Optimization of the Design of the Hard X-ray Polarimeter X-Calibur

TL;DR

This paper details the optimization process of the X-Calibur hard X-ray polarimeter for balloon flights, enhancing its design for improved sensitivity and efficiency in detecting polarized X-ray photons.

Contribution

The paper introduces a new optimized design of X-Calibur based on Monte Carlo simulations, improving detection efficiency and sensitivity for balloon-borne X-ray polarization measurements.

Findings

Achieves near-unity detection efficiency.

Sensitivity close to theoretical maximum.

Optimized design suitable for balloon and satellite missions.

Abstract

We report on the optimization of the hard X-ray polarimeter X-Calibur for a high-altitude balloon-flight in the focal plane of the InFOC{\mu}S X-ray telescope from Fort Sumner (NM) in Fall 2013. X-Calibur combines a low-Z scintillator slab to Compton-scatter photons with a high-Z Cadmium Zinc Telluride (CZT) detector assembly to photo-absorb the scattered photons. The detector makes use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity and reaches a sensitivity close to the best theoretically possible. In this paper, we discuss the optimization of the design of the instrument based on Monte Carlo simulations of polarized and unpolarized X-ray beams and of the most important background components. We calculate the sensitivity of the polarimeter for the upcoming balloon flight from Fort Sumner and for additional longer balloon flights with higher throughput mirrors. We conclude by emphasizing that Compton polarimeters on satellite borne missions can be used down to energies of a few keV.