Study of the Polarimetric Performance of a Si/CdTe Semiconductor Compton Camera for the Hitomi Satellite

TL;DR

This study evaluates the polarimetric performance of a Si/CdTe Compton camera prototype for the Hitomi satellite, demonstrating its capability to measure gamma-ray polarization with high precision and low systematic uncertainty.

Contribution

It provides the first beam test results of the SGD Compton camera prototype, validating its polarimetric performance and calibration through Monte Carlo simulations.

Findings

Modulation factor measured between 0.637 and 0.701 at 122.2 keV and 194.5 keV.

Systematic uncertainty of the modulation factor is approximately 3%.

Demonstrates the camera's potential for gamma-ray polarization measurements in space.

Abstract

Gamma-ray polarization offers a unique probes into the geometry of the gamma-ray emission process in celestial objects. The Soft Gamma-ray Detector (SGD) onboard the X-ray observatory Hitomi is a Si/CdTe Compton camera and is expected to be an excellent polarimeter, as well as a highly sensitive spectrometer due to its good angular coverage and resolution for Compton scattering. A beam test of the final-prototype for the SGD Compton camera was conducted to demonstrate its polarimetric capability and to verify and calibrate the Monte Carlo simulation of the instrument. The modulation factor of the SGD prototype camera, evaluated for the inner and outer parts of the CdTe sensors as absorbers, was measured to be 0.649--0.701 (inner part) and 0.637--0.653 (outer part) at 122.2 keV and 0.610--0.651 (inner part) and 0.564--0.592 (outer part) at 194.5 keV at varying polarization angles with respect to the detector. This indicates that the relative systematic uncertainty of the modulation factor is as small as ~3%.