Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session CF: Electroweak Interactions I |
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Sponsoring Units: DNP Chair: Geoff Greene, Oak Ridge National Laboratory Room: Gaylord Opryland Hermitage C |
Friday, October 27, 2006 9:00AM - 9:12AM |
CF.00001: An Experiment for a Precision Measurement of the Radiative Decay Spectrum of the Neutron R.L. Cooper, T.E. Chupp, K.J. Coakley, M.S. Dewey, T.R. Gentile, H.P. Mumm, J.S. Nico, A.K. Thompson, B.M. Fisher, I. Kremsky, F.E. Wietfeldt, E.J. Beise, K.G. Kiriluk, J. Byrne We have recently observed the radiative decay mode of the free neutron, in which a photon accompanies the usual beta decay products. Monte Carlo methods were used in the analysis of this observation, and these are being applied to optimize the apparatus for a precision measurement of the photon spectrum. The goal is to substantially increase the number of detected radiative decay events while better understanding the systematic effects. Increased statistical sensitivity is expected with a 12-element scintillation detector that is currently being constructed and modeled. These 12 independent channels for photon detection will allow a more thorough examination of our sources of background. Monte Carlo methods address subtle design issues regarding the charged particle detector and neutron transport. Direct photon detection with an avalanche photodiode as a potential photon detector will also be discussed. [Preview Abstract] |
Friday, October 27, 2006 9:12AM - 9:24AM |
CF.00002: Ft value for the superallowed decay of $^{32}$Ar A. Garcia, M. Bhattacharya, D. Melconian, E.G. Adelberger, H.E. Swanson, S. Triambak, A. Komives, T. Glasmacher, P.F. Mantica, A. Oros, J.L. Priscindaro, M. Steiner, B.A. Brown, M.W. Cooper, S.L. Tabor, M. Wiedeking, V. Guimaraes $^{32}$Ar produced by fragmentation at the NSCL at MSU was implanted into a Si detector. Beta-delayed protons and gammas were measured with the help of additional Si detectors and Ge detectors. We were able to determine the superallowed branch with a precision of $\approx 0.5$\%. With this information plus the half-life and endpoint from previous work we extract the $f t$ value which allows for a comparison of predicted versus measured isospin-breaking correction. Results will be presented. [Preview Abstract] |
Friday, October 27, 2006 9:24AM - 9:36AM |
CF.00003: Measurement of the $\gamma$ branches in the $\beta^+$ decay of $^{\mathrm 32}$Cl Dan Melconian, C. Bordeanu, A. Garc\'ia, J.C. Hardy, V.E. Iacob, N. Nica, H.I. Park, G. Tabacaru, L. Trache, S. Triambak, R.E. Tribble, Y. Zhai As discussed in the previous talk (A.~Garc\'ia, et al.), one of the dominant systematic uncertainties in the measurement of the $ft$ value of $^{\mathrm 32}$Ar arises from the uncertainty in the HPGe $\gamma$ efficiency. The $\gamma$s emitted in the decay of $^{\mathrm 32}$Cl cover the same range of energies and, since $\approx10\%$ of the time it is a daughter product of $^{\mathrm 32}$Ar, a precise knowledge of these branches will provide us with an \emph{in situ} calibration of our HPGe detectors. This talk will describe the experiment and results of the measurement performed at the Texas A \& M Cylcotron Institute. We have identified a number of new branches and determined the $\gamma$ yields to $<0.3\%$, generally an order of magnitude improvement from previous results. Implications for the $^{\mathrm 32}$Ar experiment will be discussed. [Preview Abstract] |
Friday, October 27, 2006 9:36AM - 9:48AM |
CF.00004: Precise Half Life Measurement of $^{10}$C V.E. Iacob, V. Golovko, J. Goodwin, J.C. Hardy, N. Nica, H.I. Park, L. Trache, R.E. Tribble We have measured the half-life of $^{10}$C as part of our program to test the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix \textit{via} $0^+\to 0^+$superallowed $\beta $ transitions. The $^{10}$C half-life has been measured twice before, with precisions of 0.10{\%} and 0.08{\%}. With our current techniques, we expect to be able to improve that precision by a factor of two. To obtain $^{10}$C, we used a $^{11}$B primary beam at 23$A~$MeV to bombard a cryogenic hydrogen target. From the reaction products, a high purity $^{10}$C beam of 18.5$A~$MeV was produced by the MARS spectrograph. The beam was then extracted in air, passed through a 0.3-mm-thick BC-404 plastic scintillator and a set of Al degraders, which had been adjusted so that the $^{10}$C nuclei stopped in the center of the 76-$\mu $m-thick aluminized-mylar tape of our fast tape-transport system. We collected $^{10}$C nuclei for 10, 15 or 20~s; then the beam was switched off and the activity was moved in less than 0.2~s to the center of a 4$\pi $ proportional counter, located in a well shielded region. The observed decays were then multi-scaled over a 400~s time span. To ensure an unbiased result, we split the experiment into several runs, each differing from the others in its discriminator threshold, detector bias or dominant dead-time setting. The analysis of these separate runs showed no systematic bias with these parameters. Our preliminary result is t$_{1/2}(^{10}$C) = 19.313(10)~s, a result with 0.05{\%} precision. [Preview Abstract] |
