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Particle Background Reduction Method and Ground Measurements

 

[Pfeffermann et al.1987]

To discriminate between real X-ray events and charged particle events - or charged particle induced events - the following anticoincidence conditions are implemented in the PSPC and in the experiment electronics.

1. Minimum ionizing particles entering the detector deposit an energy signal  keV in the proportional counter. Since this energy is well above that of any expected X-ray, these events are vetoed by the upper energy level discriminator.
2. Spurious X-ray events, originating from the detector housing or the frames of the wire grids by particle interaction, are typically absorbed by the counter gas near the edge of the detector in a five sided anticoincidence counter system. Anode A2 is the back veto counter and senses background events originating from the central mounting plate of the PSPC, as well as charged particles passing the whole detector system. The cathode strips on the edges of cathode 1 and 2 are broader (12.5 mm K1 and 10 mm K2) compared to the position sensing strips. Using the signals of these broader strips in anticoincidence provides side vetos in the x- and y-directions. The gas depth of the veto counters is sufficiently optically thick to stop spurious X-rays of  keV with an efficiency of 95%. For geometric reasons, the detection efficiency of the side veto in K1 is somewhat lower, resulting in a higher background at the lowest and highest Y-coordinates.
3. Events passing these veto conditions are additionally checked by the experiment electronics, using the signal pattern of the event on the cathode strips of K1 and K2. Events with more than 5 triggered cathode strips in one cathode or events showing signals in cathode strips which are not contiguous are excluded from further evaluation.

Ground tests carried out with a Co source measured the background rejection efficiency of  rays (1.17 and 1.3 MeV) to be 99.5%.