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

For the detection of interstellar neutral helium particles entering the inn er solar system, a novel detection technique has been developed using sputt ering of secondary ions from lithium-fluoride (LiF) surfaces on the impact of He particles. In order to be able to determine the interstellar He fluxe s, the detection efficiency had to be determined in an energy range E-kin a pproximate to 20 to 200 eV, an energy range for which data were not availab le in literature. Therefore, a calibration had to be performed in a dedicat ed accelerator system providing a neutral beam in the required energy range . While precise values could be obtained for energies above about 50 eV, si gnificant uncertainties in the efficiency remained for lower energies, due to the difficulty in determining the absolute flux of the primary beam afte r acceleration, as elastic scattering of particles out of the beam geometry during the charge exchange process becomes more important. Corrections hav e been applied using a theoretical simulation of this process. These uncert ainties 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 th e 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 tim e 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 (less than or equal to 40 eV). (C) 1999 American Institute of Physics. [S0034-6748(99)03411-5].