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    ANNUAL REPORT Center for Experimental Nuclear Physics and Astrophysics University of Washington May, 2007 Sponsored in part by the United States Department of Energy under Grant #DE-FG02-97ER41020.


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    This report was prepared as an account of work sponsored in part by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, makes any warranty, expressed or implied or assumes any legal liability or responsibility for 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, from top to bottom: John Amsbaugh, Tom Burritt and Sean McGee removing the SNO NCD’s, Michelle Leber in front of the KATRIN vacuum vessel, and the short-range gravity pendulum.


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    UW CENPA Annual Report 2006-2007 May 2007 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. CENPA is a major participant in the Sudbury Neutrino Observatory (SNO), the KATRIN tritium beta decay experiment and the Majorana double-beta decay experiment. The current program includes “in-house” research on nuclear astrophysics 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. We thank our external advisory committee, Baha Balantekin, Russell Betts, and Stu- art Freedman, for their continuing valuable recommendations and advice. The committee reviewed our program in May, 2005. Data-taking on the Sudbury Neutrino Observatory project ended November 28, 2006, concluding six highly successful years in which the solar neutrino problem was resolved and new neutrino properties measured. As the heavy water is being returned to the owners, the data from the final phase, during which 3 He-filled proportional counters were deployed in SNO, are being analyzed. A clear neutron signal is seen and significantly improved precision on the mixing angle θ12 can be expected. The completion of the main KATRIN spectrometer, its very successful vacuum test and spectacular delivery to the FZK marked the the achievement of a major KATRIN milestone. The UW played a prominent role in the commissioning of the pre-spectrometer, yielding valuable lessons on Penning traps which will be applied to the main spectrometer. Following a review in November, the DOE recently announced its decision to fund the US-KATRIN proposal. This will enable purchasing of long lead-time items to begin, with forward funding assistance being provided by the University of Washington to ease funding profiles. Our recent test of the gravitational inverse-square law showed that the law is valid at 95% confidence for length scales down to 56 micrometers. This result places a model-independent limit of 44 micrometers on the largest possible size of an extra dimension. It rules out the interpretation of the PVLAS “birefringence of the vacuum” measurement in terms of a low- mass spin-0 meson, and is inconsistent with “natural values” for the chameleon mechanism that was invented to evade experimental limits on string theory’s predicted low-mass scalar particles We have nearly completed our precision 3 He + 4 He fusion measurements, with data from counting both the prompt and the activity gamma-rays over the energy range Ec.m. = 420 - 1230 keV, and one more lower energy point to be measured. A new beamline has been constructed for our 22 Na destruction experiment, and the target chamber has been designed. We finished an analysis of data that yields the branch for the 0+ → 0+ transition in 32 Ar, which allows an experimental determination of the isospin-breaking correction and a stringent test of calculations. We have made significant improvements in our production of Ultra Cold Neutrons at Los Alamos, which has allowed us to measure the beta-decay spectrum from UCN at a


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    ii rate of approximately 2 Hz. We expect to get a determination of the beta asymmetry to approximately 2% by the end of 2007. We have completed a survey of minijet number and pt correlations on A-A collision energy and centrality, indicating that minijets form a strong contribution to RHIC A-A collisions although they are strongly altered in central collisions. We are now using the same analysis system to study elliptic flow on its own and relative to minijets, to reconsider its hydrodynamic interpretation and possible alternatives. Axion Dark Matter eXperiment: We have nearly finished construction and are planning to start commissioning this summer. We anticipate the data-taking will take about a year. The collaboration has endorsed the plan to move the experiment to the University of Wash- ington/CENPA. We plan to submit a proposal to DOE/HEP for this Phase II of the project in summer 2007 for a fall 2008 construction start. We received DOE/NA22 funding for a large-channel-count TPC for indentification of special nuclear material. The readout electronics and software will be the responsibility of CENPA. Two CENPA graduate students obtained their Ph.D. degree during the period of this report. 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 Debra Nastaj, Assistant Editor


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    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. Methods 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 7.5 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. Methods A 287, 247 (1990). The Booster is presently in a “mothballed” state.


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    UW CENPA Annual Report 2006-2007 May 2007 v Contents INTRODUCTION i 1 Neutrino Research 1 SNO 1 1.1 Status of the SNO Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 SNO NCDs 3 1.2 Stability of the NCD array during SNO phase III . . . . . . . . . . . . . . . . 3 1.3 Development of the NCD pulse simulation . . . . . . . . . . . . . . . . . . . 5 1.4 Revisiting the Deployed NCD tilts in SNO . . . . . . . . . . . . . . . . . . . 6 1.5 Resistive coupler disconnect and NCD data quality . . . . . . . . . . . . . . 7 1.6 A new external alpha counter for the SNO neutral current detectors . . . . . 8 1.7 Energy losses of protons and alphas in NCDs and energy spectra of alphas from the counter walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 KATRIN 11 1.8 Status of the CENPA contribution to the KATRIN experiment . . . . . . . . 11 1.9 Status of the KATRIN detector mechanical design . . . . . . . . . . . . . . . 13 1.10 Commissioning of the KATRIN pre-spectrometer system . . . . . . . . . . . . 15 1.11 Updates on simulation for KATRIN detector backgrounds . . . . . . . . . . 16 1.12 Electron gun for profiling silicon detectors for KATRIN . . . . . . . . . . . . 17 1.13 Development of an in-situ dead-layer measurement for the KATRIN detector 19 Majorana 21 1.14 The Majorana neutrinoless double-beta decay experiment . . . . . . . . . . . 21 1.15 Surface contamination simulations for the proposed Majorana neutrinoless double-beta decay experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . 23


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    vi 1.16 Validation of cosmic-ray muon-induced physics within the GEANT4-based simulation and analysis package MaGe . . . . . . . . . . . . . . . . . . . . . . 24 1.17 Development of technologies for low-background experiments using the MEGA cryostat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 1.18 Low-energy neutron response of germanium detectors in MaGe/GEANT4 . . 26 1.19 LArGe, Liquid Argon Compton suppressed Germanium crystal . . . . . . . . 27 1.20 Design, construction and operation of a small-scale radioactivity assay chamber 29 2 Fundamental Symmetries and Weak Interactions 30 Torsion Balance Experiments 30 2.1 Torsion balance search for spin coupled forces . . . . . . . . . . . . . . . . . . 30 2.2 Tests of the gravitational inverse-square law below the dark-energy length scale 31 2.3 Particle physics implications of our recent test of the gravitational inverse square law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.4 A new limit on a P- and T-violating force . . . . . . . . . . . . . . . . . . . . 35 2.5 Laboratory test of Newton’s second law in the limit of small accelerations . . 36 2.6 Rotating torsion balance test of the weak equivalence principle for beryllium and titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.7 Development of a pendulum to test short range spin-spin interactions . . . . 38 2.8 Continued progress toward improved torsion fibers . . . . . . . . . . . . . . . 39 2.9 New short range test of Gauss’ law for gravity . . . . . . . . . . . . . . . . . . 40 2.10 Wedge Pendulum: A new test of the gravitational inverse-square law . . . . 41 2.11 Investigations of small thermal gradient and electrostatic effects for LISA . . 42 2.12 Initial results from the APOLLO lunar-laser ranging experiment . . . . . . . 43 Weak Interactions 44 2.13 NSAC - the data acquisition and control system for the parity non-conserving neutron spin rotation experiment . . . . . . . . . . . . . . . . . . . . . . . . 44 2.14 Parity non-conserving neutron spin rotation experiment . . . . . . . . . . . . 45


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    UW CENPA Annual Report 2006-2007 May 2007 vii 2.15 Nuclear reactions of 32 Ar in Si . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2.16 100 Tc electron-capture branching ratio with a Penning Trap . . . . . . . . . . 47 2.17 Status of the ultra-cold neutron Aβ experiment at Los Alamos . . . . . . . . 49 2.18 Characterization of ultracold neutron detectors for use in the UCNA experi- ment at LANL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Quantum Optics 52 2.19 A test of quantum nonlocal communication . . . . . . . . . . . . . . . . . . . 52 3 Nuclear Astrophysics 55 3.1 Precision measurements of the 3 He(α,γ)7Be cross section . . . . . . . . . . . 55 3.2 E0 emission in α + 12 C fusion at astrophysical energies . . . . . . . . . . . . 56 3.3 Motivation for Measuring the 22 Na(p,γ)23Mg Reaction Rate . . . . . . . . . . 58 3.4 Technical developments of the 22 Na(p,γ)23Mg experiment . . . . . . . . . . . 59 4 Nuclear Structure 61 4.1 Investigation of states in 33 Cl using 32 S(p, γ) . . . . . . . . . . . . . . . . . . 61 4.2 Measurement of the absolute γ branches in the decay of 32 Cl . . . . . . . . . 62 4.3 Delayed γ branches from 32 Ar β-decay . . . . . . . . . . . . . . . . . . . . . 63 5 Relativistic Heavy Ions 64 5.1 Summary of event structure research . . . . . . . . . . . . . . . . . . . . . . . 64 5.2 Accurate centrality determination in A-A collisions down to single N-N collisions 65 5.3 Precision Glauber-model parameterizations and particle production in A-A collisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 5.4 Charge-independent angular autocorrelations in Au-Au collisions at sN N = √ 62 and 200 GeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 5.5 Particle identified two particle correlations: techniques . . . . . . . . . . . . . 68 5.6 Identified-particle two-particle correlations: observations . . . . . . . . . . . . 69 5.7 Forward-backward correlations in relation to angular autocorrelations . . . . 70


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    viii 5.8 Monte Carlo simulations of correlation structures . . . . . . . . . . . . . . . . 71 5.9 Reaction-plane-dependent correlations . . . . . . . . . . . . . . . . . . . . . . 72 5.10 Distinguishing elliptic flow from “non-flow” in heavy ion collisions . . . . . . 73 5.11 Energy and centrality dependence of elliptic flow in Au-Au collisions . . . . . 74 5.12 Is “elliptic flow” a hydrodynamic phenomenon? . . . . . . . . . . . . . . . . . 75 5.13 “Flow” phenomena in nuclear collisions and Brownian motion . . . . . . . . . 76 5.14 Opacity and chiral symmetry restoration in heavy ion collisions at RHIC: the DWEF model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 6 Electronics, Computing, and Detector Infrastructure 79 6.1 Electronic Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 6.2 Additions to the ORCA DAQ system . . . . . . . . . . . . . . . . . . . . . . . 81 6.3 Laboratory computer systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 6.4 Studies of energy losses of fast charged particles . . . . . . . . . . . . . . . . 83 7 Accelerator and Ion Sources 84 7.1 Injector deck and ion sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 7.2 Van de Graaff accelerator operations and development . . . . . . . . . . . . . 85 7.3 Physical plant maintenance, repairs, and possible upgrade . . . . . . . . . . . 86 7.4 Axion magnet cryostat: mechanical characteristics . . . . . . . . . . . . . . . 87 8 CENPA Personnel 88 8.1 Faculty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 8.2 CENPA External Advisory Committee . . . . . . . . . . . . . . . . . . . . . . 88 8.3 Postdoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . 88 8.4 Predoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . . 89 8.5 Research Experience for Undergraduates participants . . . . . . . . . . . . . . 89 8.6 University of Washington undergraduates taking research Credit . . . . . . . 89 8.7 Professional staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90


