硬X射线调制望远镜低能探测器量子效率标定

摘要

低能X射线望远镜是硬X射线调制望远镜卫星的主要载荷之一,探测器采用CCD236.探测器的量子效率会影响能谱拟合和绝对流量,有必要对其进行标定.利用55Fe放射源,以硅漂移探测器为标准探测器,标定了CCD236在Mn-Kα (5.899 keV)和Mn-Kβ (6.497 keV)能量点处的量子效率,此能段在Fe线附近,对X射线天文观测有重要价值.考虑探测器的分裂事例后,Mn-Kα和Mn-Kβ处的量子效率分别为71％和62％.在-95--30℃C工作温度范围内,CCD量子效率与温度无关.利用CCD236的结构及实测的量子效率,不考虑沟阻影响,得到耗尽层厚度为38μm.对CCD236施加不同的电压,其量子效率基本不变,表明其在两相驱动下高低电平的耗尽层厚度相等,进而说明CCD236一直工作在深耗尽状态,其耗尽层到了外延层和衬底层边界,己达最大值.%Low energy X-ray telescope,working over 0.7-15 keV energy band,is one of the main payloads in the hard X-ray modulation telescope satellite.The primary scientific objectives are to survey large sky area to investigate galactic X-ray transient sources as well as the cosmic X-ray background,and to observe X-ray binaries or black holes for studying the dynamics and emission mechanism in strong gravitational or magnetic field.The detector of low energy X-ray telescope is CCD236,a new generation of swept charge device,which has good time and energy resolution.Quantum efficiency (QE) of the detector has a crucial influence on X-ray spectrum fitting and absolute luminosity calculation.To provide valuable scientific data,QE should be calibrated in detail.In this paper,QE calibration is accomplished with respect to a silicon drift detector (SDD),using an Fe-55 radioactive source,at energy points Mn-Kα (5.899 keV) and Mn-Kβ (6.497 keV).The energies of Mn-Kα and Mn-Kβ are near that of iron-K,which is an important line in X-ray observation.Additionally,Mn-Kα and Mn-Kβ X-ray will partially pass through the depletion region of CCD236,and these energy points can be used to measure the depletion thickness.This experiment is set up in a vacuum cooling chamber.The X-ray source perpendicularly illuminates SDD and CCD236 through a small hole,whose area is far less than those of two detectors;therefore,QE measurements are irrelevant to neither the distance nor the azimuth angle between the X-ray source and the detector.For CCD236,split events should be corrected.Energy spectra of SDD and CCD236 are fitted with two Gaussian distributions,respectively,to obtain peak positions and standard variations of Mn-Kα and Mn-Kβ.With known structure of SDD,the QE of CCD236 can be calculated.QE values at Mn-Kα and Mn-Kβ are 71％ and 62％,respectively.QE and temperature are uncorrelated with each other in a temperature range from-95 ℃ to-30 ℃.According to the specific structure of CCD236 and the measured QE,without considering the effect of channel stop,the best-fit thickness of depletion region is obtained to be 38 [μm.When CCD236 is applied with different driving or substrate voltages,no obvious variation of QE is observed.It indicates that the thickness values of depletion region with high and low level voltages are equal.Furthermore,it shows that working CCD236 is deep depleted,and the thickness of depletion region will not change because it reaches its maximum,the edge of epitaxial layer and substrate layer.