Observation of a new mechanism of ultracold-neutron losses in traps

That ultracold neutrons (UCN) can be confined for a long time in traps makes it possible to use them extensively in experiments, including those aimed at studying problems of fundamental physics. It is well known that UCN losses in traps are due, in particular, to the beta decay of neutrons, as well as to their heating and capture at the trap walls. In this study, an additional mechanism of UCN losses has been observed that is associated with an approximately twofold increase in the neutron energy with a probability of about 10-5 per collision with the walls of the trap made from stainless steel or with the walls of an analogous trap that are covered with hydrogen-free Vaseline or Fomblin oil. The observed effect is not reduced to the known heating of UCNs that leads to an increases in their mean energy approximately to the energy of thermal vibrations of the walls. Owing to this small increase in energy, UCNs can penetrate, however, into a wall material, where they are heated or absorbed in an ordinary way, but, if the trap walls are thin, such neutrons can traverse them. The observed increase in the UCN energy fully accounts for the result of our previous experiment, where it was found that UCNs stored in a trap can penetrate through a thin beryllium foil. A direct study is required to find out whether the observed effect has any connections with anomalous UCN losses that have almost the same probability, but which are measured under different experimental conditions