of Washington (2003) p. 21. 3 K. Eguchi et al. (KamLAND Collaboration), Phys. Rev. Lett. 92, 071301 (2004). 4 Y. Gando et al. (Super-Kamiokande Collaboration), Phys. Rev. Lett. 90, 171302 (2003). 5 E. Kh. Akhmedov, S. T. Petcov and A. Yu. Smirnov, Phys. Rev. D 48, 2167 (1993). 6 CENPA Annual Report, University of Washington (2003) p. 22.


  • Page 29

    UW CENPA Annual Report 2003-2004 May 2004 19 2.7 Neutral-current results from the salt phase of SNO J. A. Formaggio, K. K. S. Miknaitis, R. G. H. Robertson, J. F. Wilkerson and the SNO Collaboration The Sudbury Neutrino Observatory is sensitive to solar neutrinos through three interactions of neutrinos with the heavy water in the detector volume: Charged Current (CC) Reaction: νe + d −→ p + p + e− Elastic Scattering (ES) Reaction: νx + e− −→ νx + e− Neutral Current (NC) Reaction: νx + d −→ p + n (1) Here, νe indicates electron-flavor neutrinos, and νx indicates neutrinos of any flavor. SNO’s sensitivity to both the CC and the NC interactions has allowed the experiment to probe flavor change in solar neutrinos. In September of 2003, the Sudbury Neutrino Observatory released first results from the second phase of the experiment.1 In this phase of the experiment, ultra-pure salt (NaCl) was added to the D2 O in the detector to enhance sensitivity to the NC reaction. Neutron capture on 35 Cl has a higher cross section than neutron capture on deuterium, and the gamma cascade emitted has a higher energy (8.6MeV). A consequence of the gamma cascade is that light emitted in salt neutron-capture events is distributed more isotropically around the detector than the characteristic distribution of light from Čerenkov-radiating electrons from the CC or ES interactions. The difference in isotropy of the light emitted in neutron and electron interactions in the detector allowed a statistical separation of CC and NC fluxes that did not require any assumptions about the incoming neutrino spectrum. Using 254.2 days of data taken between July 26, 2001 and October 10, 2002, the extracted neutrino fluxes reported were (in units of 106 cm−2 s−1 ): +0.06 φCC = 1.59+0.08 −0.07 (stat.)−0.08 (syst.) φES = 2.21+0.31 −0.26 (stat.) ± 0.10(syst.) φNC = 5.21 ± 0.27(stat.) ± 0.38(syst.) These results, which were obtained without constraints on the incoming-neutrino spectrum, allowed maximal mixing in the solar neutrino sector to be ruled out to 5.4σ. The best-fit neutrino-oscillation parameters in a global fit of all solar-neutrino data including the salt results were: −5 ∆m2 = 7.1+1.0 −0.3 × 10 eV 2 θ = 32.5+1.7 −1.6 degrees Progress is being made on analysis of the remaining portion of the salt data. 1 S. N. Ahmed and the SNO Collaboration, Phys. Rev. Lett. 92, 181301 (2004).


  • Page 30

    20 2.8 The day-night measurement in the salt phase of SNO J. A. Formaggio, K. K. S. Miknaitis and the SNO Collaboration Thanks to results from SNO, we now know that electron-type neutrinos from the sun are oscillating into mu- and tau-type neutrinos prior to reaching the earth. This is explained through matter-enhanced neutrino oscillations (the so-called MSW effect) taking place inside the interior of the sun. In the same way that matter effects can change the flavor composition of neutrinos passing through the sun, matter effects could also affect the flavor of neutrinos passing through the earth. We can look for this effect by comparing the flavor composition of solar neutrinos detected during the night, after they have traveled through the earth, to solar neutrinos detected during the day. If we were to detect a day-night asymmetry in the flavor composition of solar neutrinos, this could be a direct test of MSW physics. To search for a day night effect, we can construct an asymmetry parameter for electron- type neutrinos: Φe,N − Φe,D Ae = 2 (1) Φe,N + Φe,D where Φe,N and Φe,D are the night and day electron-neutrino fluxes, respectively. Detecting a positive day-night asymmetry in the electron-neutrino flux would be direct evidence for MSW physics. Neutrino-oscillation parameters in the currently favored region predict electron- neutrino day-night asymmetries between 2% and around 7%. In the first phase of SNO we measured A = 7.0 ± 4.9(stat.) ± 1.3(syst.)%. This measurement is statistically limited, so we look forward to adding the additional data from the salt and NCD phases. We are currently in the process of analyzing the salt-phase data for a day-night asymmetry. The first step in this measurement is an accurate determination of our day and night livetime, accounting for all data cleaning cuts. SNO’s livetime is measured using a 10MHz GPS- synchronized clock, which is then checked using a 5Hz pulsed global trigger. Since we do not have the ability to do regular calibrations of the SNO detector during the night, determination of systematic uncertainties in diurnal stability of the detector response must be done using in-situ techniques. By studying the differences in rates and characteristics of signals that are always present in our detector, we can limit systematic variations in parameters such as energy scale or resolution day and night that could affect the asymmetry measurement. Classes of events that are used for these studies include a slightly radioactive “hot spot” on the acrylic vessel, low-energy backgrounds in the detector materials, and spallation neutrons due to cosmic-ray muon interactions. The muon follower events are a particularly attractive event class for these studies, because they mimic the NC signal.


  • Page 31

    UW CENPA Annual Report 2003-2004 May 2004 21 SNO Neutral Current Detectors (NCDs) 2.9 NCD backgrounds and data cleaning G. A. Cox, H. Deng,∗ P. J. Doe, S. R. McGee, K. Rielage, R. G. H. Robertson, L. C. Stonehill and J. F. Wilkerson Now that the SNO NCD array has been deployed, efforts are shifting towards analysis ac- tivities such as background identification and rejection. This is a two-step process, where first non-physics backgrounds must be rejected and then the signal must be separated from physics backgrounds, such as alphas from the U- and Th-decay chains. Many non-physics backgrounds have been identified by hand-scanning through data taken during the cool-down phase and during NCD deployment. Based on the types of non-physics events, cuts are be- ing developed to remove them. One major tool for rejecting backgrounds is the fact that the NCD system has different triggers in the shaper and digitized data paths. The shaper trigger integrates the pulse over several microseconds, so it is effectively a charge threshold, whereas the digitized data trigger has a very short integration time, so it is effectively a current amplitude threshold. Physics events will trigger both paths but many different types of non-physics events will trigger only one or the other. For example, oscillatory noise may have large amplitudes but integrate to small or zero charge. Similarly, HV related events such as microdischarge may also carry a small charge, due to the short duration of the discharge. By requiring that all candidate events pass both triggers, many non-physics backgrounds can be rejected. Other types of non-physics backgrounds require special cuts, which are being developed here and by SNO collaborators. For example, cuts are being developed to reject microphonic noise induced by blasting and seismic activity in the mine where SNO is housed. Once the non-physics backgrounds are removed from the NCD data, most of the remaining events should be signal neutrons and U- and Th-chain alpha-particle emission from the NCD walls. The NCDs detect thermalized neutrons via the 3 He(n,p)3 H reaction. The neutron capture efficiency is such that we expect of order 1000 captures per year in the entire NCD array from solar-neutrino neutral-current interactions with SNO’s heavy water. In addition to the main array of 36 3 He strings, there are 4 strings filled with 4 He-CF4 gas to assess non-neutron backgrounds. Data taken from the NCDs before their deployment into SNO indicates that the intrinsic alpha background rates are on the order of 100 events per day in the entire NCD array. A substantial number of these background events can be eliminated by energy cuts, since the 3 He(n,p)3 H reaction deposits 764 keV of energy in the NCD and the alpha backgrounds can range up to about 9 MeV. In addition, many more of these background events can be cut using pulse-shape analysis of the digitized current pulses from the NCDs. A technique using pulse duration vs. energy scatter plots to separate neutrons from physics backgrounds is the default pulse-shape analysis, although other techniques are being developed, such as a semi-analytic pulse-shape fitter. ∗ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104-6396.


