spase://SMWG/Instrument/VenusExpress/ASPERA4/NPD

The Neutral Particle Detector (NPD) of the ASPERA-4 package
flown on Venus Express provides measurements of the ENA differential flux over
the energy range 100 eV - 10 keV resolving H and O with a coarse 5 deg x 30 deg
angular resolution. The sensor consists of two identical detectors each with a
9 deg x 90 deg intrinsic field of view. The measurement technique is based on a
principle similar to the Neutral Particle Imager. ENAs incident on a surface at
a grazing angle of 15 deg are reflected and cause secondary electron emission.
The secondary electrons are transported to an MCP assembly, which gives the START
signal. The reflected ENAs hit the second surface and again produce the secondary
electrons used to generate the STOP signal. The time-of-flight (TOF) electronics
give the ENA velocity. The pulse-height distribution analysis of the STOP signals
is used to provide a rough determination of the ENA mass.