# Upgrade of the ultracold neutron source at the pulsed reactor TRIGA Mainz

## TL;DR

This paper reports on the upgrade of an ultracold neutron source at TRIGA Mainz, including new components and improved transmission, resulting in a 3.5-fold increase in UCN density and validated by Monte Carlo simulations.

## Contribution

The paper introduces a comprehensive upgrade to the UCN source, including a He liquefier and enhanced neutron guides, achieving significantly higher UCN densities.

## Key findings

- UCN density increased by a factor of 3.5
- UCN storage curve matches Monte Carlo predictions
- Solid deuterium formation influenced by H2 ortho/para ratio

## Abstract

The performance of the upgraded solid deuterium ultracold neutron source at the pulsed reactor TRIGA Mainz is described. The current configuration stage comprises the installation of a He liquefier to run UCN experiments over long-term periods, the use of stainless steel neutron guides with improved transmission as well as sputter-coated non-magnetic $^{58}$NiMo alloy at the inside walls of the thermal bridge and the converter cup. The UCN yield was measured in a `standard' UCN storage bottle (stainless steel) with a volume of 32 litres outside the biological shield at the experimental area yielding UCN densities of 8.5 /cm$^3$; an increase by a factor of 3.5 compared to the former setup. The measured UCN storage curve is in good agreement with the predictions from a Monte Carlo simulation developed to model the source. The growth and formation of the solid deuterium converter during freeze-out are affected by the ortho/para ratio of the H$_2$ premoderator.