  • Page 32

    22 2.10 Deployment of an array of Neutral Current Detectors (NCDs) in SNO J. F. Amsbaugh, J. Banar,∗ T. A. Burritt, G. A. Cox, P. J. Doe, D. Earle,† J. A. Formaggio, G. C. Harper, J. Heise,∗ A. Krueger,‡ A. Krümins,§ I. T. Lawson,¶ K. Lesko,k S. McGee, B. Morrisette,† A. W. Myers, R. W. Ollerhead,¶ K. R. Rielage, R. G. H. Robertson, M. W. E. Smith, L. C. Stonehill, T. D. Van Wechel, B. L. Wall, J. F. Wilkerson and J. Wouters∗ An array of 40 proportional counters has been installed in the heavy water volume of SNO. This array consists of 36 3 He filled counters and four 4 He filled counters (as controls) arranged on a 1-m square grid about the center of SNO’s acrylic vessel (AV). The NCDs are 9 to 11 m long and are comprised of combinations of at least three counter sections of 2, 2.5 or 3 m in length welded with an Nd-YAG laser designed at UW.1 Each NCD has an open-ended electrical configuration with an 80-nsec roundtrip delay line con- tained within a housing which terminates the NCD string and at least 9-m of readout cable required to run the length of the neck of the AV to reach the NCD data acquisition system.2 Constraints imposed by the size of the cavity in which SNO sits limited the length of sections that could be deployed at one time to 5.5 m. This meant that at least one weld per string was needed at the time of deployment. After the final weld was done over the neck of the AV, but before being flown to its final position with the Remotely Operated Vehicle (ROV),3 a 60-Hz AmBe neutron source was placed near the center of each counter section of the NCD. The gain, resolution and total number of neutrons detected for each section was compared to known pre-deployment values to insure each NCD was functioning properly. The first NCD was successfully deployed on December 2, 2003. Extensive monitoring of each NCD’s data quality was conducted throughout the entire deployment period. Setbacks led to the extraction, testing and redeployment of a handful of the NCDs and caused the deployment schedule to run beyond its original end-date. The deployment phase officially ended on April 21, 2004 with the removal of the ROV from the AV. The addition of NCDs begins phase III of the SNO experiment and allows for event-by- event separation of the neutral current (NC) measurement from that of the charged current (CC). This effectively makes phase III a different experiment from the earlier phases of SNO and puts it in the unique position of being able not only to make an improved NC/CC measurement but also to confirm its own previous results. ∗ Los Alamos National Laboratory, Los Alamos, NM 87545. † Sudbury Neutrino Observatory, Lively, Ontario, Canada P3Y 1M3. ‡ Laurentian University, Sudbury, Ontario, Canada P3E 2C6. § Queens University, Kingston, Ontario, Canada, K7L 3N6 ¶ University of Guelph, Guelph, Ontario, Canada N1G 2W1. k Lawrence Berkeley National Laboratory, Berkeley, CA 94720. 1 CENPA Annual Report, University of Washington (2001) p. 37. 2 CENPA Annual Report, University of Washington (2003) p. 28. 3 CENPA Annual Report, University of Washington (2001) p. 30.


  • Page 33

    UW CENPA Annual Report 2003-2004 May 2004 23 2.11 NCD analysis tools and electronics calibrations G. A. Cox, P. J. Doe, A. L. Hallin∗ , M. A. Howe, S. McGee, K. R. Rielage, R. G. H. Robertson, L. C. Stonehill, B. Wall and J. F. Wilkerson The NCD data readout system has undergone a major change within the last year. While the hardware remained essentially unchanged, our data-acquisition software migrated from SHaRC to ORCA.1 Both programs are similar in function, but ORCA has improved capabil- ities, integrates the NCD data acquisition system with the SNO PMT system, and runs on Mac OS X. Integration with the PMT system required a significant change in the methods used to access our data. There is a two—step process where the data from ORCA are processed through two programs called “ncdbuilder” and “snobuilder.” (There is also a “pmtbuilder” program for the PMT data.) These programs combine information from the various hardware components in the NCDs (and PMTs) and reformat the “raw” data stream from ORCA into a ZDAB format that is readable by various analysis tools. There are four main programs that read ZDAB data files and produce useful information for the SNO detector operators and analysis teams: SNOSTREAM, NCDMonitor, SNOMAN, and XSnoED. NCDMonitor and SNOMAN are the programs that have been used by the NCD group for data analysis tasks. The NCDMonitor displays events and allows one to “hand—scan” the data to look for anomalous events. Our past ROOT macros are as useful as before because we use SNOMAN to process our ZDAB data files into ROOT—readable files.2 Using ROOT’s C—interpreter and a set of libraries built exclusively for SNO and NCD data, event rates, energy spectra, pulse width versus energy plots, data-acquisition efficiencies, and other information from NCD data can be extracted. The multiplexer (MUX) logarithmic amplifier (logamp) must be calibrated in order to properly analyze the data. Events of sufficient amplitude are amplified by the logamp and then digitized by the oscilloscopes. The recorded pulses must be “de—logged” to retrieve the original event pulse. The logamp transforms the pulse according to µ ¶ Vin Vout = a · log 1 + + c. (1) b Calibration of the MUX logamps is performed by injecting a known waveform into the current preamplifier at the front—end of the electronics system and performing a χ2 minimization of the digitized pulse compared to the expected transformation, Vout . The values of a, b, and c are found in this way. Inverting this transformation allows original pulses to be retrieved from the digitized pulses. The calibration pulse is generated by a Hewlett Packard 33120A Function Generator and is distributed to all NCD electronic channels by our custom—built Pulser Distribution System. An offset single sine wave has been chosen to be the calibration pulse since it can have a large range in amplitudes and is a smooth function that is unrestricted by bandwidth constraints of the preamplifiers. The calibration of the logamp has been demonstrated to work, but is still being developed into a more automated routine. Calibration of the gain and thresholds of shaper/ADC cards has not yet been done in SNO. ∗ Department of Physics, Queen’s University, Kingston, ON K7L 3N6. 1 CENPA Annual Report, University of Washington (2002) p. 70. 2 CENPA Annual Report, University of Washington (2003) p. 25


