A Triple-GEM Time Projection Chamber for Wide Field-of-View Hard X-ray Polarimetry: First Results

TL;DR

This work demonstrates the feasibility of a CYGNO-inspired triple-GEM TPC with optical readout for wide-field hard X-ray polarimetry. By reconstructing photoelectron directions in the 10–60 keV range and extracting the modulation factor through deconvolved angular distributions, the study reports angular resolutions better than $<30^{\circ}$ above 10 keV and $<20^{\circ}$ for 20–60 keV, corresponding to modulation factors $μ$ of at least $0.6$ (10 keV) and $0.8$ (20–60 keV). Geant4-based simulations further show that Ar-rich gas mixtures can boost the figure of merit $μ\sqrt{ε}$ by up to a factor of ~4 compared with the CYGNO baseline, due to higher photoelectric cross sections. The results support the feasibility of rapid, wide-FoV hard X-ray polarimetry for transient events (e.g., GRBs, solar flares), with future refinements in gas composition, timing, and 3D tracking expected to enhance performance.

Abstract

We report on the development of a large-volume, wide field-of-view time projection chamber (TPC) for X-ray polarimetry, featuring a triple-GEM amplification stage and optical readout. Originally developed within the CYGNO program for directional dark matter searches, the system employs a scientific CMOS (sCMOS) camera and a photomultiplier tube (PMT) to collect secondary scintillation light produced during charge amplification. A prototype with a cylindrical active volume (radius 3.7 cm, height 5 cm) was tested at the INAF--IAPS calibration facility (Rome, Tor Vergata) to assess sensitivity to low-energy electron directionality. We fully reconstruct electrons in the 10-60 keV range, obtain angular resolutions as good as 15°, and infer modulation factors up to 0.9. These first results demonstrate robust photoelectron tracking at tens of keV with strong modulation, indicating that photoelectric-effect polarimetry can be extended to higher energies. This capability is promising for rapid transients (GRBs, solar flares) and would broaden the astrophysical reach of X-ray polarimetry.

Figures (15)

• Figure 1: Schematic of the detector used in this work.
• Figure 2: Photograph of the detector.
• Figure 3: Geant4 simulation geometry for comparison with data.
• Figure 4: Contained electron track as seen by the camera. The source position and direction is highlighted.
• Figure 5: First step on the reconstruction algorithm, clusters of pixels over the threshold are grouped.