Sensitivity of stacked imaging detectors to hard X-ray polarization

TL;DR

This paper evaluates the potential of stacked imaging detectors for hard X-ray polarization measurements, combining analytical calculations and Monte Carlo simulations to assess their sensitivity and effectiveness.

Contribution

It provides the first detailed analysis of the sensitivity of stacked imaging detectors for hard X-ray polarization detection, integrating geometric and simulation-based approaches.

Findings

Analytical and simulation results show the potential of stacked detectors for polarization sensitivity.

Efficiency and modulation factor are comparable between analytical models and Geant4 simulations.

The study offers insights into optimizing detector design for future X-ray polarimetry missions.

Abstract

The development of multi-layer optics which allow to focus photons up to 100 keV and more promises an enormous jump in sensitivity in the hard X-ray energy band. This technology is already planned to be exploited by future missions dedicated to spectroscopy and imaging at energies >10 keV, e.g. Astro-H and NuSTAR. Nevertheless, our understanding of the hard X-ray sky would greatly benefit from carrying out contemporaneous polarimetric measurements, because the study of hard spectral tails and of polarized emission often are two complementary diagnostics of the same non-thermal and acceleration processes. At energies above a few tens of keV, the preferred technique to detect polarization involves the determination of photon directions after a Compton scattering. Many authors have asserted that stacked detectors with imaging capabilities can be exploited for this purpose. If it is possible to discriminate those events which initially interact in the first detector by Compton scattering and are subsequently absorbed by the second layer, the direction of scattering is singled out from the hit pixels in the two detectors. In this paper we give the first detailed discussion of the sensitivity of such a generic design to the X-ray polarization. The efficiency and the modulation factor are calculated analytically from the geometry of the instruments and then compared with the performance as derived by means of Geant4 Monte Carlo simulations.