Neutron Polarimetry Using a Polarized 3He Cell for the aCORN Experiment

TL;DR

This paper reports the measurement of neutron polarization in the NG-C beamline at NIST using a polarized 3He cell and AFP-NMR, achieving a polarization upper limit of 4×10⁻⁴ with 90% confidence, relevant for the aCORN beta decay experiment.

Contribution

It introduces a method to measure neutron polarization using a polarized 3He cell and AFP-NMR, providing a new upper limit for neutron polarization in this setup.

Findings

Neutron polarization was constrained to ≤ 4×10⁻⁴ at 90% confidence.

The transmission dependence on 3He polarization was used to determine neutron polarization.

A model including neutron spectrum and 3He polarization losses was fit to transmission data.

Abstract

The neutron polarization of the NG-C beamline at the NIST Center for Neutron Research was measured as part of the aCORN neutron beta decay experiment. Neutron transmission through a polarized 3He spin filter cell was recorded while adiabatic fast passage (AFP) nuclear magnetic resonance (NMR) reversed the polarization direction of the 3He in an eight-step sequence to account for drifts. The dependence of the neutron transmission on the spin filter direction was used to calculate the neutron polarization. The time dependent transmission was fit to a model which included the neutron spectrum, and 3He polarization losses from spin relaxation and AFP-NMR. The polarization of the NG-C beamline was found to be ${\mid}P_\mathrm{n}{\mid} \leq 4\times 10^{-4}$ with 90 % confidence.