Bulletin of the American Physical Society
2013 Fall Meeting of the APS Division of Nuclear Physics
Volume 58, Number 13
Wednesday–Saturday, October 23–26, 2013; Newport News, Virginia
Session DE: Neutrons I |
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Chair: Mike Huber, National Institute of Standards and Technology Room: Grand Ballroom V |
Thursday, October 24, 2013 10:30AM - 10:42AM |
DE.00001: Systematic Effects of UCN Trapping in the UCN$\tau$ Experiment Chen-Yu Liu, Daniel Salvat, Pete Walstrom There is a recent disagreement on the mean lifetime of neutron $\beta$-decay, as determined by the most precise in-beam measurement and the most precise trap-based measurements. While the precision of the trap technique has reached 1 second and the planned NIST beam experiment will soon catch up to the same level, the discrepancy remains as large as $\sim$ 8 seconds. The immediate goal for the UCN$\tau$ experiment is to understand the systematic effects of trap-based experiments, and to investigate sources of neutron loss beyond typical material interactions. We present simulations of UCN trapping in our magneto-gravitational trap, with special attention to the population of quasi-bound neutrons. We also present results of magnetic field mapping and discuss the implications of defects in the trap-as-built. [Preview Abstract] |
Thursday, October 24, 2013 10:42AM - 10:54AM |
DE.00002: Backgrounds from Neutron Capture in the NPDGamma Experiment Serpil Kucuker Dogan The NPDGamma experiment, which measures the parity-violating directional gamma asymmetry in neutron-proton capture, completed its second run cycle in May 2013 at the Fundamental Neutron Physics Beamline at the Oak Ridge Spallation Neutron Source. In the experiment an intense polarized low-energy neutron beam interacts with a liquid para-hydrogen target. Gamma rays from the capture reaction are detected by 48 CsI(Tl) current mode detectors with the 3$^{\circ}$ acceptance angle. The goal of the experiment is to measure the asymmetry with precision of $1 \times 10^{-8}$. The polarized neutrons also interact with other materials in beam, such as the beam windows and the walls of the target vessel producing a background to the signal that dilutes the PV gamma asymmetry. These materials could, in principle, have their own PV asymmetries. The Aluminum contribution in the background is the most significant, contributing about 20\% of the total signal. We have studied the backgrounds and identified their sources and the strengths. I will discuss the methods to determine the backgrounds and their effect on NPDGamma. [Preview Abstract] |
Thursday, October 24, 2013 10:54AM - 11:06AM |
DE.00003: A Statistically Optimal Algorithm for Filtering the Hydrogen Asymmetry data in the $np \rightarrow d\gamma$ Experiment Jason Fry The NPDGamma Experiment seeks to measure the parity violating gamma asymmetry from polarized neutrons captured on protons at the Spallation Neutron Source at ORNL. This parity violating asymmetry, $A_{\gamma}$, is directly related to the $\Delta$I = 1 contribution of the hadronic weak interaction and will be measured to a precision of 1$\times$10$^{-8}$. Liquid para-hydrogen production data has been taken since May 2012. As one of the parallel analyses of NPDGamma, we present a statistically optimal algorithm using a least squares fit to the 60Hz neutron pulses in order to filter, and make high level cuts on, production data. We show that this algorithm is sensitive to neutron beam intensity fluctuations, dropped pulses, background effects, and wraparound neutrons, among others. [Preview Abstract] |
Thursday, October 24, 2013 11:06AM - 11:18AM |
DE.00004: Neutron Polarimetry with Polarized 3He for the NPDGamma Experiment Matthew Musgrave The goal of the NPDGamma experiment is to measure the parity violating directional $\gamma$-ray asymmetry in the capture of polarized neutrons on protons. The neutrons are polarized with a multichannel super mirror polarizer, and the neutron spins can be flipped with an RF spin flipper (RFSF) to measure the $\gamma$-ray asymmetry for both neutron spin states. The neutron polarization and the RFSF efficiency are multiplicative corrections to the measured $\gamma$-ray asymmetry and are measured to a precision of a few percent. The large spin dependent capture cross section of polarized $^{3}$He is used to determine the neutron polarization and the RFSF efficiency. Transmission measurements are taken through a polarized glass $^{3}$He cell at various locations in the neutron beam, and the beam average neutron polarization and RFSF efficiency are determined from a weighted average of these transmission measurements. The results are compared to simulations. [Preview Abstract] |
Thursday, October 24, 2013 11:18AM - 11:30AM |
DE.00005: The Orthohydrogen Fraction of the NPDGamma Liquid Hydrogen Target Chad Gillis The NPDGamma Experiment at the SNS is measuring the parity-violating correlation $A_\gamma$ between neutron spin and gamma momentum in the reaction $\overrightarrow{n} + p\rightarrow d + \gamma$ with a projected statistically limited precision of a part per $10^{8}$. In the experiment, a polarized cold neutron beam captures on a 16.5~liter cryogenic liquid parahydrogen target. Since orthohydrogen has a large neutron spin flip scattering cross section, with a part per $10^4$ of orthohydrogen expected to produce a relative change in the beam polarization of close to a percent, it is essential for our experiment that the ortho fraction be kept low. An ortho-para conversion loop was incorporated into the target so that the ortho fraction would be held close to its thermodynamic equilibrium level of a few parts per $10^{4}$, and it is important that deviations from thermodynamic equilibrium be well understood. Theoretical and empirical arguments for placing a limit on the orthohydrogen fraction of our liquid hydrogen target will be discussed. [Preview Abstract] |
