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    UW CENPA Annual Report 2001-2002 i INTRODUCTION CENPA pursues a broad program of research in nuclear physics, astrophysics and related fields. Research activities are conducted locally and at remote sites. The current program includes “in-house” research on nuclear collisions and fundamental interactions using the local tandem Van de Graaff, as well as local and remote non-accelerator research on fundamental interactions and user-mode research on relativistic heavy ions at large accelerator facilities in the U.S. and Europe. Our Phase I determination of the astrophysical S-factor for the 7 Be(p,γ)8 B reaction has been completed and published. Phase II measurements are currently underway, and will result in a more accurate value and will provide additional data at lower proton energies. The first analysis of the neutral-current-induced breakup of deuterium by 8 B solar neutri- nos was completed for the pure heavy water phase of SNO operation (November 1999 to May 2001). A clear 5.3 σ excess of neutral-current events above what can be attributed to electron neutrinos was found, consistent with flavor transformation, neutrino oscillations, and mass. The data from the pure heavy water phase was also analyzed with respect to day and night rates and a slight asymmetry of night over day for the charged current signal (14 ± 7%) was found. In a global two-flavor analysis incorporating the total spectral shape (NC + CC) in night and day from SNO as well as other solar-neutrino data, the LMA solution is strongly favored at more than the 99.5% confidence level. Hadronic interactions of muons and their secondaries have now been incorporated into the SNO data analysis code. Electronics for the neutral-current detector array have been completed, tested and shipped to Sudbury. Data-acquisition and analysis code has been written and is under test. The upgrade to the emiT detector for investigation of time-reversal invariance violation in neutron beta decay has been completed, with new high-voltage withstand capability, analog fiber-optic links from the proton detector arrays, new surface-barrier detectors with thin and stable dead layers, and new preamplifiers. Initial tests of an experiment to search for the ground state decay of 8 B were carried out, with expected production of a magnetically analyzed radioactive beam of 8 B at a few ions per second. Study of lead perchlorate as a Čerenkov medium has been completed and the technique has been adopted for use by the OMNIS supernova detector. Initial attempts to dissolve 100 Mo in liquid scintillator are encouraging. We are within a factor of two of developing a viable technique for use in a search for neutrino-less double beta decay of 100 Mo. The hardware for welding and deployment of the NCD array has been completed and is being prepared for shipping to SNO.


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    ii The UW Electroweak Interactions group joined an international collaboration that has begun to build “KATRIN” (KArlsruhe TRItium Neutrino project), a next generation exper- iment aimed at making a precise determination of or limit on the mass of νe from tritium beta decay. CENPA faculty have been involved in efforts to establish a National Underground Science Laboratory (NUSL) in the United States. A proposal for building the laboratory at the Homestake Mine in Lead, SD was submitted in June 2001 to the National Science Foundation. Our short-range tests of the Newtonian inverse-square law, which test “large extra dimen- sions” and “fuzzy graviton” scenarios, have made substantial progress during the last year. We recently reported an upper limit of 200 micrometers on the maximum size of the largest extra dimension that couples to gravity. We are currently analyzing results taken with a new instrument that has about 50 times higher sensitivity. An even more sensitive instrument has been designed and should be taking data by the middle of the summer. A major upgrade of the 199 Hg EDM experiment has been completed and should yield a factor of three improvement in a new EDM search now ready to begin. Our event-by-event analysis of STAR data has revealed the development of complex corre- lation structures with strong centrality dependence in Au-Au collisions at RHIC energies, in contrast to results at CERN SPS energies. The dominant source of charge-independent cor- relation structure appears to be initial-state scattering, and centrality dependence seems to indicate the growth of a dissipative medium for more central events. Charge-dependent cor- relation structures are consistent with a possibly-related source opacity for the more central events. The initial HBT results from STAR have produced several surprises, which the physics community has come to call “The HBT Puzzle”. In particular, the source radius ratio ROut /RSide , expected for dynamical reasons to have a value between 2 and 10 reflecting long-duration pion source emission, instead has a value very close 1 over the whole range of the measurements, reflecting a very short emission duration and suggesting a very “hard” equation of state for the expanding system. 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, to whom inquires should be addressed, underlined. Derek Storm, Editor Barbara Fulton, 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. Meth. 122, 65 (1974). Recently adapted to an (optional) terminal ion source and a non-inclined tube #3, which enables the accelerator to produce high intensity beams of helium and hydrogen isotopes at energies from 100 keV to 5.5 MeV. Some Available Energy Analyzed Beams Ion Max. Current Max. Energy Ion Source (particle µA) (MeV) 1H or 2 H 50 18 DEIS or 860 3 He or 4 He 2 27 Double Charge-Exchange Source 3 He or 4 He 30 7.5 Tandem Terminal Source 6 Li or 7 Li 1 36 860 11 B 5 54 860 12 C or 13 C 10 63 860 ∗14 N 1 63 DEIS or 860 16 O or 18 O 10 72 DEIS or 860 F 10 72 DEIS or 860 ∗ Ca 0.5 99 860 Ni 0.2 99 860 I 0.001 108 860 *Negative ion is the hydride, dihydride, or trihydride. Additional ion species available including the following: Mg, Al, Si, P, S, Cl, Fe, Cu, Ge, Se, Br and Ag. Less common isotopes are generated from enriched material. BOOSTER ACCELERATOR See “Status of and Operating Experience with the University of Washington Superconducting Booster Linac,” D. W. Storm et al., Nucl. Instrum. Meth. A 287, 247 (1990). The Booster is presently in a “mothballed” state.


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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. Meth. 122, 65 (1974). Some Available Energy Analyzed Beams Ion Max. Current Max. Energy Ion Source (particle m A) (MeV) 1H or 2H 50 18 DEIS or 860 3He or 4He 2 27 Double Charge-Exchange Source 3He or 4He 30 7.5 Tandem Terminal Source 6Li or 7Li 1 36 860 11B 5 54 860 12C or 13C 10 63 860 * 14N 1 63 DEIS or 860 16O or 18O 10 72 DEIS or 860 F 10 72 DEIS or 860 * Ca 0.5 99 860 Ni 0.2 99 860 I 0.01 108 860 * Negative ion is the hydride, dihydride, or trihydride. Additional ion species available include the following: Mg, Al, Si, P, S, Cl, Fe, Cu, Ge, Se, Br and Ag. Less common isotopes are generated from enriched material.


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    UW CENPA Annual Report 2001-2002 v Contents 1 Fundamental Symmetries and Weak Interactions 1 Weak Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Status of the emiT data acquisition system . . . . . . . . . . . . . . . . . . . 1 1.2 Final preparations for a second run of the emiT experiment . . . . . . . . . . 3 1.3 Search for second-class currents in the mass-8 system . . . . . . . . . . . . . . 5 1.4 Search for a permanent electric dipole moment of 199 Hg . . . . . . . . . . . . 6 Torsion Balance Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.5 Sub-mm test of Newton’s inverse-square law . . . . . . . . . . . . . . . . . . . 8 1.6 A new equivalence principle test . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.7 Numerical calculation for the short-range inverse-square law test . . . . . . . 11 1.8 Computer controlled torsion fiber damping routine . . . . . . . . . . . . . . . 13 1.9 Finite element analysis of capacitance for pendulum in 1/r2 test . . . . . . . 14 1.10 Torsion balance test of CPT and Lorentz symmetries . . . . . . . . . . . . . . 15 2 Neutrino Research 16 SNO 16 2.1 Solving the solar neutrino problem at SNO: Evidence for the flavor transformation of solar 8 B neutrinos . . . . . . . . . . 16 2.2 Muon spallation neutrons at the Sudbury Neutrino Observatory . . . . . . . . 18 2.3 Electron antineutrino studies at the Sudbury Neutrino Observatory . . . . . 19 2.4 Neutron backgrounds from cosmic rays and atmospheric neutrinos in SNO . . 20 2.5 Search for solar hep neutrinos in the Sudbury Neutrino Observatory . . . . . 21 2.6 Seasonal variation of the muon flux at SNO in the deepest underground labo- ratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.7 The day-night asymmetry of the solar neutrino flux measured at SNO . . . . 24 2.8 Status and updates to the SNO data acquisition system . . . . . . . . . . . . 26


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    vi SNO NCDs 27 2.9 NCD data taking and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.10 Status of the NCD DAQ for SNO . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.11 NCD cable repair and testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.12 Neutral current detector electronics commissioning status . . . . . . . . . . . 33 2.13 Underground NCD welding prior to deployment . . . . . . . . . . . . . . . . 34 2.14 NCD deployment equipment progress . . . . . . . . . . . . . . . . . . . . . . 35 Neutrino Detectors 36 2.15 Lead perchlorate as a neutrino detection medium . . . . . . . . . . . . . . . . 36 Double Beta Decay 38 2.16 Heat capacity and thermal conductivity of molybdenum at millikelvin temper- atures for a molybdenum bolometer . . . . . . . . . . . . . . . . . . . . . . . 38 2.17 Cosmogenic backgrounds for MOON . . . . . . . . . . . . . . . . . . . . . . . 40 2.18 Search for a molybdenum-loaded liquid scintillator . . . . . . . . . . . . . . . 41 2.19 Majorana search for neutrinoless ββ decay . . . . . . . . . . . . . . . . . . . . 43 KATRIN 44 2.20 The KATRIN tritium beta decay experiment . . . . . . . . . . . . . . . . . . 44 National Underground Science Laboratory 45 2.21 National Underground Science Laboratory at Homestake . . . . . . . . . . . . 45 3 Nuclear and Particle Astrophysics 47 3.1 Astrophysical S-factor for 7 Be(p,γ)8 B . . . . . . . . . . . . . . . . . . . . . . 47 3.2 e+ e− pair emission and the 3 He+4 He→7 Be S-factor . . . . . . . . . . . . . . 48 3.3 Search for the 8 B(2+ ) → 8 Be(0+ ) transition . . . . . . . . . . . . . . . . . . . 49 3.4 Reanalysis of α+α scattering and β-delayed α spectra from 8 Li and 8 B decays. 50


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    UW CENPA Annual Report 2001-2002 vii 3.5 β-delayed α spectra from 8 Li and 8 B decays and the shape of the neutrino spectrum in 8 B decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.6 WALTA: The Washington Large-area Time-coincidence Array . . . . . . . . . 51 4 Ultra-Relativistic Heavy Ion Physics 53 HBT Physics at STAR 53 4.1 Overview of HBT physics at STAR . . . . . . . . . . . . . . . . . . . . . . . 53 4.2 Pion phase space density and “bump volume” . . . . . . . . . . . . . . . . . 55 4.3 Moment analysis of 3D HBT histograms and the Rout /Rside ratio . . . . . . . 57 4.4 Simulation of opacity effects in HBT sources . . . . . . . . . . . . . . . . . . . 59 Event by Event Physics 61 4.5 Event-by-event analysis overview . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.6 Pt fluctuations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Non-statistical < pt > fluctuations in STAR data . . . . . . . . . . . . . . . . 62 Systematic error analysis for < pt > fluctuations . . . . . . . . . . . . . . . . 63 4.7 Multiplicity fluctuations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Scale-dependent shape analysis of the minimum-bias terminus and multiplicity fluctuations at b = 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Net charge fluctuations as a function of scale and centrality . . . . . . . . . . 65 4.8 Centrality dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Initial state scattering, Glauber model and pt fluctuations . . . . . . . . . . . 66 Hijing Inclusive pt distributions and the 1D Lévy reference . . . . . . . . . . . 67 Centrality dependence of inclusive pt distribution parameters . . . . . . . . . 68 Centrality dependence of particle production . . . . . . . . . . . . . . . . . . 69 4.9 Scaling analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Scaling analysis of < pt > fluctuations . . . . . . . . . . . . . . . . . . . . . . 70 Scaling analysis of multiplicity fluctuations . . . . . . . . . . . . . . . . . . . 71


