A method for measuring deviation from Lambert diffuse scattering law of ultracold neutrons on material walls

TL;DR

This paper proposes an experimental method and simulations to measure deviations from the Lambert diffuse scattering law in ultracold neutrons, which is important for improving models of neutron behavior in material traps.

Contribution

It introduces a novel experimental approach and Monte Carlo simulations to quantify deviations from the Lambert law in UCN diffuse scattering.

Findings

Significant deviations observed in simulations for various trap geometries.

The proposed method can effectively measure the angular distribution deviations.

Results suggest the Lambert law may not always accurately describe UCN scattering.

Abstract

Modeling the motion of ultracold neutrons (UCNs) is crucial for assessing their losses, accurately measuring their lifetime, and describing other experiments. In material traps, it is necessary to account not only for specular but also for diffuse elastic reflection of UCNs from the trap walls. Typically, the Lambert cosine law is used to describe the angular distribution of diffusely scattered neutrons. However, this law lacks a rigorous theoretical derivation and is often violated. In our work, we propose an experiment to measure the deviation of the angular distribution of UCNs during diffuse scattering from the Lambert law. This deviation can be determined by the difference in the number of neutrons exiting through the central and end windows of a long narrow UCN trap. Monte Carlo simulations corresponding to a possible experiment have been performed, demonstrating a significant effect for different trap geometries.