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

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 Compton X-ray Polarimeter (CXPOL), a prototype developed at Physical Research laboratory, India, which consists of a plastic scintillator as active scatterer readout by Photomultiplier Tube (PMT) surrounded by CsI(Tl) scintillators in cylindrical array with Si photomultiplier (SiPM) readout from one side. First results of the prototype have been demonstrated in 20 to 80 keV energy range by Chattopadhyay et al. (2015). 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 surrounded by position-sensitive 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 Silicon Photomultiplier (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.