Accumulation and extraction of ultracold neutrons from a superfluid helium converter coated with fluorinated grease

TL;DR

This paper demonstrates the production and extraction of ultracold neutrons from superfluid helium coated with fluorinated grease, using a window-free method and cold neutron beams, with results aligning with theoretical predictions.

Contribution

It introduces a novel window-free extraction technique for ultracold neutrons from superfluid helium and provides experimental validation of UCN production rates against theory.

Findings

Extraction time constant was halved despite lower UCN velocities.

UCN production rate matches theoretical estimates.

Multi-phonon contribution estimated from cold neutron spectrum.

Abstract

We report experiments on the production of ultracold neutrons (UCN) in a converter of superfluid helium coated with fluorinated grease. We employed our technique of window-free extraction of accumulated UCN from the helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. The time constant for UCN passage through the same extraction aperture as in a previous experiment was a factor two shorter, despite a lower mean velocity of the accumulated UCN in the present experiments. A time-of-flight measurement of the cold neutron spectrum incident on the converter allowed us to estimate the multi-phonon contribution to the UCN production. The UCN production rate inferred from two methods agrees with the theoretical expectation.