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    viii 4.10 Two-point correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Two-point correlations on (η, φ) and source opacity . . . . . . . . . . . . . . . 72 Two-point correlations on mt ⊗ mt and 2D Lévy distributions . . . . . . . . . 73 4.11 Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Glauber model and minimum-bias distribution shape . . . . . . . . . . . . . . 75 Accessing a two-particle momentum space with joint autocorrelations . . . . 75 Statistical measure bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 4.12 Comparison of measured and calculated Landau functions for STAR-TPC . 79 5 Atomic and Molecular Clusters 80 5.1 Structure of anions containing B and N . . . . . . . . . . . . . . . . . . . . . 80 6 Electronics, Computing and Detector Infrastructure 81 6.1 Status of an advanced object oriented real-time data acquisition system . . . 81 6.2 Electronic equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 6.3 Laboratory computer systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 6.4 An alternative data acquisition system . . . . . . . . . . . . . . . . . . . . . . 84 7 Accelerator and Ion Sources 85 7.1 Van de Graaff accelerator operations and development . . . . . . . . . . . . . 85 7.2 Tandem terminal ion source . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 8 The Career Development Organization for Physicists and Astronomers 88 9 Center for Exp erimental Nuclear Physics and Astrophysics Personnel 89 9.1 Faculty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 9.2 Postdoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . 89 9.3 Predoctoral Research Associates . . . . . . . . . . . . . . . . . . . . . . . . . 90 9.4 Research Experience for Undergraduates participants . . . . . . . . . . . . . . 90


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    UW CENPA Annual Report 2001-2002 ix 9.5 Professional staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 9.6 Technical staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 9.7 Administrative staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 9.8 Part time staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 10 List of Publications from 2001-2002 93 11 Degrees Granted, Academic Year, 2001-2002 102


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    UW CENPA Annual Report 2001-2002 1 1 Fundamental Symmetries and Weak Interactions Weak Interactions 1.1 Status of the emiT data acquisition system M. A. Howe and J. F. Wilkerson The emiT data acquisition (emiTDAQ) software that will be used for the second run of the emiT experiment is rapidly evolving into its final form. It is written in C++ and is compiled with the MetroWerks compiler for running on Macintosh computers. The emiTDAQ software uses much of the code base of the successful SHaRC program (see Section 2.8) that has been in routine operation at SNO for several years. The emitDAQ application is composed of object- oriented software modules that represent each of the emiT electronic hardware modules, a control task that coordinates the actions of those modules, and dialogs for initializing, controlling, and monitoring the emiT experiment. A number of new software modules were developed for this run of emiT because the electronic hardware set has changed substantially. New hardware modules include a CEAN 755 TDC and a CEAN 862 QDC for acquisition of PMT data with timing information provided by a CENPA-built 100MHz latched clock. For digital control functions an Acromag IP320 I/O module and an Acromag IP220 DAC module have been added. The new modules enable the experiment to be run using only VME hardware instead of the mixture of CAMAC and VME hardware that was used in the first run of the experiment. Since each of these hardware objects is represented by a self contained object-oriented software object, it was possible to do systematic testing of the interaction of the various subsystems of the detector as the emitDAQ application was being developed. The figure below shows emiTDAQ being used during system validation testing. The main control object of emitDAQ is a task that coordinates the activities of all of the software modules. One of the main modules the task is a well developed run control system for testing event readout of data in the current system. As well as providing a ‘one-button’ start run capability, it also provides controls for setting the length of a run and whether a run is to be repeated. In the developmental phase of the hardware/software the run control also provides controls for taking data with sub-parts of the system, i.e. just proton singles data, just TDC data, data with or without using the latched clock, etc. As the emiTDAQ matures, this part of the event readout software will be moved into an embedded processor which will read out the data In addition to run control, the main dialog for the system shows a layout of the emiT hardware and is active in the sense that it shows a lot of information about the current state of the detector and the data rates. Maximum data rates are shown in bar graphs, but in the pictorial representation of the detector the rate for each channel is shown color coded in the place where that detector channel actually is in the detector. Clicking on a particular channel brings up a dialog that can be used to adjust the constants for that channel. By


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    2 Figure 1.1-1. A screen dump of emiTDAQ during software validation tests. clicking on other parts of the picture, dialogs for controlling or visualizing other parts of the detector can be displayed. For monitoring the data, a multi-plot object was developed that shows a histogram of the data for every ADC channel on one page. There are also multi-plots for the TDC and QDC data. These allow the operator to quickly visualize the overall state of the detector, find dead channels, gain/threshold problems, etc. The multi-plot can also dump the data from each channel into a disk file for offline analysis. The final data path is now under development. In this phase the actual event readout will be moved off the Mac and into an embedded eCPU running in the VME crate. The eCPU will monitor the latched clock for signals to proceed, read out hardware as required, bundle the data together with the timing information from the latched clock, and finally place the data into dual memory for access by the Mac for monitoring and storage.


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    UW CENPA Annual Report 2001-2002 3 1.2 Final preparations for a second run of the emiT experiment L. Grout,∗ H. P. Mumm, A. W. Myers, P. Parazzoli,† R. G. H. Robertson, K. Sundqvist,‡ T. D. Van Wechel, D. I. Will and J. F. Wilkerson The emiT experiment is a search for time-reversal (T) invariance violation in the beta decay of free neutrons. Current observations of CP(T) violation in the Kaon and B-meson systems can be accommodated within the standard model of particle physics. However, baryogenisis and attempts to develop unified theories indicate that additional sources are required. The standard model predicts T-violating observables in beta decay to be extremely small (second order in the weak coupling constant) and hence are beyond the reach of modern experiments.1 However, potentially measurable T-violating effects are predicted to occur in some non- standard models such as those with left-right symmetry, exotic fermions, or lepto-quarks.2,3 Thus a precision search for T-violation in neutron beta decay provides an excellent test of physics beyond the Standard Model. The emiT experiment probes the T-odd P-even triple correlation between the neutron spin and the momenta of the neutrino and electron, D~σn · P~e × P~ν , in the neutron beta-decay distribution. The coefficient of this correlation, D, is measured by detecting decay electrons in coincidence with recoil protons from a polarized beam of cold (2.7 meV) neutrons. Four elec- tron detectors (plastic scintillators) and four proton detectors (large-area diode arrays) are arranged in an alternating octagonal array concentric with the neutron beam. The protons produced in the decay have a relatively low energy (≤ 751 eV). While this allows for a de- layed coincidence trigger between the proton and electron it increases the complexity of the detection scheme by requiring that the protons be accelerated using high voltage electrodes. During the first run, high voltage related problems damaged electronic components, led to high background rates and ultimately produced a non-symmetric detector. Systematic effects were less effectively canceled due to the lack of full detector symmetry and a more complex data analysis scheme was required. The result, D = −0.1 ± 1.3 × 10−3 , represents a small improvement over the current world average.4 We have used an electrostatic modeling program to fully redesign the proton focusing assembly. The aim was to maintain focussing efficiency while reducing high field regions and minimizing the associated field emission, the dominate background during the first run. We have successfully constructed and tested four new electrodes based on this design. Initial tests indicate an accidental coincidence rate of approximately 10−5 Hz compared to an estimated signal of 20 Hz. ∗ Presently at MIT Lincoln Labs, Lexington, MA 02420. † Presently at Los Alamos National Laboratory, Los Alamos, NM 87545. ‡ Presently at Physics Department, University of Washington, Seattle, WA 98195. 1 M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973). 2 P. Herczeg, Progress in Nuclear Physics, W.-Y. P. Hwang, ed., Elsevier Sciences Publishing Co. Inc. (1991) p. 171. 3 E. G. Wasserman, Time Reversal Invariance in Polarized Neutron Decay, Ph.D. thesis, Harvard University, (1994). 4 L. J. Lising et al, Phys. Rev. C 62, (2000) 055501.


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    4 To reduce dead-layer proton energy-loss we have decided to use surface barrier detectors having a thickness of 20 µg/cm2 Au, a depletion region of 300 microns and an active area of 300 mm2 for the second run. As cooling of these detectors is critical to achieving acceptable energy resolution, we have installed a new liquid nitrogen based system. The system enables cooling of the detectors to approximately −50 C with the minimal requirement that the dewar be refilled once a day. In addition, fabrication of redesigned low-power preamps is nearing completion. To test the performance of our hardware a series of measurements using low energy gammas from a 133 Ba source have been made. These have demonstrated an energy resolution as good as 3 keV full width at half maximum and a background around 9 keV. Due to the complexity of the system, however, the average performance is typically around 6 keV FWHM. 4 Source Protons Beam Off 2 Beam On 100 8 6 133 4 31 keV Gammas ( Ba) Counts 2 10 8 6 Source Pile-up 4 2 1 50 100 150 200 250 300 350 bins 133 Figure 1.2-1. Ba spectrum and proton source with electrode bias at -30 kV We have also constructed a low energy, < 800 eV, low-intensity proton source to facilitate in situ characterization of our detectors5 (see figure). Using this source we have directly measured detector thickness allowing a comparison with previous measurements using alpha particles.6 Considerable progress has been made on an upgrade and simplification of the data ac- quisition system. Code that allows detailed control and monitoring of much of the hardware is nearing completion (see previous section). In addition we have chosen to time-stamp in- dividual proton and electron events allowing the use of a slow-software coincidence trigger. To this end a custom timing board with 10 ns precision has recently been constructed and tested. See Section 6.1. Beam-line preparations for the second run are currently underway at NIST. Initial mea- surements indicate that the reactor upgrade has yielded the expected factor of 1.8 increase in flux. We estimate that emiT will begin collecting data in the middle of May 2002, likely reaching the goal of D < 5 × 10−4 during the fall of 2002. 5 F. Naab, Nucl. Instrum. Methods, to be submitted. 6 CENPA Annual Report, University of Washington (2001) p. 13.


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    UW CENPA Annual Report 2001-2002 5 1.3 Search for second-class currents in the mass-8 system M. Beck,∗ R. Hazama, K. A. Snover and D. W. Storm We are searching for second-class currents in the mass-8 system by comparing the weak magnetism plus second-class tensor form factors in beta decay with the isovector M1 matrix element for the photon transitions in 8 Be. CVC relates the isovector M1 and weak magnetism. The beta decay data were measured previously.1 The form factors are extracted from the measured energy dependence of angular correlation coefficients. Preliminary results have been reported,2 but there are remaining questions about the beta detector response functions. The detectors are plastic scintillator, consisting of a large cylinder preceded by a thin, smaller diameter paddle, and surrounded by a veto annulus. We modeled the detectors’ response to electrons and to positrons using GEANT. Before including any effect of photon statistics, we find (see Fig. 1.3-1) the electron response to be described by a narrow Gaussian with a low energy tail. The positron response, as ex- pected, shows an additional high energy peak from annihilation radiation which registers in the detector. We have parameterized the GEANT response functions with a set of energy dependent parameters. Using these response functions, we plan to reanalyze the beta-decay Electrons Positrons GEANT pc = 10 MeV fit GEANT pc=10 MeV 1000 1000 fit Counts Counts 100 100 10 10 4 5 6 7 8 9 10 4 5 6 7 8 9 10 E deposited (MeV) E deposited (MeV) Figure 1.3-1. Spectra from the large plastic cylinder, for 10 MeV/c particles that pass through the paddle and do not register in the veto. Points are calculated with GEANT and the curves are the parameterized detector response functions. data to obtain the sum of weak magnetism and second-class tensor form factors. To obtain the isovector M1 matrix element for the analog transition in 8 Be, we have measured γ-spectra at three angles as a function of excitation energy in 8 Be. The 8 Be is made as a resonance with 4 He incident on a 4 He target, as has been described previously.3,4 The details of this analysis have been described previously,3 and we are presently at- tempting to resolve some discrepancies in the data analysis. ∗ Presently at Katholieke Universiteit, Leuven, Belgium. 1 Nuclear Physics Laboratory Annual Report, University of Washington (1998) p. 8. 2 M. Beck et. al., Proc. 6th Conf. on the Intersections of Particle and Nuclear Physics, 1997. AIP Conference Proceedings 412, 416. 3 CENPA Annual Report, University of Washington (2001) p. 14. 4 L. De Braeckeleer et. al., Phys. Rev. C 51, 2278 (1995).


