Calibration and Simulation of Astronomical X-ray Detectors

Abstract

To extract any astronomical information from the data obtained with a detector, detail knowledge of detector response and proper calibration is imperative. In this thesis, I present results obtained by carrying out calibration and simulations of two astronomical X-ray detectors. One of the instrument is Large Area X-ray Proportional Counter (LAXPC) which is a payload on recently launched Indian space observatory, ASTROSAT. The instrument is dedicated for timing and spectroscopic studies in energy range 3-80 keV. The second instrument is an X-ray polarimeter, POLIX, which is being developed at Raman Research Institute for a future satel- lite mission. The aim of this mission is to measure polarisation of X-ray sources in the energy range 5 - 50 keV. In case of the proportional counters, the spectrum for incident radiation is measured in terms of the height of pulses produced by the individual X-ray photons. To interpret the data, the measured pulse heights must be converted into energy values. If we know the response of the detector for photons of various incident energies, we can transform the detector output from voltage values to energy values. The aim of the first part of my thesis work was to create this transformation matrix, called as response matrix. This was created by simulating detector response in Geant4 (a simulation toolkit for passage of particles through matter) for various energies. As the LAXPC consists of three detectors, the matrix was created for each one. The accuracy of matrix is checked by fitting it with well understood Crab spectrum obtained from the detector. Currently the matrix is fitting data very well in 6 - 30 keV range. However, outside this energy range, the observed spectrum and the measured spectrum shows some dif- ference. Investigations are still in progress to address this problem. Polarisation properties of astronomical sources in the high energy bands like X-ray and gamma ray are yet to be explored. As far as X-rays are concerned, many instrument developments are going on for measurements in energy range 2 - 8 keV using photo-electron track imaging, while in 30 - 200 keV band using Compton scattering method. The only experiment which works in intermediate band (5 to 30 keV) is a Thompson scattering X-ray polarimeter under development at Raman Research Institute, Bangalore for a future small satellite mission of the Indian Space Research Organization (ISRO). The detector geometry of the instrument is fixed with four square proportional counter detectors as four sides with a scatterer in the middle and a collimator on top. The aim of the second, important part of the thesis was to calibrate position determination from the ratio of the pulse height and to carry out simulations for given instrument configuration to optimise scatterer material, position and to find out modulation factor for the instrument. The simulations were carried out in Geant4. We report that best choice of scatterer material is Lithium-Beryllium combination kept at mid-way between centre and bottom of the detector. The modulation factor obtained for this configuration is 0.44. Also, Minimum Detectable Polarisation dependence on different intensities and different exposure time for Crab like sources for this modulation factor has been discussed.