Unconventional trapping of ultracold neutrons

In unconventional storage experiments we filled ultracold neutrons (UCN) in to a Fomblin-grease coated trap and then immediately removed the UCN from t he storage volume by an absorber, until their residual density in the trap was measured to be negligible. When subsequently the absorber was withdrawn a significant number of UCN of higher energies emerged from the trap. Thei r appearance cannot be attributed to heating or cooling of residual UCN. Fu rther experiments were performed to investigate the origin of these UCN whi ch we call 'late UCN'. We noticed that application of a magnetic field grad ient at the trap wall as well as a replacement of Fomblin grease on the sur face by Fomblin oil gave rise to small but measurable alterations of storag e behavior. These phenomena an consistent with the hypothesis of temporary adhesion of a few UCN to a rough wall. (C) 1999 Elsevier Science B.V. All r ights reserved.