NICE: an instrument for direct mass spectrometric measurement of interstellar neutral gas

The direct measurement of the neutral interstellar gas requires a very sensitive neutral particle imaging instrument in the energy range of 10 eV-1000 eV For successful detection and identification, the neutral particles have to be ionized first, which will be accomplished via surface ionization. This method is successfully employed in the Low Energy Neutral Atom imager (LENA) instrument on the IMAGE spacecraft launched on 25 March 2000, which still operates well. We present the laboratory prototype of the Neutral Interstellar Composition Experiment (NICE), a neutral particle mass spectrometer dedicated to the measurement of interstellar gas, and will discuss its instrumental characteristics. Performance is evaluated with emphasis on the neutral to negative ion conversion for hydrogen and oxygen and the collection of these ions by the mass spectrometer. Measurements of the detection efficiency of the prototype for primary neutral hydrogen and oxygen atoms are presented. Several conversion surfaces, conductive and insulating, were investigated and all are potential candidates for a next generation neutral particle imaging instrument.