  • Page 34

    24 2.12 PMT calibrations in SNO during the NCD phase A. Hallin,∗ I. Lawson,† N. S. Oblath and R. G. H. Robertson The purpose of the SNO Photomultiplier Tube array is to measure the timing and number of photons within SNO. Naturally, the ability of the PMTs to make those measurements needs to be calibrated. This is done using a pulsed nitrogen/dye laser system. A 10-cm diameter diffuser ball is placed near the center of the detector. The light provided by the diffuser ball (laserball) is nearly isotropic (to ≈10%). With the laserball at the center of the detector, the light travel time to each PMT is the same, and refraction effects at the boundaries between the heavy water and the acrylic, and between the acrylic and the light water can be ignored. The intensity of the laser pulses is reduced so that, at most, only one photon is detected by any PMT. In the NCD phase, the PMT calibrations are complicated by the fact that the NCDs cast shadows on the PMT array. This problem is solved by performing the calibration with the laserball at multiple locations. The goal is that every PMT will be illuminated by the laserball when it is at one or more of the positions. Having the laserball off-center requires making a timing correction in the calibration soft- ware, since the distances between the laserball and the PMTs are not equal. Furthermore, the light spends different amounts of time in the heavy water and light water regions, de- pending on the exact path between the laserball and the PMT. The correction consists of determining the timing change between light traveling from the center of the detector to the PMT and light traveling from the off-center laserball to the PMT, and that time is subtracted off the PMT signal time before the calibration is made. Fig. 2.12-1 shows the result of the timing correction test. The laserball was 75 cm from the center of the detector. It is also Figure 2.12-1. The prompt light peak of the PMT signals as a function of azimuthal angle before and after the timing correction. necessary to determine which combinations of laserball positions allow for all of the PMTs to be calibrated in the shortest amount of time. We have written a 2-dimensional simulation to find the ideal combination of two laserball runs where the laserball is within 1 m of the center along the x-axis of the detector. The preliminary result shows that placing the laserball at +45 cm and −45 cm, with a tolerance of approximately 4 cm, provides the greatest overall coverage of the detector. ∗ Queens University, Kingston, Ontario, Canada K7l 3N6. † University of Guelph, Guelph, Ontario, Canada, N1G 2W1.


  • Page 35

    UW CENPA Annual Report 2003-2004 May 2004 25 2.13 Upgrade to the NCD electronics for SNO G. A. Cox, S. McGee, A. W. Myers, K. Rielage, R. G. H. Robertson, L. C. Stonehill, T. D. Van Wechel and J. F. Wilkerson A number of upgrades have been made to the SNO NCD electronics since original commis- sioning underground.1 The electronics for the NCDs are composed of two main systems: a digitized data path utilizing 40 multiplexer channels that can be sent to two different oscilloscopes and 40 shaper channels that return a total integrated charge for the event. The NCD cable connections to the electronics channels must be able to withstand sig- nificant strain during deployment as the NCDs are moved into position. All forty of the deployed NCD cables’ dryend connections were replaced in November 2003 with a newly designed connector to ensure both excellent mechanical strength and electrical grounding. These replacements also lowered the overall noise pickup in these cables. A new NCD preamp was designed in Summer 2003. This new design utilizes several additional JFETs in parallel and allows for the preamp to have an impedance match to the NCD cable. This close impedance match results in a signal with much lower noise. The new design also increased the bandwidth by a factor of three. These changes result in a much cleaner neutron signal compared to the older design as shown in Fig. 2.13-1. Fifty-five new preamps were constructed and tested and are now in use on the SNO NCD system. Figure 2.13-1. NCD neutron source spectrum with old preamp compared to new preamp An NCD trigger card was also designed and constructed to allow the NCD events to be integrated into the SNO master trigger card with unique global trigger numbers. Several other modifications were made to high voltage readback electronics, multiplexer controller boards and multiplexer threshold readback electronics. An improved NCD trigger card is currently being designed to include a 10-MHz clock for timing information when the SNO master trigger card is offline. Other future upgrades include the replacement of all non- commercial electronics boards. 1 CENPA Annual Report, University of Washington (2002) p. 33.


  • Page 36

    26 KATRIN 2.14 The KATRIN neutrino-mass experiment T. H. Burritt, P. J. Doe, G. C. Harper, J. A. Formaggio, M. A. Howe, M. Leber, S. R. McGee A. W. Myers, K. R. Rielage, R. G. H. Robertson, T. D. Van Wechel and J. F. Wilkerson The KArlsruhe TRItium Neutrino (KATRIN) experiment1 is a next-generation tritium β-decay experiment designed to measure the mass of the neutrino with sub-eV sensitivity. The experiment should be an order of magnitude more sensitive than the best terrestrial measurements carried out to date, and hence should both improve our understanding of neutrino properties as well provide important constraints on models of nuclear and particle physics, cosmology, and astrophysics. With the current design, KATRIN expects a sensitivity to neutrino mass of 0.20 eV (90% CL) and would expect to observe a neutrino mass with a mass of 0.35eV at the 5 sigma significance level. The experiment will be constructed adjacent to the Tritium Laboratory Karlsruhue at the Forschungszentrum Karlsruhe (FZK). The current goal is for the KATRIN to become operational during 2008. The University of Washington in collaboration with FZK intends to provide the primary detector system as well as the real-time data acquisition system. During the past year, research and development at UW has progressed on several fronts, with emphasis on the pre-spectrometer system which is expected to begin operation in 2004. • We are designing and fabricating the pre-spectrometer inner electrode system. • A high-vacuum detector test stand was assembled and commissioned. The stand in- cludes a UV photoemissive monoenergetic electron gun that can produce electrons with energies up to 30 keV. Preliminary backscattering studies have been completed. • A Monte Carlo simulation effort to investigate energy loss and backscattering issues has been initiated. • An interim electronics and data-acquisition system for the KATRIN pre-spectrometer, was delivered to FZK last fall. This system consists of 64 channels of peak detect, shaper ADCs based in a VME bus. For the shaper ADCs we used a slightly modified design from our custom emiT/NCD electronics. The data-acquisition software is built in our recently developed Macintosh OS X Object-oriented Real-time Control and Acquisition (ORCA) system. In the coming year we expect our efforts on KATRIN to increase as our SNO NCD con- struction and deployment efforts have ended. We will concentrate on detector development, with some additional work on issues concerned with calibration and the accurate characteri- zation of potential systematic uncertainties. 1 The present collaboration: Univ. of Bonn; CCLRC Daresbury Laboratory; Joint Institute for Nuclear Research, Dubna; Univ. of Applied Sciences (FH) Fulda; Forschungszentrum Karlsruhe; U. of Karlsruhe; Johannes Gutenberg-Univ., Mainz; Institute for Nuclear Research, Moscow; Nuclear Physics Institute, Prague; Rutherford Appleton Laboratory; University of Wales Swansea; Center for Experimental Nuclear Physic and Astrophysics, Univ. of Washington.


