CPM Calibration


The calibration of CPM involves gamma ray energy calibration for the Qualification Module (QM) and Flight Module (FM). Since the output from CPM contains only counts above a set threshold, spectrum of the incoming radiation can be obtained by successively changing the threshold and getting an integral spectrum. This integral spectrum can be converted to a conventional energy spectrum by simple differentiation. During all environmental tests and satellite integration tests, the record of background counts for a threshold of 0.5 MeV is kept and the requirement is that this background counts should be stable within 50%.

Energy Spectrum
In the laboratory tests, the pulse output from the CPM module was directly fed to an MCA
and spectrum was obtained for Cs137 (662 keV). The energy spectrum was then obtained
by differentiating the integral spectrum by giving successive LLD commands.
The spectra obtained by these two methods are given in the Fig. (a) and (b) above.

The QM detector calibration and FM detector calibration was performed in the integrated state with the package. The detector's stability with environment temperature and linearity in Gamma ray spectrum was tested.

 

Stability with environment
The detector was tested in thermovac with Cs137 source mounted on it. After achieving vacuum of 1.07 X 10-5 mBar, temperature was brought down to 0ºC and then it was taken gradually upto 50ºC. Spectra was observed at 0ºC, 10ºC, 20ºC, 30ºC, 40ºC, 50ºC temperatures. For reference prethermovac spectra was taken at ambient with Cs137 source. After completion of thermovac test, calibration of CPM QM unit was carried out with Ba133, Cs137 and Eu152 radio sources and linearity is checked.

 

Linearity in Gamma ray spectrum
Post themovac, Linearity for the gamma ray spectrum was tested using Ba133, Cs137 and Eu152 radioactive sources both for QM and FM The energies are 356 KeV (Ba), 662 KeV (Cs), 1112 KeV (Eu) and 1408 KeV (Eu).  The reposnse is linear with a slope of 0.45±0.05 LLD channel/keV and an intercept of -58±33 LLD channels.  The peak position was found to be stable correct to 5% in the temperature range 0 – 40 C and decreased by 10% at 50 C. The system was also found to be stable when a large number of low energy (6 keV) photons are bombarded on the detector, demonstrating that a stable particle rate measurement is possible even in the presence of intense solar flares.