摘要
采用蒙特卡罗程序Geant4模拟2~10 keV线偏振X射线光子在几种常用工作气体中的极化光电过程,明确了光电子出射位置、方位角分布与入射光子偏振方向、能量之间的响应关系。光电子的出射方向在入射光子偏振方向上的分布概率最高,且出射光电子的方位角分布可近似为余弦平方函数。光子能量增大时,各角度光电子计数不同程度地减少,但都呈现出在方位角为0或π(-π)时有极大值的统计规律。此外,揭示并量化了气体厚度、气体组成、气体体积分数之比和光子能量对探测效率的影响规律。气体厚度越大、平均原子序数越大,则探测效率越高。光子能量增大会导致探测效率降低,而对于由Xe或Ar组成的工作气体,当光子能量大于某壳层电子结合能时,由于相应壳层电子开始被弹射出,探测效率会有一定程度的提高。这些结果可为X射线偏振探测器的结构设计提供理论依据和数据支持。
Objective X-ray polarization detection is an important means to study the astrophysical properties of intense X-ray sources such as black holes,pulsars,and related gamma-ray bursts.The development of X-ray polarization detectors with excellent performance is the technical basis for related research.Early X-ray polarization detectors were mainly Thomson scattering polarimeters and Bragg polarimeters.However,due to the low modulation factor and narrow detection energy range,the ideal polarization measurement results were not obtained.In 2001,Costa et al.proposed a new way of X-ray polarization detection using the photoelectric effect,in which the X-ray polarization information was obtained by imaging the photoelectron track produced by X-ray photons through a gas detector.The polarimetric photoelectric process is the key physical process for the detector to realize polarization detection.It is of great significance to clarify the photon-gas interaction process and the distribution law of emitted photoelectrons for further understanding the working mechanism of the detector.The polarimetric photoelectric process is an important research content in the development of this type of Xray polarization detector.Different types of gases have various properties,which will affect the particle transport in the polarimetric photoelectric process and further leads to different detection efficiencies.Therefore,it is necessary to simulate the polarimetric photoelectric process under different conditions.This can provide a theoretical basis and data support for the structure design of X-ray polarization detectors.Methods We simulate the polarimetric photoelectric process of 2-10 keV linearly polarized X-ray photons in several commonly used working gases by the Monte Carlo code Geant4.The selected working gas combinations include He+C_(3)H_(8),Ne+CF_(4),Ne+DME,Ar+CH_(4),Ar+CO_(2),Xe+CO_(2),CF_(4)+C_(4)H_(10),and DME+CO_(2).The response relationship of the emission position and azimuthal angle distribution of photoelectron with the polarization direction and energy of the incident photon is discussed.Moreover,the effects of gas thickness,gas component,gas ratio,and photon energy on the detection efficiency are analyzed.Results and Discussions First,the response relationship of the emission position and azimuthal angle distribution of the photoelectron with the polarization direction and energy of the incident photon is clarified.The emission direction distribution probability of the photoelectron is the largest in the polarization direction of the incident photon,and the azimuthal angle distribution can be approximated as a cosine squared function.With the increase in photon energy,the counts of photoelectrons at each angle decrease in different degrees,but all of them show a statistical law that the maximum values occur when the azimuthal angle is 0 orπ(-π)(Fig.6).Moreover,the effects of gas thickness,gas component,gas ratio,and photon energy on the detection efficiency are revealed and quantified.For 2 keV photons entering into 90%Ne+10%DME gas mixture,when the gas thickness is small,the detection efficiency increases rapidly with the increase in gas thickness,from less than 0.1 at 0.1 cm to 0.64 at 1 cm(Fig.7).When the gas thickness increases to 3 cm,the detection efficiency is greater than 0.9.Then,with the increase in gas thickness,the detection efficiency gradually approaches 1.For the CF_(4)+C_(4)H_(10),Ne+CF_(4),Ne+DME,DME+CO_(2),and He+C_(3)H_(8),the detection efficiency decreases with the increase in photon energy,and the large average atomic number of gas can lead to a high detection efficiency(Fig.8).While for the Xe+CO_(2),Ar+CO_(2),and Ar+CH_(4),when the photon energy is greater than the binding energy of certain shell electrons of Xe or Ar atoms,the detection efficiency will be improved to a certain extent because the corresponding shell electrons begin to be ejected.In addition to the Ar+CO_(2)which is affected by the electron emission in K-shell,the detection efficiency in each energy range can be effectively improved by increasing the proportion of gas with high atomic number(Fig.9).Conclusions We simulate the polarimetric photoelectric process of 2-10 keV linearly polarized X-ray photons in several commonly used working gases by the Monte Carlo code Geant4.The response relationship of the emission position and azimuthal angle distribution of the photoelectron with the polarization direction and energy of the incident photon is clarified.The emission direction distribution probability of the photoelectron is the largest on the polarization direction of the incident photon,and the azimuthal angle distribution can be approximated as a cosine squared function.With the increase in photon energy,the counts of photoelectrons at each angle decrease in different degrees,but all of them show a statistical law that the maximum values occur when the azimuthal angle is 0 orπ(-π).Moreover,the effects of gas thickness,gas component,gas ratio,and photon energy on the detection efficiency are revealed and quantified.The larger gas thickness and larger average atomic number can lead to higher detection efficiency.In addition,the increase in photon energy can result in a decrease in detection efficiency.However,for the working gases composed of Xe or Ar,when the photon energy is greater than the binding energy of a certain shell electron,the detection efficiency will be improved to a certain extent because the corresponding shell electrons begin to be ejected.The results in this paper can provide some theoretical basis and data support for the structure design of X-ray polarization detectors.In the actual selection of working gases,the drift properties of electrons in gases,the effect of photoelectron drift and diffusion on track thickness and length,and the reconstruction efficiency of the track reconstruction algorithm should also be considered.
作者
郑人洲
强鹏飞
盛立志
闫永清
Zheng Renzhou;Qiang Pengfei;Sheng Lizhi;Yan Yongqing(State Key Laboratory of Transient Optics and Photonics,Xi'an Institute of Optics and Precision Mechanics,Chinese Academy of Sciences,Xi'an 710119,Shaanxi,China)
出处
《光学学报》
EI
CAS
CSCD
北大核心
2024年第3期321-328,共8页
Acta Optica Sinica
基金
国家自然科学基金(42327802,62271483)
国家重点实验室基金(SKLIPR2021)
陕西省自然科学基础研究计划重点项目(2023-JC-ZD-40)。
关键词
X射线光学
X射线偏振探测
气体探测器
极化光电过程
探测效率
X-ray optics
X-ray polarization detection
gas detector
polarimetric photoelectric process
detection efficiency