  • Page 37

    UW CENPA Annual Report 2003-2004 May 2004 27 2.15 KATRIN electron gun testing of detector dead layers P. J. Doe, T. Gadfort,∗ J. A. Formaggio, G. C. Harper, M. A. Howe, M. Leber, S. McGee, K. Rielage, R. G. H. Robertson, J. F. Wilkerson and the KATRIN Collaborators The KATRIN1 (KArlsruhe TRItium Neutrino) experiment will measure the electron energies from tritium beta decay. By examining the endpoint of the electron energy spectrum near 18.6 keV for any deviation, an upper limit of 0.35 eV can be placed on the neutrino mass (or detection at the 5σ level). The experiment consists of a tritium source, a small pre- spectrometer followed by a 10-m diameter retarding-field magnetic-electrostatic analyzer, and a large (approximately 10 cm x 10 cm) silicon detector. The UW group is responsible for several aspects of this experiment including the DAQ system, the design and building of electrodes for the pre-spectrometer, and the design and characterization of the detector and associated front-end electronics. This section discusses the progress of the detector development and characterization. In order to determine the deviation at the electron energy-spectrum endpoint with small systematic uncertainty it is necessary to utilize a detector that is well characterized. Some of the most crucial pieces of information needed are the accurate determination of the detector’s dead layer and the number of events backscattering off the detector. To perform these measurements a monoenergetic electron gun was built2 capable of producing electrons with energies up to 30 keV. These electrons can be used to determine the dead layer of various sold state detectors by using the tilt method.3 The electron gun is complete and has used to test several small silicon photodiode detec- tors including the Hamamatsu S3204-9. This detector has a 1.8 cm x 1.8 cm active window. The dead layer was measured to be 107 ± 10 nm. A second detector (Hamamatsu S3590-09 windowless Si PIN photodiode) with an active area of 1.0 cm x 1.0 cm was used to examine electrons scattered off the first detector as a function of electron angle of incidence. Currently the detector test set-up is being redesigned to accommodate detectors up to 10 cm x 10 cm in size. A precision X-Y translation stage is being purchased capable of operating via stepper motors in the high-vacuum environment of the electron gun. This translation stage will be capable of rotating around the axis of the detector to perform dead- layer measurements at any location on the detector. The cooling system for larger detectors is also being designed to operate the detectors below 0o C. In addition, the electronics and data acquisition systems are being upgraded to support a Firewire to VME connection for communication between the two systems. A full 64-element detector will be tested and characterized for use during the testing of the KATRIN pre-spectrometer scheduled for late 2004 at Karlsruhe. ∗ Dept. of Physics, UW 1 CENPA Annual Report, University of Washington (2002) p. 44. 2 CENPA Annual Report, University of Washington (2003) p. 34 and p. 65. 3 R. L. Williams and P. P. Webb, IEEE Trans. Nucl. Sci., NS-9 (Number 3), June 1962, pp. 160-166.


  • Page 38

    28 2.16 KATRIN pre-spectrometer electrode T. H. Burritt, P. J. Doe, R. G. H. Robertson and J. F. Wilkerson Both the pre-spectrometer and the main spectrometer vacuum vessels are equipped with an internal electrode. The purpose of this electrode is to prevent electrons, originating from the walls of the vacuum vessel, from entering the spectrometer and constituting a source of background. These electrons may be produced by cosmic rays or natural radioactivity in the vessel walls. It is necessary that the internal electrode have very low mass and be compatible with the ultra high vacuum requirements of the spectrometer. The University of Washington will provide the internal electrode for the pre-spectrometer shown in Fig. 2.16-1. Figure 2.16-1. The Pre-Spectrometer Electrode, resting on its cradle in the pre- spectrometer vacuum vessel. The electrode consists of a central, low mass, wire barrel. At each end of the barrel is a cone made of thin sheet metal. These cones are located in high field regions where the use of wires could result in breakdown. The electrode must be capable of surviving bake-out to 350o C and be compatible with vacua of 10−11 Torr. In order to test the design concepts, a test electrode has been built to study the various fabrication concepts and techniques proposed for the final pre-spectrometer electrode. This electrode is shown in Fig. 2.16-2. Figure 2.16-2. The test electrode resting on its cradle. It is designed to fit in the Karlsruhe vacuum test chamber. The test electrode has shown that it is capable of withstanding the bake-out temperatures and maintaining wire tension and dimensional tolerance. It is being shipped to Karlsruhe to be subjected to vacuum tests. The fabrication drawings for the pre-spectrometer are undergoing final review. It is expected that the electrode will be shipped to Karlsruhe in late October 2004.


  • Page 39

    UW CENPA Annual Report 2003-2004 May 2004 29 Majorana 2.17 CENPA contributions to the Majorana experiment P. J. Doe, V. M. Gehman,∗ K. Kazkaz, R. G. H. Robertson and J. F. Wilkerson The Majorana experiment1 is a 76 Ge-based search for neutrinoless double-beta (0νββ) decay. In the past, CENPA has been involved with construction of related experiments, development of the data-acquisition system, and shielding simulations.2 This year we have built on those projects and extended our involvement with the wider collaboration. The construction of MEGA3 has continued over the past year at Pacific Northwest Na- tional Laboratory. One two-pack is fully assembled, and the parts for the second two-pack are in hand. CENPA is developing an automated evaluation system for the two-packs as well as a system for monitoring state-of-health of the installed detector. Two preamplifiers have been evaluated for their use in the Majorana experiment: the PGT-RG11 and the Amptek A250. The preamps are evaluated on the basis of various criteria, including response time, quality of the response curve, exponential return to baseline, intrinsic resolution, FET power dissipation, linearity of energy dependence, and bandwidth. Both of these preamps display acceptable performance, though the A250 has much cleaner response, with no real overshoot or ringing. It also has better intrinsic resolution. Three more preamps remain to be evaluated. The simulation work done last year has been absorbed by a larger, all-encompassing simulation effort organized by Lawrence Berkeley National Laboratory. The Majorana col- laboration has decided to adopt the GEANT44 simulation package. The simulation has been broken down into five primary efforts: database, background models and event generation, signal output, geometry, and the physics list. The last two of these efforts are the responsi- bility of CENPA. The first geometry that is being created within the Majorana simulation framework is of what is known as a “clover detector,” manufactured by Canberra. The clover simulation is used as a simple first case to determine how the various parts of the simulation will cooperatively interact. The simulation data will initially include event generators for both 232 Th and 68 Ge, and will be compared to experimental data obtained from the clover detector at Los Alamos National Laboratory. The GEANT4 physics list incorporates low-energy electromagnetic and neutron interac- tions applicable to a low-background environment, as well as high-energy hadronic interac- tions resulting from cosmic-ray spallation. Our tuning of the physics list takes place using ∗ Presently at Los Alamos National Laboratory Los Alamos, NM 87545. 1 http://majorana.pnl.gov 2 CENPA Annual Report, University of Washington (2003) p. 32. 3 K. Kazkaz et al., MEGA: A Low-Background Radiation Detector, IEEE Transactions on Nuclear Science, accepted for publication June, 2004. 4 http://wwwasd.web.cern.ch/wwwasd/geant4/geant4.html


