Precision Neutron Polarimetry and a Measurement of Parity Violating Asymmetry in N-P Capture.

Abstract

This dissertation describes developments and applications that advance the utility of polarized neutrons in fundamental nuclear physics applications including study of the hadronic weak interaction in nuclei and beta decay. Weak interactions between nucleons have the potential to provide new information on quark-quark correlations in the strongly interacting limits of QCD. Parity violation is a unique signature of the weak interactions and can be used to probe weak interactions amid the much stronger electromagnetic and strong interactions. An experiment which makes use of this property is the NPDGamma experiment, which aims to measure the pion-nucleon coupling constant in $n+prightarrow d+gamma$. This experiment measures the directional asymmetry in gamma-ray emission, $A_gamma$, with respect to the neutron spin. In the first run $A_gamma$ has been found to be (-1.1$pm$2.1)$times$ 10$^{-7}$. The $^3$He polarizer set up used for the NPDGamma experiment was developed in Michigan. We also developed an analysis method for monitoring the neutron polarization. The details of the $^3$He spin filter and the polarization analysis are discussed.

During the polarization analysis of the NPDGamma experiment, $^3$He polarization decay was observed when the high intensity cold neutron beam was incident on alkali-metal-spin-exchanged polarized $^3$He cells. This decay in the $^3$He polarization was attributed to a decrease of alkali-metal polarization, which led us to directly measure the alkali-metal polarization over a range of neutron fluxes at the Los Alamos Neutron Science Center and Institut Laue-Langevin in Grenoble, France. The data revealed a new alkali-metal-spin-relaxation mechanism that appears to scale as the square root of neutron flux. Discussions of the experiments and the analysis are presented.

Motivated by $beta$-decay correlation coefficients measurements, such as the measurement of proton asymmetry in neutron decay we further developed precision polarimetry. A 0.1% measurement of the proton asymmetry requires better than 0.1% measurement of neutron polarization. An experiment was developed to measure the polarization of a pulsed neutron beam to 0.1% using a $^3$He spin filter as analyzer. Details of the experiment and the analysis are presented.