Characterization of scatterers for an active focal plane Compton polarimeter

TL;DR

This paper presents the first experimental characterization of an active Compton polarimeter for hard X-ray polarimetry, demonstrating its feasibility and sensitivity with a low energy threshold and potential for astrophysical observations.

Contribution

It provides experimental measurements of tagging efficiency and evaluates the sensitivity of a novel polarimeter design for hard X-ray polarimetry.

Findings

Demonstrated feasibility of polarimetry with a 20 keV threshold.

Estimated a 10% Minimum Detectable Polarization for a 10 mCrab source.

Characterized scattering materials for polarimeter sensitivity.

Abstract

In this work we present an active Compton scattering polarimeter as a focal plane instrument able to extend the X-ray polarimetry towards hard X-rays. Other authors have already studied various instrument design by means of Monte Carlo simulations, in this work we will show for the first time the experimental measurements of "tagging efficiency" aimed to evaluate the polarimeter sensitivity as a function of energy. We performed a characterization of different scattering materials by measuring the tagging efficiency that was used as an input to the Monte Carlo simulation. Then we calculated the sensitivity to polarization of a design based on the laboratory set-up. Despite the geometry tested is not optimized for a realistic focal plane instrument, we demonstrated the feasibility of polarimetry with a low energy threshold of 20 keV. Moreover we evaluated a Minimum Detectable Polarization of 10% for a 10 mCrab source in 100 ks between 20 and 80 keV in the focal plane of one multilayer optics module of NuSTAR. The configuration used consisted of a doped p-terphenyl scatterer 3 cm long and 0.7 cm of diameter coupled with a 0.2 cm thick LaBr3 absorber.