  • Page 40

    30 a very simple single-crystal simulation with a thorium source. We compared the simulation data to data taken by a coaxial germanium surface detector completely encompassed by a six-inch lead shield in Seattle. The experimental setup is not considered “low-background”, e.g., no measures were taken to reduce either 40 K contamination or radon within the shield. Fig. 2.17-1 shows the comparison between the simulated and experimental data. The two spectra have been normalized by the integrated counts in the well-modeled region between 1800 keV and 2640 keV. The curves do not match in the entire spectrum as a result of a deficiency of peaks in the simulation event generator. The validation of the physics list, however, comes about by comparing peak-to-Compton ratios, observing the existence of single- and double-escape peaks, and comparing the height of the annihilation peaks between curves. In this sense, the physics list is deemed trustworthy. The quality of the simulated hadronic interactions resulting from high-energy muons and fast neutrons has not yet been evaluated. 1 -1 10 -2 10 10-3 -4 10 10-5 10-6 0 500 1000 1500 2000 2500 Figure 2.17-1. Spectra from simulated and experimental data from 232 Th. The lighter grey curve is the experimental data, and is narrower because of higher statistics. The vertical axis is an arbitrary scale resulting from the renormalization.


  • Page 41

    UW CENPA Annual Report 2003-2004 May 2004 31 2.18 A Joint Institute for Advanced Detector Technology P. J. Doe, J. H Gundlach, H. S. Miley,∗ R. G. H. Robertson and J. F. Wilkerson The University of Washington (UW) and Pacific Northwest National Laboratory (PNNL) have a history of successful collaborations. At the institutional level, a Joint Board has been established between UW and PNNL to facilitate and foster such collaborative efforts. This board oversees and governs official Joint Institutes and Programs that the institutions have identified as meriting strategic support. For the past four years our Electroweak Interactions group at CENPA and the PNNL Radiological and Chemical Sciences group have been collaborating together on a number of projects, including the Majorana next generation double-beta decay experiment and the investigation of advanced ultra-low background counting capabilities one would need at a Deep Underground Science and Engineering Laboratory (DUSEL). In March of 2004 we presented the concept of a new UW-PNNL Joint Institute for Advanced Detector Technology to a meeting of the Joint Board. This idea was extremely well received and the Board has requested that we submit a detailed formal proposal at its next meeting. We have identified three main thrusts for this Joint Institute (JI): • Developing Advanced Detectors for both applied (National and Homeland security) and basic physics (next-generation double-beta decay, and dark matter) experiments. There is a surprisingly large amount of overlap between these two research areas. It is envisioned that the JI would quickly reach beyond the initial core groups and engage experts in engineering, materials science, and chemistry. It is expected that this area will grow into a strong interdisciplinary effort. • Developing a DOE proposal for an Underground Ultra Low Background Counting Fa- cility (LBCF). This would be an effort aimed at developing a national facility based at DUSEL. Such a facility would support research into both next-generation basic physics detectors and national and homeland security applications. We envision that this proposal will have additional partners from both universities and other national laboratories, such as LBNL, LANL, and LLNL. The LBCF is currently a key component of our DUSEL - Cascades proposal. • Working cooperatively to build the Majorana double beta decay detector. Both PNNL and UW have significant expertise in this area. UW brings knowledge on building and operating experiments in a clean, ultra-low background underground environment, most recently having deployed the specially fabricated neutral-current detectors at SNO. The PNNL group has extensive experience with germanium detectors, double beta decay, and ultra sensitive underground counting experiments. We anticipate submitting a detailed proposal to the Joint Board in the fall of 2004. ∗ Pacific Northwest National Laboratory, Richland, WA 99352.


  • Page 42

    32 3 Nuclear Astrophysics 3.1 Precise measurement of the 7 Be(p,γ)8 B S-factor A. R. Junghans,∗ E. C. Mohrmann, K. A. Snover, T. D. Steiger,† E. G. Adelberger, J. M. Casandjian,‡ H. E. Swanson, L. Buchmann,§ S. H. Park,§ A. Zyuzin§ and A. M. Laird§ We have completed our precision measurements of the 7 Be(p,γ)8 B cross section at astro- physically interesting energies.1 Results were obtained with 3 different targets, spanning the center-of-mass energy range E = 116 to 2460 keV. Our latest results, obtained with a 340-mCi 7 Be target, incorporated several improvements over our previously published exper- iment.2 Our new measurements lead to S17 (0) = 22.1 ± 0.6(expt) ± 0.6(theor) eV b based on data from Ēc.m. = 116 to 362 keV, where the central value is based on the theory of Descouvemont and Baye, and the theoretical error estimate is based on the fit of 12 different theories to our low-energy data. This result is in excellent agreement with our previously published value. We find that all modern direct results for S17 (0) are mutually compatible. We recommend a “best” value, S17 (0) = 21.4 ± 0.5(expt) ± 0.6(theor) eV b, based on the mean of all modern direct measurements below the 1+ resonance. We also present S-factors at 20 keV which is near the center of the Gamow window: the result of our measurements is S17 (20) = 21.4 ± 0.6(expt) ± 0.6(theor) eV b, and the recommended value is S17 (20) = 20.6 ± 0.5(expt) ± 0.6(theor) eV b. S17 (0) values have also been determined using indirect techniques; namely Coulomb dis- sociation and peripheral heavy-ion transfer. Mean values determined from these indirect measurements lie lower than the mean determined from direct experiments. In addition the slope (energy dependence) of S(E) versus E inferred from Coulomb dissociation is steeper than the slope observed in the direct measurements.1 These differences represent an important unresolved problem. Recently, we refit the region of the 3+ , E = 2183 keV resonance with an improved (quadratic polynomial) background, and determined Γγ = 101 ± 51 meV for the ground- state transition, corresponding to a reduced transition strength B(M1) = 0.38 ± 0.19 W.u. (Weisskopf units). The reduced M1 strength for the ground-state transition from the 1+ , E = 630 keV resonance is 2.66 ± 0.13 W.u. (see Table IV of ref.1). These estimates, made assuming pure M1 decay (and negligible resonance inelasticity) may be compared to 0.29 ± 0.13 W.u. and 2.8 ± 0.9 W.u. for the mirror transitions in 8 Li, respectively.3 ∗ Permanent address: Forschungszentrum Rossendorf, Dresden, Germany † Present address: Cymer, Inc., San Diego CA 92127. ‡ Present address: GANIL, Caen, France. § TRIUMF, Vancouver, Canada. 1 A. R. Junghans et al., Phys. Rev. C 68, 065803 (2003). 2 A. R. Junghans et al., Phys. Rev. Lett. 88, 041101 (2002). 3 http://www.tunl.duke.edu/nucldata/ourpubs/p08 2002.shtml. The shell model predicts small isoscalar matrix elements for these transitions (D. J. Millener, private comm.).