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    UW CENPA Annual Report 2006-2007 May 2007 ix 8.8 Technical staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 8.9 Administrative staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 8.10 Part time staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 9 Publications 92 9.1 Published papers: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 9.2 Papers submitted or to be published 2006: . . . . . . . . . . . . . . . . . . . . 95 9.3 Invited talks, abstracts and other conference presentations: . . . . . . . . . . 97 9.4 Ph.D. degrees granted: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101


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    UW CENPA Annual Report 2006-2007 May 2007 1 1 Neutrino Research SNO 1.1 Status of the SNO Project J. F. Amsbaugh, G. A. Cox, J. A. Detwiler, P. J. Doe, C. A. Duba, G. C. Harper, M. A. Howe, S. McGee, A. Myers, N. S. Oblath, R. G. H. Robertson, T. D. Van Wechel, B. A. VanDevender and J. F. Wilkerson The Sudbury Neutrino Observatory (SNO) detector took its last production data on Novem- ber 28, 2006, concluding an extraordinarily successful physics experiment that began in November, 1999, and demonstrated that the solar neutrino problem was due to a new neu- trino property, flavor conversion. SNO ran in three distinct configurations or phases; pure heavy water, heavy water with dissolved salt, and heavy water with discrete neutron detectors (‘NCDs’) deployed in it. Production running began November 27, 2004 with 3 He-filled proportional counters de- ployed in the heavy water. There were 36 ‘strings’ of individual counters filled with 3 He and another four filled with 4 He for investigation of backgrounds. The total deployed length of counter was 398 m. Two calendar years of good data have been recorded, with a live-time fraction of neutrino data about 63%. Calibrations made up another 30% of the data, and the remaining 7% consisted of maintenance or shutdown time. The fraction of neutrino plus calibration data selected for analysis was 89%. These fractions were significantly higher than for previous phases. The day after data acquisition ended, a magnitude 4.1 earthquake struck the Sudbury area, and caused significant effects. Some disused drifts in the Garson mine collapsed. In the Creighton mine, there were minor rockfalls, but no serious damage or injury. It initially appeared that the SNO detector emerged unscathed, but five strings of NCDs (I1, J7, M1, M7, and N2) no longer functioned properly. When removal of NCD strings from the heavy water began, it was seen that gas was leaking from string N2 into the water. Inspection with the remotely-operated vehicle (ROV) showed that the five strings had collapsed near the anchor. Remarkably, however, four of them popped back into their circular shape and regained normal function once they were raised out of the water. It was fortunate that this event did not occur during SNO’s NCD data-taking phase. The ‘undeployment’ process essentially reversed the deployment process of 2003-2004. The calibration hardware and glove box above the neck of the vessel were removed, and the ROV was lowered into the water. Teams of personnel from CENPA, SNO, Guelph, Oxford, Queen’s, and Los Alamos began removing the strings, carrying out neutron calibrations at the center of the vessel with an AmBe source encapsulated in the shuttle float that is part of the mechanism to install and remove NCDs. The counter strings, 9 to 11 m in length in SNO, were cut into sections of 5 m or less at the location of welds by means of a pipe cutter. The sections are now stored in racks in the corridor leading to the SNO cavity pending a decision on their possible future use in another application. (It was hoped that they could be incorporated into a lead-based supernova detector called HALO, but the future of HALO


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    2 is uncertain.) Two strings, K2 and K5, had ‘hot spots’ observable in Cherenkov light, and special plans were made to study these strings on removal. The composition of the hot spots, whether Th, U, or something else, determines the neutron background to be associated with them. The strings were removed without allowing them to come into contact with anything at the locations of the hot spots, and were stored in a tent in the deck clean room. A 1-m long clamshell proportional flow counter was constructed at CENPA to allow the external alpha activity to be studied. The counter has good energy resolution, position resolution, and azimuthal angle resolution in order to better characterize the hot spots. Difficulties, described elsewhere in Sec. 1.6, were experienced commissioning this External Alpha Counter (EAC). Some strings exhibited a problem termed RCD, or resistive-coupler disconnect, in which the 325-Ω resistive coupler that matches the characteristic impedance of the NCDs to that of the cables was making poor electrical contact. (see Sec. 1.5). The cable bells were opened on those strings, and loose contacts were observed in K2 and M8, cases with clear RCD symptoms in the data. String J3, which produced non-physics events at low energies that resembled neutrons, was also found to have a loose contact (at the cable side of the resistive coupler, unlike K2 and M8). Other suspect strings, I7, K7, L3, and N4, were opened at the cable bell but did not have loose contacts. The removal of the NCDs was completed successfully on January 18, 2007. Removal of the heavy water began, and by the end of March, 467 tonnes had been taken to the surface. In parallel, distillation of concentrated brine left from the second phase of running with NaCl in the heavy water began. At the end of March, 7 tonnes out of a total of 35 had been distilled. The array ran successfully, producing a clear neutron signal that is approximately of the magnitude expected from neutral-current disintegration of deuterium by solar neutrinos. Data are being analyzed under blindness protocols designed by the UW group. In those protocols, a concealed fraction of events is dropped from the data, and some muon-follower neutrons are added back in, before the data are studied by analysts. Data analysis is proceed- ing along two separate paths. The objective in both approaches is to separate the neutron events from other types of event (alphas, betas, ‘non-physics’ events) and to minimize the overall statistical and systematic uncertainty. One method includes a set of semi-empirical parameters optimized for the separation by various means such as Fisher Discriminants, Boosted Decision Trees, and polynomial fits. The other approach models each pulse in detail based on the physical processes involved in creating and amplifying the ionization signals. The latter approach is also the basis of the Monte Carlo being developed to serve both anal- ysis communities. It is the approach being followed by the UW group, and a description of its status is contained in Sec. 1.3.


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    UW CENPA Annual Report 2006-2007 May 2007 3 SNO Neutral Current Detectors (NCDs) 1.2 Stability of the NCD array during SNO phase III S. McGee The Neutral Current Detector (NCD) array at the Sudbury Neutrino Observatory (SNO) was a collection of 40 9-11 m long ultra-clean, sealed 3 He and 4 He proportional counters known as NCD strings. Due to limitations on access to the experimental site, it was necessary to construct the NCD strings from three or four 2.0, 2.5 and 3.0 m sections of individual NCD counters immediately before installation into the active region of the SNO detector. Measures were taken to minimize counter-to-counter gain variations during the NCD gas fill but slight variations were unavoidable. To optimize the energy resolution of an NCD string, the individual counters that comprised an NCD string were selected so their gains were well matched. Figure 1.2-1. Plot of the change of gain of the individual counters relative to the string average gain. The majority of the counter relative gains changed very little during the 26 month run time of Phase III. The counter with a relative gain change of -0.5 is the one counter known to have been leaking 3 He from its active region. This forced the other three counters in that string to have a markedly higher (0.16) relative gain change. During installation of the NCD array, an AmBe source was placed at the center of each counter section to check electrical continuity of the string and individual counter health before relocation to its assigned location.1 The source was placed close enough to the counter so approximately 95% of the neutrons captured on the string were captured in the desired 1 CENPA Annual Report, University of Washington (2004) 22.


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    4 counter. The AmBe source allowed for events to be registered by both the 3 He and 4 He proportional counters (γ’s and n’s). Calibrations were done periodically at SNO with neutron-emitting sources attached to a lanyard system. However, this did not allow for the gain of the individual counters of an NCD string to be determined due to limitations in the proximity of source placement and the dispersion of neutrons emitted from the sources. To check individual counter health and stability and the accumulate a large sample of neutron pulses from each counter section, the initial calibration procedure was repeated during the removal of the NCD strings. During the installation calibrations, the same preamplifier was used to calibrate each NCD counter. However, during the removal calibration, it was desirable to use the preamplifier that had been used with the respective string during the entire NCD run period so the pulse shapes acquired during the calibration could be directly applied in comparison to run-time events. Therefore, to compare the gains of the individual counters at the beginning and the end of Phase III, the gains of the counters relative to the average gain of the entire strings are compared. Deviations in relative gain would be a clear indication of a change in the gain of an individual counter unless, of course, all counters changed in the same way. However, this kind of change would have been seen in the normal calibration of the array and it was not. Fig. 1.2-1 shows the change in the relative gain of each individual 3 He counter section with respect to the average gain of the entire string. The majority of the counter sections showed good stability with a deviation of 0.32%. One counter was known to be damaged and leaking 3 He (into a field-free region of the string) and can be seen as an outlier in Fig. 1.2-1 at -0.5. The other three counters on that string are pushed to a higher fractional difference by this leaking counter, appearing near 0.16, but are themselves still tightly grouped. Not included in this plot are results from the counters of five 3 He strings that could not be calibrated due to damage incurred during a large earthquake that shook SNO the day after data-taking was completed.


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    UW CENPA Annual Report 2006-2007 May 2007 5 1.3 Development of the NCD pulse simulation N. S. Oblath, R. G. H. Robertson and H. S. Wan Chan Tseung The events from the Neutral Current Detection (NCD) Array at SNO are current pulses where the integral of the pulse is approximately proportional to the amount of energy deposited in the counter. The shape of the pulse is a result of the energy deposition as a function of ion energy, the geometry of the ionization track within the counter, and a variety of other effects. We want to be able to produce a realistic Monte Carlo data set to show that we understand our detectors and with which we can test our pulse-shape analysis methods. Furthermore, we want to use the neutron-capture and alpha pulse models to fit data pulses. By fitting data pulses with both models we can separate the alpha background pulses from the neutron-capture signal pulses. Over the past year we have made great progress in improving the models we use for neutron-capture and alpha pulses and the way in which those models are implemented in the SNO Monte Carlo software, SNOMAN. These changes have included: • Developing a simulation of multiple scattering for the ions in the gas based on the methods used in the SRIM software package,1 • Improving our implementation of the ion energy deposition, • Creating a Monte Carlo simulation of electron drift from scratch to determine the average electron drift speed and diffusion as a function of radius within the counter, • Adding gas-gain fluctuations and realistic electronics noise, • Measuring the ion-drift time constant and the energy needed to produce an electron-ion pair, • Overhauling the electronics and trigger simulations to represent the real system more accurately and to integrate it with the simulation of the SNO PMT system. A qualitative comparison is shown in the figure below, with data on the left and a simu- lated pulse on the right. Quantitative comparisons are underway while the last few model improvements are nearing completion. 1 J. F. Ziegler, et al., in The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press (1985).


