Cryogenic irradiations, a more realistic study of the impact of radiation on detectors in space

电荷耦合器件(CCD)长期以来一直是空间天文任务的首选探测器。当放置在轨道上时，这些设备可能会经历恶劣的辐射环境。入射到探测器上的辐射会破坏硅晶格，产生缺陷，从而形成“陷阱”。形成的陷阱可以捕获从器件外部检测到的信号，导致CCD中的拖尾和信号损失，以及CMOS传感器中的电荷损失和明亮像素，以及增加的噪声源，如暗电流。在发射任何任务之前，至关重要的是详细研究辐射的影响，以便在开发新的读出技术和校正算法的同时使用适当的屏蔽来减轻剩余的损害。目前，标准做法规定，器件的辐射测试是在室温下进行的，尽管探测器在轨道运行期间将保持低温，温度往往低至-120摄氏度。辐照后产生的空位在器件中迁移，直到它们达到稳定状态，但任何特定缺陷结构的稳定性都强烈依赖于温度；室温下存在的陷阱可能与器件受到冷照射并保持低温时的陷阱有很大不同。关于“低温辐射”的文献很少，但有报道称，与更现实的低温辐射相比，室温下辐射的影响相差2-3倍，这种差异(增加或减少的影响)的方向因设备类型而异。欧几里得任务的目标是绘制暗物质和能量图，它依赖于对辐射对探测器的影响的全面和彻底的了解，以便对辐射进行修正，将影响减少300倍。哈勃能够进行大约30倍的修正，比欧几里得的高精度测量要求低一个数量级。欧空局的SOUE任务将需要充分了解木星附近恶劣的电子主导辐射环境对cmos传感器的影响，以及欧空局柏拉图任务的大型ccd将导致信号电荷在辐射损坏的大片硅上的转移。低温辐照带来许多挑战，因为设备必须始终保持低温(因此在真空下)，包括在辐照期间和辐照后的几周或几个月内。电子成像中心(CEI)在过去12个月里开发了设备和技术，使我们能够进行第一次低温照射。早期的结果表明，低温结果与室温下的结果有很大的不同，包括存在不同的陷阱种类和种群密度。

Charge-Coupled Devices (CCDs) have long been the detectors of choice for space astronomy missions. When placed in orbit, the devices can experience a harsh radiation environment. The radiation incident on the detector will act to damage the silicon lattice, creating defects that lead to the formation of "traps". The traps formed can capture signal that is being detected out of the device, leading to smearing and signal loss in CCDs and charge loss and bright pixels in CMOS sensors as well increasing sources of noise such as dark current.Before any mission is launched, it is vital that the impact of radiation is studied in detail such that appropriate shielding is used along with the development of novel readout techniques and correction algorithms to mitigate the damage that remains. Currently, standard practice dictates that radiation testing of devices is carried out at room temperature, despite the fact that the detectors will be kept cold during operation in orbit, with temperatures often as low as -120 degrees Celsius.The vacancies created after irradiation migrate through the device until they reach a stable state, but the stability of any particular defect structure is strongly dependent on temperature; the traps present at room temperature may be dramatically different to those present when a device is irradiated cold and kept cold. Literature on the so called "cryogenic irradiation" is sparse, but there are reports of a factor of 2-3 difference between the impact of radiation at room temperature compared to the more realistic cryogenic irradiation, with the direction of this difference (increased or decreased impact) varying between different device types.ESA's Euclid mission, aiming to map dark matter and energy, is dependent on a complete and thorough understanding of the impact of radiation on the detectors to allow for correction against radiation to reduce the smearing by up to 300 times. Hubble is able to correct by approximately a factor of 30, one order of magnitude less than that required for Euclid's high precision measurements. ESA's JUICE mission will require a full understanding of the impact on CMOS sensors of the harsh electron-dominated radiation environment near Jupiter and the large CCD's baselined for ESA's Plato mission will lead to the transfer of signal charge across large areas of radiation damaged silicon.Cryogenic irradiations present many challenges as the devices must be kept cold (and therefore under vacuum) at all times, including whilst irradiating and for a period of weeks or months after the irradiation. Equipment and techniques have been developed within the Centre for Electronic Imaging (CEI) over the past 12 months that have allowed our first cryogenic irradiations to take place. Early results demonstrate that there is a dramatic difference between the cryogenic results and those performed at room temperature, including the presence of different trap species and densities of populations.