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    6 199 1.4 Search for a permanent electric dipole moment of Hg W. C. Griffith, M. D. Swallows,∗ M. V. Romalis† and E. N. Fortson∗ The measurement of a finite permanent electric dipole moment (EDM) on an atom or el- ementary particle would reveal a new source of CP violation beyond the standard model. Currently, the most precise limit on an EDM is given by our search for the EDM of 199 Hg. In the experiment, spin polarized Hg vapor, contained in quartz cells, is placed in parallel magnetic and electric fields, and the Larmor spin precession frequency is measured. The direction of the electric field is frequently reversed, and a nonzero EDM would manifest itself as a frequency shift correlated with the direction of the electric field. Our result of |d(199 Hg)| < 2.1 × 10−28 e cm,1 was obtained using a two vapor-cell setup, which allowed for cancellation of frequency noise due to magnetic field fluctuations common to both cells. We have since been developing a four vapor-cell version of the experiment. The additional cells will allow for cancellation of magnetic field gradient noise, and will also improve our under- standing of systematic effects by allowing us to measure magnetic fields due to charging and leakage currents. All upgrades to the apparatus allowing it to accommodate four vapor cells have been completed. These included constructing new electrodes and a larger cell-holding vessel, and setting up optics and detectors for the additional light beams. Also, the data acquisition and analysis software was modified to accommodate the additional signals. HV HV Figure 1.4-1. Cutaway view of the cell-holding vessel. High voltage (± 10 kV) is applied to the middle two cells with the ground plane in the center, so that the electric field is opposite in the two cells. The outer two cells are enclosed in the HV electrodes (with light access holes shown for the bottommost cell), and are at zero electric field. A uniform magnetic field is applied in the vertical direction. ∗ Physics Department, University of Washington, Seattle, WA 98195. † Princeton University, Princeton, NJ 08544. 1 M. V. Romalis, W. C. Griffith, J. P. Jacobs and E. N. Fortson, Phys. Rev. Lett. 86, 2505 (2001).


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    UW CENPA Annual Report 2001-2002 7 Several batches of new Hg-vapor cells have been prepared for use in the experiment. The cells used in our previous measurement contained a buffer gas composition of 90% N2 and 10% CO and their spin coherence time tended to degrade after long term UV exposure. This was probably caused by damage to the paraffin cell wall coating from collisions with Hg atoms in the metastable 63 P0 state. While CO is effective in quenching Hg to the ground state, N2 tends to quench to the metastable state. To avoid this behavior, the new cells use pure CO buffer gas, which has led to longer and more stable spin relaxation times.2 200 Transverse Relaxation Time (sec) cell #36: CO 150 100 cell #23: N 2 + CO 50 0 0 1 2 3 4 5 6 UV Exposure (days) Figure 1.4-2. Effect of UV exposure on transverse spin relaxation time. The relaxation time for the cell containing 90% N2 and 10% CO quickly drops below 100 seconds, while in the cell containing pure CO the relaxation time initially increases and then is stable. We also investigated a possible source of noise due to light shifts affecting the atomic magnetization direction. Spin polarization is achieved through optical pumping with circu- larly polarized light directed perpendicular to a magnetic field of 20 mG and chopped at the Larmor frequency of the 199 Hg spins. If the chopping frequency does not match the Larmor frequency exactly, then a small amount of Hg magnetization is rotated into the vertical di- rection (parallel to the magnetic field) due to the Zeeman light shift. Later, the probe beam, which is linearly polarized but acquires a small circular polarization by passage through the spin polarized vapor, produces a light shift that rotates the vertical magnetization back into the plane of precession, but phase shifted from the main component of magnetization, leading to a change in the measured Larmor frequency. We combat this effect by taking several steps to reduce the buildup of vertical magnetization. We installed a lock system for the laser that allows us to set the wavelength of the pump light at the point of zero light shift. Furthermore, we have taken measures to ensure that the chopper frequency better matches the Hg Larmor frequency in each of the four cells. Previously, the frequencies in the four cells differed by a part in 104 due to magnetic field gradients. We have installed gradient compensation coils so that the four precession frequencies match to a part in 106 . The chopper frequency is then set precisely to the measured Larmor frequency using a function generator. Overall, these improvements have resulted in a factor of three improvement in our sta- tistical sensitivity per unit time, and we are presently preparing to start accumulating data towards a new measurement of the EDM of 199 Hg. 2 M. V. Romalis and L. Lin, submitted to Phys. Rev. A.


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    8 Torsion Balance Experiments 1.5 Sub-mm test of Newton’s inverse-square law E. G. Adelberger, M. C. Feig, J. H. Gundlach, B. R. Heckel, C. D. Hoyle,∗ D. J. Kapner, U. Schmidt† and H. E. Swanson Since our last report,1 we have continued testing the gravitational inverse-square law at even shorter distances. Our experiments are principally motivated by higher-dimensional string theories that predict deviations from the gravitational inverse-square law at short distances,2 and by attempts to understand the observed cosmological constant that speculate about similar modifications.3 These deviations are typically parameterized as an addition of a Yukawa term to the Newtonian potential, m1 m2 V (r) = −G( )(1 + αe−r/λ ). (1) r The basic geometry of our experiment has remained the same, consisting of a thin metal disc with an azimuthally symmetric array of holes, suspended from a 20µm diameter tungsten fiber. A thin attractor disc, with a geometry similar to that of the pendulum disk rotates beneath the pendulum. A third, thicker disc, is attached to the bottom of the thin attractor so that its holes are out of phase with the upper attractor, canceling the Newtonian torques between the thin attractor and the pendulum. To improve the sensitivity of our experiment to these potential deviations, we consider the following simplified scaling of the torque produced by a Yukawa potential term. αGρ1 ρ2 Aλ4 e−s/λ τy ∝ (2) s where ρ1 and ρ2 are the densities of the two thin discs, A is the total area of the holes in a disc, and s is the vertical separation between the pendulum and attractor. To increase the signal from a Yukawa interaction, we can increase the densities of the discs, increase the areas of the holes, and decrease the separation. We have done all of these things. By using molybdenum for both the pendulum and attractor, we increased the product of the densities by a factor of four over our published result.4 We added a second row of holes to the pendulum and attractor to increase the interacting area, and installed a class 10,000 clean room to reduce the dust contamination that was limiting our vertical separation. We have also come up with a satisfactory way to eliminate electrostatic potentials in our experiment. We have always isolated our pendulum and attractor electrostatically by stretching a 10µm thick gold-coated beryllium-copper membrane between them. In the past ∗ Presently at University of Trento, Italy. † Presently at Physikalisches Institut, Heidelberg, Germany. 1 CENPA Annual Report, University of Washington (2001) p. 2. 2 See, for example, N. Arkani-Hamed et al., Phys. Lett. B 429, 263 (1998). 3 G. Dvali et al., hep-th/0202174 v1. 4 C. D. Hoyle et al., Phys. Rev. Lett. 86, 1418 (2001).


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    UW CENPA Annual Report 2001-2002 9 we have deposited a gold coating via resistive evaporation onto the pendulum surface to elim- inate any electrostatic wells that may exist between the pendulum disc and the membrane. For our most recent data, we did not deposit this coating on the molybdenum, as we feared that the chromium adhesion layer might introduce some unknown magnetic effect. Without this gold coating, we noticed two effects. First, our pendulum period varied with height above the membrane, belying a height-dependent electrostatic potential. This is to be expected, as there is a small contact potential difference between molybdenum and gold. We found that we could eliminate this effect by applying a reverse voltage. A less benign effect was the increase in noise as we moved our pendulum very close to the membrane. This could be due to small motions of the pendulum relative to the membrane. The reverse voltage served to reduce this effect as well. To avoid this problem, we now gold coat our pieces with a sputtering technique. By first sputtering our substrate (“etching”), we remove any thin oxide layers, the usual culprit in poor adhesion. We then sputter gold onto this fresh surface. We have found that sputtered layers of gold are more durable on our substrates than those from resistive evaporation. With existing data taken with a 22-fold symmetric system, we have improved our sen- sitivity substantially; pending analysis should show our experiment to be sensitive to large extra dimensions with sizes as small as 100µm. Figure 1.5-1. (α,λ) Parameter space.The heavy solid line shows existing Eöt-Wash limits, while the dashed line shows our expected sensitivity from existing data, if a null result is found.


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    10 1.6 A new equivalence principle test E. G. Adelberger, T. W. Butler, K.-Y. Choi, J. H. Gundlach, B. R. Heckel, D. J. Kapner, S. M. Merkowitz,∗ U. Schmidt† and H. E. Swanson Practically every attempt at formulating quantum gravity, as for example string, or M- theory, predicts new, subtle, gravitational effects. Precision experimental tests of gravity, as for instance equivalence principle (EP) tests, have therefore emerged as important tests of fundamental physics. Theoretical predictions for an EP-breakdown are tantalizingly close to the sensitivity of modern torsion balances. We have over the past few years constructed a new continously rotating torsion balance. The instrument is in operation undergoing systematic tests. Its key features include the fol- lowing: High rotation rate: A fast rotation rate will minimize statistical noise which has predomi- nantly 1/f-character. The instrument is rotated with a direct-driven air-bearing turntable, stabilized with a tight feedback loop to a high-resolution angle encoder. For angle encoder non-linearities of up to the 9th harmonic of a revolution, we establish a correction function by operating the turntable at two different speeds. From the response of the pendulum at these two speeds we calculate the harmonic correction function. Tilt elimination: A turntable rotation axis misalignment from vertical produces a rotation of the pendulum at the signal frequency. We eliminate the axis tilt using an active leveling mechanism.1 The system functions well, leaving an unresolved periodic tilt of less than 3 nrad. Vibration isolation: The balance’s day-time noise performance is worsened by vertical building vibrations. We built a vibration isolater for the torsion-fiber top attachment with 3 leaf springs. An eddy current damper damps the vertical motion and pendulum motion. Day and nighttime noise performances are now equal. New pendulum design: We have built an aluminum pendulum with eight 5-g test bodies, seated in cones and held on by a small screw. The repeatability of the vertical test body placement is better than <5 µm. We have sets of Al, Be and Ti test bodies. Gravity gradient compensation: The instrument is surrounded by precisely machined compensators to eliminate the ambient gravity gradient with l=2, m=1,2 and l=3, m=1. The uncompensated gravity gradients were measured with special test bodies that augmented the corresponding pendulum moments. To minimize the pendulum’s gravitational moments, the compensators were rotated about the pendulum so that they add to the ambient gradient. We are currently operating the instrument with a composition dipole. Once the instru- ment has come into an equilibrium state, our statistical uncertainty in one day of operation is ≈2 nrad, corresponding to 1.2 × 10−12 cm/s2 . ∗ Presently NASA/GSFC, Greenbelt, MD 20771. † Presently at Physikalisches Institut, Heidelberg, Germany. 1 CENPA Annual Report, University of Washington (2001) p. 4.


