Thermal excitations in helium as a function of temperature and pressure

The low energy roton-phonon excitations have been studied extensively in helium. However, the spectrum at thermal energies has not been characterised. With the development of high pressure time of flight inelastic neutron scattering measurements using helium as the pressure transmitting medium, it is clear that the excitations arising from the helium are of vital significance to understanding the excitations in the actual samples being studied. We have observed this to be the case in our previous measurements on Ammonium Sulfate, in which a strong signal was attributed to the helium. To accurately analyse that data it is vital that we obtain a model for the helium excitations. These results therefore are essential for the PhD work of ISIS funded student Bernet Meijer, and will be of great use to the wider user community.