A charged particle monitor (CPM) is included as a part of ASTROSAT payloads to control the operation of the LAXPC, CZTI, SXT and SSM. Even though the orbital inclination of the satellite will be 8 deg or less, in about 2/3rd of the orbits, the satellite will spend a considerable time (15 - 20 minutes) in the South Atlantic Anomaly (SAA) region which has high fluxes of low energy protons and electrons. The high voltage will be lowered or put off using data from CPM when the satellite enters the SAA region to prevent damage to the detectors as well as to minimize ageing effect in the Proportional Counters.
A Scintillator Photodiode Detector (SPD) with a Charge Sensitive Preamplifier will be used to detect the charged particles.
In the ASTROSAT CPM, a 10 x 10 x 10 mm cube of CsI (Tl) crystal (wavelength = 550 nm) with Teflon reflective material is coupled to the same area window of a Si-PIN diode. The incident charged particle energy is converted into light in CsI, and the light seen by the photodiode is converted into an electrical pulse with the help of a charge sensitive pre-amplifier (CSPA). The electrical signal is then passed through a Lower Level Discriminator (LLD) with a threshold level commandable from ground. The output is made available to all other instruments on board, and is also recorded as a part of the satellite housekeeping data.
Some salient features of the CPM are:
Scintillator : 1cm x 1cm x 1cm CsI (Tl) crystal Light collector :
Photodiode with pre-amp
Window : 1 mil Kapton Low energy threshold : 1.2 MeV Time resolution : 5 s Expected count rate : 1 s-1 (non-SAA) Max. count rate : 1000 s-1 Overall size : 18cm x 15cm x 5cm Weight : 2 kg Power requirement : 2.3 W