Studies of the linearity of the readout module assembling using the eXTP-GPD chips

Background The Gas Pixel Detector (GPD) is a key part of the Polarimetry Focusing Array (PFA) sub-system onboard the enhance X-ray Timing and Polarimetry mission (eXTP). The GPD can accurately provide the polarized angle and degree for incoming soft X-rays (requirements of minimum detectable polarization <1% (10⁶ s, 1 mCrab)) from celestial objects under extreme conditions of density, gravity and magnetism.Purpose Paste the GPD chips on a matched ceramic package is one of the most important procedures for the GPD assembly, and the parallelism between the top surface of an ASIC chip and the bottom surface of a Gas Electron Multiplier (GEM) foil can have impact on the distribution of electric field within transfer region and reconstruction of photoelectric trajectories, resulting in deterioration of the GPD sensitivity. This study quantifies the influence and gives a reasonable expectation of the parallelism.Methods A simulation framework of the GPD is constructed to quantify the influence on the GPD performance and values of the parallelism are measured by a gauge with a position resolution of 0.1μm.Results: The difference of degree of modulation is less than 1% comparing the parallelism with a value of 20 μm to a value of 0μm in response to both polarized and unpolarized X-rays. Meanwhile, the value of parallelism can reach 6.9 ±3.4μm (0.0176° ± 0.0087°) by testing and verifying.Conclusions The impact on the GPD performance can be less than 1% as the parallelism with an order of 20 μm, and we also give a reasonable measuring method and verify the controllability and feasibility of the parallelism with an order of 20 μm for pasting the ASIC chip on the surface of a ceramic package.