Low temperature testing and neutron irradiation of a swept charge device on board the HXMT satellite

We present the low temperature testing of an SCD detector, investigating its performance such as readout noise, energy resolution at 5.9 keV and dark current. The SCD's performance is closely related to temperature, and the temperature range of -80 ℃ to -50 ℃ is the best choice, where the FWHM at 5.9 keV is about 130 eV. The influence of the neutron irradiation from an electrostatic accelerator with fluence up to 1×10⁹ cm^-2 has been examined. We find the SCD is not vulnerable to neutron irradiation. The detailed operations of the SCD and the test results of low temperature are reported, and the results of neutron irradiation are discussed.     

面向脉冲星导航的聚焦型X射线探测器测试标定方法研究

通过脉冲星辐射信号特征研究和空间观测需求分析,提出了一种面向导航应用的X射线探测器测试方法.首先推导了X射线光子欠探测概率公式,分析了不同星源流量及不同探测器时间分辨率下对光子探测能力的影响.通过数值模拟方法建立了脉冲到达时间与脉冲轮廓相似度的关系.处理了我国硬X射线调制望远镜的Crab脉冲星观测数据,研究了不同能段脉冲轮廓差异.其次,系统地研究了面向导航应用的X射线探测器测试及处理方法,并利用地面测试系统完成了一款自主研发的聚焦型X射线探测器测试工作.通过数据分析得到,聚焦型探测器本底噪声为3.63×10-5ph/(cm~2·s~(-1)),工作能区为0.2～22.7keV,时间分辨率为4.17μs,空间响应约为5′,能量非线性为0.52%,能量分辨率优于200eV@5.7keV,典型探测效率为39.18%@4.51keV.聚焦型X射线探测器在弱脉冲信号及强背景噪声下,均能还原出Crab脉冲星脉冲轮廓,在2 400s内能够探测到辐射流量弱于背景噪声10倍的脉冲信号.结果表明,该款聚焦型探测器性能优秀,能够满足导航脉冲星(如PSR B1509)的空间观测需求,也验证了测试方法的可行性.     

Measurements of charge transfer ef f iciency in a proton-irradiated swept charge device

Charged Coupled Devices(CCDs) have been successfully used in several low energy X-ray astronomical satellites over the past two decades. Their high energy resolution and high spatial resolution make them a perfect tool for low energy astronomy, such as observing the formation of galaxy clusters and the environment around black holes. The Low Energy X-ray Telescope(LE) group is developing a Swept Charge Device(SCD) for the Hard Xray Modulation Telescope(HXMT) satellite. A SCD is a special low energy X-ray CCD, which can be read out a thousand times faster than traditional CCDs, simultaneously keeping excellent energy resolution. A test method for measuring the charge transfer efficiency(CTE) of a prototype SCD has been set up. Studies of the charge transfer inefficiency(CTI) with a proton-irradiated SCD have been performed at a range of operating temperatures. The SCD is irradiated by 3×108cm-210 MeV protons.     

Design and optimization of the readout system for X-ray CCDs

A readout system for X-ray CCDs based on an improved architecture is presented; by optimizing several critical circuit blocks along the analog signal chain, the conflict between the readout speed and readout noise is greatly alleviated. Using CCD47-10 as its target CCD, the readout system has achieved 8.6e^- readout noise and 142 eV FWHM at 5.9 keV Mn m K_α under a pixel rate of 80 kHz. Also its performance of imaging has been investigated.     

硬X射线调制望远镜低能探测器量子效率标定

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~(55)Fe放射源,以硅漂移探测器为标准探测器,标定了CCD236在Mn-K_α(5.899 keV)和Mn-K_β(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-K_α和Mn-K_β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,已达最大值.     

爱因斯坦探针后随X射线聚焦镜的粒子污染仿真

X射线聚焦镜是一种对粒子污染物颗粒极其敏感的设备,通常依靠控制地面各流程的污染物积累来限制其影响.本文根据粒子污染物的特点,结合Mie散射理论构建了聚焦镜片上的粒子污染颗粒物与入射X射线光子的相互作用模型.在此基础上,结合蒙特卡罗法进行光线追迹工作,构建出爱因斯坦探针后随X射线聚焦镜的粒子污染仿真程序.通过仿真计算得到入射X射线光子与粒子污染物相互作用时的反应截面与散射角分布函数.这两个量与粒子污染物的尺度相关联,本文将聚焦镜片样本上粒子污染颗粒尺度分布的测量结果进行拟合,得到粒子污染尺度分布.根据上述结果进行仿真计算,得到粒子污染物密度与聚焦镜的有效面积和角分辨的关系,并使用防污染监测数据和爱因斯坦探针后随X射线聚焦镜性能测试数据验证了仿真结果的可靠性.对爱因斯坦探针后随X射线聚焦镜粒子污染的仿真实现了对粒子污染物影响的定量分析,明确了粒子污染物对聚焦镜各方面性能的具体影响,为防污染工作提供了理论支持.     