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    6 1.4 Revisiting the Deployed NCD tilts in SNO J. F. Amsbaugh, T. H. Burritt and J. Heise∗ Buoyancy should align each neutral current detector, NCD, vertically above its anchor point (AP), but the readout cable at the top of the NCD can pull it out of plumb. The global view camera (GVC), being near the central axis, can only resolve an extreme tilt. At the end of NCD deployment, the GVC and a laser range finder (LRF) were used to estimate these tilts.1 Similar measurements were done before removing the NCDs. Additional measurements were made with the mounting plate shifted by 7.1 cm. This moved obscured NCDs into the GVC view and provides triangulation. The LRF and GVC angular readouts were replaced, as the ones previous used were inaccurate. The LRF projects two beams at a known included angle and crossing range enabling triangulation. This is illustrated in Fig. 1.4-1. GVC images of the LRF spots on the NCDs were taken at higher zooms increasing the contrast and resolution. The spot separation measurements need to be corrected for an estimated barrel distortion for the wide angle zooms, mostly the M and N NCD strings. The LRF pointing direction angle is also read out and we use the AV suspension ropes to get the arbitrary offset. The LRF model needed about 5 cm of displacement from the AV axis to minimize the residuals, indicating the mount is not level or the 900-cm long LRF pole is slightly bent. Accuracy is ±0.3◦ , consistent with beam width, rope size, and distance. We waited longer for GVC and LRF motions to damp out before taking images with better results. At GVC installation, we discovered the top pole was bent. This particular bend increased the circle swept by the camera center as the pan angle is changed and couples GVC pan and tilt angles when looking from NCD bottom to top, even for a plumb NCD. This requires a more complicated model to analyze GVC top-bottom results. Analysis and a report will be done soon. Figure 1.4-1. The laser range finder pointed at NCD string I4 and the GVC pointed at M4. Readout cables truncated. ∗ Sudbury Neutrino Observatory, Lively, ONT, Canada P3Y 1M3. 1 CENPA Annual Report, University of Washington (2005) 30.


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    UW CENPA Annual Report 2006-2007 May 2007 7 1.5 Resistive coupler disconnect and NCD data quality T. H. Burritt, G. A. Cox-Mobrand, S. McGee, R. G. H. Robertson and J. F. Wilkerson During the NCD electronics calibrations, parameters which describe the logarithmic ampli- fication of pulses before they are digitized were observed to deviate significantly from their previous average values for NCD strings 1 (M8) and 31 (K2). Calibration pulses propagate along the NCD cable in the direction opposite of physics signals from the NCD. These pulses observe an impedance mismatch at the junction between the NCD and NCD cable that causes pulse reflections. An examination of the calibration pulses which measured the logamp pa- rameter deviation revealed that the pulse reflection caused by this impedance mismatch was missing from the signal. The NCD and NCD cable are coupled by the “resistive coupler” and based upon this evidence, it is theorized that the resistive coupler disconnected (RCD) from either the NCD or the NCD cable. If true, RCD poses a serious threat to the determination of the detector live-time and NCD data quality. In response to the RCD theory, a new electronics calibration was devised: the NCD RCD Electronics calibration (NRE). The NRE calibration was initiated in June, 2006, and involves injecting a narrow (78 ns) square wave into the front-end electronics and examining the ensuing pulse reflections. By measuring the amplitude difference between two of the reflection peaks, the presence of the missing reflection can be inferred, since the total injected charge is conserved. The NRE pulse is injected into each NCD electronics circuit once every forty minutes. The missing pulse reflection has not been observed on any other NCDs besides string 1 and 31. (See Fig. 1.5-1). 16 Peak Amplitude Difference (arb. units) 1400 14 1200 12 1000 10 800 8 6 600 4 400 2 200 00 0 5 10 15 20 25 30 35 40 NCD Figure 1.5-1. The difference in peak amplitudes for NRE calibration pulses. NCD strings 1 and 31 indicate a different pulse reflection behavior. Other attempts to observe RCD behavior have been performed: power spectrum tests, event rate tests, and local bench-tests that attempt to mimic the RCD condition. Tests to measure the time-between-event distribution of various types of noise events have been initiated. A decrease in the rate of these events could be interpreted as a condition for RCD.


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    8 1.6 A new external alpha counter for the SNO neutral current detectors N. Gagnon,∗ S. McGee, A. Myers, R. G. H. Robertson, B. A. VanDevender and T. D. Van Wechel A new External Alpha Counter (EAC) was constructed at CENPA to count low-level ra- dioactivity on the surfaces of the SNO Neutral Current Detectors (NCDs). This radioactivity produces backgrounds in the SNO detector. If the particular isotopes responsible for these backgrounds can be identified and their strengths determined, the overall systematic uncer- tainty in SNO’s neutral current flux measurement will be significantly reduced for the NCD phase. The EAC is being commissioned at SNO, with counting of NCDs expected to begin in April 2007. 1200 V -100 V A diagram of the EAC, viewed from the elec- tronics end. The NCD being counted rests in the hole in the middle. The dotted lines show how the anodes are shorted together at the far end. The counter is normally operated with one pair of anodes grounded and the in- terlaced pair at high voltage. The EAC is a simple multi-wire proportional counter, sketched in the figure. Its total length is 48 in. The total length includes 3.06 in at the “electronics end” which is filled with wire-binding posts, preamplifiers, and signal and high-voltage connections. Another 2.06-in enclosure at the “far end” contains binding posts and short-circuit jumpers that connect each alternate pair of anode wires. The remaining 42.88-in active volume is filled with P5 gas (95 % Ar, 5 % Methane). The counter is designed so that the symmetric upper and lower halves can be easily separated, allowing insertion of an NCD into a 5-cm diameter opening in the endplates. The NCD is surrounded by a concentric 10 cm diameter ring of 8 active anode wires. The anode wires are made of 25 µm diameter resistive Stablohm 800. Using resistive anode wires with preamps at each end allows a determination of the longitudinal location of events by a simple charge-division algorithm. Concentric with the NCD and the anode ring is another ring with diameter 16.8 cm consisting of 24 50 µm diameter copper wires. These wires are held at a small negative potential to suppress events occuring in the outer volume of the EAC. Nominal conditions for sensitivity to alpha particles with energies in the range from 5– 10 MeV are found to occur for an anode voltage Va = 1200 V. However, at these voltages, it was observed that the chamber discharged at a very high rate near the ends of the active volume. The discharges have been temporarily suppressed by coating the anode wire binding posts with an insulating silicone grease. An improved design for the insulating posts is in progress. ∗ Sudbury Neutrino Observatory, Lively, ONT, Canada P3Y 1M3.


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    UW CENPA Annual Report 2006-2007 May 2007 9 1.7 Energy losses of protons and alphas in NCDs and energy spectra of alphas from the counter walls H. Bichsel, R. G. H. Robertson and H. S. Wan Chan Tseung The first step in the determination of the pulse shapes in the NCD is a calculation of the track structure of the particles which produce the ionization in the NCD gas. In order to calculate track structures, the collision cross sections σ(E) differential in energy loss E for fast charged particles in the gas must be known for both components of the gas (85% by volume of 3 He and 15% of CF4 at a pressure of 2.5 atm). The Fermi Virtual Photon method (FVP) is used to obtain σ(E).1 The photo-absorption data for He were obtained from the literature.2,3,4 The total number of collisions per unit length Σ(T ) (T is the kinetic energy of a proton) for the gas mixture is 0.8/µm at T = 600 keV and about 3/µm at T = 50 keV. For He-ions the Σ(T ) are approximately 4Σ(T ) of protons with the same speed (i.e. Tα ∼ 4 Tp ). In order to calculate the structure of the particle tracks in the gas with 1 µm segments, a MC calculation with single collisions must therefore be made. A second problem is the determination of the spectra of the energy deposition in the gas by alpha particles from the wall. Consider alpha particles with energy T0 from a very thin layer in the Ni-wall. If they travel a distance x to the point where they enter the gas, their mean energy T is given implicitly by R(T ) = R(T0 ) − x. They have an energy spectrum f (T ; T0) similar to a Gaussian due to straggling in energy loss. An estimate of the widths of these Gaussians can be made from the literature.5 Other details are given in Section 6.4 of this report. As an example, a spectrum f (T ) of the residual energies below 1 MeV in the gas for 5.4 MeV alphas is given by the solid line in Fig. 1.7-1. Straggling is assumed to be Gaussian, with a full-width-at-half maximum given by the Bohr approximation (approximately 200 keV), reduced by a correction for atomic binding of the electrons. In order to show the importance of straggling, a second function is shown with a straggling-width of 0.01 of the Bohr straggling. Alpha particles are assumed to be produced in an infinitesimal layer every 22 nm inside the Ni, while the FWHM of the Gaussian (∼ 2 keV) is equivalent to 2 nm. Therefore separate straggling functions are seen. 1 see http://faculty.washington.edu/hbichsel for details. 2 Joseph Berkowitz, Atomic and Molecular Photo Absorption, Academic Press (A division of Harcourt) 2002. 3 L. C. Lee, E. Phillips and D. L. Judge, J. Chem. Phys. 67, 1237 (1977). 4 I. B. Smirnov, Nucl. Instrum. Methods A 554, 474-493 (2005). 5 C. Tschalär, Nucl. Instrum. Methods 61, 141 (1968) and 64, 237 (1968).


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    10 Figure 1.7-1. Energy deposition spectrum in counter gas by alpha particles from the wall. The solid line represents Gaussian straggling with FWHM of 200 keV; the separate functions are calculated with FWHM of 2 keV. Straggling tends to decrease the rise of the spectrum with decreasing energy T . For the ionization, a further decrease will occur because of the dependence of W on T .