  • Page 43

    UW CENPA Annual Report 2003-2004 May 2004 33 3 3.2 He(α, γ)7 Be γ-ray background analysis C. Bordeanu, J. Manor,∗ P. N. Peplowski, K. A. Snover and D. W. Storm We are planning a precision measurement of the 3 He(α, γ)7 Be cross section by detecting both the prompt capture γ-rays and the delayed γ-rays from 7 Be decay, using an α beam incident on a 3 He gas cell. Before the gas cell can be constructed, it is necessary to test materials for possible use as the entrance foil, cell liner and beam stopper. Due to the small 3 He(α, γ)7 Be cross section, it is important to limit both the prompt and delayed γ-ray background coming from interactions of the beam with materials in the gas cell. We irradiated Ni, Co, Cu, Nb, Ta, Pt and Au test materials from Goodfellow (labelled ‘G’) and Alfa Aesar (labelled ‘A’), and measured both prompt and delayed γ-ray backgrounds. Prompt γ-ray background resulting from the α beam interacting with the entrance foil and/or beam stopper can cause difficulties if it interferes with the 3 He(α, γ)7 Be capture γ- ray peaks. Prompt background measurements were analyzed to determine which materials have the lowest background in the energy range of the 3 He(α, γ)7 Be primary and secondary capture γ-rays. Gamma-ray spectra were measured in the range Eγ = 0.5 - 5 MeV, with results for selected energy bins shown in Fig. 3.2-1. 100.00 400-500keV 2.0-3.0MeV 10.00 Counts/uC 1.00 0.10 0.01 Co-AE Ni-AE Ni-G Cu-AE Ta-AE Ta-G Pt-AE Pt-G Au-AE Au-G 99.995 99.994 99.98 99.9999 99.95 99.9 99.997 99.99 99.9975 99.99 Figure 3.2-1. Prompt γ-background from 3.5 MeV α-bombardment of various materials, observed in a 50% Ge detector 3 cm from the target. Yields in Counts per µC of 4 He+ are shown for energy bins Eγ = 400 - 500 keV and 2.0 - 3.0 MeV. Delayed γ-ray background from 7 Be decay may result from contamination reactions.1 For a 4 He beam, contaminant 7 Be production can occur via 6 Li(d,n)7 Be and 10 B(p,α)7 Be, if there is a proton or deuteron contaminant in the beam and Li or B in the 3 He gas cell stopper or ∗ Graduated in March 2004. 1 M. Hilgemeier, H. W. Becker, C. Rolfs, H. P. Trauvetter and J. W. Hammer, Z. Phys. A 329, 243 (1988).


  • Page 44

    34 liner. Since the cross sections for these contamination reactions are five orders of magnitude larger than the cross section for 3 He(α, γ)7 Be, small amounts of contamination may result in significant 7 Be production. We performed tests with our terminal ion source to determine the molecular p and d contamination in our 3 MeV 4 He beam using the accelerator 90◦ analyzing magnet. A small satellite beam at slightly higher rigidity than 4 He+ was observed, corresponding to D+ + 2 +DH2 . The presence of deuterium in this satellite was verified by observing β-delayed α-particles from the 7 Li(d,p)8 Li reaction. The intensity of this satellite peak was 0.1% to 1% of the 4 He+ beam, depending on the preparation of the ion source. This satellite beam was intermittently transmitted to the target chamber by small downward fluctuations in the accelerator voltage. In separate measurements, we determined the 7 Be production from 0.75-MeV proton and 1.5-MeV deuteron irradiation of our test materials, shown in Fig. 3.2-2. On the basis of these measurements, we should be able to avoid contaminant 7 Be production and achieve low prompt γ-background during α-bombardment by using a Ni, Co or Ta gas cell stopper. 1000 Deuteron Irradiation Proton Irradiation 100 N(7Be)/uC 10 1 0.1 Co-A Ni-G Cu-A Nb-G Ta-G Pt-G Au-G 99.995 99.98 99.999 99.9 99.9 99.99 99.99 Test Material Figure 3.2-2. Number of 7 Be atoms N(7 Be) produced per µC of irradiated 0.75 MeV proton and 1.5 MeV deuteron beam on various test materials. Error bars indicate statistical uncertainty. Error bars with no histogram indicate upper limits.


  • Page 45

    UW CENPA Annual Report 2003-2004 May 2004 35 3.3 Search for the 8 B(2+ ) → 8 Be(0+ ) ground state transition M. K. Bacrania, D. Crompton, R. G. H. Robertson, D. W. Storm and S. Uehara SNO and Super-Kamiokande are sensitive to high-energy (> 5 MeV) solar neutrinos pro- duced by the decay of 8 B. The decay of 8 B proceeds primarily through the allowed 2+ → 2+ transition to the 3-MeV broad excited state in 8 Be. The resulting neutrino spectrum has an endpoint energy of approximately 14 MeV. It is also possible for 8 B to decay to the ground state of 8 Be through a second-forbidden transition (2+ → 0+ ) with an endpoint of approxi- mately 17 MeV. Solar neutrinos produced via this decay branch would be a background to both measurements of 8 B neutrino spectrum shape and measurements of the hep neutrino spectrum. This branching ratio is expected to be very small, but to date, no experimental verification has been published. As reported last year,1 we produce beams of 15-MeV 8 B by irradiating a 3 He gas target with a 36-MeV 6 Li beam. The 8 B is magnetically separated and implanted into a 500µm silicon PIN detector. The 8 B decay produces a β + and the prompt decay of 8 Be produces a pair of α particles. The β + is detected in a scintillator, providing a coincidence trigger. For 8 Be excitation energies below 7 MeV, both α particles come to rest inside the Si detector. Decay via the 2+ → 2+ branch results in a broad α-energy spectrum centered around 3 MeV, and decay to the ground state results in a line centered at 92 keV. In 2003, we improved beam collimation and detector positioning in order to reduce back- grounds from stray 8 B deposited near the detector. We also improved our 6 Li ion source which allowed us to produce higher beam currents over a longer period of time. Our current limit (1σ) for the branching ratio of the 8 B(2+ ) →8 Be(0+ ) is < 1 × 10−4 . This limit was obtained by subtracting from our 6 Li + 3 He spectrum a 6 Li + 4 He spectrum obtained under identical conditions, and dividing the net number of counts in a 60-100 keV region of interest by the total number of detected 8 B decays. Our limit is an order of magnitude smaller than the value which would affect solar neutrino measurements. We are currently studying the response of our PIN photodiode detector to low-energy alpha particles. We scattered alpha particles with incident(scattered) energies between 300(130) keV and 1000(440) keV from a carbon foil into our detector. To correct for the detector dead layer, measurements were made with the detector at different angles relative to the incident alpha particles. The detector was calibrated using 30-keV x-rays and 81-keV gamma rays from a 133 Ba source. Based on the measured alpha peak shape and position, we hope to be able to determine the pulse-height defect and peak-shape parameters as a function of alpha-particle energy in order to better define our low-energy region of interest. While we are able to detect alpha particles coming from the 2+ → 2+ transition, our current PIN detector calibration is not precise enough for us to determine the 8 B neutrino spectrum from this data. We are currently investigating whether alpha particles from the reaction 29 Si(n,α)26 Mg can be used as a precise in situ calibration for our PIN detector in the 3-5 MeV energy range. 1 CENPA Annual Report, University of Washington (2003) p. 37.


