Radiation damage in detectors: what really happens to silicon imaging detectors in orbit?

The Euclid mission, launched in July 2023, is an ESA space mission with the objective of mapping the geometry of the Universe to better understand dark matter and dark energy. While in space, like all spaceborne instrumentation on any satellite, Euclid is bombarded with highly energetic particles, mainly from the Sun, which will slowly degrade the focal-plane detectors and thus have a have a major impact on the scientific data unless fully understood and corrected for.

The Centre for Electronic Imaging (CEI) at the Open University has been involved in the Euclid mission since its conception as part of the Euclid VIS instrument development work, carrying out detector characterisation and leading the radiation damage testing of the focal-plane imaging detectors; detectors designed and produced by Teledyne e2v. As part of this work, the CEI has developed the trap-pumping technique, which allows for the characterisation of single defects in the silicon lattice caused by the space radiation environment that leads to the degradation of the science images returned.

This new technique, originally developed in the CEI a decade ago, allows for a much deeper understanding of the actual damage the devices are subject to when in space and will be performed on a daily basis as part of the in-orbit calibrations. This will be the first time that the technique has been used in space, opening up a new era in the understanding of radiation damage to detectors in-orbit.

As part of the data processing and calibration routines for the Euclid VIS instrument, the data from trap pumping will be analysed to provide parameters for the image correction algorithms. However, the data are able to tell us much more about the space radiation environment, radiation damage processes, and how accurately our current ground-based testing can replicate the conditions in-orbit.

Through a deeper analysis of the in-orbit data, not funded under the current UKSA/STFC grants, coupled with targeted ground-based experiments in the laboratory, we will for the first time be able to directly analyse the radiation damage processes in space.

For the Gaia mission, a previous STFC CASE PhD student in the CEI in collaboration with Te2v, investigated the differences between pre-launch predictions of radiation damage and what happened in-orbit using the basic calibration data available. Their findings have shown very interesting correlations with device batches, as well as fine-tuning the calibration process used in the RVS instrument.

However, a first proper investigation of radiation damage while in-orbit in this new studentship will help our fundamental understanding, not just for CCDs but also for future missions using CMOS image sensors, DEPFETs and other technologies.

Having access to a highly accurate and much deeper analysis technique in trap pumping gives the power to analyse the performance in great detail and investigate many open questions that can't be answered from ground testing, important to all future X-ray and visible space missions:

How does a 5 minute on-ground irradiation test performed before launch compare to a steady rate of radiation dose increase over 5-years in-orbit?

How does a NIEL scaled 200MeV dose (i.e. bombarding a detector on the ground at a single energy) compare to the bombardment received in-orbit from a wide spectrum of particle types and energies? How does annealing of radiation-induced defects in space compete with the rate of new defect production?