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    UW CENPA Annual Report 2001-2002 11 1.7 Numerical calculation for the short-range inverse-square law test E. G. Adelberger, B. R. Heckel, D. J. Kapner and U. Schmidt∗ Our short-range test of Newton’s inverse-square law requires a high precision calculation of the expected Newtonian torques acting on our pendulum. The “test bodies” of our pendulum and attractor are azimuthally symmetric arrays of holes in flat metal discs. To calculate the torques on our pendulum, we must start by calculating the gravitational force between two cylinders. In our planar geometry, it is the x-component of the force between the cylinders that produces a torque along the axis of our torsion fiber. This component is computed as the six-dimensional integral: Z Z x1 − x2 F (dx, dz)x = Gρ1 ρ2 d3 r1 d3 r2 (1) [(x1 − x2 )2 + (y1 − y2 )2 + (z1 − z2 )2 ]3/2 where the limits for r1 ,r2 are the physical boundaries of the two cylinders. We work in a Cartesian system; the x and z integrations are performed analytically, while the y integra- tions are performed numerically using a Romberg integration scheme. The results of two independent calculations of Fx vs. dx, one written in Mathematica, one written in C, agree within their fractional precisions (< 10−6 ). Figure 1.7-1. Coordinate System for Calculating the Force between two cylinders. Knowing Fx , we then calculate the harmonic components of the torque on the pendulum as a function of attractor rotation angle. For our current geometry, a 22-fold azimuthally symmetric system, our first harmonic occurs with a frequency 22 times that of the attractor rotation, while the second harmonic has twice that frequency, etc. Our attractor actually consists of two stacked discs. The upper, thin, disc produces a Newtownian torque on the pendulum, plus whatever torque arises from a short-range Yukawa force. The lower, thicker attractor has holes that are placed out of phase with the upper attractor, serving to cancel Newtonian torques on the pendulum. The cancellation is not perfect, but reduces the expected Newtonian signal by a factor of 50. Several parameters are needed to calculate these harmonics. For each plate, the para- meters are the hole radii in the discs, the vertical spacing to the pendulum (dz), and the ∗ Presently at Physikalisches Institut, Heidelberg, Germany.


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    12 Figure 1.7-2. The x component of the force between two Molybdenum cylinders of radii 2.4 and 1.6mm as a function of dx. The solid line is for dz = 50µm while the dashed line is for dz = 1050µm. horizontal offset between the pendulum and attractor axes (dR). To combine the torques from the two plates, we have additional parameters: the gap between the attractor discs (t), and the angle between the hole patterns of the two discs (θ). We then have the Newtonian torque, τN (rp , rua , rla , t, θ, dR, dz) (2) where rta , rba are the radii of the holes of the upper and lower attractor. Figure 1.7-3. The first three harmonic components of the torque for ideal parameters of our 22-fold symmetric system.


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    UW CENPA Annual Report 2001-2002 13 1.8 Computer controlled torsion fiber damping routine E. G. Adelberger, S. Beckman,∗ M. C. Feig, B. R. Heckel, D. J. Kapner and H. E. Swanson Because the pendulum fiber for the sub-millimeter experiment is so weak, even small influ- ences such as seismic effects, pressure bursts, or accidental contact with the attractor plate can cause the pendulum to twist wildly. The energy in the twist motion must be damped out and the pendulum brought back onto the detector before data taking can resume. The damping can be done manually, but the process is tedious and time consuming. Greatly complicating matters is the fact that the detector is only 0.1 degrees wide, but the pendu- lum often acquires an amplitude several times this value. We have written a C program to perform the damping more efficiently and accurately. The damping program interacts with the apparatus in two ways. First, it reads informa- tion from the detector for one full period of the pendulum to determine its current motion. This portion of the code is very similar to the data acquisition routines used during experi- ments except that the sampling rate is set much higher, 20 points per second, to account for the higher speed of the pendulum. Second, the program can alter the pendulum’s motion via a motor that rotates the point of suspension of the fiber. The motor does not directly affect the current angular position of the pendulum, but rather changes the equilibrium angle of the oscillation. The most complicated portion of the code is the motion fitting routine. The data are handled in a variety of different ways depending on two parameters: the number of good data points and the number of times the pendulum crosses through the detector during the period. Using whichever method, a fit is performed to determine the amplitude and phase of the oscillation and the offset of the equilibrium position. From this information, two adjustments are calculated, one to be made at the maximum of the oscillation and one at the minimum, which should return the pendulum to a stationary position centered on the detector. These changes are then sent to the motor. The damping program has been tested out in a variety of situations and is typically able to reduce the amplitude by a factor of 50, and thus the energy by a factor of 2500, in a single run. If the initial oscillation is especially large, the program can be run iteratively until the motion is damped sufficiently. The limiting factor is the step size of the motor, 0.001 degrees. This is more than adequate, however, to ensure that the pendulum remains on the detector for the entire period. A single run of the damping routine takes approximately two periods, one period for data taking and one period for the adjustments at the maximum and minimum. Depending on the specific fiber used, the pendulum period is about 500 seconds, so the program can effectively damp the pendulum in eight to 16 minutes. And since the process is automated, the only time commitment for the user is to begin a run and check back on the results at the end. ∗ Department of Physics, University of California, Berkeley, CA.


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    14 1.9 Finite element analysis of capacitance for pendulum in 1/r2 test E. G. Adelberger T. W. Butler and D. J. Kapner We are in the process of using finite element analysis to calculate the capacitance between the short-range torsion pendulum and the thin beryllium-copper screen that electrostatically isolates it from the attractor mass. This calculation will be used to determine more accurately the separation between the pendulum and attractor mass. The short-range experiment tests the gravitational inverse square law by monitoring the interaction between the pendulum and attractor mass at a number of different heights. Con- sequently, the height of the pendulum above the attractor is one of the most significant parameters in the experiment. The height is obtained from measurements of the capacitance between the pendulum and electrostatic screen. In order to use these measurements to ob- tain heights, we must know the functional dependence of capacitance on height. Capacitance measurements were made at a number of different heights above the screen. While we were unable to directly measure the absolute height of the pendulum, the measurements did give the shape of the capacitance versus height (CH) curve. The absolute height was then deter- mined by a fitting procedure. The fitting procedure relied on an analytical expression for the functional dependence of capacitance on height that included a number of approximations to account for edge effects and finite thickness. In addition, the expression assumed that any contribution to the capacitance resulting from the pendulum support frame and the sur- rounding electrostatic shield was independent of the separation between the pendulum and the screen that is held at equipotential with the beryllium-copper screen. Thus it is desirable to verify the result determined from this semi-empirical procedure in an independent manner. It was proposed that finite element analysis (FEA) could be used to calculate an absolute CH curve for the pendulum and electrostatic screen. The advantage of FEA is that it requires much less geometric approximation than analytical methods. I have begun an effort to use ANSYS, a widely distributed commercial FEA program, to make calculations of the capacitance of the short-range geometry. I have been able to use ANSYS to reproduce capacitance values of geometric configurations that admit exact ana- lytical solutions. I have also constructed an accurate model which includes the short-range pendulum, the pendulum support frame, the beryllium-copper screen, and the surrounding electrostatic shield. ANSYS is able to take advantage of the 22-fold symmetry of the pendu- lum, which greatly reduces computational time and increases the accuracy of the calculations. Preliminary calculations have been made which show good agreement with the semi-empirical capacitance values.


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    UW CENPA Annual Report 2001-2002 15 1.10 Torsion balance test of CPT and Lorentz symmetries E. G. Adelberger, B. R. Heckel, J. H. Gundlach and M. White The standard model of particle physics is invariant under Lorentz and CPT symmetries. However, in an extended theory that combines the standard model with gravity, Lorentz and CPT symmetries may be spontaneously broken. Colladay and Kostelecky1 have developed a consistent picture of new particle couplings that arise if CPT and Lorentz symmetries are broken. They pointed out that torsion balance measurements with a spin polarized pendulum provide the most sensitive test for these symmetry violations in the electron sector of their model. In previous work, we developed a spin-polarized pendulum by stacking four rings of per- manent magnets.2 Each ring had the form of a hollow octagon with four consecutive magnets made from Alnico V material and the remaining four from SmCo5 . The magnetization in the Alnico comes entirely from electron spin polarization while that in SmCo has a substantial contribution from the orbital magnetization of the Sm ions. The assembled octagons were magnetized as a unit, forcing the magnetization to run inside the octagon, leaving only a small leakage magnetic field outside of the octagon. The stacked octagons were placed inside of a magnetic shield to complete the spin pendulum. We estimated that there were 8 × 1022 uncompensated spins in the assembled pendulum, with a leakage field of less than 0.2 mGauss outside of the shield. The pendulum was operated in the EotWash II apparatus to search for new pseudo-scalar fields. To search for CPT and Lorentz symmetry violation, we have rebuilt the spin pendulum. The octagon segments were magnetized individually to the same level of magnetization, making the leakage field from the assembled stack small enough to eliminate the need for an entire magnetic shield. Instead, two layers of magnetic shielding foil now encircle the pendulum, resulting in a reduction by over a factor of two of the pendulum mass. The leakage field from the new spin pendulum is 0.7 mGauss at a distance of 3 cm from the pendulum. The 120-g pendulum is now supported by a 30-micron diameter W fiber, increasing the sensitivity to torques by a factor of 7 over our original spin pendulum. The largest systematic error in the original experiment came from spurious signals associated with the tilt of the rotation axis of the apparatus. The “feetback” leveling system, described in the 2001 Annual Report3 has eliminated tilt as a limitation for the new measurements. The new spin pendulum has been mounted in the EotWash II apparatus and data col- lection has begun. The signature of CPT and Lorentz symmetry violation is a torque on the pendulum that couples to an axis fixed in space. We rotate the entire apparatus with a period of 1600 sec. to produce a signal that would have the same period. Preliminary data exceeds our original sensitivity by a factor of five and we anticipate an overall increase by a factor of 30 for this measurement, corresponding to an anomalous coupling to spin of less than 10−21 eV. 1 D. Colladay and V. A. Kostelecky, Phys. Rev D. 55, 6760 (1997); ibid. 58, 116002 (1998). 2 M. G. Harris, Ph.D. thesis, University of Washington, 1998. 3 CENPA Annual Report, University of Washington (2001) p. 4.


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    16 2 Neutrino Research SNO 2.1 Solving the solar neutrino problem at SNO: Evidence for the flavor transformation of solar 8 B neutrinos K. M. Heeger and the SNO Collaboration For more than 30 years, experiments have detected neutrinos produced in the thermonuclear fusion reactions which power the Sun. These reactions fuse protons into helium and release neutrinos with an energy of up to 15 MeV. Data from these solar neutrino experiments were found to be incompatible with the predictions of solar models. More precisely, the flux of neutrinos detected on Earth was less than expected, and the relative intensities of the sources of neutrinos in the sun were incompatible with those predicted by solar models. With the recent measurements of the Sudbury Neutrino Observatory (SNO), it has finally become possible to test the solar model predictions and the particle properties of neutrinos independently. The measurements at the Sudbury Neutrino Observatory (SNO) show that the neutrino flux produced in the 8 B → 8 Be∗ + e+ + νe beta-decay reaction in the Sun contains a significant non-electron type component when measured on Earth. This measurement is the first direct evidence for the flavor transformation of solar neutrinos. This neutrino flavor conversion indicates that neutrinos have mass. Together with the oscillation signature in atmospheric neutrino studies, these results are strong evidence for mixing in the lepton sector and new physics beyond the Standard Model. Located 2-km underground in an active nickel mine in Sudbury, Ontario, the Sudbury Neutrino Observatory is a water Čherenkov detector specifically designed to study the prop- erties of solar neutrinos. It consists of a spherical acrylic tank filled with 1000 tonnes of heavy water and surrounded by 7000 tonnes of light water to shield it from b ackgrounds. The choice of D2 O as a target material makes the SNO detector unique in comparison with other solar neutrino detectors. It allows SNO to measure both the total flux of solar neutrinos as well as the electron-type component of the neutrino flux produced in the Sun. Solar neutrinos from the decay of 8 B are detected via the charged-current reaction on deuterium (νe + d → p + p + e− ) and by elastic scattering off electrons (νx + e− → νx + e− ). Some 9,500 photomultiplier tubes (PMTs) are used to record the Čherenkov signature of these neutrino interactions. The charged-current reaction is sensitive exclusively to νe while the elastic-scattering reaction also has a small sensitivity to νµ and ντ . Neutrinos also interact through the neutral-current reaction (νx + d → p + n + νx ). The neutron produced in the NC interaction thermalizes in the heavy water and captures on deuterium, emitting a characteristic 6.25-MeV γ. All three interaction rates have been measured in SNO. SNO has been online since November 1999 taking production data, calibration data, and background measurements. The current SNO results are based on 306.4 live days of data taken between November 2, 1999, and May 27, 2001. Using the characteristic radial and


