ASPERA-3: Start and Stop Surfaces for the NPD Sensor
The Neutral Particle Detector (NPD) is part of the Analyzer of Space Plasma and Energetic Atoms ASPERA-3 instrument  on the Mars Express Mission of ESA to be launched in 2003. The instrument will investigate the Martian magnetosphere and its interaction with the solar wind. As Mars does not have an intrinsic magnetic field, the solar wind may directly interact with the Martian ionosphere and upper atmosphere where energetic neutral atoms (ENA) are produced by charge exchange between neutral background atoms and solar ions. These ENA do no longer interact with the magnetic field and follow ballistic trajectories.
The NPD sensor will detect these neutral particles. The sensor consists of two identical units, each of which is a one-dimensional pinhole camera with a 90° field of view. In each unit charged particles are removed by an electrostatic deflection system also used as collimator. A neutral particle would release secondary electrons upon reflection of the start surface. These secondary electrons are collected at the start MCP. After reflection the particle hits the stop surface again releasing secondary electrons. These electrons are detected using a stop MCP allowing a time-of-flight measurement of the particle (Figure above). The ENA flux is expected to consist mainly of oxygen and hydrogen. The mass of the particles will be inferred from pulse-height analysis of the start and the stop secondary electron yield. This will make it possible to distinguish between the two main species H and O .
For a high overall performance the NPD detector requires a start and a stop surface with a high secondary electron yield upon particle impact. The start surface should also exhibit a high particle reflection efficiency and has to be very smooth, a feature not needed for the stop surface. All these properties should be stable with time. Using the ILENA experiment we were able to evaluate suitable surfaces for this sensor.