  • Page 46

    36 4 Nuclear Structure 4.1 β − ν correlation in A=8 and neutrino spectrum from 8 B J.M. Couture,∗ A. Garcı́a and S. Sjue Due to the presence of gluon exchanges between quarks, the weak currents between nucleons are more complicated than the corresponding between leptons. The axial vector current between nucleons, for example, is: ft fp (p|Aµ |n) = i(up | − fa γµ γ5 − σµν qν γ5 + i qµ γ5 |un ) (1) 2M 2M The term with ft is called Second Class Currents (SCCs) and should vanish if Time-Reversal Invariance and Charge Symmetry hold. Presently the limits on SCCs are still not very good: the recent fruit of > 20 years of work by a group at Osaka yields an upper limit of |ft /fa | < 0.15.2 If enough accuracy could be reached to measure ft /fa at the ≈ 0.1% level, the experiments would be sensitive to the up-down quark mass difference. In the A = 8 system SCCs show through the angular correlations in the decay, in which the dominant terms are: ve 2 Ee − Eν 2 Ee + Eν I W (ê, ν̂, α̂) ≈ 1 − ê · α̂ν̂ · α̂ ∓ b/Ac(ê · ν̂) + (d /Ac ± ft /fa )(ê · ν̂) (2) c 3 M 3 M where the upper (lower) sign corresponds to β −(+) decays and b, c, and dI are the weak- magnetism, Gamow-Teller, and first-class induced pseudotensor form factors. We have measured the energy of the two alpha particles emitted following the β decays of 8Band 8 Li. The data taking took place over the last couple of years at Notre Dame. J. Couture is presently doing calculations to properly calibrate the spectra and analyzing the data to extract the β − ν correlation. From the data we plan to extract limits on ft /fa and to get an improved measurement of the (first class) pseudo-induced tensor, dI . On a separate report we describe ancillary calculations that are being done regarding the response function of the Si detectors (see Section 4.2). In a previous experiment performed at Notre Dame3 we deduced the shape of the 8 B neu- trino spectrum from the alpha spectrum following the beta decay of 8 B. A later publication4 claims disagreements on the shape of the alpha spectrum. Unfortunately the original data taken at Notre Dame and the tools for the analysis were lost. S. Sjue in collaboration with the rest of us has started calculations that will allow us to extract the neutrino spectrum from 8 B using the new data and calibrations obtained at Notre Dame. ∗ Department of Physics, University of Notre Dame, Notre Dame, IN 46556. 2 Phys. Rev. C 65, 015501 (2002). 3 Phys. Rev. Lett. 85, 2909 (2000). 4 Phys. Rev. Lett. 91, 252501 (2004).


  • Page 47

    UW CENPA Annual Report 2003-2004 May 2004 37 4.2 Response function of Si detectors for α particles H. Bichsel, J. M. Couture∗ and A. Garcı́a In order to extract the neutrino spectrum from 8 B and the beta-neutrino correlation in the decay of 8 Li and 8 B from the measured energy of the alpha particles (see Section 4.1) following the decays of 8 Li and 8 B, we need to understand the response of our α counters in the range 1MeV ≤ Eα ≤ 9MeV. We have made calibration determinations at Eα ≈ 3.2 and 5.5 MeV, but calculations are essential to interpolate between them. We have performed Monte-Carlo simulations of the ionization function of our Si counters for α particles. We consider two effects which cause an asymmetry in this function: 1) energy losses in dead layers of detector and sources; and 2) energy transfered to Si atoms in nuclear collisions that does not generate further ionization. 1000 1000 148Gd 241Am 100 100 10 10 1 1 0.1 0.1 2750 3000 3250 5000 5250 5500 148 241 Figure 4.2-1. Measured versus simulated (shaded) response functions for Gd and Am sources. Alpha particles are tracked through Si taking into account energy-loss straggling. The Rutherford scattering collisions that determine the tails for the ionization function occur near the end of the range of the alpha particle, when the cross section for nuclear scattering becomes significant compared to the cross section for electronic energy loss. The figure shows measured ionization functions using 148 Gd and 241 Am sources compared to our calculations where we assumed the cross section for nuclear scattering is twice the value given by the Rutherford cross section. We are still working on understanding the differences. ∗ Department of Physics, Notre Dame University, Notre Dame, IN 46556.


