Comment to the article "Temperature dependence of ultracold neutron loss rates" E.Korobkina et al., PRB 70,035409

TL;DR

This paper critiques previous measurements of ultracold neutron loss rates, suggesting that observed temperature dependence is likely due to trap leakage and surface conditions rather than intrinsic neutron-surface interactions.

Contribution

It provides a reanalysis of experimental data indicating that trap leakage and surface slit variations, not hydrogen presence, explain the temperature dependence of UCN losses.

Findings

Losses at 10K are higher than previously reported.

Leakage through trap slits likely causes observed temperature dependence.

Hydrogen presence on surface has minimal impact on loss rates.

Abstract

In work [1] ("Temperature dependence of ultracold neutron loss rates" E.Korobkina et al., PRB 70,035409) results of measurement of temperature dependence of losses of ultracold neutrons (UCN) in a range from 4 K to 300 K at UCN storage in a copper trap are presented. At interpretation of experimental data it was artificially considered only the difference of losses rate 1/tau(T)= 1/tau_exp(T)- 1/tau_exp(10K), (tau - UCN storage time in the trap). It has been accepted for temperature dependence of losses which changes from 0 to 3.3*10-4 per one collision of UCN with a trap surface. However, the analysis of raw experimental data shows that in a trap at temperature 10 K there is losses 1.0*10-3 per collision. They are considerably (in 3 times) more than discussed temperature dependence and almost 10 times more than losses due to capture cross section on copper. It is the most probable that these losses are connected with leakage of UCN through a slit of trap shutter. Change of the size of a slits on 25% at change of temperature from 300K to 10K can quite explain discussed temperature dependence. Certainly, hydrogen is present on a surface, but as it was shown at work [2] (V.P.Alfimenkov et al., JETP., Vol. 55, No. 2 25 Jan.1992) temperature dependence in 3 times lower, even on the undegased surfaces than on the degassed and deuterated surface in work [1]. At last, deuteration of a trap surface allows almost completely to suppress temperature dependence connected with presence of hydrogen [2]. The conclusion of work [1] is that hydrogen is localized on a surface in the form of a film, instead of distribution in the surface substance. This conclusion contradicts results of measurement of energy dependence of UCN losses in work [2]. More detailed analysis of work [1] and works [2] is presented below.