Development And In-Orbit Validation Of Widefield X-Ray Polarimeter

X-ray polarization detection is a critical technology for uncovering astrophysical processes.It plays a vital role in studying the geometric structure,magnetic field configuration,particle acceleration mechanisms,and radiation mechanisms of transient sources such as Gamma-Ray Bursts（GRBs）.Considering the random outbursts,short duration,and high instantaneous flux characteristic of transient sources,the Low-Energy Polarization Detector（LPD）payload of the Chinese Space Station’s POLAR-2 experiment is specifically designed with a 90°field of view.The detector utilizes a high-sensitivity,wide-field,large-area sky survey strategy to accurately measure the X-ray polarization of GRBs and their early afterglows.To meet the technical requirements of LPD payload observations,this paper presents the development of a high-sensitivity,large-area splicing X-ray polarization detector unit.This unit is based on the photoelectric effect principle and incorporates micropattern gas detector technology.To adapt to the space environment,the detector employs advanced assembly technology to achieve vacuum sealing of the working gas.Verification of wide-field X-ray polarization observation by the detector is accomplished in orbit using the Cube Sat platform.The purpose of this study is to develop a novel high-sensitivity X-ray polarization detector,providing directly applicable wide-field X-ray polarization survey technology for the LPD payload.Considering the fluctuating nature of X-ray flux from transient sources,a novel gas electron multiplier,GMCP,has been successfully developed and integrated into the micropattern gas detector for the first time.Through hydrogen reduction treatment,the GMCP has been endowed with bulk resistivity characteristics,effectively mitigating the impact of charging-up on gain performance.This results in the detector achieving gain stability better than 5%during sudden changes in the counting rate.In conducting X-ray polarization measurement research based on the photoelectric effect,we combined Thick-Gas Electron Multiplier（THGEM）and GMCP detectors with Topmetal-II chips.Due to its finer structure,GMCP demonstrated higher polarization measurement sensitivity compared to THGEM.Given the high sensitivity and absence of charging-up effects in GMCP polarization detectors,we have confirmed the superiority of combining GMCP with Topmetal-II pixel chips as X-ray polarization detectors.This new type of gas detector has been named the Gas Microchannel Plate Pixel Detector（GMPD）.To meet the requirements for space applications,this paper presents the design of a sealed gas,compact GMPD detector.The assembly process involved rigorous control over material selection,device cleaning and storage,vacuum baking for outgassing,and gas filling procedures.Advanced assembly techniques such as brazing and laser welding were employed to control the leakage rate within 7.6×10^-11 Torr·L/s,with the main contaminant（H₂O）partial pressure reduced to 7.0×10^-8Torr,providing a solid foundation for the long-term stable operation of the detector in space.Performance tests demonstrated that the GMPD detector exhibited excellent short-term stability（better than 5%）conditions,with stable gains and energy resolution maintained over a period of 370days.The polarization measurement modulation factor reached 41.28%（@4.5 ke V）,and the position resolution was 187.7μm（@6.4 ke V）.The development of the Cosmic X-ray Polarimetry Detector（CXPD）Cube Sat payload leverages the mature Cube Sat platform for in-orbit validation.The payload integrates key components,including the anti-coincidence detector,GMPD detector,electronic system,and mechanical support structure.At its core,the GMPD detector captures photoelectron track images and provides essential energy and polarization information.During the development process,the mechanical,thermal,and electromagnetic compatibility of the payload were optimized to ensure high reliability in space operations.An X-ray polarization calibration platform was constructed in the laboratory to provide comprehensive calibration for the GMPD detector,establishing a database of key performance parameters.Mechanical vibration and thermal vacuum tests were conducted to simulate the extreme environments of space,verifying the stability and reliability of the payload.The CXPD Cube Sat,successfully launched on June 7,2023,completed in-orbit verification of the GMPD detector assembly technology,anti-coincidence detector,and electronic system.It also conducted wide-field X-ray polarization observation experiments.The CXPD Cube Sat effectively verified the mission of wide-field X-ray polarization detection.The research results presented in this paper provide key technical support for the development of the LPD payload for the Chinese space station.