CPM Design Details

The Charge Particle Monitor is an auxiliary payload on ASTROSAT. The main objective of CPM is particle count rate measurement above an energy threshold. The detector used for the Charge Particle Monitor is a CsI(Tl) scintillation detector. The specifications of CsI(Tl) are as mentioned in the table below. CsI(Tl) is preferred over more common NaI(Tl) which is hygroscopic in nature. A Si-PIN diode is placed on the surface of the scintillation crystal.


Scintillator Type Density Refractive Index Light Output Decay Constant (ns) Wavelength (nm) Principal Application
CsI(Tl) Crystal 4.31 1.788 95 1100 580 Heavy Particle, γ


CPM Logic

CPM Logic The Detector module consists of Ceasium Iodide Thalium activated [CsI(Tl)] crystal along with Si-Pin Photodiode and charge sensitive preamplifier (CSPA). The energy of charge particle striking the crystal gets converted into photon (light energy). Photodiode converts this photon energy into electrical energy by generating electrons corresponding to energy of incident photons. CSPA amplifies this small electrical signal to a measurable millivolt (mV) range. The pulse corresponding to every energy particle from detector is passed through post amplification, where milli-volt signal is converted to volt level.

These amplified pulses are passed through a Low Level Discriminator (LLD) circuit to cut-off noise level and select the Charge Particle signals above a defined value of 1MeV. The LLD reference voltage value is programmable from ground station through telecommand. The pulse signal from LLD circuit output is digitized and fed to a counter. The count rate is transmitted through telemetry to ground station. This count rate is compared with a preset value set from ground station through tele-command. Whenever count rate is greater than preset value the output is activated. The output is de-activated whenever count rate is less than preset value.


Design Details

The CPM count rate is transmitted through telemetry to ground station. It is also available for onboard users in serial format as Data, Clock and a Latch. Maximum three users can be supported. This count rate is compared with a preset value set from ground station through telecommand. Whenever count rate is greater than a preset value the output is activated. The output is deactivated whenever count rate is less than preset value. To avoid false triggering, output is activated/deactivated only after 3 successive confirmation of count rate. This SAA warning outputs are available for users. Maximum eight users can be supported. The energy threshold of CPM is nominally set at 500 keV and this can be changed unto 10 MeV with a few keV steps. The count threshold for the automatic onboard sensing of SAA can be set from 1 to 16384 counts with step of 1 count and the integration time can be set from 1 s to 32 s with a step of 1 s. Additionally, there is a command facility to automatically step up the energy threshold so that, at stable non-­SAA regions, integral energy spectrum can be obtained.