High-Efficiency Resonant RF Spin Rotator with Broad Phase Space Acceptance for Pulsed Polarized Cold Neutron Beams

TL;DR

This paper presents a highly efficient RF resonant spin rotator for pulsed cold neutron beams that preserves beam phase space and achieves near-perfect polarization reversal over a broad energy range, enhancing experimental capabilities.

Contribution

The paper introduces a novel RF resonant spin rotator with 98% efficiency that maintains neutron beam phase space and broad energy acceptance, improving polarization control in neutron experiments.

Findings

Achieved 98% spin-flip efficiency on resonance.

Operates effectively over neutron energies from 3.3 to 18.4 meV.

Preserves neutron beam phase space with high precision.

Abstract

We have developed a radio-frequency resonant spin rotator to reverse the neutron polarization in a 9.5 cm x 9.5 cm pulsed cold neutron beam with high efficiency over a broad cold neutron energy range. The effect of the spin reversal by the rotator on the neutron beam phase space is compared qualitatively to RF neutron spin flippers based on adiabatic fast passage. The spin rotator does not change the kinetic energy of the neutrons and leaves the neutron beam phase space unchanged to high precision. We discuss the design of the spin rotator and describe two types of transmission-based neutron spin-flip efficiency measurements where the neutron beam was both polarized and analyzed by optically-polarized 3He neutron spin filters. The efficiency of the spin rotator was measured to be 98.0+/-0.8% on resonance for neutron energies from 3.3 to 18.4 meV over the full phase space of the beam. As an example of the application of this device to an experiment we describe the integration of the RF spin rotator into an apparatus to search for the small parity-violating asymmetry A_gamma in polarized cold neutron capture on para-hydrogen by the NPDGamma collaboration at LANSCE.