Ultracold neutron depolarization in magnetic bottles

TL;DR

This paper investigates how lateral motion of ultracold neutrons in magnetic traps causes depolarization, impacting neutron lifetime measurements and polarized neutron decay experiments, using extended semi-classical and quantum analyses.

Contribution

It extends previous models by including lateral neutron motion, revealing its dominant role in depolarization loss in magnetic storage traps.

Findings

Lateral neutron motion causes significant depolarization.

Depolarization estimates for neutrons reflected on magnetic mirrors.

Implications for neutron lifetime and polarization experiments.

Abstract

We analyze the depolarization of ultracold neutrons confined in a magnetic field configuration similar to those used in existing or proposed magneto-gravitational storage experiments aiming at a precise measurement of the neutron lifetime. We use an extension of the semi-classical Majorana approach as well as an approximate quantum mechanical analysis, both pioneered by Walstrom et al. [Nucl. Instr. Meth. Phys. Res. A 599, 82 (2009)]. In contrast with this previous work we do not restrict the analysis to purely vertical modes of neutron motion. The lateral motion is shown to cause the predominant depolarization loss in a magnetic storage trap. The system studied also allowed us to estimate the depolarization loss suffered by ultracold neutrons totally reflected on a non-magnetic mirror immersed in a magnetic field. This problem is of preeminent importance in polarized neutron decay studies such as the measurement of the asymmetry parameter A using ultracold neutrons, and it may limit the efficiency of ultracold neutron polarizers based on passage through a high magnetic field.