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    UW CENPA Annual Report 2001-2002 17 Neutral Elastic Charged June 2001 Current (NC) Scattering (ES) Current (CC) 2.0 BPB00) 8B from CC SNO+ESSK CC shape (SSM BP00) constrained ESSNO 1.5 Signal (SSM CCSNO Neutrino Signal 1.0 SSM April 2002 5.3 σ NCSNO 0.5 ESSNO CC shape unconstrained CCSNO 0.0 νe+ νµ+ντ νe+ 0.15 (νµ+ντ) νe Figure 2.1-1. SNO’s solar neutrino flux measurements in units of standard solar model predictions (BPB00). About 2/3 of the active solar neutrino flux consists of flavors other than νe . The difference between the total flux of 8 B neutrinos and the νe flux provides evidence at the 5.3 σ level for the flavor transformation of solar neutrinos. solar angle distributions as well as the energy spectrum of γ’s from neutron capture events on deuterium, the neutrino candidate event set is resolved into contributions from charged- current interactions, elastic scattering, and neutron events. Backgrounds from radioactivity in the D2 O and H2 O are measured by regular low-level radioassays of U and Th decay chain products and from a lower-threshold neutrino signal analysis. In common with all previous solar neutrino experiments, SNO observes a reduced flux of νe from the Sun compared to solar-model predictions. A comparison of the charged-current and neutral-current interaction rates is used to test the hypothesis of neutrino flavor transformation. The charged-current reaction on deuterium is sensitive exclusively to νe while the neutral-current interaction is sensitive to νµ and ντ , as well as νe . Under the assumption of no spectral distortions in the CC spectrum the difference between the CC and NC interaction rates is more than 5.3 σ. This is clear evidence for the non-electron flavor component of the solar neutrino flux. Without the constraint on the CC spectrum SNO makes a model-independent determination of the 8 B flux. The measured total flux of solar 8 B neutrinos is in good agreement with solar model predictions. We note that the total 8 B neutrino flux deduced in June 2001 from SNO’s measurement of the charged-current interaction and Super-Kamiokande’s measurement of the elastic scattering of 8 B neutrinos is in excellent agreement with SNO’s neutral-current measurement published this year. The results of this work are summarized in Fig. 2.1-1 and have been submitted for publication.1 1 Q. R. Ahmad et al., LANL arXive, nucl-ex/0204008, nucl-ex/0204009.


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    18 2.2 Muon spallation neutrons at the Sudbury Neutrino Observatory Q. R. Ahmad,∗ J. A. Formaggio, R. Hazama, J. L. Orrell, R. G. H. Robertson, M. W. E. Smith and J. F. Wilkerson Muons produced by cosmic-ray interactions in the Earth’s atmosphere reach the SNO detector even at its 6010 m water equivalent depth. Muons both Čerenkov radiate and interact electromagnetically with nuclei as they pass through the SNO detector. Continuing previous work,1 spallation neutrons produced by the passage of high energy muons have been studied. The passage of muons through the SNO detector is distinct from the neutrino interactions. The amount of Čerenkov light produced by a muon is orders of magnitude greater than that produced by the resultant electrons of neutrino interactions. This higher energy signal allows for easy separation of the muons from neutrinos. The ability to separate out the muons allows for the selection and/or removal of muon spallation products which produce subsequent events. It has been shown that the muon spallation products are predominately neutrons by comparing the events which follow muons with single neutron Monte Carlo calculations and also with data from neutron calibration sources. The capture of a neutron on a deuteron, d(n, γ)t, produces a mono-energetic gamma ray with an energy of 6.25 MeV. This mono-energetic gamma can be used as a calibration source. Spallation neutrons provide a constant, uniformly distributed, and container-less calibration source which has been used to study SNO’s low energy response for both energy scale and temporal variations. One use of this neutron data set is as a “blindness” tool. Once the spallation neutrons are flagged, they can be reinserted into the neutrino data set. This has the effect of increasing the apparent neutral current (NC) component of the solar neutrino flux. This blindness scheme was instituted for the current salt phase of SNO as one way of hiding the true NC number from the analyzers. Muon spallation neutrons have also been studied in support of a day vs. night neutrino analysis.2 The goal of the study was to determine if there are day-night asymmetries in the rate of muon induced spallation neutrons. It was determined that the spallation neutron rate had a day-night asymmetry A = 2(φD − φN )/(φD − φN ) = 2.2 ± 5.9%, which is consistent with zero. These neutrons were also used to compare the low energy response of the detector day vs. night - another potential asymmetry. Recent work has begun the process of comparing theoretical predictions of neutron pro- duction from muons to that which SNO measures. In addition Monte Carlo algorithms are in the early stages of integration in the SNO Monte Carlo and analysis code framework. It is expected that muons and their spallation products will remain a fertile non-neutrino physics topic. ∗ Presently at Sapient Corporation, Cambridge, MA 02142. 1 CENPA Annual Report, University of Washington (2001) p. 23. 2 arXiv.org pre-print: nucl-ex/0204009.


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    UW CENPA Annual Report 2001-2002 19 2.3 Electron antineutrino studies at the Sudbury Neutrino Observatory S. R. Elliott, C. E. Okada,∗ J. L. Orrell, R. G. H. Robertson and J. F. Wilkerson Electron antineutrinos, ν̄e , interact in SNO’s heavy water via a charged-current weak inter- action with deuterium nuclei, d: ν̄e + d → e+ + n + n Q = −4.03 MeV (CCν̄e ) The positron, e+ , and neutrons, n, produced by this reaction can give detectable signals in SNO. If the positron has sufficient energy, it will Čerenkov radiate as it passes through the heavy water. Each neutron can capture on another deuterium nucleus releasing a 6.25 MeV gamma-ray which will also produce a detectable signal. Reaction CCν̄e provides a distinctive signature which is separable from the “background” of other processes occurring in the SNO detector. The distinctive signature is the time coincidence of SNO events due to each of the three product particles. In Table 2.3-1 we list sources of electron antineutrinos and give a preliminary estimate of the CCν̄e event rate in SNO.1 One background to the CCν̄e signal is an accidental coincidence ν̄e Source # CCν̄e (kT·Yr)−1 Comment Atmospheric 10 Potential 1st Φatmo atmo measurement ν̄e /Φνe Nuclear Power Reactors 2 Precisely calculable Supernova Relic 0.1 Terrestrial 0 Below CCν̄e threshold Solar <100 Based on current best limits Table 2.3-1. Preliminary estimates of the CCν̄e rate in SNO. of uncorrelated SNO events. The accidental coincidence rates are given by racc.,2 = r 2 tw racc.,3 = r 3 tw 2 (for rtw ¿ 1) where racc.,2 (racc.,3 ) is the rate of accidental coincidences of 2(3) SNO events in a given time window, tw , with a single event rate r, after instrumental cuts are applied. A preliminary analysis determined that the SNO single event rate (after cuts) was 2.61×10−4 events/second. The neutron’s mean capture time on deuterium in pure heavy water is ≈40 ms. The coinci- dence window, tw , is chosen to be large compared to the mean neutron capture time and 500 ms is a good preliminary choice. Using these values we determine the accidental coincidence rates: racc.,2 = 3.4 × 10−8 Hz racc.,3 = 4.4 × 10−12 Hz These values support the belief that an electron antineutrino search in SNO’s data is feasible even though the expected CCν̄e event rate is low. ∗ Institute of Nuclear and Particle Astrophysics and Nuclear Science Divison, Lawrence Berkeley National Laboratory Berkeley, CA. 1 References: The atmospheric & nuclear power reactor numbers are internal SNO calculations, the super- nova relic numbers are from M. Kaplinghat et al., Phys. Rev. D 63, 043001 (2001), and the solar number is based on a limit from M. Aglietta et al., JETP 63, 791 (1996).


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    20 2.4 Neutron backgrounds from cosmic rays and atmospheric neutrinos in SNO Q. R. Ahmad,∗ J. A. Formaggio, R. Hazama, J. L. Orrell, R. G. H. Robertson, M. W. E. Smith and J. F. Wilkerson The Sudbury Neutrino Observatory (SNO) is a unique second-generation solar neutrino ex- periment specifically designed to measure both the energy and flavor composition of neutrinos emanating from the sun. By using heavy water as its primary target, SNO is able to distin- guish between charged-current and neutral-current neutrino interactions and thus uniquely determine the flavor content of solar neutrinos in a model-independent way. SNO is sensitive to the neutral-current reaction via the capture of neutrons on deuterium (2 H(n,γ)3 H), releasing a gamma ray of 6.25 MeV. Because the detector is sensitive to all free neutrons within it, a precise determination of all neutron-induced background is essential in measuring the neutral-current rate in SNO. The majority of background events come from low-level radioactivity present in the detector; mainly uranium and thorium decay-chain daughters (214 Bi and 208 Tl). However, a non-negligible background comes from external sources, such as cosmic-ray activity, nuclear reactor neutrinos, and atmospheric neutrinos. Spallation productions from high-energy cosmic-rays constitute a potentially serious back- ground to the neutral-current measurements, since a large number of neutrons are released as a muon passes through the detector. Current measurements estimate a neutron production rate of approximately 4.14 (Eµ /GeV)0.74 × 10−6 neutrons/(µg cm−2 ).1 Most of the muons which produce spallation products are tagged by the Čerenkov light produced. However, muons which travel through and/or interact in the surrounding rock have no tagged signa- ture and constitute a potential silent background to the neutral current measurement. An additional background arises from atmospheric neutrinos interacting in the detector. These events are indistinguishable from solar neutrinos and represent an irreducible background. Muon spallation, muon interactions in the rock, and atmospheric neutrinos typically in- volve interactions above 20 MeV. To properly model neutrons and hadrons produced in these interactions, a full high-energy hadron transport scheme was added to the SNO Monte Carlo.2 Simulations of silent backgrounds determined that atmospheric neutrino events in the heavy water are the most significant, producing about 14.6 ± 2.8 neutrons per year that capture on deuterium. This translates into a 0.7 ± 0.2% contribution to the total neutral current signal. The utilities developed to access this background have a wide range of other uses in SNO physics. The hadron transport code will be used also in atmospheric neutrino measurements, muon spallation measurements, and exotic neutron-anti-neutron oscillation searches. ∗ Presently at Sapient Corporation, Cambridge, MA 02142. 1 Y.-F. Wang et al., Phys. Rev D 64, 013012 (2001). 2 Applications and Software Group, CERN, “GEANT: Detector Description and Simulation Tool,” CERN Program Library Report Q123.


