Development of CCD and CMOS detector technology for X-ray spectroscopy

CCD技术在过去已经发展用于直接x射线探测，并且相当成熟，已经在x射线天文任务中飞行，如XMM， Swift和钱德拉。在这些CCD x射线探测器中，已经实现了所谓的“fano限制”性能，这意味着传感器的内在能量分辨率受到系统读取噪声和硅中的fano限制射击噪声的组合的限制，这是一个基本的物理限制。例如，这使得在6 keV下的半最大全宽（FWHM）分辨率达到~130 eV，并已用于为EPIC和Swift返回包含光子光谱信息的图像的非色散成像仪，具有很高的检测效率。这些设备确实有局限性，特别是最大计数率能力（受需要通过少量输出节点传输整个图像的限制），以及在空间质子损伤下的电荷传输效率，这会降低固有能量分辨率。除了空间科学之外，这些探测器还被用于一系列地面应用，从同步加速器研究和自由电子激光器，到工业x射线光谱学和生物医学成像。未来，对x射线天文学的需求可能会分成两大类：非色散成像，如XMM/EPIC或IXO的宽视场成像仪，其聚焦通量可能非常高；以及色散仪器，如XMM/RGS或IXO上的光栅光谱仪（XGS）。在过去的一年里，CEI为ESA领导了IXO上XGS的研究，并且很好地奠定了未来这项工作的基础，并且通过ESA/NASA/JAXA系统推进了这种大型天文望远镜的任务概念。色散型仪器目前选择的检测器仍然是CCD，而对于高通量应用（具有大光学），像素阵列更受青睐。我们目前有两名stfc资助的博士生，一名研究XEUS（现在的IXO） （Tutt）的x射线ccd，另一名研究e2v的新CMOS技术用于空间应用（Dryer），特别关注这些新传感器的空间辐射损伤的影响。然而，在CMOS博士学位的过程中，学生开发了使用单色x射线进行校准的测试技术，初步结果很有希望，在室温下产生~250 eV分辨率（传统的ccd可能需要冷却到-60℃以下才能达到类似的性能）。CCD技术更加成熟，提供了一个先进的参考（低噪声和良好的能量分辨率，像素匹配良好的应用，高检测效率，合理的辐射硬度）。较新的CMOS技术不太成熟，包括更高的噪声，非常小的像素，但具有非常高的帧速率，复杂的窗口读出模式的可能性和进一步的像素内信号处理的可能性。此外，在学生学习期间，e2v的未来发展将看到更高效率，更低噪音设计的发展，这应该更适合x射线应用。在这个博士学位的过程中，我们将推荐一位研究和开发x射线探测的候选人，用于未来的天文学应用，同时使用CCD和CMOS技术。该学生将使用现成的ccd作为他们工作和理解技术和仪器要求的基础，但将花费60 - 60%的时间用于开发CMOS成像仪技术，以实现真正有用的成像仪，能够对x射线光子进行成像光谱，用于科学应用。我们预计所做的工作将有益于其他科学领域，如同步加速器研究、太阳物理学和核聚变研究。该学生可以利用最近与瑞士PSI合作的博士生项目，获得同步加速器光束线。

CCD technology has been developed in the past for direct X-ray detection and is quite mature, having been flown on X-ray astronomy missions such as XMM, Swift and Chandra. In these CCD X-ray detectors, what is referred to as 'Fano-limited' performance has been achieved, meaning that the intrinsic energy resolution of the sensor is limited by a combination of the system read noise and the Fano-limited shot noise in silicon, which is a basic physical limit. This enables for example a full width at half maximum (FWHM) resolution of ~130 eV at 6 keV, and has been used in the non-dispersive imagers returning images that include photon spectroscopic information for EPIC and Swift with high detection efficieny. The devices do have limitations, particularly the maximum count rate capability (restricted by the need to transfer the whole image through a small number of output nodes), and in charge transfer efficiency under space proton damage which degrades the intrinsic energy resolution. Beyond Space Science, these detectors have found use in a range of terrestrial applications ranging from synchrotron research and free electron lasers, to industrial X-ray spectroscopy and bio-medical imaging. In future, the requirements for X-ray astronomy might be seen to diverge into two main classes, non-dispersive imaging as in the case of XMM/EPIC or the wide field imager for IXO, where the focused flux could be very high, and in dispersive instruments such as XMM/RGS or the grating spectrometer (XGS) on IXO. Over the last year, the CEI has lead the study of the XGS on IXO for ESA and is well placed to build upon this work in the future and the mission concept for such large astronomy telescopes progresses through the ESA/NASA/JAXA systems. The current detector of choice for the dispersive-type instrument is currently still the CCD, whilst for high throughput applications (with large optics), the pixel array is more favoured. We currently have two STFC-funded PhD students, one studying X-ray CCDs towards XEUS (now IXO) (Tutt), and another studying e2v's new CMOS technology for space applications (Dryer), particularly concentrating on the impact of space radiation damage in these new sensors. During the course of the CMOS PhD however, the student has developed test techniques using monochromatic X-rays for calibration, and the initial results are promising, yielding ~250 eV resolution at room temperature (where conventional CCDs may need to be cooled to below -60 degrees C to achieve similar performance). The CCD technology, being more mature, provides a state of the art reference (low noise with good energy resoution, pixels well matched to the application, high detection efficiency, reasonable radiation hardness). The newer CMOS technology is much less mature, comprising higher noise, very small pixels, but with very high frame rates, the possibility of complex windowing readout modes and further in-pixel signal processing possibilities. In addition, future developments at e2v during the course of the studentship will see the development of higher efficiency, lower noise designs, which should become much more suitable for X-ray applications. In the course of this PhD, we would propose a candidate who studied and developed X-ray detection for future astronomy applications using both CCD and CMOS technologies. The student would use the readily available CCDs as a foundation for their work and understanding of the technology and instrument requirements, but would spend >60% of their time on the development of CMOS imager technology toward a truly useful imager capable of performing imaging spectroscopy on X-ray photons for science applications. We would anticipate that the work performed would be of benefit to other science areas such as synchrotron research, solar physics, and fusion research. The student could take advantage of a recent PhD-student programme in collaboration with PSI, Switzerland, for access to synchrtron beamlines.