Assembly and test of the gas pixel detector for X-ray polarimetry

Abstract

The gas pixel detector (GPD) dedicated for photoelectric X-ray polarimetry is selected as the focal plane detector for the ESA medium-class mission concept X-ray Imaging and Polarimetry Explorer (XIPE). Here we show the design, assembly, and preliminary test results of a small GPD for the purpose of gas mixture optimization needed for the phase A study of XIPE. The detector is assembled in house at Tsinghua University following a design by the INFN-Pisa group. The improved detector design results in a good uniformity for the electric field. Filled with pure dimethyl ether (DME) at 0.8 atm, the measured energy resolution is 18% at 6 keV and inversely scales with the square root of the X-ray energy. The measured modulation factor is well consistent with that from simulation, up to ∼0.6 above 6 keV. The residual modulation is found to be 0.30% ± 0.15% at 6 keV for the whole sensitive area, which can be translated into a systematic error of less than 1% for polarization measurement at a confidence level of 99%. The position resolution of the detector is about 80 µm in FWHM, consistent with previous studies and sufficient for XIPE requirements.

PolarLight: a CubeSat X-ray polarimeter based on the gas pixel detector

“…The GPD for PolarLight is based on the design introduced by the INFN-Pisa group [5], and is similar to the one reported in Li et al [11]. It is a 2D gas proportional counter with pixel readout to measure the track image of photoelectrons emitted following the absorption of X-rays.…”

“…The GPD polarimeter has the advantage of compactness and low mass, and can be operated at roomtemperature with a total power of about 2 W. These indicate that it can easily fit into a CubeSat spacecraft. Thus, based on the GPD tested in the lab [5,11], we modified the design and interface to be compatible with a 6U CubeSat developed by Spacety Co. Ltd, trying to test the technique in orbit (see Figure 1). The detector occupies a standard unit of the CubeSat and is named Polarimeter Light (PolarLight), but it is not the only payload of the CubeSat.…”

eXTP: Enhanced X-ray Timing and Polarization mission

“…The ASIC, a development of INFN-Pisa, has 105600 pixels at 50 μm pitch, and it is at its third generation of development [14]. A prototype of the detector has been recently assembled and tested at Tsinghua University following a design by the INFN-Pisa group [11]. Thanks to an improved design a very careful manufacturing a good uniformity of the electric field has been obtained.…”

“…The systematic error for polarization measurement is less than 1% (at a confidence level of 99%). We refer to [11] for more details. The energy range of the PFA is 2-10 keV, and the time resolution is 500 μs.…”

“…The energy range of the PFA is 2-10 keV, and the time resolution is 500 μs. The sensitivity is about 5 μCrab for an exposure of 10 4 s. The composed picture has been adapted from [11]. The GPD prototype has been extensively tested and the performance is excellent.…”