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    UW CENPA Annual Report 2006-2007 May 2007 11 KATRIN 1.8 Status of the CENPA contribution to the KATRIN experiment T. H. Burritt, P. J. Doe , G. C. Harper, M. A. Howe, M. J. Leber, A. W. Myers, R. G. H. Robertson, B. A. VanDevender, T. D. Van Wechel, B. L. Wall and J. F. Wilkerson. 2006 has been an eventful year for the KATRIN experiment, culminating in the delivery of the main spectrometer vessel. Construction of the vessel was completed in August, 2006. After successful leak checking, the vessel was pumped down using a fore pump to 1.5 x 10−3 mbar in 48 hours. A single 2800 l/sec turbo-molecular pump was then used to reduce the pressure to 3 x 10−7 mbar in approximately 20 hours. This is a remarkable achievement for such a large vessel that had not been baked out. After a 9000 km, 60 day voyage the vessel was delivered to the Forschungszentrum, Karlsruhe, and installed in the new spectrometer hall on 29 November, 2006. Subsequently, the vessel has received its thermal insulation and has been attached to the heating/cooling system in preparation for further commissioning. Commissioning of the pre-spectrometer began in September, 2006. This activity involves two CENPA contributions, the inner electrode system, used to suppress backgrounds orig- inating from the wall of the vessel, and the ORCA data acquisition system. The commis- sioning exercise was very educational. Significant problems were encountered due to vacuum breakdown conditions that occur in the presence of crossed magnetic and electric fields and to the formation of penning traps. A theoretical understanding of the vacuum breakdown mechanism and the formation of penning traps has been developed. The ability to vary the potential on different parts of the electrode proved to be a valuable diagnostic tool in identi- fying and understanding the source of the problems. Modifications are underway both to the pre-spectrometer and the main spectrometer to prevent such vacuum breakdown occurring. The ORCA data taking system performed flawlessly during the commissioning exercise. The pre-spectrometer commissioning made use of a 64 pixel, PIN diode array. Prior to use, the detector was studied using the CENPA, variable energy, electron gun. An important property of the detector is the thickness of the dead layer at the entrance window of the device. The thickness of this layer, which can change due to surface contamination or due to a change in the properties of the device, must be monitored throughout the KATRIN data taking. Two different techniques were investigated. One technique involves varying the angle of incidence of the electron beam. This effectively varied the thickness of the dead layer the beam traversed, allowing an estimate of the layer thickness at normal incidence. Unfortunately, this technique cannot be used in the actual KATRIN experiment, since the angle of incidence cannot be varied. A second technique, involving varying the beam energy, was tested. Both techniques were within reasonable agreement and we are optimistic that the variable energy technique will work well for the KATRIN experiment. This work is described in Sec. 1.13 of this Report. The electromagnetic design of the overall KATRIN experiment underwent significant changes in 2006. It was determined that the transport magnets that connect the main


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    12 spectrometer to the detector system could be discarded without effecting the physics goals of the experiment, thus realizing considerable savings. The detector magnet is in close proximity to the 6-Tesla pinch magnet at the output of the main spectrometer, subjecting both magnets to considerable mechanical loads. It was the recommendation of the magnet experts that the magnets should be designed and commissioned as a pair. As a result we have requested quotes for the pair of magnets as well as quotes for individual magnets. The new electromagnetic design resulted in a new flux tube profile which required slight modification of the internal dimensions of the detector housing. With the removal of the transport magnets system, the detector is now in more direct contact with the main spectrometer, and therefore the quality of the vacuum in the detector system becomes more critical. In addition, due to the extensive fringe fields associated with the magnets, a new pumping system consisting of cryopumps and non-evaporable getters has been designed. This new vacuum system has been modeled by our KATRIN colleagues at the Accelerator Science and Technology Center, (ASTeC) at the Daresbury Laboratory and found to be compatible with the 10−11 mbar requirement of the main spectrometer. The design of the detector system is close to being finalized. Specifications for the detec- tor and the magnet system are complete and preliminary bids have been received for both. As the design matures, modeling of the detector backgrounds has become more detailed, in- cluding realistic modeling of the structure of the detector housing using levels of radioactivity obtained by assay of candidate materials. The effects of intrinsic radioactivity and cosmic ray interactions in the magnet have also been incorporated into the model. This work is described in Sec. 1.11 of this Report. In November the US KATRIN proposal was subject to a DOE Review. The physics goals, technical approach and the strength of the collaboration received strong reviews. At the request of the committee, valuable improvements were made to the management plan. It is anticipated to receive a response by mid-April 2007. If approved, the tentative DOE funding profile will not support immediate purchase of long lead-time, critical path items, such as the magnet system. To address this problem the University has agreed to provide forward funding upon receiving notification of DOE approval. To meet this schedule we rely heavily on the diverse talents of the CENPA staff.


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    UW CENPA Annual Report 2006-2007 May 2007 13 1.9 Status of the KATRIN detector mechanical design T. H. Burritt In September, 2006 it was realized that considerable savings could be achieved by removing the transport magnets between the main spectrometer and the detector without impacting the physics goals. This optimization of the electromagnetic design in turn required modification of the detector housing design and vacuum pumping systems. The resultant mechanical design is shown in Fig. 1.9-1. Figure 1.9-1. The general arrangement of the detector vacuum housing. The detector and pinch magnets are ghosted in, the flux tube is shown on the left. Detector signals are extracted through the port on the right. The pumping system now makes use of cryopumps for both the extreme high vacuum (XHV) region and the medium vacuum region that houses the electronics. The XHV cry- opump will be augmented by a non-evaporable getter (NEG) pump to improve hydrogen pumping. The switch from ion pumps to cryopumps was made because the latter are more compatible with high magnetic field environments. Investigations are continuing to determine whether the cryopumps, which contain moving parts, can be replaced by pulse tube coolers which have no moving parts in the cold head and are ideal for high field operation. The cosmic-ray veto and radiation shield are being designed by our colleagues at MIT. A simple bolt circle on each face of the detector magnet will allow the massive shield and veto to be attached. MIT is also responsible for the calibration system, which consists of a radioactive source and a mono-energetic electron gun that can be scanned over the face of the detector. In order to allow the gun a full range of movement it was necessary to provide square ports as can be seen in Fig. 1.9-1. Working in conjunction with the engineers at the Forschungszentrum, Karlsruhe (FZK),


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    14 a rail system has been designed which supports the magnets and detector system and allows straightforward alignment of the components. This system is being installed at the FZK. The readout electronics will be provided by the FZK. A scheme for mounting and cooling the detector and the preamplifiers has been approved by the FZK electronics engineering group. The temperature of the detector will be maintained at approximately 120 K. Pre- liminary tests of the detector cooling scheme have been conducted. Connections between the detector, which sits in the XHV region, and the preamplifiers, which sit in the medium vacuum region, is by means of gold plated pogo-pins. Between 10 - 30 W of heat will need to be removed from the preamplifiers. This will be achieved using liquid nitrogen supplied from a “chicken feeder” system. A commercial company has been identified to provide this system. The front-end electronics sit in the medium vacuum region at the same potential (up to 30 kV) as the post acceleration electrode to which the detector is attached. The scheme for extracting these high potential signal lines from the vacuum chamber can be seen in Fig. 1.9-1. Fine tuning of the design is currently underway with the goal of producing the detector design manual and holding a final design review at the end of June 2007.


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    UW CENPA Annual Report 2006-2007 May 2007 15 1.10 Commissioning of the KATRIN pre-spectrometer system L. Bornschein,∗ F. Glück,∗ M. L. Leber and J. F. Wilkerson The Karlsruhe Tritium Neutrino experiment (KATRIN) plans to precisely measure the tri- tium beta-decay electron energy spectrum near the endpoint in order to directly probe the mass of neutrinos in the degenerate region. The highest energy beta-decay electrons from gaseous molecular tritium will be energy analyzed using a 10-m diameter Magnetic Adiabatic Collimation and Electrostatic (MAC-E) spectrometer. This main spectrometer is preceded by a similar but smaller pre-spectrometer that serves as a pre-filter to reduce the number of electrons that enter the main spectrometer. Commissioning of the pre-spectrometer’s electric and magnetic systems began in late 2006. Since the design of the main spectrometer is similar, lessons learned at the pre-spectrometer test-setup apply to both spectrometer systems. In order to measure the transmission function, a photo-electric electron gun was mounted on one end of the pre-spectrometer and Multi-Channel Plate detector on the other. In this configuration the high voltage (up to 35 kV), superconducting magnets (up to 4.5 Tesla), vacuum system, and intrinsic backgrounds could also be tested. In November of 2006, the transmission function was successfully measured at 1 and 1.5 kV. At higher electric and mag- netic fields problems were observed in the form of electric breakdown. In order to characterize the problems, a silicon detector was mounted in place of the electron gun. Four measuring devices were used: • Multi-channel plate detector • Silicon detector • Voltage and leakage current read-back of high voltage power supplies • Ion gauges reading tank vacuum Electric breakdown occurred when event rates spiked in both detectors, voltage read-back dropped, power supply leakage current increased, and the tank vacuum increased. Vacuum breakdown was observed with voltage settings above 20 kV and no magnetic fields other than earth’s natural field. Penning discharges were observed with moderate electric and magnetic fields, starting around 6 kV and 0.1 Tesla. A very high, constant electron back- ground proportional to the electric field setting was observed with no magnetic fields, but it was demonstrated that a more negative potential on the inner electrode could shield these electrons. In January of 2007, the fields behind the ground cone of the pre-spectrometer were simu- lated for the first time, confirming Penning traps near the entrance of the pre-spectrometer which could explain the Penning discharge. New electrodes will be manufactured and in- stalled in April 2007 which remove the Penning traps. These new electrodes will also shield sharp edges which could field emit and are the likely the source of the vacuum breakdown. The entrance region to the main spectrometer has also been simulated and electrodes will be designed to prevent similar Penning traps. ∗ Forschungszentrum Karlsruhe, Institut für Experimentelle Kernphysik, Postfach 3640, 76021 Karlsruhe, Germany.


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    16 1.11 Updates on simulation for KATRIN detector backgrounds P. J. Doe, J. Formaggio,∗ M. L. Leber, R. G. H. Robertson and J. F. Wilkerson In order to reach the projected neutrino mass sensitivity of 0.2 eV, KATRIN’s detector-related backgrounds must be limited to 1 mHz in the energy region of interest. GEANT4-based simulations continue to be used to estimate these backgrounds. As the detector design has evolved and progressed, so have the background simulations. From the silicon detector to the superconducting magnet, the backgrounds from radioactivity in the construction materials, cosmogenics, and interactions of cosmic rays have been estimated. Fig. 1.11-1 shows the simulation geometry. Estimates indicate that the design goal of 1 mHz in KATRIN’s region of interest will be met. Figure 1.11-1. Drawing of the simulated detector design. The largest contribution to the detector-related background comes from the feed-through insulators that allow the electronic signals to exit the highest vacuum region. Beta-decay electrons from these insulators are guided by the magnetic fields directly to the detector just like signal electrons. Cleaner materials must be found for these insulators. Another large contribution is secondaries from cosmic muons. High energy muons pass through the detector leaving energy in the minimum ionizing peak around 120 keV. KATRIN’s region of interest will fall between 15 and 50 keV, depending on the post-acceleration setting. In this region the dominant backgrounds from cosmic ray muons are secondary photons produced by interaction in the passive shield of copper and lead. Simulations show this component can be reduced by placing an active shield of plastic scintillator outside the passive shield. Although the simulation geometry is not an exact replica of the current design, small changes in dimensions are not expected to change background estimates drastically. The electronics will need to be included and verification will be necessary, but the design seems to be within background specifications. ∗ Presently at Massachusetts Institute of Technology, Building 26-568, 77 Massachusetts Ave, Cambridge, MA 02139.