Friday, October 27, 2006 9:48AM - 10:00AM |
CF.00005: New measurement of the parity-violating asymmetry $A_\gamma$ in neutron-proton capture. Rob Mahurin The NPDGamma experiment aims to measure the parity-violating correlation $A_\gamma$ between the neutron spin and photon direction in the capture of polarized cold neutrons on hydrogen, to a precision $5\times 10^{-9}$, 10\% of the expected value. This asymmetry measures the neutral weak hadronic coupling without the complicating effects of nuclear structure. In summer 2006 the collaboration will have commissioned its liquid parahydrogen target at the Los Alamos Neutron Science Center. I will present a preliminary analysis of the hydrogen data. [Preview Abstract] |
Friday, October 27, 2006 10:00AM - 10:12AM |
CF.00006: Parity Violating Gamma-Ray Asymmetries in Compound Nuclei from Polarized Cold Neutron Capture in the NPDGamma Experiment Michael Gericke In an effort to measure the strength of the neutral weak hadronic N-N coupling the NPDGamma collaboration has completed constructing and commissioning an experiment to measure the parity violating up-down asymmetry in the angular distribution of gamma rays with respect to the neutron spin direction in the reaction n + p $\rightarrow$ d + gamma. The asymmetry has a predicted size of 5e-8 and we will measure it to 10\%. The small size of the asymmetry requires the precise determination of effects from neutron capture on all other materials found in the experiment. We introduce a statistical approach to estimate the expected root mean square (RMS) of the asymmetry in the integrated gamma spectrum from the capture of cold polarized neutrons on various medium A targets. Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states and it is important to verify that this effect will not produce a large background asymmetry in the hydrogen signal. [Preview Abstract] |
Friday, October 27, 2006 10:12AM - 10:24AM |
CF.00007: Performance of Current-Mode Ion Chambers as Beam Monitors in a Pulsed Cold Neutron Beam for the NPDGamma experiment R. Chad Gillis The NPDGamma collaboration has built and commissioned an apparatus to measure the parity-violating gamma asymmetry A in the low energy np capture process $n+p\to d+ \gamma$. The asymmetry in question is a ~ $10^{-8}$ correlation between the spin of the incident (polarized) neutron and the outgoing 2.2 MeV gamma ray. A set of purpose-built, 3He-filled ionization chambers read out in current mode is used to monitor the incident neutron flux, the beam polarization, and the transmission of the liquid para-hydrogen target during the NPDGamma measurements. As will be described in the talk, these beam monitors are simple, reliable, low-noise detectors that have performed excellently for NPDGamma. We have verified that the beam monitor signals can be interpreted to reproduce the known time-of-flight dependence of beam flux from the LANSCE pulsed cold neutron source, and that the neutron beam polarization can be measured at the 2\% level from direct measurements of the transmission of the beam through the beam polarizer. [Preview Abstract] |
Friday, October 27, 2006 10:24AM - 10:36AM |
CF.00008: Overview of the NPDGamma Experiment J. David Bowman The NPDGamma Experiment aims to measure the directional asymmetry of the gamma ray direction and the neutron spin when polarized neutrons capture on protons. The asymmetry results from the weak parity-violating hadronic interaction. The asymmetry is expected to be small $\sim$ 10$^{-7}$, however the measured asymmetry can be expressed in terms of the coupling strengths of the meson-exchange model of the hadronic weak interaction. The measured asymmetry is strongly dominated by pion exchange and the measurement will determine the pion coupling, $f$. The first phase of the experiment is being tested at Flight Path 12 at the Los Alamos Neutron Scattering Center. The goal of this phase of the experiment is to measure the asymmetry with an accuracy of 10$^{-7}$. After the first phase is complete, the experiment will be moved to the Fundamental neutron Physics Beam at the Spallation Neutron Source and measure the asymmetry with a combined statistical and systematic uncertainty. All components of the experiment, beam, beam monitors, $^{3}$He spin-filter neutron polarizer, guide field, radio-frequency spin rotator, liquid para-hydrogen target, and cesium ioidide gamma detector have been tested and installed. I will give an overview describing the functions of these components, their design goals, and their expected and measured performance. [Preview Abstract] |
Friday, October 27, 2006 10:36AM - 10:48AM |
