NICE: Neutral Interstellar gas Composition Experiment

Introduction

The solar system is immersed in a large cloud of low density gas, the Local Interstellar Medium (LISM). The interaction between the solar wind and this plasma manifests itself in the buildup of the heliosphere. The heliospheric cavity is shielded from the inflow of interstellar ions, since the solar wind plasma is highly magnetized compared to the LISM. Only neutral particles can enter the heliosphere. Due to the motion of the solar system relative to the interstellar medium these neutral particles penetrate deeply into the solar system [1]. The interstellar neutrals have a speed of about 10 to 70km/s relative to a spacecraft in earth or solar orbit. In a neutral particle instrument these neutrals would first be ionized and then measured using conventional mass spectrometry used in space research. Surface ionization was identified as the only viable ionization technique to meet the requirements concerning ionization efficiency for the energy range of 10eV to 1keV within the limitations imposed by the resources (space, weight, power, etc.) available on a satellite [2,3,4,5].

To get information about the composition of the neutral particles an ionization surface is combined with a time-of-flight spectrometer. The prototype of the Neutral Interstellar Composition Experiment (NICE) was built to demonstrate the performance of this approach.

Principle of the sensor

After removal of charged particles in the entrance system the remaining neutral atoms would be ionized upon reflection at grazing angle of a conversion surface. The produced negative ions are postaccelerated in an extraction lens to energies above 10keV to ensure a high detection efficiency in the time-of-flight section mounted at the exit of the extraction lens. The extraction lens focuses particles with different primary energies to different positions at the lens exit. This makes it possible to infer the primary particle energy even after postacceleration. The main requirements for the conversion surface are low angular scattering and high ionization efficiency .