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    UW CENPA Annual Report 2001-2002 21 2.5 Search for solar hep neutrinos in the Sudbury Neutrino Observatory K. M. Heeger and the SNO Collaboration The Sudbury Neutrino Observatory (SNO) is designed to measure the flux of solar neutrinos and to determine the shape of the solar neutrino spectrum. Neutrinos from the beta-decay of 8 B dominate the solar neutrino spectrum between 5-15 MeV. The shape of the neutrino energy spectrum from a single beta-decaying source is well known and independent of solar physics to an accuracy of 1 part in 106 . A measurement of the shape of the solar neutrino energy spectrum is a direct test of the minimal electroweak model and can be used to constrain models of neutrino flavor transformation. Near the 8 B endpoint the solar neutrino spectrum is very sensitive to any underlying background, including instrumental effects and neutrinos with energies above 15 MeV from other sources than 8 B-decay. In a rare branch of the pp-chain in the Sun, 3 He and proton fuse forming the reaction 3 He + p → 4 He + e+ + νe . This hep process produces the highest energy solar neutrinos with an energy of up to 18.77 MeV. Fig. 2.5-1 shows Monte-Carlo simulations of the observed energy spectrum of 8 B and hep neutrinos in the SNO detector as predicted by Standard Solar Model calculations. The number of photomultiplier hits, NHIT, is the basic energy measure of events. Monte-Carlo 150 hep (100 x SSM) 8 B (SSM) Number of Events 100 50 0 80 100 120 140 160 180 NHIT Figure 2.5-1. Energy (NHIT) spectrum of 8 B neutrinos and hep neutrinos in the Sudbury Neutrino Observatory as predicted by Standard Solar Model calculations. The hep spectrum is scaled by a factor of 100. With a maximum energy of 18.77 MeV, neutrinos produced in the 3 He + p → 4 He + e+ + νe (hep) reaction are the highest energy solar neutrinos. In the past, the measurements of the energy distribution of recoil electrons created by 8 B and hep neutrinos scattering off electrons in the Super-Kamiokande detector raised significant experimental and theoretical interest in the observation and calculation of the hep neutrino flux.1 One possible interpretation of the Super—Kamiokande measurement suggested a flux of hep neutrinos ≥ 20 times larger than the best theoretical estimate at the time. Standard Solar Model (SSM) calculations for the flux of hep neutrinos predict a flux of 9.3 × 103 cm−2 1 J. N. Bahcall et al., Phys. Lett. B 436, 243 (1998).


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    22 s−1 compared to 5.05 × 1010 cm−2 s−1 for the much more abundant 8 B neutrinos. The SSM predictions of the hep flux are based on low-energy cross-section calculations with an S-factor of S SSM (hep) = (2.3 ± 1.3) × 10−20 keV-b. The rate of the hep reaction is so small that it does not affect solar modeling and other solar model predictions. The reliable estimation of the hep cross-section has been a long-standing challenge in nuclear physics. Modern evaluations of this S-factor using approaches based on effective field theory (EFT)2 or standard nuclear physics approaches (SNPA)3 yield S-factors in the range between 8.6 − 10.1 × 103 cm−2 s−1 . These calculations yield a hep flux prediction that is ∼ 4 times higher than Standard Solar Model calculations. Therefore, a measurement of the flux of hep neutrinos at the Sudbury Neutrino Observatory is of interest to astrophysics and nuclear physics alike. It is also an important part of a detailed shape analysis of the solar neutrino energy spectrum measured at SNO. In the first phase of the experiment, SNO has measured the interaction rates of solar neutrinos with pure D2 O. The results deduced from SNO’s measurements include the total active flux of 8 B neutrinos. It was found to be in good agreement with Standard Solar Model predictions. Low backgrounds and the efficient discrimination of instrumental effects have also allowed SNO to measure the energy spectrum of solar neutrinos and to perform a rare event study in the energy region between 15-30 MeV. Above the 8 B endpoint high energy backgrounds and instrumental backgrounds are estimated to contribute less than 0.8% and 0.5% to the measured interaction rate. The physics background from atmospheric neutrino interactions is estimated to be small in comparison. As part of the SNO solar neutrino analysis, we have analyzed data from the pure D2 O phase for high-energy solar neutrinos as well as lowest-energy atmospheric neutrinos. Calibra- tion of the solar neutrino energy spectrum is based on a tagged 6.13-MeV 16 N γ-source below the 8 B endpoint and 19.8-MeV γ’s from a pT source above the 8 B endpoint. Using analysis techniques similar to the ones developed for the measurement of the interaction rates of 8 B neutrinos with deuterium the flux of hep neutrinos in the Sudbury Neutrino Observatory has been determined. Candidate events for atmospheric-neutrino induced muons are found and techniques for the robust and unique identification of low-energy atmospheric neutrino events are under development. The results of this work are being prepared for publication. 2 T. S. Park et al., LANL arXive, nucl-th/0110084. 3 L. E. Marcucci et al., Phys Rev C 63, 015801 (2001).


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    UW CENPA Annual Report 2001-2002 23 2.6 Seasonal variation of the muon flux at SNO in the deepest underground laboratory R. Hazama and the SNO Collaboration SNO is the most northerly underground neutrino detector in the world (the geographical po- sition is 46.5◦ N (57.2◦ N magnetic) in latitude) and the geomagnetic field guides cosmic rays into the earth’s atmosphere and determines the minimum momentum of cosmic ray primaries that can reach the top of the atmosphere above the detector (the geomagnetic cutoff is 40 GeV/c for protons).1 Besides, SNO lies under rock overburden of 6010 m water equivalent. Hence, the muon energy threshold at the SNO site is ∼4 TeV and larger than at all the other sites, such as MACRO (1.3 TeV)2 , Super-Kamiokande (1.7 GeV), and AMANDA (500 GeV). For high energy muons, air expansion caused by an increase in atmospheric temperature (see Fig. 2.6-1) leads to increased decay of the parent mesons and thus to a positive correlation between muon intensity and atmospheric temperature. As the atmospheric temperature in- creases, the height of the atmosphere increases, density of the air decreases, and fractionally more pions/kaons decay to muons before interacting. SNO’s great depth is the most ap- propriate for this correlation study. At shallow depths, this correlation is small because the threshold energy is so low that interactions are unimportant in the cascades. This seasonal variation is the observation of a known phenomenon and is thus a test of the achievable sta- bility of the SNO detector. Further, it can be used as a continuous monitor during the entire day and night running time without any disruption to normal data taking, unlike deployable devices. Monthly Variations in Temperature Radiosonde(Pickle lake ON, Canada) 12 10 8 6 4 2 ∆Teff(K) 0 −2 −4 −6 −8 −10 −12 −2 −1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Month, 2000 Figure 2.6-1. Monthly variations in the effective temperature, 4Tef f = Tef f − < Tef f >, where Tef f is the mean of the monthly effective temperature distribution and < Tef f >= 219.4 K is the mean effective temperature for the data set (Nov’99-Dec’00). The temper- ature data of the upper atmosphere were provided by the nearest available meteorological observatory to SNO; Pickle Lake site (51.4◦ N −90.2◦ E). You can see a ±4% variation. 1 C. E. Waltham, Initial Observation of Through-Going Muons in SNO, SNO internal report. 2 M. Ambrosio et al., Astropart. Phys 7, 109 (1997).


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    24 2.7 The day-night asymmetry of the solar neutrino flux measured at SNO M. W. E. Smith and the SNO Collaboration As solar neutrinos pass through the earth, matter effects might enhance the oscillation from one flavor to another. This effect is insignificant when the sun is above the horizon (day) but can be quite substantial when the sun is below the horizon (night). One can quantify the effect by forming the day-night asymmetry parameter φN − φD A=2 φN + φD where φN and φD are the measured night and day neutrino fluxes. The flux of electron neutrinos is measured primarily by the charged-current interaction with deuterium, although some additional sensitivity is provided by the elastic-scattering (ES) reaction. The total flux of all active neutrinos is measured primarily by the neutral- current reaction with deuterium, with the ES reaction again providing a little information. If the electron neutrinos are oscillating only to active flavors then the asymmetry in the total neutrino flux Atot is expected to be zero. Allowing Atot to float, SNO has measured1 Ae = 12.8±6.2+1.5 +2.4 −1.4 %, Atot = −24.2±16.1−2.5 %, with a correlation of -0.602 between the two measurements. The joint confidence ellipses for Ae , Atot are shown in Fig. 2.7-1. We note that at ≈ 68% c.l., the measurements permit Atot = 0. By forcing this constraint, SNO measures Ae = 7.0 ± 4.9 ± 1.3%. The previous measurement from the SuperK experiment used only the ES reaction, which constrains only a linear combination of Ae and Atot . This result is shown as a diagonal band in Fig. 2.7-1. The width of the band is determined by the SuperK error, while the slope of the band is determined by SNO data. 1 The SNO Collaboration, nucl-ex/0204009.


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    UW CENPA Annual Report 2001-2002 25 Figure 2.7-1. Joint probability contours for Ae and Atot . The points indicate the results when Atot is allowed to float and when it is constrained to zero. The diagonal band indicates the 68% joint contour for the Super-K AES measurement.


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    26 2.8 Status and updates to the SNO data acquisition system A. A. Hamian,∗ M. A. Howe, P. Harvey† and J. F. Wilkerson The Sudbury Neutrino Observatory data acquisition (SNO DAQ) system is designed to pro- vide continuous readout of the detector’s photomultiplier tubes (PMTs) with a minimum of dead time. Since the SNO DAQ system has been described extensively in past annual reports,1 only a brief overview of the most significant updates is provided here. In August 2001, the computer running the SNO Hardware Acquisition Real-time Control program (SHaRC) failed and was replaced with one of the spare monitoring computers. As it was becoming clear that the DAQ computers were rapidly aging, it was decided to retire all of the 250MHz PPC Macs and replace them with newer models. Four machines were shipped to site, two 733MHz G4s and two 450 MHz G3s. The faster machines were used to replace both the underground DAQ system and the above ground test stand DAQ system. The other two are being used as the operator control stations. In addition, the SBS 617 PCI to VME controller was replaced with a SBS 622 controller. The new controller has the advantage of using fiber optics to decouple the VME crate from any electronic noise generated by the DAQ computer. The upgrades were carried out in December 2001. To run SHaRC on the new G4s the code that probes the bus for cards and builds the device registry was rewritten from the ground up to work with the new G4 bus structure. It is now more robust against future changes and is backward compatible with older machines. After SHaRC begun running on the G4, a number of eCPU/Mac VME hardware excep- tions began to occur during periods of intense Mac-VME bus activity. Since the issue was resolved, no VME hardware exceptions have be reported and the overall stability of the DAQ system running on the new machines has been excellent. Another enhancement to SHaRC was the introduction of the capability to edit the stan- dard run types directly from within SHaRC and to save the run types into files which can be reloaded on demand. This capability extends to the different sources, so that each source can have multiple custom setup files. This has proven to be extremely helpful for the calibration group which regularly adjusts the trigger thresholds. In addition, a new type of ECA calibration task was added which allows the operator to use an external pulser to generate very low rate pedestal pulses for doing calibrations during normal running to help diagnose various hardware/calibration problems. The PMTs in the pedestal set can be set from a group of randomly selected tubes or can be specified from a file. The SNO DAQ system continues to perform reliably, and there are no major upgrades foreseen in the coming year. ∗ Presently at Avocent Corporation, Redmond, WA 98052. † Queens’s University, Kinston, Ontario, Canada. 1 Nuclear Physics Laboratory Annual Report, University of Washington (1997) pp. 20-23; (1998) pp. 18-20; (1999) pp. 16-18; (2000) p. 19; CENPA Annual Report (2001) p. 25.