CF.00009: $^{3}$He polarizer performance in the NPDGamma experiment Monisha Sharma The ability to polarize neutrons in a broad energy range and measure the neutron polarization with absolute accuracy using a $^{3}$He polarizer has opened up opportunities in Nuclear and other branches of Physics. $^{3}$He polarizers are based on the transmission of neutrons through the polarized $^{3}$He gas which has a strong spin dependent absorption of neutrons. A $^{3}$He polarizer is currently in use in the NPDGamma experiment on FP12 at LANSCE. In a $^{3}$He polarizer, $^{3}$He polarization is produced by the Rb spin exchange method. With the previous cell, BooBoo, used in the experiment maximum $^{3}$He polarization of 57{\%} was obtained and the neutron polarization of 80{\%} was obtained at 5 {\AA}. For the final run of the experiment a new cell, Pebbles, which is considered to be a better cell as compared to BooBoo will be used. The set up for the polarizer has been improved to obtain a better polarization. Improvements include greater laser power, achieved by combining two fiber-coupled laser diodes into a single fiber and a new oven for heating the cell, which will allow more stable and higher temperatures. In this work, we will present the behavior of the polarizer and the neutron polarization in the NPDGamma experiment. [Preview Abstract] |
Friday, October 27, 2006 10:48AM - 11:00AM |
CF.00010: Determination of the ortho/para ratio in the LH$_{2}$ target for the NPDGamma experiment Libertad Barr\'on-Palos In the NPDGamma experiment to measure the parity violating asymmetry $A_{\gamma}$ in the distribution of the gamma rays emitted in the capture of polarized neutrons by protons ($\vec{n} + p \rightarrow d + \gamma$), an important issue is to ensure that the neutrons retain their polarization as they travel into the hydrogen target. For that purpose, a 16-liter LH$_{2}$ vessel, held at 17-18 K and 15 psi pressure, is used as the target. Under these conditions, the concentration of para-hydrogen is 99.97\%, so that the effect of polarization loss by the incoherent scattering of neutrons in ortho-hydrogen is reduced. Due to the large difference in the scattering cross-section for low energy neutrons in both hydrogen species, changes in the ortho/para ratio can be monitored through the study of the transmitted neutron flux. In this work, we present a model for the transmission of neutrons through the apparatus and hydrogen target, as well as the effect of different ortho/para ratios in the data of the 2006 NPDGamma commissioning run. [Preview Abstract] |
Friday, October 27, 2006 11:00AM - 11:12AM |
CF.00011: Evaluation of Systematic Effects for the NPDGamma Experiment William M. Snow The NPDGamma experiment proposes to search for the parity-odd correlation between the neutron polarization and the direction of gamma emission in polarized neutron-proton capture with a sensitivity at the level of $10^{-8}$ for the amplitude of the asymmetry. An extensive set of systematic effects could contaminate the asymmetry if not properly suppressed. In the course of several measurements conducted on FP12 at LANSCE, the NPDGamma collaboration has placed upper bounds on some systematic effects, has demonstrated methods of discovering other systematic effects in a time short compared to the running time required to achieve the statistical accuracy goal, and has verified that yet other classes of possible systematic effects are adequately suppressed by the design and properties of the apparatus. In this talk we will present an extensive list of possible systematic effects and show our current upper bounds on their size. [Preview Abstract] |
Friday, October 27, 2006 11:12AM - 11:24AM |
CF.00012: The Liquid H2 target for NPDGamma Jiawei Mei The NPDGamma experiment requires a liquid hydrogen target. The target must possess a large volume (17 liters) to produce n-p capture events, be bubble-free to reduce noise in the current-mode gamma array, exist as parahydrogen at 17K to suppress neutron depolarization, be constructed of low Z nonmagnetic materials to pass 2.2 MeV gammas and maintain magnetic field uniformity, and produce negligible parity violation from capture gammas other than hydrogen. It also must incorporate safety features such as triple-confinement of the hydrogen, a relief path which can respond to all credible accident scenarios, and various warning and alarm systems. We will describe the design and performance of the NPDGamma hydrogen target system. [Preview Abstract] |
Friday, October 27, 2006 11:24AM - 11:36AM |
CF.00013: The NPDGamma Motion System for Detector Array Alignment Christopher Crawford One of the major systematic uncertainties of the NPDGamma experiment is the Mott-Schwinger effect (the electromagnetic spin-orbit interaction between the neutron magnetic moment and the nuclear charge). It is a parity conserving asymmetry of the same order of magnitude as the parity violating asymmetry being measured. The two asymmetries can be separated since they are 90 degrees out of phase in the azimuthal angle with respect to the neutron beam. A method of measuring the effective detector alignment to the required precision for this separation will discussed along with prelimary results. [Preview Abstract] |
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