ASPERA-3: Neutral Particle Detector
The Neutral Particle detector sensor consists of two identical
detectors, each of which is a pinhole camera. In each detector the
charged particles with energies up to 70 keV, electrons and ions, are
removed by the deflection system which consists of two 90° sectors
separated by a 4.5 mm gap. Apart from being ON or OFF the deflection
system can be operated in the alternative mode. The ENA beam emerging
from the 4.5 x 4.5-mm pin - hole hits the START surface under the
grazing angle 20° and causes the secondary electron emission. By a
system of collecting grids, the secondary electrons (SE) are
transported to one of two MCP assemblies giving the START signal for
the time-of-flight (TOF) electronics.
Depending on the azimuth angle the collection efficiency varies from 80% to 95%. The incident ENAs are reflected from the START surface near-specularly. Since the charge state equilibrium is established during the interaction with the surface, the emerging beam contains both the neutral and ionized (positive and negative) components. To increase the total efficiency, no further separation by the charge is made. As proven by the ion tracing, there is very little disturbance to the reflected atomic ions leaving the START surface with an energy above 80 eV, introduced by the START electron optics. Therefore particles of all charge states - negative, neutral, and positive - will impact the second surface, the STOP surface, and again produce secondary electrons which are detected by one of the three MCP assemblies giving the STOP signal. The time of flight over the fixed distance of 8 cm defines the particle velocity. The STOP MCPs also give the azimuthal direction. Since the SE yield depends on mass for the same velocity, the pulse height distribution analysis of the START signals and independent analysis of the STOP signals provide the estimation of ENA mass. Each event is stored in the array START MCP charge x STOP MCP charge x time-of-flight x direction.
The UV suppression in NPD is based on the coincidence of START / STOP signals. To increase the particle reflectivity, it is considered to use very smooth (roughness is of the order of 5 - 10 Å) metal surfaces. On the other hand the STOP surface is proposed to be made of graphite (roughness around 100 nm) covered by MgO. This combination has a very high secondary electron yield, low photoelectron yield and high UV absorption. Both proposed surfaces are stable and do not require special maintaining.
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