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Journal of Experimental and Theoretical Physics Letters

A new escape channel for ultracold neutrons in traps

Peter W. Geltenbort, Валерий Викторович Несвижевский, D. G. Kartashov, Е. В. Лычагин, Al. Yu. Muzychka, G. V. Nekhaev, Valery N. Shvetsov, Alexander V. Strelkov, А. Г. Харитонов, Anatolii P. Serebrov, R. R. Tal'Daev, J. M. Pendlebury

Published Aug 1, 1999• 9 citations

Abstract

A surprising new escape channel for ultracold neutrons (UCNs) in traps was reported recently. It could be relevant to the long-standing puzzle of the “too high” loss rate of UCNs from traps, which has yet to be completely understood and eliminated. In the present work we positively identify the new phenomenon and investigate it in detail. The escape of UCNs from traps is due to rare events in which their energy undergoes a small increase (∼10 −7 eV). The reason for such an energy gain and its impact on the physics of UCN storage is still to be investigated.

DOI: 10.1134/1.568148

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The capture of trapped ultracold neutrons (UCNs) by closed hydrogenous impurities within a solid coating of the trap is discussed as a possible cause of observed anomalously large losses of UCNs in solid material UCN traps. Then significant losses of UCNs arise only if resonances occur in the UCN-impurity scattering amplitude. For a large size impurity, higher partial waves in the UCN-impurity interaction are important, and they are taken into account in the present paper. The method of the calculation is applicable to irregular shape impurities as well. A small distortion of an impurity shape, if it splits the resonance, can increase the UCN losses by a few times. UCN losses in the beryllium trap are calculated assuming they are due to the UCN capture by ice spherical impurities within the coating of the trap walls. Both s- and p-wave resonances contribute significantly to the UCN losses considered. As an example, observed anomalous large UCN losses are achieved if the average radius of the impurity is about 600 Angstroms and the impurity density is about 3*10^{14}/cm^3. A distortion of the spherical shape of the impurity could increase the UCN losses and therefore decrease the impurity density.

Ultracold neutrons (UCN) with energy up to the Fermi potential were stored for the first time in a volume coated with diamondlike carbon. As a function of the UCN energy and wall temperature of the storage volume, we measured the wall loss parameter \ensuremath{\eta} and obtained $\ensuremath{\eta}=(3.1\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at

T=290

K and $\ensuremath{\eta}=(1.8\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\cdot\else\textperiodcentered\fi{}{10}^{\ensuremath{-}4}$ at

T=115

K.