Development of a one-dimensional position sensitive detector for Compton X-ray polarimeters

Abstract

The scientific potential of X-ray polarimetry has long been recognized, but the challenges in measuring polarization have left it largely unexplored, particularly in the hard X-ray regime. While tremendous advancement has been made in soft X-ray polarimetery, the lack of sensitive hard X-ray polarimeters and polarisation measurements continues to limit our understanding of high-energy astrophysical processes. With the development of hard X-ray mirrors, it is now possible to develop a sensitive focal plane hard X-ray polarimeter. One such effort is CXPOL, a prototype developed at PRL, India, which consists of a plastic scintillator as active scatterer readout by PMT surrounded by CsI(Tl) scintillators in cylindrical array with SiPM readout from one side. First results of the prototype have been demonstrated in 20 to 80 keV energy range. The sensitivity of the instrument can be significantly enhanced using faster and better light yield scintillator like NaI as absorbers. Further, the use of a position-sensitive scatterer and absorbers, can also provide spectroscopic information by measuring the interaction position along the length and from the known energy depositions in the detectors. Position sensitive detectors are also helpful in mitigating the systematic effects introduced by the off-axis events in the polarisation measurements. Here, we demonstrate the detection sensitivity in the 100x20x5 mm^3 NaI(Tl) scintillator absorber readout on both ends by SiPM arrays operating in co-incidence. In this work, we characterize the first prototype of this detector system and investigate the variation in energy and position resolution, and light output with irradiation position along the length of the detector. The two end readout in co-incidence also reduces the overall SiPM background per absorber by an order of magnitude, further enhancing the polarimetric sensitivity of the instrument.

Figures (11)

• Figure 1: Left figure shows the variation of relative number of photons at SiPM with distance considering different reflectivity in simulations and overplotted with the experimental values for CsI(Tl) scintillator absorber of size (150$\times$5$\times$5 mm$^{3}$) with one end readout by a SiPM. Right figure shows the variation of the number of photons with irradiation position at 20, 60 and 122 keV for NaI(Tl) scintillator absorber of size (100$\times$20$\times$5 mm$^{3}$) with two end readout by the SiPMs.
• Figure 2: The figure on the left is the schematic of the scattering geometry of the proposed spectro-polarimeter. The right side figure shows the schematic of the absorber detector (100$\times$20$\times$5 mm$^{3}$) with two end readout by the SiPMs.
• Figure 3: Block diagram of the electronic chain employed for the experimental setup
• Figure 4: Panel a.: Experimental setup for the position measurement consisting of detector setup, front-end, back-end readout electronics, and power supply. Panel b.: NaI(Tl) scinitllator is packaged in the aluminium cover with SiPM array at both ends, and CSPA+Shaper is shown. Panel c.: The bare scintillator readout by an array of 3 SiPMs at both ends is shown.
• Figure 5: ADC distribution detected at two ends when irradiated at the center using an Am$^{241}$ X-ray source.