Resonance capture by hydrogenous impurities and losses of ultracold neutrons in solid material traps

TL;DR

This paper investigates how hydrogenous impurities within solid UCN traps can cause resonance-enhanced neutron losses, with calculations showing how impurity size, shape, and resonances influence the loss rates.

Contribution

It introduces a method to calculate UCN losses due to resonant capture by impurities of arbitrary shape, considering higher partial waves and shape distortions.

Findings

Resonances significantly increase UCN losses at specific impurity sizes.

Impurity shape distortions can amplify UCN losses by a few times.

Calculated losses match observed anomalies with realistic impurity parameters.

Abstract

The capture of trapped ultracold neutrons (UCNs) by closed hydrogenous impurities within a solid coating of the trap is discussed as a possible cause of observed anomalously large losses of UCNs in solid material UCN traps. Then significant losses of UCNs arise only if resonances occur in the UCN-impurity scattering amplitude. For a large size impurity, higher partial waves in the UCN-impurity interaction are important, and they are taken into account in the present paper. The method of the calculation is applicable to irregular shape impurities as well. A small distortion of an impurity shape, if it splits the resonance, can increase the UCN losses by a few times. UCN losses in the beryllium trap are calculated assuming they are due to the UCN capture by ice spherical impurities within the coating of the trap walls. Both s- and p-wave resonances contribute significantly to the UCN losses considered. As an example, observed anomalous large UCN losses are achieved if the average radius of the impurity is about 600 Angstroms and the impurity density is about 3*10^{14}/cm^3. A distortion of the spherical shape of the impurity could increase the UCN losses and therefore decrease the impurity density.