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 1.2). 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 (see Chapter 3). The start surface should also exhibit a high particle reflection efficiency (see Chapter 4) and has to be very smooth, a feature not needed for the stop surface. All these properties should be stable with time.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland