Spectroscopic study of scattering of UCN by liquid HE4

We report on our first recent measurements of the energy spectrum of upscattered Ultra-Cold-Neutrons in liquid He4 at temperatures around 1 Kelvin. The measurements were made on the Superthermal UCN Source at the ILL [1]. This source produces UCN’s in liquid He4 by down-scattering (phonon-emission) of 10 A neutrons. The UCN’s produced in this way are stored in the He4 container, since they are totally reflected from the walls. The UCN’s can gain energy through scattering by Phonons [3] and Rotons [3], and hence leave the container. We have measured the spectrum of these upscattered neutrons by using a pseudo statistical chopper [4]. The first preliminary measurements show a discrepancy with the spectrum expected on the basis of phonon and roton scattering.