First ultracold neutrons produced at TRIUMF

TL;DR

This paper reports the successful installation and testing of a new ultracold neutron source at TRIUMF, achieving significantly higher neutron yields and densities, with detailed experimental and simulation analysis confirming key physical behaviors.

Contribution

First ultracold neutron source at TRIUMF using a superfluid-helium converter, demonstrating high neutron yield and detailed characterization.

Findings

Extracted up to 325,000 ultracold neutrons in one minute

Achieved ultracold-neutron density of 5.3 cm$^{-3}$

Confirmed UCN upscattering rate proportional to T$^7$

Abstract

We installed a source for ultracold neutrons at a new, dedicated spallation target at TRIUMF. The source was originally developed in Japan and uses a superfluid-helium converter cooled to 0.9$\,$K. During an extensive test campaign in November 2017, we extracted up to 325000 ultracold neutrons after a one-minute irradiation of the target, over three times more than previously achieved with this source. The corresponding ultracold-neutron density in the whole production and guide volume is 5.3$\,$cm$^{-3}$. The storage lifetime of ultracold neutrons in the source was initially 37$\,$s and dropped to 24$\,$s during the eighteen days of operation. During continuous irradiation of the spallation target, we were able to detect a sustained ultracold-neutron rate of up to 1500$\,$s$^{-1}$. Simulations of UCN production, UCN transport, temperature-dependent UCN yield, and temperature-dependent storage lifetime show excellent agreement with the experimental data and confirm that the ultracold-neutron-upscattering rate in superfluid helium is proportional to $T^7$.