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    UW CENPA Annual Report 2006-2007 May 2007 17 1.12 Electron gun for profiling silicon detectors for KATRIN T. H. Burritt, P. J. Doe, G. C. Harper, J. A. Mitchell,∗ B. A. VanDevender, B. L. Wall and J. A. White The monoenergetic electron source first developed in 20031 to profile electron backscattering with respect to incident angle of the large area silicon detectors to be used in the KATRIN experiment was modified in 20052 and 2006.3 It has been modified to accommodate large (10-cm diameter) multi-pixel detectors and to measure the dead layer of these detector arrays. The apparatus uses an electron gun and an einzel lens to produce a tight, focussed electron beam of a few Hz to a few kHz. Scanning a smaller, 10-mm square PIN diode detector it was found that the beam spot was larger than anticipated by the ion optics program SIMION4 and, rather than a sharp, 2 mm diameter beam spot there appeared to be a 2 mm core with a halo that extended the beam diameter to 3.5 mm. We are attempting to remedy the problem by the approach shown in Fig. 1.12-1. Figure 1.12-1. KATRIN monoenergetic electron gun modified cathode It is intended that the gun produce low energy electrons by UV photoemission from a stainless steel surface. A UV grade silica fiber and hypodermic needle collimator direct photons from a mercury arc UV lamp onto a 1 mm diameter spot on the cathode emission surface. The electrons are accelerated through a potential that can be varied up to −30 kV. The electron beam is focussed to a spot on the device under test (DUT) 0.6 m from the emission surface by an einzel lens operating at about half of the accelerating potential. We now believe that there is enough internal reflection in the hypodermic needle collimator and scattering from its edges to produce the halo observed. As shown in the figure, the cathode is now made of aluminum with a 0.5-mm diameter slug of stainless steel pressed into the ∗ Presently at nanoMaterials Discovery Corp, 2121 N 35th St #201, Seattle, WA 98103. 1 CENPA Annual Report, University of Washington (2003) 65. 2 CENPA Annual Report, University of Washington (2005) 40. 3 CENPA Annual Report, University of Washington (2006) 12. 4 SIMION 3D, version 6.0, David A. Dahl, Idaho National Engineering Lab.


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    18 center. The aluminum oxidizes rapidly to form a thin film of Al2 O3 . This alumina film has a work function of 11 eV, substantially higher than the work function of stainless steel which is approximately 4 eV. With the UV wavelength used we believe that the electrons will only be emitted from the stainless steel slug. This modification has yet to be tested. A radiative cooling system was tested on a 10 cm diameter silicon detector wafer made by the Washington Technology Center.5 Five E-type chromel-constantan thermocouples were used to measure the temperatures of the DUT and the detector cooling ring. We were able to shock-cool the cooling ring to -100◦ C without damage to the wafer using cold nitrogen vapor produced by forcing gaseous nitrogen through the internal liquid nitrogen dewar which also serves as a vacuum cold trap. The wafer temperature equilibrated at about −80◦ C uniformly across its surface after about 50 minutes. New feedthrus were purchased for the recent tests. These include a quad BNC grounded feedthru and a quad E-type thermocouple feedthru. These have been installed, leak tested, and used. The stand for the translation table was rotated by about 2◦ to compensate for dis- placement of the electron beam caused by the bottom flange warping when the gun mounting flange was welded to it. 5 Washington Technology Center, 300 Fluke Hall, Box 352140, Seattle, WA 98195-2140.


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    UW CENPA Annual Report 2006-2007 May 2007 19 1.13 Development of an in-situ dead-layer measurement for the KATRIN detector T. H. Burritt, P. J. Doe, C. Fredericks, H. Gemmeke,∗ G. C. Harper, M. A. Howe, A. W. Myers, R. G. H. Robertson, M. Steidl,∗ B. A. VanDevender, T. D. Van Wechel, B. L. Wall, S. Wüstling∗ and J. F. Wilkerson In order to develop an in-situ technique to measure the dead-layer thickness of the KATRIN detector, we have measured the dead layer of a CANBERRA1 64 pixel PIN diode array using a technique that utilizes a variable energy electron beam. The results obtained using this “energy” method was then compared to the results obtained using the more conventional angle method described in Knoll.2 The energy method makes use of the fact that average energy loss in a material is char- acterized by the Bethe-Bloch function dE dx (Ee− ) multiplied by the thickness t of the material. Therefore, energy deposition in a detector can be modeled as the incident energy Ee− less the energy deposited in the dead layer. By varying the energy of incident electrons on a detector, we can compare the incident energy of the electrons with the energy collected in the detector. The plot of energy recorded by the detector (ADC bin) as a function of incident electron energy can be fitted to the equation: dE Edeposited = C ∗ (E − T) (1) dx where C is an ADC bin to energy conversion parameter, dE dx is the energy dependent Bethe- Bloch function and T is the thickness of the dead-layer. Leaving T and C as free parameters, a value for the dead layer thickness can be obtained. The dead-layer measurements were conducted in a stainless steel vacuum chamber3 with an electron gun attached at its base.4 The CANBERRA PIN diode array was mounted on a X-Y translation stage and a rotary table. This allowed us to change both the beam spot position and the incident angle. Each pixel of the array was connected to a voltage amplifier, the output of which was converted to a digital signal via a VME based ADC shaper card. The data readout was handled by the OS X based ORCA5 program. The comparison of the two methods shows an average difference of 10 ± .3 nm between them. (See Fig. 1.13-1). The source of this systematic discrepancy between the two methods is currently being investigated. However, even with this discrepancy the agreement between the two methods is sufficient for the energy method to be used in the KATRIN experiment. ∗ Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany. 1 CANBERRA, 800 Research Parkway, Meriden, CT 06450. Available: http://www.canberra.com/ 2 G. Knoll, Radiation Detection and Measurement, 3rd ed. Wiley (2000). 3 CENPA Annual Report, University of Washington (2005) 40. 4 CENPA Annual Report, University of Washington (2006) 27. 5 M. A. Howe, G. A. Cox, P. J. Harvey, F. McGirt, K. Rielage, J. F. Wilkerson and J. M. Wouters, Sudbury Neutrino Observatory Neutral Current Detector acquisition software overview, IEEE Transactions on Nuclear Science, 51:878-883 (2004).


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    20 Figure 1.13-1. A comparison of the dead layer thickness of the 64 pixels obtained via the angle and energy methods


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    UW CENPA Annual Report 2006-2007 May 2007 21 Majorana 1.14 The Majorana neutrinoless double-beta decay experiment J. F. Amsbaugh, J. A. Detwiler, P. Doe, A. Garcı́a, M. Howe, R. A. Johnson, M. G. Marino, S. McGee, R. G. H. Robertson, A. G. Schubert, B. A. VanDevender and J. F. Wilkerson Neutrinoless double-beta decay (0νββ) provides the physics community with the opportunity to build on our successes in understanding the neutrino and crafting a new standard model. Observation of 0νββ would demonstrate that neutrinos are Majorana particles, indicate that Lepton number is not conserved, and provide a measure of the effective Majorana mass of the electron neutrino. For the first time we can mount experiments that probe the neutrino mass region below the upper limits set by direct kinematical searches (tritium) and suggested by observational cosmology, while planning scaled approaches that can address the lower bounds of mass defined by neutrino oscillation experiments. Determining if neutrinos are Dirac or Majorana particles is one of the most important questions facing the physics community today. The Majorana experiment aims to answer this question. Our proposed method uses the well-established technique of searching for 0νββ in high- purity Ge diodes that play both roles of source and detector. The technique is augmented with recent improvements in detector technology, and advances in controlling intrinsic and external backgrounds. Progress in signal processing from segmented and modified-electrode Ge-diode detectors offers significant benefits in rejecting backgrounds, and providing ad- ditional handles on both signals and backgrounds through multi-dimensional event recon- struction. Development of sophisticated Cu electroforming methods allow the fabrication of ultra-low-background materials required for the construction of next generation detectors. The Majorana collaboration is directing its efforts toward an R&D phase in which 50-100 kg of high-resolution intrinsic germanium detectors, most of which will be enriched to 86% in 76 Ge, will be deployed in an ultra-low-background electroformed Cu cryostat. This R&D module will be located deep underground within a low-background shielding environment. This effort is aimed at demonstrating that backgrounds can be pushed low enough to justify a future ton-scale 0νββ experiment in 76 Ge. The R&D will also address the choice of Ge detector technology, and cost and schedule drivers for a ton-scale experiment. A parallel Eu- ropean experimental effort in 76 Ge 0νββ decay, the GERDA experiment, is proceeding with a technique in which bare Ge diodes are immersed in liquid cryogen that shields and cools the crystals but can also potentially be instrumented as a veto. The Majorana and GERDA collaborations have expressed intent to merge collaborations for a ton-scale experiment using the best technology demonstrated in the initial phase. The collaborations openly exchange information, and are cooperating on several fronts including the joint development of a Monte Carlo simulation framework, MaGe. These experiments will either conclusively establish the KKDC1 claim of double-beta decay, or will significantly improve lifetime limits. A ton-scale experiment in 76 Ge is potentially sensitive enough to address effective Majorana masses over most of the allowed parameter space for an inverted neutrino mass hierarchy. The goals of this effort are consistent with recent recommendations from the DNP/DPF/DAP/DPB 1 H. V. Klapdor-Kleingrothaus et al., Phys. Lett. B 586, 198 (2004).


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    22 Joint Study on the Future of Neutrino Physics 2 and the conclusions on 0νββ reported by the Neutrino Scientific Assessment Group. 2 S. J. Freedman and B. Kayser, physics/0411216 (2004).


