Abstract
Throughout a typical Earth orbit a satellite is constantly bombarded by radiation with trapped and solar protons being of particular concern as they gradually damage the focal plane devices throughout the mission and degrade their performance. To understand the impact the damage has on CCDs and how it varies with their thermal history a proton radiation campaign has been carried out using a CCD280. The CCD is irradiated at 153 K and gradually warmed to 188 K in 5 K increments with Fe55 X-ray, dark current and trap pumping images taken at 153 K after each anneal step.The results show that despite the trap landscape changing throughout the anneal it has little impact on parallel charge transfer inefficiency. This is thought to be because most traps are unaffected and a lot of those that do anneal only move from the continuum between distinct trap species and into a nearby divacancy trap “peak” whose emission time constant is similar enough to still impact the CTI.In terms of using a CCD280 or similar devices in a mission the CTI being unaffected by thermal annealing up to 188 K means that any CTI correction needed as the radiation damage builds up does not have to take into account the thermal history of the focal plane. However, it is possible that a significant amount of annealing will occur at temperatures greater than 188 K and care should be taken when a mission is operating in this range to gather accurate pre-flight data.
Highlights
This paper focuses on the impact of isothermal annealing on the DC, trap landscape and the parallel CTI by carrying out DC, trap pumping and X-ray CTI measurements over 153 to 188 K which covers, with margins, the approximate range the SMILE focal plane will vary across throughout an orbit
It is important to assess how this impacts the trap landscape by carrying out the temperature cycling described in Figure 1 to give a representation of how the device is likely to respond to any temperature variations experienced throughout a space mission
Throughout the course of a typical earth orbit mission the focal plane detectors are damaged by proton radiation which over time causes a degradation in their performance
Summary
The back illuminated 1552 x 1280 pixel CCD280 manufactured by Teledyne-e2v and used throughout this campaign is an example of the test vehicle for the PLATO mission and is functionally very similar to the larger 4510 x 4510 pixel CCD270 flight devices, which have been cut from the same wafer to provide uniformity, the specifications of the CCD280 are summarised in Table 1 [5]. The PLATO CCDs have served as a precursor to the SMILE devices which have subsequently been optimised to maximize their efficiency for the low flux, low energy signals the SXI will detect. The CCD280 has a store area which covers half of the device and can be run in either full frame or frame transfer mode. The device is irradiated in two steps up to the 70 % EOL fluence predicted from the modelling of the radiation environment for the SMILE mission. A characterization stage is indicated in Figure 1 at each horizontal section and it shows that after irradiating the CCD temperature is lowered to 143 K and characterized before gradually increasing up to 188 K in 5 K steps. After each warming stage above the irradiation temperature the device is re-characterized at 153 K to assess the impact of any annealing that has taken place. Each stage lasts for approximately 5 days before the temperature changes, after the 173 K warming stage the CCD is held at 153 K for 15 weeks with a characterization before and after
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