Calibration of the Gamma-RAy Polarimeter Experiment (GRAPE) at a Polarized Hard X-Ray Beam

TL;DR

The paper details the calibration and testing of the GRAPE hard X-ray polarimeter prototype using a polarized beam, demonstrating promising modulation factors for future astrophysical polarization measurements.

Contribution

It presents the first calibration of the GRAPE prototype with a polarized X-ray beam, validating its design and simulation predictions for astrophysical polarization detection.

Findings

Modulation factors of ~0.46 at 69.5 keV and ~0.48 at 129.5 keV

Good agreement between experimental results and Monte Carlo simulations

Successful calibration of the prototype with polarized X-ray beam

Abstract

The Gamma-RAy Polarimeter Experiment (GRAPE) is a concept for an astronomical hard X-ray Compton polarimeter operating in the 50 - 500 keV energy band. The instrument has been optimized for wide-field polarization measurements of transient outbursts from energetic astrophysical objects such as gamma-ray bursts and solar flares. The GRAPE instrument is composed of identical modules, each of which consists of an array of scintillator elements read out by a multi-anode photomultiplier tube (MAPMT). Incident photons Compton scatter in plastic scintillator elements and are subsequently absorbed in inorganic scintillator elements; a net polarization signal is revealed by a characteristic asymmetry in the azimuthal scattering angles. We have constructed a prototype GRAPE module containing a single CsI(Na) calorimeter element, at the center of the MAPMT, surrounded by 60 plastic elements. The prototype has been combined with custom readout electronics and software to create a complete "engineering model" of the GRAPE instrument. This engineering model has been calibrated using a nearly 100% polarized hard X-ray beam at the Advanced Photon Source at Argonne National Laboratory. We find modulation factors of 0.46 +/- 0.06 and 0.48 +/- 0.03 at 69.5 keV and 129.5 keV, respectively, in good agreement with Monte Carlo simulations. In this paper we present details of the beam test, data analysis, and simulations, and discuss the implications of our results for the further development of the GRAPE concept.