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    UW CENPA Annual Report 2006-2007 May 2007 23 1.15 Surface contamination simulations for the proposed Majorana neutrinoless double-beta decay experiment J. A. Detwiler, R. A. Johnson, M. G. Marino, A. G. Schubert and J. F. Wilkerson Previous-generation experiments looking for the neutrinoless double-beta decay(0νββ) of 76 Ge have set a lower limit on the half-life at 1.9 × 1025 years. Next-generation experiments will increase their sensitivity to this very rare decay by substantially lowering signals from radioactive backgrounds. This report summarizes progress on understanding the effects of surface contamination backgrounds for high-purity germanium detectors within the Majorana neutrinoless double-beta decay experiment. The thin dead layers on the p+ contact of a HPGe crystal detector(typically ∼ 0.3−1.0µm) allows for alpha radiation outside the detector to deposit energy within the active region of the crystal. Typical energies of alpha particles originating from the 238 U and 232 Th decay chains range from ∼ 4 − 9 MeV. Depending on how much energy it loses in the dead layer of a crystal an alpha could deposit just enough energy in the active volume to mimic the signal from 0νββ(2039 keV for 76 Ge). Alpha radiation originating from the surfaces of detectors is therefore a possible source of background. Surface contamination simulations are performed using MaGe, a C++ based simulation and analysis toolkit developed jointly between the Majorana and GERDA collaborations. The simulations consist of decays of radioactive isotopes on the surfaces of the proposed Majorana neutrinoless double-beta decay experiment. These simulations yield efficiencies for an initial decay to land near the 0νββ region of interest energy for 76 Ge(2037-2041 keV). The simulated isotopes include the 238 U and 232 Th decay chains, with particular attention paid to the long-lived radon daughter 210 Po and its 5.3 MeV alpha. Validation of the simulation is being pursued with the “WIPPn” detector. This detector has a distinctive peak at 5.3 MeV corresponding to the alpha from 210 Po(figure Fig. 1.15-1). Further work is planned to incorporate pulse-shape analysis into MaGe. Pulse-shape analysis is expected to allow for higher discrimination of surface events, and is important to be included in a realistic simulation. WIPP-n Background Counts/Day/keV 0.09 0.08 WIPP-n Data 0.07 MaGe 210Po Simulation 0.06 0.05 0.04 0.03 0.02 0.01 02500 3000 3500 4000 4500 5000 Energy [keV] 210 Figure 1.15-1. Comparison of the 5.3 MeV α peak from Po simulation with background data taken from the WIPPn detector.


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    24 1.16 Validation of cosmic-ray muon-induced physics within the GEANT4- based simulation and analysis package MaGe J. A. Detwiler, R. A. Johnson, M. G. Marino, A. G. Schubert and J. F. Wilkerson Understanding the background created by cosmic-ray muon-generated neutrons involves the simulation of their creation, propagation, and interaction with the detector as well as the ver- ification of all associated code. New simulations have been performed modeling two relevant experiments to verify neutron production and propagation in GEANT4. The NA55 experiment at CERN measured the double differential cross section for neu- trons emitted at 45, 90 and 135 degrees from a 190 GeV muon beam incident upon three different materials: graphite, Cu, and Pb.1 The shapes of the double-differential cross section for both simulation and experiment were compared and found to be roughly in agreement, though this agreement worsens at back-scattering angles (i.e. 135 deg). A comparison of total fluences is given in Table 1.16-1. Table 1.16-1. Calculated and measured neutron productions for the NA55. Material Exp. Fluence [n/µ/g/cm2 ] Calc. Fluence [n/µ/g/cm2 ] Ratio Graphite 6.8×10−5 3.3×10−5 2.1 Copper 1.2×10−4 4.8×10−5 2.5 Lead 3.5×10−4 5.9×10−5 5.9 An experiment performed at the Stanford Linear Accelerator Center (SLAC)2 involved a 28.7 GeV electron beam incident upon an aluminum beam dump. The neutron time- of-flight and energy spectra were measured outside a steel shield and a concrete shield of variable width. A comparison between experiment and GEANT4 simulation indicated an over-attenuation of neutrons within concrete. A correction method was applied to mitigate this discrepancy. These results are presented in Fig. 1.16-1. Neutron Yield (electron-1) Geant4 (98.9 ± 5.3) 10-3 FLUKA (115 ± 5) 10-4 Measurement (124 ± 4) Reweighted G4 (116.8 ± 5.9) 10-5 10-6 10-7 10-8 0 200 400 600 800 1000 Shield Width (g/cm2) Figure 1.16-1. A comparison of calculated (GEANT4 and FLUKA) and measured total neutron fluence (E≥ 6 MeV) for varying shield width. Exponential fits to all sets of points are included and the values of the characteristic length (λ) are noted in parentheses in units of g/cm2 . 1 V. Chazal, et al. Nucl. Instrum. Methods A 490, 334 (2002). 2 S. Taniguchi, et al. Nucl. Instrum. Methods A 503, 606 (2003).


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    UW CENPA Annual Report 2006-2007 May 2007 25 1.17 Development of technologies for low-background experiments using the MEGA cryostat J. A. Detwiler, R. A. Johnson, M. G. Marino, A. G. Schubert and J. F. Wilkerson Low-background experiments such as those searching for neutrinoless double-beta decay or dark matter demand advances in current technologies to achieve their background goals. The MEGA cryostat is an electroformed-Cu cryostat located underground at the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM.1 Work was completed to populate this cryostat with three working high-purity germanium (HPGe) crystals. To record pulses from a HPGe detector suitable for pulse-shape analysis it is important to place front-end electronics near the detector. Any material placed near the detector creates a potential increase in background. A low-background front-end package (LFEP) has been developed to satisfy the requirements of background and signal readout.2 Characterization of these LFEPs, including noise measurements and bandwidth determination, were performed. The LFEPs were deployed into the MEGA cryostat along with two new crystals to bring the total crystal population to four. Tests were performed on the installed crystals including monitoring time required for sufficient cooldown (i.e. through the semi-conductor transition of Ge), measuring operating voltages and reading out pulses. To demonstrate the functionality of the crystals and LFEPs, test data were taken with and without a Th source. Pulses seen and a spectrum from the source run are shown in Fig. 1.17-1 and in Fig. 1.17-2. Figure 1.17-1. A triple coincidence from three working detectors in the MEGA cryostat. Thorium Source Spectrum Counts 400 350 300 250 200 150 100 50 00 500 1000 1500 2000 2500 3000 Energy [keV] Figure 1.17-2. Spectrum from one channel taken during a Th source run. 1 K. Kazkaz, et al. IEEE Trans. on Nucl. Sci. 51, 1029 (2004). 2 T. Hossbach and C. Aalseth, APS Meeting, Maui, HI, Sept 2005.


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    26 1.18 Low-energy neutron response of germanium detectors in MaGe/GEANT4 J. A. Detwiler, R. A. Johnson, M. G. Marino, A. G. Schubert and J. F. Wilkerson The Majorana1 and Gerda2 collaborations jointly developed MaGe, a GEANT43 - and ROOT4 - based simulation framework. This paper describes a simulation used to study MaGe’s ability to model low-energy neutrons in germanium detectors. The simulation was based on an ex- periment performed at Los Alamos National Laboratory (LANL). Comparison between the simulation results and experimental data can be used to verify MaGe/GEANT4. During the experiment, energy spectra were collected from each of the four n-type germa- nium crystals in a Canberra clover detector. The detector was surrounded by lead shielding, and an AmBe neutron and gamma source was located outside of the shield. A slab of polyethylene neutron moderator was placed within the lead shield between the source and detector. The simulation results were generated with MaGe, using the GEANT4 software package to model the physics processes. ROOT was used to analyze the results. The AmBe source, clover detector, lead shield, and polyethylene moderator were modeled in the simulation. A comparison between the experimental and simulated results appears in Fig. 1.18-1. The MaGe/GEANT4 package captures many of the features of the experimental energy spectrum. The results of the simulation provide an understanding of the strengths and deficiencies of the MaGe/GEANT4 package. Experimental Data Geant4 Results 1 Counts/s/keV 10-1 10-2 0 500 1000 1500 2000 2500 3000 Energy [keV] Figure 1.18-1. Comparison between experimental data and simulation results of energy spectra collected by the clover detector. 1 White Paper on the Majorana Zero-Neutrino Double-Beta Decay Experiment. 2 The Gerda Collaboration, http://www.mpihd.mpg.de/ge76/gerda lngssc mar06.pdf. 3 S. Agostinelli, et al., GEANT4 - A Simulation Toolkit, Nucl. Instrum. Methods A 506, 250 (2003). 4 R. Brun, et al., ROOT - An Object Oriented Data Analysis Framework, Nucl. Instrum. Methods A 389, 81 (1997).


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    UW CENPA Annual Report 2006-2007 May 2007 27 1.19 LArGe, Liquid Argon Compton suppressed Germanium crystal C. E. Aalseth,∗ J. F. Amsbaugh, P. J. Doe, J. L. Orrell,∗ R. G. H. Robertson and J. F. Wilkerson. We have continued the development of the LArGe apparatus1 to study background suppres- sion in a high purity germanium crystal diode. The technique uses scintillation light from the surrounding liquid argon (LAr) created when a Compton-scattered gamma-ray escapes the crystal and electron scatters in the LAr. Our effort has been to finish the design and manufacture the new apparatus, which improves the efficiency of the Compton shield by increasing the surrounding LAr from 0.5 to 3 radiation lengths(X0 ). A pressurized annular liquid nitrogen (LN2 ) tank near the top of the dewar maintains the LAr in a liquid state. The dewar holds 780 kg of LAr and the LN2 tank volume is 78 l. The crystal can be put in or removed from the LAr by a slide mechanism down the center of the dewar made of low background materials. An airlock is provided so the crystal can be replaced without contamination or loss of LAr. Also, the dewar can be cooled and filled with LAr without the delicate crystal in place. The degraded crystal previously used has been successfully refurbished. The front-end components of the preamplifiers are mounted close to the HPGe to optimize the noise performance. Four hemispheric 20-cm photomultiplier tubes (PMTs) are mounted in the LN2 tank’s inner cylinder with their photocathodes in the LAr. The PMTs bias bases mount outside in air, and a 16-pin feed through assembly connects to each PMT. A blue LED2 in the bottom provides typical PMT output pulses with < 10-nsec rise time and 25-nsec width comparable to single photoelectron pulses of < 7 nsec and 20 nsec. Three LEDs have been thermally cycled in LN2 20 times and kept cold for 2 weeks without degradation or failure. The LAr emits scintillation light peaked at 128 nm which is wavelength shifted by a non-metallic visible light reflector, ESR,3 lining the sides and bottom of the dewar. The reflector was not coated with additional wavelength shifter due to concerns about adhesion, crystal contamination and the fact that the X0 = 0.5 test setup4 achieved a factor of 2 suppression. Except for the reflector, its support and the LED, all components mount to or hang from the top plate of the dewar. The top plate has lift points and several extra ports for future uses, viewports, et cetera. Before the initial cooldown and fill, a few external cryogenic lines need to be made and the cold tests of the crystal mechanism and contacts need to be done. The device is illustrated in Fig. 1.19-1. ∗ Pacific Northwest National Laboratory, Richland, WA 98352. 1 CENPA Annual Report, University of Washington (2006) 25. 2 HLMP-CB15-R0xx. 3 also known as VM2000, from 3M Corp. 4 J. L. Orrell, C. E. Aalseth, J. F. Amsbaugh, P. J. Doe, T. W. Hossbach, July, 2006, submitted for publica- tion to Nucl. Instrum. Methods A, nucl-ex/0610018.


