Sputtering efficiency of LiF surfaces on impact of low energy neutral helium (20–80 eV): Calibration of the interstellar neutral helium instrument on ULYSSES

For the detection of interstellar neutral helium particles entering the inner solar system, a novel detection technique has been developed using sputtering of secondary ions from lithium-fluoride (LiF) surfaces on the impact of He particles. In order to be able to determine the interstellar He fluxes, the detection efficiency had to be determined in an energy range E kin ≈20 to 200 eV, an energy range for which data were not available in literature. Therefore, a calibration had to be performed in a dedicated accelerator system providing a neutral beam in the required energy range. While precise values could be obtained for energies above about 50 eV, significant uncertainties in the efficiency remained for lower energies, due to the difficulty in determining the absolute flux of the primary beam after acceleration, as elastic scattering of particles out of the beam geometry during the charge exchange process becomes more important. Corrections have been applied using a theoretical simulation of this process. These uncertainties have subsequently been eliminated, using the inflight measurements of neutral He during the ULYSSES mission. Due to the orbital parameters of this spacecraft, the relative energy of the interstellar He particles in the instrument’s reference system varies in the range 20–80 eV. Assuming that the interstellar He flow is stationary, i.e., it shows only negligible time variations, constant count rates would be expected over this energy range. The energy dependence in the actually observed count rates has been used to refine the efficiency calibration at low energies (⩽40 eV).