A liquid helium target system for a measurement of parity violation in neutron spin rotation

TL;DR

This paper describes the design and implementation of a liquid helium target system for precise measurement of parity-violating neutron spin rotation, addressing challenges of magnetic isolation and system stability.

Contribution

It introduces a specialized nonmagnetic cryostat and target system enabling sensitive parity violation measurements in helium with minimal magnetic interference.

Findings

Successful construction of a nonmagnetic liquid helium target system

Achieved conditions suitable for detecting spin rotations of order 10^{-6} rad

Demonstrated system stability and magnetic shielding effectiveness

Abstract

A liquid helium target system was designed and built to perform a precision measurement of the parity-violating neutron spin rotation in helium due to the nucleon-nucleon weak interaction. The measurement employed a beam of low energy neutrons that passed through a crossed neutron polarizer--analyzer pair with the liquid helium target system located between them. Changes between the target states generated differences in the beam transmission through the polarizer--analyzer pair. The amount of parity-violating spin rotation was determined from the measured beam transmission asymmetries. The expected parity-violating spin rotation of order $10^{-6}$ rad placed severe constraints on the target design. In particular, isolation of the parity-odd component of the spin rotation from a much larger background rotation caused by magnetic fields required that a nonmagnetic cryostat and target system be supported inside the magnetic shielding, while allowing nonmagnetic motion of liquid helium between separated target chambers. This paper provides a detailed description of the design, function, and performance of the liquid helium target system.