Wolter-I型聚焦镜热变形数值研究

为了给聚焦镜热控及支撑结构优化设计提供依据,对爱因斯坦探针项目中的后随X射线望远镜的镜片组进行了三维全尺寸建模与有限元分析.研究了轴向、径向两种温度梯度,以及有、无支撑结构对镜片组形变的影响,并对面形误差与温差范围的关系进行了探究.结果表明:对于无支撑结构,轴向温度梯度下面形误差与半径关系接近线性关系,径向温度梯度下则接近分段二次关系;支撑结构发生热变形时,会使镜片组产生与之对应的面形误差,令同相圆度误差转变为异相圆度误差,并使镜片组整体面型误差峰谷值增加32.25%~123.01%,均方根值增加4.13%~5.14%;对于有支撑结构,热致面形误差与温差成正比,温差每增加1℃,轴向温度梯度下,面形误差峰谷值与均方根值分别增加7.76μm、1.12μm,而对于径向温度梯度则分别增加9.67μm、1.60μm.聚焦镜片热致面形误差在一定情况下与镜片尺寸、温差成线性关系,并受到支撑结构变形的显著影响.     

EP-FXT聚焦镜真实表面状态的性能模拟方法

后随X射线望远镜(follow-up X-ray telescope, FXT)是爱因斯坦探针卫星的主要载荷之一.为了获取高信噪比的数据,实现对观测天体的高精度定位, FXT使用Wolter-Ⅰ型X射线聚焦光学系统,该系统一直是X射线空间天文观测中的重要设备.根据Wolter-Ⅰ型的聚焦原理,结合实际的加工特点,利用蒙特卡罗模拟算法对影响光学成像质量的几个关键参量,如表面粗糙度、面形误差进行了模拟,结合模拟结果对各参量的作用效果进行了分析.之后利用PANTER实验室提供的聚焦镜性能测试结果对模拟方法进行了验证,同时对面形误差参量进行了限制.最终聚焦镜结构热控件半能量宽度(half energy width, HEW)模拟与实测结果基本一致.该模拟过程可以很有效地应用于聚焦镜加工工艺的摸索,为FXT的聚焦镜测试和标定工作提供参考.结合实测标定数据,该模拟方法生成的有效面积、渐晕和点扩散函数等可用于在轨观测标定数据库.     

Proton irradiation effect on SCDs

The Low Energy X-ray Telescope is one of the main payloads on the Hard X-ray Modulation Telescope satellite.Swept charge devices(SCDs)are selected as detectors for the Low Energy X-ray Telescope.As SCDs are sensitive to proton irradiation,irradiation tests were carried out on the HI-13 accelerator at the China Institute of Atomic Energy.The beam energy was measured to be 10 MeV at the SCD.The proton fluence delivered to the SCD was 3×108protons/cm2over two hours.By comparing the performance before and after irradiation,it is concluded that proton irradiation affects both the dark current and the charge transfer inefficiency of the SCD.The energy resolution of the proton-irradiated SCD is 212 eV@5.9 keV at-60?C,while it before irradiated is 134 eV.Moreover,better performance can be reached by lowering the operating temperature of the SCD in orbit.     

A digital CDS technique and its performance testing

Readout noise is a critical parameter for characterizing the performance of charge-coupled devices (CCDs), which can be greatly reduced by the correlated double sampling (CDS) circuit. However, a conventional CDS circuit inevitably introduces new noise since it consists of several active analog components such as operational amplifiers. This paper proposes a digital CDS circuit technique, which transforms the pre-amplified CCD signal into a train of digital presentations by a high-speed data acquisition card directly without the noisy CDS circuit, then implements the digital CDS algorithm through a numerical method. A readout noise of 3.3 e^- and an energy resolution of 121 eV@5.9 keV can be achieved via the digital CDS technique.