Polarimetric evaluation of CdTe-Timepix3 detector module for THOR-SR Compton telescope

Gamma-ray polarimetry for the sub-MeV to lower-MeV band, an unexplored regime, requires the development of polarimeters that utilize novel space instrumentation with enhanced timing, imaging, and polarimetric capabilities, in addition to a robust spectroscopic response. THOR-SR is an astrophysics technology demonstrator to fly aboard the ESA vehicle Space Rider on a two-month mission. The main instrument is an all-sky Compton telescope composed of four layers, each layer with four modules based on 2.0 mm thick CdTe-Timepix3 detectors.

Herein, we evaluate the polarimetric performance of a 2.0 mm thick CdTe-Timepix3 detector with 256 × 256 pixels (55 μm pixel size, 1.98 cm² sensitive area, and featuring the Timepix3 chip). For gamma-ray beams, 100 % linearly polarized with energies within 100−300 keV, we explored a method for reconstructing Compton events based on Compton scattering kinematics. Using a selection process, we examined several empirical factors that influence the statistics in the azimuthal angular distribution of scattered gamma rays. Then, we present the response to unpolarized and polarized gamma rays, including beams with very close polarization angles. Finally, an estimate of the THOR-SR telescope's polarization sensitivity in Crab observations is presented.

The features of the CdTe-Timepix3 detector enable its operation as a fine Compton polarimeter, ensuring high modulation factors of ≈0.6 and a significantly greater Compton detection efficiency compared to other 2.0 mm thick pixelated CdTe/CZT polarimeters. The laboratory polarimeter exhibits a sub-degree angular sensitivity, capable of distinguishing polarization angles separated by <1.0°, and reveals a residual statistical polarization fraction of ≈0.03. Minimum detectable polarization <0.3 is foreseen in 20-day Crab observations with the THOR-SR Compton telescope.