High-efficiency resonant rf spin rotator with broad phase space acceptance for pulsed polarized cold neutron beams

P N Seo ,G S Mitchell,H Zhu,S J Freedman,M Sharma,W D Ramsay,S A Page,H Nann,W S Wilburn,R C Gillis,F W Hersman,S Santra,W M Snow,S I Penttilä,T E Chupp,T B Smith,G L Jones,R Mahurin,M Dabaghyan,J D Bowman,B Lauss,L Barrón‐Palos ,M Leuschner ,Michael Gericke ,M C Kandes ,Geoffrey L Greene ,Jia‐Wei Mei ,Chris Crawford ,Thomas R Gentile ,Marian Stamp Dawkins ,A Bacci ,S K Lamoreaux ,Michael G Mason

doi:10.1103/physrevstab.11.084701

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.

Highlights

Reversal of the neutron polarization direction in sensitive nuclear, particle, and condensed matter physics experiments using polarized cold neutron beams is a simple method to control systematic uncertainty
As the continued increase in the brightness of pulsed spallation neutron sources makes polarized neutron measurements possible with a new level of statistical accuracy, there is a need to improve and develop neutron spin flippers to accommodate the demands of these new types of experiments by taking advantage of the pulsed nature of the neutron source where time-offlight measurement is used to determine the neutron energy, to do so over an increased beam phase space, and to control possible sources of systematic uncertainties
It is an active device in the sense that the amplitude of the rf magnetic field is varied in phase with the pulsed neutron source in such a way that the condition for rotating the neutron spins by radians in the rotator is always met for each subsequent neutron velocity class as it reaches the device

Summary

Introduction

Reversal of the neutron polarization direction in sensitive nuclear, particle, and condensed matter physics experiments using polarized cold neutron beams is a simple method to control systematic uncertainty. In this paper we describe the design and detailed characterization of a neutron spin flipper which we term a spin rotator It is a resonant device based on performing NMR on a polarized neutron beam which moves through an orthogonal combination of static and rf magnetic fields in no DC field gradient. It is an active device in the sense that the amplitude of the rf magnetic field is varied in phase with the pulsed neutron source in such a way that the condition for rotating the neutron spins by radians in the rotator is always met for each subsequent neutron velocity class as it reaches the device. These achievements represent a qualitative advance in the art of neutron polarimetry and polarized neutron beam phase space manipulation

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Conclusion