Measurement of the energy dependence of the neutron loss per bounce function on reflection from oil and grease surfaces using monochromatic ultracold neutrons

We report a technique for determining directly the energy dependent UCN loss per bounce function μ(E) for material surfaces using monochromatic UCN. Results of such measurements on Fomblin oil and grease surfaces are presented for neutrons with energies in the range 5 to 106 neV, in the temperature range 283–308 K. The results show that these materials exhibit a loss per bounce function in good agreement with that predicted by the theory of UCN scattering from an abrupt one dimensional complex potential step. Some evidence for very small energy changes at reflection was found for neutrons with energies close to the critical energy of the step. Values of the loss parameter (2f) = 4.7(0.2) and 3.7(0.2) × 10−5 (at 294 K) were deduced for the oil and grease surfaces respectively. The (2f;) parameter of the oil was found to be strongly temperature dependent.