Development of a Parity-Violating Neutron-Capture Experiment (NDT) for the Spallation Neutron Source (SNS): Measurement of Neutron Depolarization in D20

Grants and Contracts Details


Symmetry plays a key role in understanding our physical universe. Of particular interest is the symmetry of reflection in a mirror, called parity. Almost every physical process in nature is indistinguishable from its mirror image counterpart. However, the weak nuclear interaction looks fundamentally different in the mirror. This violation of parity, fir st discovered in 1957, has been verified in high-energy scattering experiments worldwide, and can be explained by the standard model of particle physics. In contrast, characterization of the hadronic weak interaction (HWI), the weak nuclear force between protons and neutrons, is relatively obscure due to the complicated internal structure of the proton and neutron. The difficulty is compounded because the HWI is masked by the strong nuclear force, which is 1 ~7 times larger in magnitude. Statistics on the order of 1017 events with unprecedented control of systematic uncertainties are essential for sensitivity to the parity-violating effects of the HWT. However, there are a number of motivations for investigating the FIWI: a) The HWI is a unique probe of correlations between quarks in the proton and neutron. b) Characterization of the HWI is needed for parity-violation tests of models of atomic nuclei. c) This data is needed for the analysis of other nuclear and high-energy experiments. Hadronic structure is a priority of the nuclear physics agencies, emphasized in the Long Range Plan [1]. The purpose of this proposal is to perform a pilot study to demonstrate feasibility of running a neutron capture experiment, NDTy, at the Spallation Neutron Source (SNS), at Oak Ridge National Laboratory. The recently completed SNS is the premier source of pulsed neutrons in the world. It is ideal for studying the HWI by virtue of its intense neutron beam with low background noise and pulsed time structure, useful for the reduction of systematic errors. We have proposed several complementary experiments at the SNS Fundamental Neutron Physics Beamline (FnPB) to measure parity-violation in independent reactions and isolate the six independent parameters of the HV,TI. One such experiment in which I am collaborating, NPD7, will run in 2009 as the first experiment at the FnPB. I am co-spokesperson of a follow-up experiment NIDTy [2}, the subject of this proposal, conditionally approved to run at the FnPB. The NDT7 experiment will measure the correlation between the direction of the emitted y-ray and the neutron spin axis in the nuclear reaction (n + d -, t + y). It will use a similar setup as the NPD7 experiment (n + p -> d + y), but with a solid deuterium (D2O) target instead of liquid hydrogen (H2). This experiment will be more difficult to perform than NPDy because of the low neutron capture rate and neutron depolarization in deuterium. In order to prove the feasibility of this experiment, we must: a) Demonstrate the retention of neutron polarization in the deuterium target. b) Show that background noise can be reasonably suppressed. We propose to demonstrate these requirements during Summer 2008 at the Los Alamos Neutron Science Center (LANSCE), at Los Alamos National Laboratory (LANL). We will measure the average depolarization of neutrons in liquid D20 using Compton polarimetry of y-rays emitted from the same reaction. The equipment we are requesting will allow us to carry out this pilot study and gain full approval to run the experiment at the SNS. The experimental setup for the demonstration run is shown in Figure 1. We will use equipment from a precision polarimetry experiment currently running at LANSCE, including: a 3He neutron spin polarizer, a radio-frequency spin flipper, a 3He neutron spin analyzer, three beam monitors, and a magnetic field. The D2O target is available, and two Compton polarimeters have been constructed at DePauw University, except for one of the high purity germanium (FIPUe) detectors, highlighted Figure 1. We request funds to help acquire this second HPGe detector ($5990) plus the associated cryostat ($1886) and electronics ($2800), totaling $10,676, in order to complete the experimental setup. Matching funds will be contributed by the Department of Physics, and by university startup funds.
Effective start/end date7/1/085/31/09


  • Oak Ridge Associated Universities: $5,000.00


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