  • Page 48

    38 35 4.3 A Monte Carlo simulation of Cl(n, γ) lines A. Garcı́a, G. J. Hodges, S. A. Hoedl and S. Triambak One of the calibration reactions in our quest to obtain the excitation energy of the lowest T = 2 state1 in 32 S is 35 Cl(n, γ). A 200-nA beam of protons with incident energy Ep = µg 1.912 MeV from the Terminal Ion Source was incident on a thick Li2 O target (≈ 500 cm 2 ) so that all the protons lost energy to below threshold2 Ep = 1881 keV to produce neutrons via 7 Li(p, n). The neutrons are moderated by a 4 cm thick slab of paraffin before being absorbed by a 8×103 cm3 volume of NaCl. The experimental setup is as shown in Fig. 4.3-1. HPGe detector Borax 512 Paraffin Experimental data Simulation is shaded part Counts (log scale) 256 Protons at 1.912 MeV Salt 128 7Li Target 64 7700 7750 7800 7850 7900 7950 HPGe detector Energy (keV) Figure 4.3-1. Experimental Setup. Figure 4.3-2. Peak at 7.79 MeV. The neutrons moving towards the detector are absorbed by ≈ 15 cm of borax after being moderated by ≈ 8 cm of paraffin to protect the high purity Ge detectors from neutron dam- age. A Monte Carlo simulation was performed to account for elastic scattering and absorption of the neutrons in the paraffin before capture on the 35 Cl nuclei. Following neutron capture, the moving compound nuclei emit gammas that are detected by the HPGe detectors. An addi- tonal radiation transport program called PENELOPE3 was used to simulate the interaction of the photons with the Ge crystal. Using both these simulations, we obtain the Doppler effects on the calibration lines as detected by the Ge detectors. Fig. 4.3-2 shows a compar- ision of simulation to actual experimental data. In addition, we compared the ratios of the areas of the primary peaks to their first and second escape peaks in the simulated spectra to the experimental data to check the geometry of the detector used in the simulations. The preliminary results are shown in the table below. Eγ Sim. Ratio Exp. Ratio Sim. Ratio Exp. Ratio (MeV) 1st /Peak 1st /Peak 2nd /Peak 2nd /Peak 8.578 1.33 1.27 0.45 0.53 5.715 0.74 0.85 0.38 0.46 Our preliminary simulations show that the average Doppler shift on the 8.578 MeV line (highest gamma energy) is ≈ 0.16 keV. 1 CENPA Annual Report, University of Washington (2003) p. 39. 2 R. Ratynski and F. Käppeler, Phys. Rev. C 37, 595 (1988). 3 J. Sempau et al., Nucl. Instrum. Methods in Phys. Res. B 132, 377 (1997).


  • Page 49

    UW CENPA Annual Report 2003-2004 May 2004 39 4.4 A stringent test of the Isobaric Multiplet Mass Equation using 31 P(p, γ) E. G. Adelberger, A. Garcı́a, G. J. Hodges and S. Triambak The Isobaric Multiplet Mass Equation (IMME) relates the masses of the levels of a isospin multiplet in the following way: M (Tz ) = a + bTz + cTz 2 It has been known to work successfully for 21 of the 22 examined quartets, the only deviation being the A = 9 multiplet which required an additional cubic term and also happens to be the most accurately known one.1 Due to its success, the IMME has been widely used to deduce masses and level energies of members of multiplets where direct measurements are difficult, for example in determining the Doppler broadening of beta delayed protons and the Ft value in the beta decay of 32 Ar, which is a member of the T = 2 quintuplet. In our effort to determine the mass of the lowest T = 2 state in 32 S ≈ 0.3 keV, we hope to be able to test the IMME up to its most stringent limits for T = 2 (since the masses of the other members are well known) and check the validity of the use of this equation to deduce masses at the levels of accuracy required.2 A proton beam from the Terminal Ion Source at Ep = 3285 keV impinged on our implanted 31 Pwater cooled target to populate the lowest T = 2 state in 32 S. We observed the decaying gammas in HPGe detectors positioned at ±90◦ to the target to minimize Doppler effects on the gamma energies (which is the largest source of systematic uncertainties). For our energy calibration, two reactions, 35 Cl (n, γ) and 27 Al(p, γ) were used. These produce very precisely known gammas3,4 in the energy range 0-8 MeV. In the former, the neutrons were produced by 7 Li(p, n) at Ep = 1912 keV. The protons were incident on a thick Li2 O target that was evaporated on a Ta backing. This produces a fairly collimated cone of neutrons5 with energies upto ≈ 110 keV which were then slowed down by a 4-cm thick slab of paraffin before absorbing in ≈ 8 × 103 cm3 volume of NaCl. Monte Carlo simulations are being performed to investigate Doppler and recoil effects in both these calibration reactions. In addition, at all times during data acquisition we used a 3.5-µCi 56 Co gamma source that emitted gammas in the range 0-3.5 MeV. Our preliminary results for the excitation energy of the lowest T = 2 state gives us EX ≈ 12046.02 keV ± 0.30 keV which disagrees with the previously measured6 value of 12045 ± 0.4 keV. 1 W. Benenson and E. Kashy, Rev. Mod. Phys. 51, 527, (1979). 2 CENPA Annual Report, University of Washington, (2003), p. 39. 3 B. Krusche et al., Nucl. Phys. A386, 245, (1982) 4 P. M. Endt et al., Nucl. Phys. A510, 209 (1990). 5 W. Ratynski and F. Käppeler, Phys. Rev. C 37, 595, (1988). 6 M. S. Antony et al. in Proceedings of the International Conference on Nuclear Physics, Berkeley, 1980, Lawrence Berkeley Laboratory, Berkeley, CA, 1980, Vol. 1.


  • Page 50

    40 5 Relativistic Heavy Ions 5.1 Summary of event-structure analysis T. A. Trainor Event-structure analysis characterizes the evolution of heavy ion collisions from first contact to kinetic decoupling on the basis of final-state hadron fluctuations and correlations. From studies to date two central issues have emerged: 1) Dynamical evolution in A-A collisions of minimum-bias partons from initial production to final fragmentation, including different aspects of parton coupling to the produced color medium, and 2) local structure of the hadronization process in p-p and A-A collisions. In particular, understanding parton dy- namics and coupling to the color medium in A-A has required a full study of minimum-bias partons and their hadron fragments (minijets) in p-p collisions as a reference system. The present elements of this program are Two-component soft/hard analysis of pt distributions from p-p collisions Two-particle correlations on yt ⊗ yt and η ⊗ φ in p-p collisions Minijets as velocity structures: hpt i fluctuation scaling in Au-Au collisions Collision energy dependence of hpt i fluctuations Minijet dissipation and two-particle pt correlations in Au-Au collisions Minijet angular deformation on (η, φ) in Au-Au collisions Hadronization geometry in p-p and Au-Au collisions Charge-independent fluctuations and minijet structure Charge-dependent fluctuations and hadronization geometry Joint autocorrelations from fluctuation scaling by inversion The principal results of this program are detailed characterization of soft and hard com- ponents of two-particle correlations in p-p collisions providing essential new information on in vacuo parton fragmentation, minijets in Au-Au collisions as velocity structures and the strong coupling of partons to an axially-expanding color medium in those collisions, a better understanding of the dominant role of minijets in producing the complex velocity structure we observe in A-A collisions at RHIC and evolution of hadronization geometry from 1D in p-p collisions to 2D in central Au-Au collisions. Centrality dependence of various correlation structures reveals an ordered sequence of dynamical changes with increasing A-A centrality: string melting, parton energy loss along the thrust axis, strong parton coupling of low-pt partons to an expanding bulk medium. Those results have depended on the development of several novel analysis techniques including direct construction of precision pair-ratio auto- correlations, derivation of the integral equation that relates fluctuations to correlations and inversion of that integral equation to extract autocorrelation distributions from fluctuation scale dependence.

  • View More

Get the full picture and Receive alerts on lawsuits, news articles, publications and more!