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    UW CENPA Annual Report 2001-2002 27 SNO NCDs 2.9 NCD data taking and analysis T. V. Bullard, G. A. Cox, S. R. Elliott, K. M. Heeger, A. Hime,∗ R. G. H. Robertson, M. W. E. Smith, L. C. Stonehill, J. F. Wilkerson and J. M. Wouters∗ With the recent installation of the nearly complete NCD electronics and data acquisition system, we are now taking data on 30 of the 96 channels available. In comparison to the 4 channels of data that were previously being acquired, the amount of data we are now able to take has significantly increased for the “cooldown” phase of the NCD’s. With this increase in data and the new structure of the data stream, a number of changes have been made, and are currently being made, to the software analysis routines. Among these changes was the development of a new program called AnalystConv, which reads in the new data stream and converts it into a file containing the digitized data and other parameters that the Analyst program uses to analyze the data. A zero level cut has also been implemented to cut out non-ionizing events that are caused by electronic noise. This cut is based on the maximum time gap between two consecutive baseline crossings in an event. A cut of 0.6 microseconds has shown to have zero sacrifice of neutron and alpha events, about 98% efficiency in cutting out microphonic and oscillatory noise events, and 100% efficiency in cutting out baseline and noise spike events. Its efficiency for cutting out micro-discharge events has not been fully characterized, but it is estimated to be about 80% with the ability to cut out more if additional cut parameters are implemented. The data analysis projects that have been in progress this year attempted to address the contamination issues of both the “background-free” neutron region of phase space and the region targeted for determining the bulk alpha activity in the counters due to the 238 U and 232 Th content in the NCD’s. The first of these projects involved the assembly of two short counters with high levels of 210 Po contamination. One counter has a high 210 Po level in the end cap region, while the other has it in the mid-body of the counter. The goal of this study was to characterize the risetime vs. energy distributions of alphas originating from the end cap region where the electric field is weak, which might mimic neutron events that would otherwise be distinct from the distributions of bulk and surface alphas. It was determined that end-effect alpha events do indeed contaminate both the neutron region and the bulk alpha region. A conservative upper limit of 0.14 end effect events per day will occur in the neutron region at the estimated time of deployment (March 2003). This limit would give 18 events over the first year of running the entire NCD array, resulting in a contamination that is a few percent of the expected neutral current signal. The contamination of the bulk alpha region will require further studies and data taking with the end effect counters now that the 210 Po contamination has decayed away and can be separated from the bulk activity. The second data analysis project was the “Water Wall” study. In this experiment, data was taken from a few NCD strings that were set up in a water enclosure underground at SNO to measure and compare the thermal and fast components of the neutron flux from ∗ Los Alamos National Laboratory, Los Alamos, NM 87545.


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    28 the surrounding norite rock. In performing this analysis, we encountered and identified a number of problems with the current analysis procedures that become more apparent at lower energies, and we have not been able to get conclusive results about the data. The problems with the current data analysis methods that have been identified are mostly attributed to the log-amp calibration procedures and parameters used, as well as to the method used to determine the onset time and duration of an ionizing event. An automated log-amp calibration routine is currently being designed and implemented to address the first issue. Studies are also underway to create a more robust method of determining the onset time and duration of an event, as well as to implement a “moments analysis” to provide more complete information about each event. Once these problems are resolved, data analysis will resume. The priority for data analysis during the remaining time in the cool-down phase of the NCD’s is to determine the intrinsic backgrounds that will affect our ability to measure the neutral current signal in SNO using the NCD’s. The data taking will also be geared to- wards verifying and characterizing the electronics and data acquisition system. Both of these characterizations will provide a large part of the information needed to assess readiness for deployment.


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    UW CENPA Annual Report 2001-2002 29 2.10 Status of the NCD DAQ for SNO G. A. Cox, M. A. Howe and J. F. Wilkerson The data acquisition system for the Neutral Current Detectors (NCDDAQ) at the Sudbury Neutrino Observatory (SNO) is near full data taking capability and complete system mon- itoring. The NCDDAQ, written in C++ , is based on the SNODAQ software within the SHaRC framework.1 The main goal of the NCDDAQ is to provide real—time system monitor- ing and initialization for the entire array of NCD electronics,2 data acquisition controls, and an output data stream, all within an intuitive graphical interface. The NCDDAQ features single—button run—initialization, a Hardware Wizard for simple setup, high-voltage controls, individual low-level electronic component controls, an alarm system, and a database for elec- tronic information storage.3 Below is a figure of the NCDDAQ main control window along with some monitoring tools Figure 2.10-1. A screenshot of the NCDDAQ The NCDDAQ now has an output data stream, which, after full development, will be inserted into the main SNO data stream. In the final configuration, most of the data-taking processes will be handled by an eCPU in the VME crate. The eCPU will load the data stream into dual port memory where it can be dispatched to analysis and monitoring tools. The data stream includes data from three main sources: the ADC Shaper cards, the NCD Multiplexers (MUX), and two digital oscilliscopes. The NCDDAQ can correlate data between the MUX and the oscilliscopes and combines them into a single event. The inclusion of the ADC data will occur with finalization of the GTID board and related software. Other minor additions to NCDDAQ include a replay function, data stream filters, pulse generator control over GPIB, improved digital oscilloscope controls, NCD array monitoring 1 Nuclear Physics Laboratory Annual Report, University of Washington (1997) pp. 20-23; (1998) pp. 18-20; (1999) pp. 16-18. 2 CENPA Annual Report, University of Washington (2001) pp. 32-33. 3 CENPA Annual Report, University of Washington (2001) pp. 34-35.


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    30 tools, multiple ADC Shaper data monitoring, and other areas to improve ease of use and configuration. The replay tool allows the user to replay an entire data file as if it were happening in real—time. The data filters parse data from the stream as specified by user input. The pulse generator will be used extensively in future calibration routines. Once fully developed, a multitude of calibration waveforms will be programmable into the pulse generator. The added NCD monitoring tools allow visual inspection of information on the array of NCD strings, such as event rates, thresholds, and gains.


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    UW CENPA Annual Report 2001-2002 31 2.11 NCD cable repair and testing T. H. Burritt, P. J. Doe, S. R. Elliott, J. A. Manor and R. G. H. Robertson SNO Neutral Current Detectors are to be to be deployed in the heavy water in an array of 96 strings. Each string is connected to the data acquisition system by a custom made coaxial cable of eight to fifteen meters length. These cables were designed to be slightly positively buoyant in heavy water, to match the impedance of the detector strings and were constructed of carefully chosen, low-radioactivity material. The cable is attached to the detector strings by way of a bell-shaped, nickel coupler. The cable end is inserted into the nickel bell and the shield braid is soldered to the nickel. Voids in the bell were then filled with silicone potting compound and adhesive heat shrink was applied to the junction between the nickel and the cable. The complete wet-end assembly is shown in figure Fig. 2.11-1. The heat shrink and the potting compound form primary and secondary water barriers. Due to the unique design of the cable and bell connections, tests are currently being conducted to ensure their long-term integrity at high voltage while underwater. Figure 2.11-1. The cable wet-end showing the nickel bell housing. The silicone potting and heat shrink that form the water barrier can also be seen. The cables must satisfy stringent requirements. They must withstand a minimum of 5 years exposure to the ultra-pure heavy water and must be electrically very quiet at their operating voltage of 1.835 kV. Spurious electrical discharges could represent a source of background to the anticipated neutron capture signal of 10 events per day. To ensure that the cables satisfy these criteria we devised the following series of tests. One form of electrical noise, micro-discharge at high voltage, can mimic the signal of a


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    32 neutron capture in the 3 He detector. The completed cable assemblies are therefore being micro-discharge tested at 2.4kV for a minimum of 12 hours. A cable with a micro-discharge rate of less that 0.25 per hour is considered acceptable. Studies have shown that this will result in a spurious trigger rate for the whole array of approximately 12 per hour, at the operating voltage of 1.835 kV. This rate is compatible with our ability to identify and reject micro-discharge events. After passing the micro-discharge tests, the cables are placed in a water tank. The water in this tank is purified until it has a resistance of 10 MΩ, degassed, chilled to 11 ◦ C, and raised to 2 atmospheres pressure. These conditions approximate those of the heavy water in which the cables will be deployed. When placed in the tank the cables are bent to a 10-inch diameter. This represents the most strenuous forces that any part of the cable will be subjected to once it is installed in the SNO detector. The cables are required to remain watertight during a 2-week exposure in the water test tank and to pass a subsequent micro-discharge test. Before deployment of the array, all 96 cables will have to pass these tests.


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    UW CENPA Annual Report 2001-2002 33 2.12 Neutral current detector electronics commissioning status G. A. Cox, P. J. Doe, C. A. Duba, A. W. Myers, R. G. H. Robertson, L. C. Stonehill, T. D. Van Wechel and J. F. Wilkerson The Sudbury Neutrino Observatory (SNO) has the ability to probe both the neutral- and charge-current neutrino flux through the use of heavy water. SNO needs to differentiate between neutrons and other events in order to distinguish potential neutral-current neutrino flux. The Neutral Current Detector (NCD) array is set to be placed within SNO in less than a year and has the potential to provide extremely accurate neutron recognition. The low expected rate of neutral-current events generated by solar neutrinos necessitates event-by- event recognition while the high potential neutron rate from galactic supernovae requires a fast data acquisition system. The NCD electronics were designed and built with both of these goals in mind. Dual data paths provide the low-rate scenario with digitized data while allowing for high-rate data taking. Each string of NCD counters has a pair of independent thresholds, one level for low-rate pulse digitization and one for high-rate signal integration. The NCD electronics also provide 8 individually controllable high voltage power supplies for selective distribution amongst the NCDs 96 strings. The bulk of construction and shipping is complete, and the NCD electronics has entered its commissioning phase as we bring the NCD array online in preparation for deployment. Currently, there are two complete running NCD systems, the primary one underground at SNO, and the secondary one at UW. A redundant third system, planned for above-ground placement in Sudbury, will be completed and shipped shortly. The underground system at SNO is currently running with 30 strings hooked into 5 NCD Shaper/ADC cards, 3 multiplexer (MUX) boxes, and two digitizing scopes. An SBS 618 VME controller connects the Shaper/ADCs to the control computer, whereas the scopes are in direct GPIB communication. After acquiring an event, the control computer saves the events to disk in a platform-independent data format. At the end of each run, the control computer then ships the data record back to CENPA. At this time, the underground system is mostly monitoring background events from the cavity wall of the SNO control room. With the help of several specially designed counters and a protective water encasement, we are exercising the full running of the system while we quantify expected background rates. During this phase, we are also sensitive to time- dependant neutron fluxes, such as might indicate or mimic a nearby supernova. In the upcoming months before deployment, the primary system will be constantly moni- toring the array, acquiring data, and shipping it for analysis at CENPA. During this commis- sioning phase, we will be finalizing software modules that facilitate simple array operation and monitoring. We will also use this time to calibrate the sensitivity and backgrounds of the NCD array.


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    34 2.13 Underground NCD welding prior to deployment J. F. Amsbaugh, T. A. Burritt, P. J. Doe and B. Morissette∗ As reported last year,1 we have completed construction and testing of the laser welding equipment needed for neutral current detector (NCD) deployment in the Sudbury Neutrino Observatory (SNO) detector. Some minor improvements and modifications have been made to aid in the use of the equipment and handling of the NCDs. The NCD welding will occur in two stages. First the so called pre—deployment welding, in which individual NC detectors are welded into the largest segments that will fit into the room above the SNO detector. The NCD anchors and cable ends are also welded to the segments during pre—deployment. It is conservatively estimated that pre—deployment welding will take about 3 months. The goal is to minimize the time the SNO detector is off for the second stage, NCD deployment. The NCDs will likely be deployed in spring 2003. Before any technical activity is allowed at the SNO detector, a review is required to satisfy the SNO Project Site Policies and Procedures.2 A review panel has been formed and has begun work. Equipment documentation, safety reports, assembly procedures, welding procedures, electrical inspection, and manpower schedules have been submitted. These will be finalized and approved by the review. A test concerning the electrical interference (EMI) of the laser welder on the SNO detector will be done. Any EMI will have to be eliminated, since the pre—deployment welding occurs while the SNO detector is running. A roll-around rack for transporting the NCD to and from the NCD storage area is already underground. It will be used to rearrange the NCDs in the storage rack in the proper order for the pre—deployment welding in April 2002. Also at this time the NCDs will be checked for failures by running them in the rack with the NCD data acquisition system. We will ship the remaining pre—deployment welding equipment to the SNO site in mid May 2002. After inspection, it will be taken underground for EMI tests as soon as possible. We anticipate completing the review in June 2002 so we can begin pre—deployment welding after the schedule maintenance shutdown, mid September 2002. ∗ Sudbury Neutrino Observatory, Lively, Ontario, Canada P3Y 1M3. 1 CENPA Annual Report, University of Washington (2001) p. 37. 2 Section No. 4.1.3, Development and Approval Process for Technical Activities.


