Characterization of ultracold neutron production in thin solid deuterium films at the PSI Ultracold Neutron source

TL;DR

This study quantitatively characterizes ultracold neutron production in thin solid deuterium films at PSI, demonstrating the negligible impact of impurities and confirming the accuracy of simulations against experimental measurements.

Contribution

It provides the first detailed measurement and simulation-based validation of UCN production in thin solid deuterium films at PSI.

Findings

UCN yield is unaffected by low impurity levels at 5 K.

Measured UCN production aligns with simulation predictions.

Thin film deuterium effectively produces ultracold neutrons for experimental use.

Abstract

We determined the ultracold neutron (UCN) production rate by superthermal conversion in the solid deuterium (sD$_2$) moderator of the UCN source at the Paul Scherrer Institute (PSI). In particular, we considered low amounts of less than $20\,$mol of D$_2$, deposited on the cooled moderator vessel surfaces in thin films of a few mm thickness. We measured the isotopic ($ c_\text{HD} < 0.2 \, \% $) and isomeric ($ c_\text{para} \le 2.7 \, \% $) purity of the deuterium to conclude that absorption and up-scattering at $5\,$K have a negligible effect on the UCN yield from the thin films. We compared the calculated UCN yield based on the previously measured thermal neutron flux from the heavy water thermal moderator with measurements of the UCN count rates at the beamports. We confirmed our results and thus demonstrate an absolute characterization of the UCN production and transport in the source by simulations.