Thursday, October 24, 2013 11:30AM - 11:42AM |
DE.00006: Enhanced UCN Polarimetry in the UCNA Experiment Eric Dees The UCNA experiment uses the decay of trapped ultracold neutrons (UCN) to measure the angular correlation A between the emitted electron's momentum and the direction of the neutron's spin. Therefore, a key systematic uncertainty for determination of the A parameter is the average polarization of the decaying UCN. The experiment is designed to couple ``in situ'' measurements of the equilibrium depolarized fraction to each of the roughly one hour beta-asymmetry data-taking cycles. The recent addition of a neutron reflecting shutter has improved the statistical precision with which the depolarized fraction can be measured, and eliminated the largest systematic correction required for the method used prior to 2011. Previously, this background of residual UCN was a leading source of uncertainty in quantifying the depolarized fraction inside the trap, but by increasing the signal to background ratio, our sensitivity is enhanced by a factor of four from previous years. Methods for maximizing the utility of the shutter, as well as characterizing residual systematics, will be discussed in relation to polarimetry data collected during the 2011/2012 UCNA run cycle. [Preview Abstract] |
Thursday, October 24, 2013 11:42AM - 11:54AM |
DE.00007: Simulation of Cold Neutron Experiments using GEANT4 Emil Frlez, Joshua Hall, Melinda Root, Stefan Baessler, Dinko Pocanic We review the available GEANT4 physics processes for the cold neutrons in the energy range 1--100 meV. We consider the cases of the neutron beam interacting with (i) para- and ortho- polarized liquid hydrogen, (ii) Aluminum, and (iii) carbon tetrachloride (CCl$_4$) targets. Scattering, thermal and absorption cross sections used by GEANT4 and MCNP6 libraries are compared with the National Nuclear Data Center (NNDC) compilation. NPDGamma detector simulation is presented as an example of the implementation of the resulting GEANT4 code. [Preview Abstract] |
Thursday, October 24, 2013 11:54AM - 12:06PM |
DE.00008: Simulation of UCN transport in the UCN$\tau$ Experiment Evan Adamek, Chen-Yu Liu, Daniel Salvat, Nathan Callahan The UCN$\tau$ experiment aims to measure the neutron $\beta$-decay lifetime to 1 s total uncertainty and beyond by trapping ultracold neutrons (UCN) in a magneto-gravitational trap, in which UCN undergo no material interactions with the walls of the trap. To investigate UCN transport in the experiment, we have built Monte-Carlo simulations of the full-scale experiment using GEANT4. We have modeled the delivery of UCN to the trap with a highly accurate transport geometry. The model is bench-marked against the experimental data collected in the early 2013 run. The simulation is used to compare proposed geometry upgrades to enhance the efficiency of UCN delivery (planned for the late 2013 run). In addition, work is underway to expand the scope of simulation to include $\beta$ and $\gamma$ detection, with the goal of modeling our in-situ UCN detector using the technique of neutron activation on a large-surface Vanadium foil. Here we present the results of this simulation effort. [Preview Abstract] |
Thursday, October 24, 2013 12:06PM - 12:18PM |
DE.00009: Abstract for Guide coating and evaluation techniques for Ultracold Neutron transport Xinjian Ding UCN are produced by cold neutron flux down-scattering in a solid deuterium source in Area B of Los Alamos National Laboratory and then transported to the experimental decay volume of the UCNA experiment and to other UCN experiments through a sequence of guide tubes. These tubes are coated with diamond-like carbon (DLC) films to maintain UCN polarization and maximize material potential. We will briefly review the UCNA guide system, the requirements for UCNA guides, and the pulsed-laser deposition (PLD) process we use to produce diamond-like carbon (DLC) films. There will be a discussion of some new analysis techniques for both in situ PLD coating and surface structure of the DLC film that are utilized in the UCN experiments. We will present both present and future research and development in UCN guide coating techniques and materials. [Preview Abstract] |
Thursday, October 24, 2013 12:18PM - 12:30PM |
DE.00010: Radioactive gases for calibration of the UCNA Experiment Brittney VornDick Calibration procedures using activated Xe and Ar now play an important role in the UCNA experiment. Both species are frozen into the ultracold neutron (UCN) production volume of the Los Alamos UCN source and activated primarily by the cold neutron flux inside the production volume. Natural Xe with an absorption cross section of roughly 24 barns is activated in the UCN source and then transferred into a LN2-cooled storage volume coupled to the UCNA spectrometer. The $^{135}$Xe $(\frac{3}{2}+)$, with a lifetime of about 9 hours, which has a 912.5keV-endpoint beta decay, is of greatest importance to UCNA. Activated Xe is leaked into the decay volume, providing a uniform source of beta-radiation filling the acceptance of the beta-detectors, which is used to map the position-dependence of the beta detectors. Natural Ar is utilized to provide an absolute calibration of the cold neutron flux. $^{41}$Ar has a half-life of 109 minutes and a strong gamma-ray line at 1294 keV. The gamma activity is measured using an HPGe detector, and, after correcting for the measured line efficiency and resonance production rates, provides the absolute cold neutron flux in the source volume. Details of the calibration procedures and characteristic results will be presented. [Preview Abstract] |
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