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    UW CENPA Annual Report 2001-2002 35 2.14 NCD deployment equipment progress J. F. Amsbaugh, M. Anaya,∗ T. A. Burritt, P. J. Doe, G. C. Harper, J. Wilhelmy∗ and J. Wouters∗ The development of the equipment needed to deploy the neutral current detectors (NCDs) into the heavy water acrylic vessel (AV) of the Sudbury Neutrino Detector (SNO) is complete. A previous progress report1 listed glove box glove mount revisions, laser welding fixture (WF), WF mount, pre—deployment welding bench, and hauldown mechanism redesign. Outstanding items were finishing the gantry crane, the neckview camera system and equipment leaching tests. The gantry crane is a commercial2 all-aluminum construction A-frame unit with 2000-lb capacity, equipped with an aluminum manual winch3 with automatic brake. The crane cross beam and winch spool have been hard anodized. The completed assembly was load tested, shipped, taken underground at the SNO site. It has been assembled and verification that it can reach all required positions is expected soon. The neck view camera system uses a remote focus, aperture, and zoom lens4 coupled to a monochrome 1/2 inch CCD video camera.5 The light source uses super bright LEDs, 3 white, 3 yellow and 6 green, which can be switched on or off in groups. The camera, lens and LEDs are in a sealed, suitably clean enclosure that can be mounted in one of seven positions during use. This mount also provides manual camera pan of 360◦ and tilt of 120◦ . Bench tests assure the system can read the NCD cable labels at the appropriate range of distances. The equipment is well along for having the deployment occur in spring of 2003. We are working on the documentation, procedures, training materials, and so forth that will be re- quired for the activity review in late summer or fall of 2002. Training at the LANL test pool in equipment use must start in fall 2002 to meet the deployment date. The only unre- solved major issue at present is whether the radon emanation and leaching of the deployment equipment is sufficiently low to allow placing into the AV. ∗ Los Alamos National Laboratory, Los Alamos, NM 87545. 1 CENPA Annual Report, University of Washington (2001) p. 37. 2 Model 1ALU1208B, Spanco, Morgantown, PA 19543. 3 Model CMA-1760, Jeamar Winches, Inc., Buffalo, NY, 14206. 4 COMPUTAR Model H6Z0812M, Chugai Boyeki Corp., New York, NY. 5 Model TM-200, Pulnix America, Inc., Sunnyvale, CA.


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    36 Neutrino Detectors 2.15 Lead perchlorate as a neutrino detection medium M. K. Bacrania, P. J. Doe, S. R. Elliott and L. C. Stonehill Due to its apparent transparency, large interaction cross section, and relatively low cost, lead perchlorate Pb(ClO4 )2 is an attractive candidate for a Čerenkov neutrino detector. Neutrino interactions with lead may occur by either the charged-current (CC) or neutral-current (NC) reactions: νe + 208 Pb ⇒ 208 Bi∗ + e− (CC) ⇓ 208−y Bi + xγ + yn (1) 0 νx + 208 Pb ⇒ 208 Pb∗ + νx (NC) ⇓ 208−y Pb + xγ + yn At 30 MeV the CC cross section with lead is about 600 times that of carbon and the NC cross section is about 100 times that of carbon. The signature of a CC interaction consists of a prompt electron followed by gamma rays and neutrons. The signature of a NC interaction consists only of gamma rays and neutrons with no prompt electron. The number of neutrons depends on the energy of the interacting neutrino. Because lead perchlorate solutions contain an appreciable amount of hydrogen, the neutrons quickly thermalize and are captured by the 35 Cl which then emits an 8.6 MeV gamma ray. The gamma ray is detected by subsequent Compton scattered electrons. To determine if a lead perchlorate Čerenkov detector can be built we investigated the optical properties of the solution. Studies using a spectrophotometer revealed that there are no obvious absorption lines between wavelengths of 250 to 600 nm. We constructed a special apparatus to measure the attenuation of 460 nm light in lead perchlorate solution. Initial measurements yielded attenuation lengths of less than half a meter, which is insufficient to build a large lead perchlorate Čerenkov neutrino detector. Diluting the solution away from saturation while heating and stirring improved the attenuation length somewhat. To remove scattering particles we filtered the solution using a series of filter pore sizes from 5.0 microns down to 0.2 microns. This resulted in an attenuation length of just over 4 meters. This improvement is sufficient for a reasonably sized Čerenkov detector and suggests that additional filtering might further increase the attenuation length. The work we have done with lead perchlorate solution has shown that it is a viable medium for a Čerenkov neutrino detector. The OMNIS collaboration, which is planning a next-generation lead-based neutrino detector, has chosen to incorporate lead perchlorate into


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    UW CENPA Annual Report 2001-2002 37 their detector proposal. Thus, although no further work with lead perchlorate solution is planned at CENPA, we have made a valuable contribution to the field of neutrino physics. A report of our work has been submitted to Nuclear Instruments and Methods and can be found at xxx.lanl.gov under nucl-ex/0202013.


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    38 Double Beta Decay 2.16 Heat capacity and thermal conductivity of molybdenum at millikelvin temperatures for a molybdenum bolometer P. J. Doe, S. R. Elliott, R. Hazama, R. G. H. Robertson, O. E. Vilches,∗ J. F. Wilkerson and D. I. Will Molybdenum is in principle a good candidate for the high energy resolution bolometer1 and this has been investigated further at UW. Recently in the context of an ultracryogenic resonant-mass gravitational wave detector, in the temperature range of 0.2 K to 1 K, the spe- cific heat of commercial 99.5 % purity annealed polycrystalline molybdenum are reported.2 We calculated the expected energy resolution of molybdenum at millikelvin temperature by using these recent experimental data of heat capacity. We extrapolated the specific heat below 0.2 K and this is fairly good agreement with the data in Fig. 2.16-1. For supercon- ductors at low enough temperature the relevant heat capacity is the Debye(lattice) term, not the electronic term. Thus, this is also shown as dotted line in Fig. 2.16-1. The estimated specific heat of molybdenum at 5 mK by these two extrapolation is 1.2×10−1 µJ/g/K and 3.0×10−8 µJ/g/K for the electronic plus lattice term and the only lattice term, respectively. Now we can compare our estimation with the calculation of E. Fiorini and T. O. Niinikoski3 at 5 mK. The estimated energy resolution at 5 mK for a mass of 1 kg is 4EFWHM = 4.5 - 8900 eV, while their estimated energy resolution for a mass of 1 kg at 5 mK is 47 eV. There are three available data for the thermal conductivity of molybdenum. The temper- ature dependence of the thermal conductivity of single crystal molybdenum in the supercon- ducting state and in the normal state is measured4 in the temperature range of 0.1 - 300 K. All data are summarized and compared in Fig. 2.16-2. You can see these are not consistent with each other and it shows roughly two order difference. In order to get the expected time constant at millikelvin temperature(T ), we extrapolated the thermal conductivity(k) data using the assumption of a linear dependence of k on T and which give us a good agreement. (The linear coefficient 1/A is associated with electron-defect.) This is justified by the ex- pectation that the effects of electron-phonon scattering to contribute less than 2 % to the normal metal thermal conductivity below 1 K. It is noted that the typical steeper falls can be seen with decreasing temperature below 0.5 K and 0.9 K for the full circles and the squares, respectively. The thermal conductivity exhibits this behavior in the superconducting state for the normal metal. Each extrapolation gives us the thermal conductivity at 5 mK and the results are summarized in Table. 1 with the time constant τ , which is obtained by the internal thermal resistance R and the heat capacity c. ∗ Department of Physics, University of Washington, Seattle, WA 98195. 1 CENPA Annual Report, University of Washington (2001) p. 46. 2 W. Duffy, Jr., J. Appl. Phys., 81, 6675 (1997). 3 E. Fiorini and T. O. Niinikoski, Nucl. Instr. Methods 224, 83 (1984). 4 A. Waleh and N. H. Zebouni, Phys. Rev. B 4, 2977 (1971).


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    UW CENPA Annual Report 2001-2002 39 Reference Form Purity Tc Temp. range 1/A κ τ % K K W/cm/K2 W/cm/K sec/g A. C. Mota - - - 4-300 0.135 0.00068 0.0016-3.9e-10 Duffy et al. polycrystalline 99.95 0.50 0.1-1.0 0.56 0.0028 0.00038-9.5e-11 Waleh et al. single crystal - 0.903 0.4-1.0 15.6 0.078 0.000014-3.4e-12 Table 2.16-1. Measurements of 1/A and κ and τ of Mo. The value of τ depends on the heat capacity(c) and crystal’s size (in this case, bar of 1.2 cm diameter and 10 cm long. Cp(j/g/K) -3 10 -4 10 -5 10 -1 10 1 10 T(K) Figure 2.16-1. Specific heat of molybdenum vs temperature in the range of 0.2 K to 10 K. The dashed line is the Debye-Sommerfeld equation, which includes both the lattice (Debye) term and the electronic term. The dotted line is only the second (Debye) term. 10 2 k(W/cm/K) 10 1 -1 10 -2 10 -1 2 10 1 10 10 T(K) Figure 2.16-2. Results referring to the whole set of measurement of the thermal conductivity of molybdenum. Full circles: Duffy Jr. et al., squares: Waleh et al., open circles: A. C. Mota.


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    40 2.17 Cosmogenic backgrounds for MOON P. J. Doe, S. R. Elliott, R. Hazama, R. G. H. Robertson, J. F. Wilkerson and D. I. Will By using the program COSMO,1 which calculates the production of all radionuclides by nucleon-induced reactions in a given target, we surveyed entire cosmogenic production for MOON detector2 on the earth with a reference of Table of Isotopes.3 The variation of spallation, evaporation, fission and peripheral reaction cross sections with nucleon energy, target and product charge and mass numbers, as well as the energy spectrum of cosmic ray nucleons near the Earth’s surface are incorporated in this program. We can categorize the background as a correlated (delayed) one and an accidental one for solar neutrino and double beta decay studies, respectively in Table 2.17-1. As for solar neutrino backgrounds, 90 Sr will be the major cosmogenic backgrounds, but enrichment of 100 Mo is very effective for the reduction of an order of 3. We can apply a chemical purification, too. Considering the raw event rate, 88 Zr and 95 Zr will be much higher and by an accidental coincidence these backgrounds sneak into the energy window of solar neutrino signals. However, these radioactivities are short lived and enrichment of 100 Mo is quite effective about an order of 4 reduction. While, 91 Y is the major cosmogenic backgrounds for neutrinoless double beta decay (0νββ). The accidental sum of two beta-rays from 91 Y is 3088 keV and close to the Q-value of 0νββ (3034 keV). This can be reduced by a chemical purification. Enrichment is also effective. It is noted here the activity of 91 Y will be 10−2 after 1 year of storage at underground. Signal half-life Raw rate Effective T1/2 (/34 ton n Mo − 3 ton100 Mo/day) (/34ton n Mo − 3 ton100 Mo/day) 0ν, 0.05 eV 0.25 0.035 7 Be 0.33 0.12 pp 0.99 0.21 Correlated 90 Sr 28.78 yr 1200 − 6.66 4.3e-4 − 2.4e-6 99 Nb 15 sec 398 − 363 0 Accidental 95 Zr 64.0 day 8.0e+5 − 8500. 0.89 − 1e-4 88 Zr 83.4 day 4.9e+5 − 347. 0.80 − 0 91 Y 58.51 day 1.7e+5 − 201 0.001 − 0 60 Co 5.27 yr 166. − 1.9 0 Table 2.17-1. Expected rates for the major cosmogenic backgrounds for MOON. Prompt event rates of 1 year cosmic radiation on the earth. 1 C. J. Martoff and P. D. Lewin, Comp. Phys. Comm. 72, 96 (1992). 2 R. Hazama et al., AIP Conference Proceedings, Volume 610, page 959-963, ed by E. Norman et al., New York, 2002, International Nuclear Physics Conference (INPC2001), July 2001, Berkeley, CA. 3 8th ed., edited by R. B. Firestone and V. S. Shirley, Wiley, New York, NY 1996.

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