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

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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.