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    28 Figure 1.19-1. A three quarters section of second LArGe apparatus revealing the HPGe crystal, track mechanism, LN2 tank, PMTs, top manipulator chamber, and air lock.


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    UW CENPA Annual Report 2006-2007 May 2007 29 1.20 Design, construction and operation of a small-scale radioactivity assay chamber J. A. Detwiler, P. J. Doe, R. A. Johnson, W. R. Ketchum,∗ M. G. Marino, A. S. Reddy,† A. G. Schubert, B. A. VanDevender and J. F. Wilkerson A new low-background radiometric assay chamber has been built at CENPA, using two germanium detectors. The chamber should allow us to make initial determinations of the viability of candidate materials for use in low-background experiments (e. g., KATRIN and Majorana). The system will provide a testing ground for many issues relating to germanium detectors, such as detector handling issues, mounting techniques, and cryostat design con- siderations. The work described in this article was completed by undergraduate students as part of the Research Experience for Undergraduates program. The two detectors are identical intrinsic germanium detectors manufactured by ORTEC. The active detector volumes are cylinders, 70.9 mm in length and 65.1 mm in diameter. Each of these is contained within an aluminum casing that is attached to a liquid nitrogen cryostat. The assay chamber consists of both active and passive shielding. Two scintillator paddles actively shield the detector from cosmic rays and any nuclear events associated with them. If energy is deposited simultaneously in both paddles, a trigger is sent to the electronics system to veto any signal that simultaneously occurs in the germanium detectors. Signals occurring in the germanium that are not accompanied by a cosmic veto are read out by the ORCA data-acquisition system. Furthermore, the detectors are housed in a large lead structure to shield background radiation. The figure shows a comparison of data taken with a single detector in the absence of a source, with the detector inside and outside of the lead house. In the background spectrum outside of the house, the energy resolution is about 1.0 keV at 1460.8 keV (the 40 K line) and 1.5 keV at 2614.5 keV (the 208 Tl line). We can see that the background is reduced by a factor somewhere between 10 and 100. This reduction could be enhanced by several modifications to the setup. Possible improvements to the shielding include the use of larger, more efficient scintillator paddles, an inner copper metal shield, and N2 gas pumping through the chamber. ∗ Presently at University of Oklahoma, 660 Parrington Oval, Norman, OK 73019-0390. † Presently at North Dakota State University, 1301 12th Avenue North, Fargo, North Dakota 58105


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    30 2 Fundamental Symmetries and Weak Interactions Torsion Balance Experiments 2.1 Torsion balance search for spin coupled forces E. G. Adelberger, C. E. Cramer and B. R. Heckel We use a torsion pendulum containing 9.7 × 1022 polarized electron spins to search for feeble long-range interactions that couple to a particle’s intrinsic spin. Initial constraints on a cou- pling between electron spins and a possible Lorentz and CPT-violating background field fixed in space and between electron spins and unpolarized matter in the earth and sun, noted in last year’s Annual Report, have recently been published.1 Interpreted as a constraint on non- commutative geometries,2 these results place an upper bound of 3 × 10−58 m2 (corresponding to an energy scale of ≈ 1013 GeV) on the minimum observable area in such geometries. Since then, we have eliminated a significant source of systematic error in lab-fixed signals, used the rotation of the earth to measure the pendulum’s spin content, and initiated a search for forces coupling two polarized spins together. The spin pendulum is inside a torsion-balance apparatus that sits on a turntable rotating about a vertical axis at a constant angular velocity. Consequently, its affinity for a preferred direction in space appears as a modulation of its angular position at the rotation frequency. The amplitude of this modulation is itself modulated both daily and annually as the ori- entation of the fiber axis changes with the earth’s rotation and orbit around the sun. We have used these additional modulation frequencies to place tight constraints on couplings to astronomical sources and background fields fixed in space. Couplings to sources fixed in the lab frame are modulated only at the turntable frequency. When analyzing these signals, we found a significant non-zero signal with larger than statistical scatter. The signal can be un- derstood as a gyrocompass effect: the pendulum’s net angular momentum from the polarized electron spins couples to the Earth’s rotation to produce a steady torque along the fiber axis twisting the spin dipole southward. To investigate the lab-fixed systematic error, we used a dummy pendulum with no significant gravitational moments and no spin dipole. We found a spurious signal associated with the orientation of the dummy pendulum with respect to the damper plates at the fiber attachment point. To eliminate this effect, we have installed a ball-in-cone fiber attachment scheme that allows us to change the angle of the pendulum with respect to the fiber. Tests with the ball-cone and dummy pendulum have shown that any residual systematic effects are unresolvable. With the ball-cone and spin pendulum in place, we have measured the gyrocompass effect and thus determined that the pendulum’s angular momentum results from 9.7 × 1022 polarized electron spins. We are currently extending our exploration of spin-coupled forces to effects that depend on the relative orientation of two spins. Such forces could arise from the exchange of axion- like pseudoscalar particles or the Nambu-Goldstone bosons associated with the spontaneous 1 B.R. Heckel et al., Phys. Rev. Lett. 97, 021603 (2006). 2 I. Hinchliffe, N. Kersting and Y. L. Ma, Int. J. Mod. Phys. A19, 179 (2004). (hep-ph/0205040).


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    UW CENPA Annual Report 2006-2007 May 2007 31 breaking of Lorentz symmetry. We have constructed kg-scale spin sources using SmCo5 magnets with a soft iron return yoke. These sources are mounted approximately 30 cm from the spin pendulum, outside the torsion-balance apparatus, allowing us to probe interaction with ranges greater than 30 cm. In the coming year, we will construct smaller spin sources that can be placed closer to the pendulum to improve our sensitivity to these interactions at shorter ranges. 2.2 Tests of the gravitational inverse-square law below the dark-energy length scale E. G. Adelberger, T. S. Cook, J. H. Gundlach, B. R. Heckel, C. D. Hoyle,∗ D. J. Kapner† and H. E. Swanson Theoretical speculations about new short-range phenomena arising from “large” extra dimen- sions,1,2 more than 1 time dimension,3 “fat gravitons”4 and forces from string-theory scalar particles5 have motivated our interest in studying gravity at the shortest accessible length scales. Furthermore, the measured6 dark energy density ρd ≈ 3.8 keV/cm3 corresponds to a distance λd = h̄c/ρd ≈ 85 µm that may represent a fundamental length scale of gravity.7,8 4 We have a continuing program of developing instruments that probe the inverse-square law at ever shorter length scales. We recently completed and published9 our inverse-square law test with the “42-hole” pendulum/attractor system that verified the inverse-square law at Yukawa length-scales down to 56µm. Our results, along with previous work,10,11,12,13,14 are shown in Fig. 2.2-1. Our results constrain, at 95% confidence, the size of the largest extra dimension to be less than 44 µm, and require the dilaton mass to be greater than 3.5 meV. This experiment has essentially reached a practical limit of our “multihole” designs. We are now building two new inverse-square instruments, which are described in Sec. 2.9 and 2.10 of this Report. ∗ present address: Humboldt State University, One Harpst St., Arcata, CA 95521-8299. † present address: Kavli Institute for Cosmological Physics, University of Chicago, Chicago IL 60637. 1 S. R. Beane, Gen. Relativ. Gravit. 29, 945 (1997). 2 G. Dvali, G. Gabadadze, M. Kolanović and F. Nitti, Phys. Rev. D 65, 024031 (2001). 3 G. Dvali, G. Gabadadze and G. Senjanovı́c, hep-ph/9910207 (1999). 4 R. Sundrum, Phys. Rev. D 69, 044014 (2004). 5 E. G. Adelberger, B. R. Heckel and A. E. Nelson, Ann. Rev. Nucl. Part. Sci. 53, 77 (2003). 6 C. L. Bennet, et al., Astrophys. J. Supp. Ser. 148, 1 (2003). 7 S. R. Beane, Gen. Relativ. Gravit. 29, 945 (1997). 8 G. Dvali, G. Gabadadze, M. Kolanović and F. Nitti, Phys. Rev. D 65, 024031 (2001). 9 D. J. Kapner, et al., Phys. Rev. Lett. 98 021101 (2007). 10 C. D. Hoyle, et al., Phys. Rev. D 70 042004 (2004). 11 R. Spero, et al., Phys. Rev. Lett. 44,1645 (1980); J. K. Hoskins, et al., Phys. Rev. D 32, 3084 (1985). 12 J. C. Long, et al., Nature 421, 922 (2003). 13 J. Chiaverini, et al., Phys Rev. Lett. 90, 151101 (2003). 14 S. J. Smullin, et al., Phys. Rev. D 72, 122001 (2005).


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    32 Figure 2.2-1. Constraints on Yukawa violations of the gravitational 1/r 2 law. The shaded region is excluded at the 95% confidence level. Heavy lines labeled Eöt-Wash 2006, Eöt- Wash 2004, Irvine, Colorado and Stanford show experimental constraints from this work, Ref. 10, Ref. 11, Ref. 12 and Refs. 13 and 14, respectively. Lighter lines show various theoretical expectations summarized in Ref. 5.


