| Author(s) and Co-Author(s) with Affiliation: Deepak Kumar Painkra(Physical Research Laboratory, Ahmedabad- 380009, India), N.P.S. Mithun(Physical Research Laboratory, Ahmedabad- 380009, India), Arpit Patel(Physical Research Laboratory, Ahmedabad- 380009, India), Hitesh Adalja(Physical Research Laboratory, Ahmedabad- 380009, India), B S Bharath Saiguhan(Physical Research Laboratory, Ahmedabad- 380009, India), Shivam A(Physical Research Laboratory, Ahmedabad- 380009, India), Bhavesh R. S. Nehra(Physical Research Laboratory, Ahmedabad- 380009, India), Ajay Rateesh(Physical Research Laboratory, Ahmedabad- 380009, India), C.S. Vaishnava(Physical Research Laboratory, Ahmedabad- 380009, India), M Shanmugam(Physical Research Laboratory, Ahmedabad- 380009, India), Santosh V. Vadawale(Physical Research Laboratory, Ahmedabad- 380009, India) |
| Abstract: X-ray polarization measurements provide critical insights into the physical processes in astrophysical sources involving compact objects, where extreme conditions prevail, such as strong gravity, intense magnetic fields, high temperatures. While significant advances have been achieved in the soft X-ray regime in recent years, hard X-ray polarimetry has remained largely unexplored. The techniques employed for polarimetry in soft X-rays and hard X-ray differ and Compton-scattering-based polarimeters are suitable for energies above ~20-30 keV.
A typical Compton polarimeter uses a low-Z scatterer surrounded by high-Z absorbers to measure the azimuthal distribution of scattered photons, from which polarization information can be derived. Coincidence of detection of interactions in the active scatterer with the absorbers reduces the background and improves the sensitivity. Low energy detection limit of the active scatterer detector system determines the lowest energy photons that can be measured with the polarimeter. For proposed Solar Hard X-ray Polarimeter (SHXP), we employ plastic scintillator scatterer coupled to PMT as the active scatters and 12 NaI (Tl) scintillators coupled with SiPMs as the absorbers. In addition to the detector characteristics, the noise performance of the scatterer readout electronics is also critical in limiting the lowest photon energy that can be detected.
Here, we present the design and development of the plastic scintillator detector system and its readout electronics. The electronics chain comprises a preamplifier and shaper circuit for improved charge collection, bandwidth, and SNR, along with a voltage divider and filter for stable PMT biasing at 1000 V. An FPGA and comparator-based system enables coincidence detection between scatterer and absorber events. Initial characterization of the scatterer is performed using Compton scattering with Am-241 (59.54 keV) and Cd-109 (22 keV) sources, where a CdTe detector mounted on rotating arm records the scattered photons in coincidence mode. Detailed design aspects and experimental results will be presented.
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