Track Reconstruction And Simulation Optimization For Soft X-ray Polarimeter

X-ray polarization measurement is an important frontier in the research of astrophysical detection technology.Guangxi Key Laboratory of Relativistic Astrophysics cooperating with the Chinese Academy of Sciences and other Chinese universities develops basic research on X-ray polarization detection technology.In order to detect the polarization information of soft X-rays from transient sources such as gamma-ray bursts,the Soft X-ray Polarimeter(SXP)projecting on the Chinese Space Station was proposed,which aims at the further study of fundamental scientific issues such as central engine,jet structure,burst environment and radiation physics of transient sources.In the first chapter of this thesis,the scientific significance,the detection technology and the progress of related international and domestic projects are reviewed.In the second chapter,the scientific goal and the overall situation for the SXP are introduced.Then,the third and fourth chapters focus on our research results.In Chapter 3,we discuss track reconstruction and simulation optimization for the SXP.X-ray photons produce photoelectrons through the photoelectric effect,which are amplified by a gas detector to generate an electron track that can be collected.The polarization information of the X-ray source can be measured by collecting its track image and reconstructing it with an algorithm to determine the initial emission angle at the time of photoelectron generation.We use a joint simulation of multiple software to obtain two-dimensional track images that are close to the experimental results,and these track images with known truth values are used to optimize our algorithm.The SXP reconstruction process includes the events selection and the reconstruction of the remaining events.The algorithm uses the Bragg peak at the tail of the electron track to find the position of the head of the track,and then mathematically calculates the initial emission angle of the photoelectrons.We find that in the 7-10 ke V energy range,with a modulation factor about57% for 100% polarized X-ray source,the algorithm is highly extended and competent under non-ideal conditions(including the detector wobble,the non-zero azimuth Angle of the source,the partially polarized sources,the background effects,etc.).The results of the experimental data reconstruction verify the feasibility of the algorithm.It provides a theoretical foundation for further optimization and upgrading of the algorithm for more accurate screening and processing of experimental tracks.In Chapter 4,we introduce the design optimization and background analysis for the SXP.The GEANT4 software package is used to build a mass model of the SXP.We provide guidance for SXP design by simulating detection efficiency.Base on this model,the effect of the background particles in the space environment on the detector is studied.The background level of the SXP is estimated by the theoretical background model and observations data from satellites on orbit,and determine that existing shielding measures can ensure the relatively low level of the background counts to the SXP.Based on the existing structure,we find that the minimum observation time which is required to measure polarization below 19% in the case of Crab is 20 kiloseconds.The conclusion of the simulation provides a guarantee for the feasibility of the SXP.