Bulletin of the American Physical Society
APS April Meeting 2013
Volume 58, Number 4
Saturday–Tuesday, April 13–16, 2013; Denver, Colorado
Session H9: Neutron Physics |
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Sponsoring Units: DNP Chair: Nadia Fomin, Los Alamos National Laboratory Room: Governor's Square 11 |
Sunday, April 14, 2013 10:45AM - 10:57AM |
H9.00001: Overview and status update on the UCNtau experiment Chris Cude Recent measurements of the free neutron beta-decay lifetime using trapped Ultra-Cold Neutrons (UCN) have yielded results with high precision ($\sim $ 1 s), but with significantly varying central values. To resolve this discrepancy, we've initiated an effort to measure the beta-decay lifetime using UCN in a magneto-gravitational trap at the Los Alamos Neutron Science Center. A permanent magnet Halbach array traps polarized neutrons, eliminating material interactions which can reduce the UCN storage time. The experiment will also employ a vanadium activation measurement using a beta-gamma coincidence technique to count the surviving UCN in the trap independently of neutron phase space distribution. We will present an overview of the experiment and a status update. [Preview Abstract] |
Sunday, April 14, 2013 10:57AM - 11:09AM |
H9.00002: Current Status of the NPDGamma Experiment Mark McCrea The NPDGamma experiment aims to measure the parity violating asymmetry in the direction of gamma ray emission from polarized cold neutrons capturing on an unpolarized proton target. The goal is to measure this asymmetry with an accuracy of $10^{-8}$ to allow comparison to theoretical predictions of the strength of the hadronic weak interaction between nucleons. A liquid para-hydrogen proton target is used to capture the polarized neutron beam. The gamma ray emission direction is measured using an array of 48 CsI(Th) detectors with a $3\pi$ acceptance angle. The neutron spin direction is reversed in a repeating eight step sequence chosen to control for time dependent systematic effects during data taking. NPDGamma has completed commissioning and is currently taking hydrogen data in the Fundamental Neutron Physics Beamline at the Spallation Neutron Source. [Preview Abstract] |
Sunday, April 14, 2013 11:09AM - 11:21AM |
H9.00003: Polarized Cold Neutron Capture in $^{27}$Al Septimiu Balascuta The NPDGamma Experiment at the Spallation Neutron Source at ORNL is measuring the parity-odd correlation between the neutron spin and the direction of the emitted photon in the capture of cold neutrons on a 16-liter liquid parahydrogen target. The goal is to determine the strength of the weak nucleon-nucleon interaction. One of the main background contributions comes from the gamma rays produced by neutrons captured in the Al walls of the target vessel. To quantify this effect a commissioning experiment measured the parity-odd and parity-even asymmetries in the angular distribution of the gamma rays from the capture of polarized cold neutrons in a solid Al target. A status of the analysis of this experiment will be presented. [Preview Abstract] |
Sunday, April 14, 2013 11:21AM - 11:33AM |
H9.00004: Status of Neutron Beta Decay Asymmetry Studies from the UCNA Experiment David Phillips II The UCNA experiment measures the neutron $\beta$ decay asymmetry parameter $A(E)$ using bottled polarized ultracold neutrons (UCN). UCN are produced from a pulsed spallation solid deuterium source coupled to the 800 MeV proton beam at LANSCE. The UCN spin states are selected via a 7 T polarizing field and an adiabatic fast passage spin flipper. The polarized UCN are then transported to a 1 T 2$\times$2$\pi$ spectrometer where the emitted electrons are measured. In the Standard Model, the leading order value of $A(E)$, $A_0$, is a function of the axial-vector to vector coupling ratio $\lambda \equiv g_{\rm A}/g_{\rm V}$, providing complementary data to the physics probed by measurements of the neutron lifetime $\tau_{\rm n}$. When taken together with $\tau_{\rm n}$, measurements of the beta decay asymmetry permit a nuclear structure independent determination of the CKM matrix element $V_{\rm ud}$. This talk presents an overview of the UCNA experiment, the status of the analysis of our 2011 dataset, the work performed in 2012 and the path forward. [Preview Abstract] |
Sunday, April 14, 2013 11:33AM - 11:45AM |
H9.00005: The UCNB experiment: progress toward the measurement of electron-proton coincidences from the beta decay of polarized, ultracold neutrons Sky Sjue The UCNB experiment will measure the neutrino asymmetry parameter, B, from the decay of polarized neutrons at the Los Alamos ultracold neutron (UCN) source. The protons will be accelerated above the noise threshold by applying -30 kV bias simultaneously to the detectors and data acquisition. Source data has been taken with Si detectors and data acquisition floated to -14 kV. Fabrication is in progress on a full detector assembly designed to instrument 128 pixels of a Si detector with 11.5-cm active diameter, floated to -30 kV, with cryogenic cooling to maintain a temperature of 100 K at the detector and front end electronics. We will present an overview on the status of implementation of the full system in the superconducting solenoid magnet spectrometer at the Los Alamos UCN source. [Preview Abstract] |
Sunday, April 14, 2013 11:45AM - 11:57AM |
H9.00006: aCORN: A precision measurement of the electron-antineutrino correlation in neutron decay Fred Wietfeldt The aCORN experiment uses a unique method to determine the electron-antineutrino correlation (``a" coefficient) in neutron decay from an asymmetry between fast and slow recoil proton groups. High precision proton spectroscopy, which has limited previous experiments to the 5\% uncertainty level, is not needed. aCORN is now running on the NG-6 neutron physics end position at the NIST Center for Neutron Research (NCNR). The goal of the current run is 2\%. In late 2013 aCORN will be moved to the new high-flux NG-C end position at the NCNR for a second run with a goal of 0.5\%. A brief status report will be presented. [Preview Abstract] |
Sunday, April 14, 2013 11:57AM - 12:09PM |
H9.00007: Limits on the Fierz Interference term for the free neutron Kevin Hickerson We report on efforts to extract limits on the Fierz interference term for the free neutron, $b_n$, using ultracold neutrons (UCN). The UCNA experiment at LANSCE, designed to measure the neutron beta asymmetry, uses a $4\pi$ spectrometer to measure the electron energy dependence of neutron beta decay and the electron asymmetry in polarized beta decay. The $b_n$ parameter can skew the peak of the beta decay spectrum and modifies the energy dependence of the asymmetry. Thus using the observed energy dependence of the decay distribution and the asymmetry, we can place limits on $b_n$ and test consistency with the Standard Model in which $b_n$ is negligibly small. [Preview Abstract] |
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