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    UW CENPA Annual Report 2006-2007 May 2007 33 2.3 Particle physics implications of our recent test of the gravitational inverse square law E. G. Adelberger, B. R. Heckel, S. Hoedl, C. D. Hoyle,∗ D. J. Kapner† and A. Upadhye‡ Our recent test of the gravitational inverse-square,1 summarized in the previous entry of this report, has interesting particle physics implications that were described a second publication.2 We mention here two of the issues. 1). The PVLAS collaboration3 studied the propagation of optical photons through a vacuum containing a strong transverse B field and saw an effect they interpreted as evidence for a new spin-zero particle that, through a second-order process, mixes with the photon in a magnetic field. The sign of the observed rotation required the new particle to be a scalar (as opposed to pseudoscalar) boson,4 and the magnitude required 1.0 meV ≤ mφ c2 ≤ 1.5 meV 1.7 × 10−6 GeV−1 ≤ gφγγ ≤ 5 × 10−6 GeV−1 . (1) This φγγ vertex generates, by a 2nd-order electromagnetic process, an effective√scalar in- teraction between two protons, which to leading order is estimated to be5 gSp /( 4πh̄c) ∼ gφγγ (α/π)mp . This force must show up as a violation of the inverse-square law with a range of about 160 µm. Our limits on such deviations require gφγγ ≤ 1.6 × 10−17 GeV−1 , which is inconsistent with Eq. 1 by a factor of ∼ 1011 . 2.) Chameleons, scalar fields that couple to themselves and to matter with gravitational strength,6 were invented to escape the strong experimental bounds on very light scalar par- ticles. Chameleon exchange leads to an effective potential density7 1 γ β Veff (φ, x) = m2φ φ2 + φ4 − ρ(x)φ , (2) 2 4! MPl where γ characterizes the strength of the self interaction, β characterizes the coupling of the scalar field to matter, and MPl is the reduced Planck mass. The “natural” values of β and γ are ≈ 1. In the presence of matter with density ρ, a massless chameleon field acquires an effective mass so that only a small amount of material near the surface contributes to a long-range force.6,7,8 For ρ = 10 g/cm2 and β = γ = 1, this skin thickness is about 60 µm. Using the method outlined in Ref. 7, we calculated the expected chameleon signal in our apparatus. Our data strongly exclude a substantial region of parameter space around the natural values β ≈ 1 and γ ≈ 1. (See Fig. 2.3-1) ∗ present address: Humboldt State University, One Harpst St., Arcata, CA 95521-8299. † present address: Kavli Institute for Cosmological Physics, University of Chicago, Chicago IL 60637. ‡ Department of Physics, Princeton University, Princeton, NJ 08544. 1 D. J. Kapner, et al., Phys. Rev. Lett. 98, 021101 (2007). 2 E. G. Adelberger, et al., Phys. Rev. Lett. 98, 131104 (2007). 3 E. Zavattini, et al., (PVLAS Collaboration), Phys. Rev. Lett. 96, 110406 (2006). 4 E. Zavattini, et al., INFN-LNL-213 (2006). 5 E. Massó and C. Rizzo, hep-ph/0610286. 6 J. Khoury and A. Weltman, Phys. Rev. Lett. 93, 171104 (2004). 7 A. Upadhye, S. S. Gubser and J. Khoury, hep-ph/0608186 (2006). 8 B. Feldman and A. E. Nelson, hep-ph/060307.


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    34 Figure 2.3-1. 2σ constraints on the chameleon parameter β as a function of γ from the Kapner et al. data. The shaded area is ruled out at 95% confidence.


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    UW CENPA Annual Report 2006-2007 May 2007 35 2.4 A new limit on a P- and T-violating force E. G. Adelberger, B. R. Heckel, H. Hess, S. A. Hoedl and H. E. Swanson Our torsion pendulum search for an axion-like particle offers a significant improvement over the most recent such measurement.1 The axion is the result of the hypothesized Peccei-Quinn symmetry and is a favored cold dark matter candidate. Its mass is constrained by the known flat geometry of the universe to be heavier than 1 µeV, and is constrained by the neutrino flux from SN1987A to be lighter than 10000 µeV.2 Note that microwave cavity searches probe for light axions (ma ∼ 1µeV) and have sufficient sensitivity to see the expected cosmological axion flux.3 A torsion-pendulum based search is possible because the axion mediates a macroscopic pseudo-scalar potential (∝ ΘQCD ) between polarized and unpolarized fermions. By observing the motion of a planar torsion pendulum (source of unpolarized fermions) positioned near the pole faces of an energized ferromagnet, we can observe such a force. Figure 2.4-1. A scale diagram of one of our pendulums positioned in between the magnet pole faces; A face-on view of the pendulum; A picture of our axion-pendulum apparatus showing the magnet and cooling lines. In the past year, we have completed the con- struction of our apparatus and tested its per- formance with two different pendulums. We are presently in the processes of identifying and mitigating systematic errors of the second pen- dulum design. Once these systematic errors are controlled, we believe the apparatus will be able to put a limit on a macroscopic parity and time violating force which is 100 trillion times more restrictive than Hammond et al. Figure 2.4-2. Our expected exclusion bounds for an axion mass of 2 meV, although this is compared with recent experimental searches not sensitive to conventional axion models. and the expected coupling for ΘQCD < 10−9 . 1 G. D. Hammond et al., Phys. Rev. Lett. 98, 081101 (2007). 2 G. Raffelt hep-ph/0611350 3 L. D. Duffy et al., Phys. Rev. D 74, 012006 (2006).


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    36 2.5 Laboratory test of Newton’s second law in the limit of small accelerations K.-Y. Choi, J. H. Gundlach, S. Schlamminger and C. D. Spitzer Newton’s second law F = ma is one of the most fundamental equations of classical physics. Its validity at low acceleration scales is generally assumed despite the lack of experimental tests. Interest in violations of F = ma for low accelerations has been spurred by Milgrom1 in 1983. He proposed a change of Newton’s second law to explain the observed flatness of galactic rotation curves. In this so-called MOdified Newtonian Dynamics (MOND), Newton’s second law becomes F = ma2 /a0 for accelerations of a << a0 . Newton’s second law is recovered in case of a > a0 . Milgrom showed that with a0 = 1.2 × 10−10 ms−2 a reasonable fit to the observed rotation curves for many galaxies could be achieved. Other observations accentuate the need to test Newton’s second law at small accelerations. The Pioneer spacecrafts 10 and 11 exhibit an unmodeled acceleration of aP = 9 × 10−10 ms−2 towards the sun at distances larger than 15 AU, and the Hubble acceleration, given as a product of the speed of light and the Hubble constant, is aH = 7 × 10−10 ms−2 . We used a torsion balance to test Newton’s second law for small acceleration. At small excursions from its equilibrium position a torsion pendulum is subject to small accelerations. Consequently, the validity of Newton’s second law can be tested by measuring the natural frequency of a torsion pendulum as a function of its amplitude. A violation of F = ma, similar to the one proposed by Milgrom, would lead to an increase of the natural frequency for smaller amplitudes. Fig. 2.5-1 shows the acceleration averaged over the active pendulum as a function of force derived from Hooke’s law. m/s2) 5 We find the data to be in good res. 0 -14 -5 agreement with Newton’s second (10 law down to accelerations as small 10 -11 as 5 × 10−14 ms−2 . This is three orders of magnitude smaller than a acceleration (m/s ) 2 -12 previous experiment.2 10 MOND requires accelerations in all dimensions to be small, a con- 10-13 dition which cannot be found on Earth, and hence our test does not directly constrain MOND. How- 10-15 10-14 10-13 10-12 -3 0 3 ever, our test sets constraints on force (N) res. (10-15 N) any theoretical formalism by re- Figure 2.5-1. The measured acceleration as a function quiring it to reproduce our result. of force. The right and top panels show the residuals. 1 M. Milgrom, Astrophys. J. 270, 365 (1983). 2 A. Abramovici and Z. Vager, Phys. Rev. D 34, 3240 (1986).


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    UW CENPA Annual Report 2006-2007 May 2007 37 2.6 Rotating torsion balance test of the weak equivalence principle for beryllium and titanium E. G. Adelberger, K.-Y. Choi, J. H. Gundlach, S. Schlamminger and T. A. Wagner The equivalence of gravitational and inertial mass exists as a fundamental assumption in General Relativity. However, attempts to create a theory including both gravity and the standard model typically violate the equivalence principle.1 To test the equivalence principle, we used a rotating torsion balance to measure the differential acceleration between different composition test masses. The test masses were arranged in a dipole configuration on the pendulum, so that a signal would occur at the rotation frequency of the apparatus. We measured the difference in acceleration for beryllium and titanium to the North ∆aN,Be−T i = (−0.8 ± 3.0) × 10−15 ms−2 and to the West ∆aW,Be−T i = (−1.3 ± 3.4) × 10−15 ms−2 . We are now repeating our measurement with beryllium and aluminum test masses. By analyzing our measurement with respect to different sources, we set limits on the violation of the equivalence principle at ranges from one meter to infinity. Using our earth fixed result, we constrain the parameter ηBe−T i = ∆a/a = (0.35 ± 1.31) × 10−13 . The galaxy provides an additional interesting source, since approximately one quarter of the total acceleration of the solar system toward the center of the galaxy is caused by dark matter.2 We find no difference in the acceleration due to dark matter between beryllium and titanium, giving ηDM,Be−T i = (0 ± 6) × 10−5 . Figure 2.6-1. The two components of the mea- Figure 2.6-2. The averaged acceleration in the sured acceleration, corrected for tilt and grav- North and West direction as a function of side- ity gradients, with respect to a zero mark on real time. The dashed line represents a space the pendulum frame. A violation of the equiv- fixed hypothetical signal of 20 × 10−15 ms−2 . alence principle would appear as a difference The solid line is the best fit of ∆a = (0.0 ± in the means (lines) of the two data sets. 3.0) × 10−15 ms−2 . 1 T. Damour, Class. Quantum Grav. 13, A33 (1996). 2 G. L. Smith, et al., Phys. Rev. D 61, 22001 (2000).


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    38 2.7 Development of a pendulum to test short range spin-spin interactions E. G. Adelberger, B. R. Heckel and W. Terrano Spin-0 Goldstone bosons generically produce new spin-coupled forces1,2 that would not be observable between ordinary, unpolarized bodies.3 Several torsion pendulums designed to investigate such interactions are already in operation.4 We have been developing a new design that is specifically optimized to study spin-spin interactions between electron spins for ranges down to a millimeter. To this end, we will build a 10-fold symmetric pendulum of alternating high (Alnico) and low (SmCo5 ) spin density wedges as illustrated in Fig. 2.7-1. By carefully matching the magnetizations, we can build a complete magnetic circuit, so that the magnetic flux is contained within the ring to minimize spurious magnetic forces. Using a similarly constructed ring as an attractor, we will operate at separations of around 1 cm. This will allow us to look at relatively short range forces compared to earlier experiments. Even for infinite range forces, the advantageous geometry should allow us to improve on the current experimental limits for the couplings of new forces by four orders of magnitude, since dipole-dipole interactions typically have a 1/r 3 potential. Figure 2.7-1. Drawing of 10 fold symmetric pendulum plate. Dark regions correspond to high spin content, (Alnico) and light regions correspond to low spin content(SmCo5 ). 1 J. E. Moody and F. Wilczek, Phys. Rev. D 30, 130 (1984). 2 N. Arkani-Hamed, H. C. Cheng, M. Luty and J. Thaler, arXiv:hep-ph 0407034. 3 D. Chang, R. N. Mohaputra, and S. Nussinov, Phys. Rev. Lett. 55, 2835 (1985). 4 CENPA Annual University of Washington (2006) 21.

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