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Electron radiation can wreak havoc in spacecrafts – and save lives in radiotherapy

Electrons emanating from our sun, can follow intricate trajectories in space governed by the magnetic fields generated in planets. This phenomenon creates radiation belts around Earth, and in particular around Jupiter. The magnetic field of Jupiter is strong, and creates a radiation environment that contains large amounts of particles at very high energies.

This presents challenges to the space missions heading towards Jupiter, such as the European Space Agency’s (ESA) Jupiter Icy Moons Explorer mission (JUICE) that was launched on April 14, 2023. Radiation can cause failures in electronic equipment, damage to materials, and other unwanted effects on for instance scientific measurements. Different particles, depending on their mass, charge, and energy, can cause a wide range of issues in various equipment. The key to being prepared and to choosing the best components for the mission, is to test.

Radiation testing of electronic components are commonly performed using particle accelerators. This is routinely done at the radiation effects facility RADEF at JYU, where there is an electron accelerator called a Clinac in addition to proton and heavy ion test stations. The Clinac is capable of producing electrons with energies up to 20 MeV. It was used in this thesis project alongside the electron test station VESPER at CERN to investigate what kind of effects high-energy electrons can induce in modern memory devices that potentially are used in space missions like JUICE. The studies found that single high-energy electrons could induce bit-flips and stuck bits in the tested memories. These types of effects have been observed before, caused by heavier particles such as protons, but not from electron radiation.

These effects can corrupt data stored on the memory. Bit-flips means a change in the stored bit value from a ‘1’ to a ‘0’ or vice versa. On the other hand, in a stuck bit the state of the memory cell is read out with the same data value (either a ‘1’ or a ‘0’), no matter which data value was actually written there. Single event effects (SEE) caused by electrons in electronic components have potential of becoming increasingly prevalent as devices and integrated circuits gets smaller, and the operating voltages lower, which in turn allow smaller perturbations and charge depositions to result in device malfunctions.

The Clinac hosted now by RADEF was originally used at a hospital for radiotherapy. In this PhD project, not only the potential radiation effects on electronics in space exploration equipment was investigated, but also ways of monitoring the radiation dose delivered by such radiation therapy accelerators. The dosimetry systems that were investigated for this purpose were based on optical fibers, where doped silica glass rods exhibiting radiation-induced luminescence (RIL) were attached to optical fibers to guide the generated light for collection and analysis.

These dosimetry systems have benefits in that they can be made in small active volumes, sub-millimeter sized, and they have no active electrical operation in the sensing region and are largely unaffected by electric and magnetic fields. Three different silica glass sample types were studied in this work, doped with Ce3+, Cu+, and Gd3+-ions respectively. It was found that these types of samples could be used to accurately monitor the ionizing dose delivered by the pulsed electron beam from the Clinac, with the deposited dose per pulse varying over three orders of magnitude.

A drawback of these tested systems is that parasitic light can be emitted, apart from the RIL. One such parasitic light emission is Cherenkov radiation, which can interfere with the detection of the RIL. In this work, the two types of light emission (Cherenkov and RIL) were separated using their different emission properties in terms of wavelength, and the longer fluorescence time of the RIL, so that the desired RIL could be extracted separately. This thesis work marks a step forward in understanding the properties of this type of optical fiber-based dosimetry systems, and towards enabling their use in clinical settings.

This thesis work has been a part of the RADSAGA project, funded by the European Commission through the Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 721624.

M.Sc. Daniel Söderström's dissertation "Radiation effects on SDRAMs and optical-fiber based dosimetry of high-energy electrons" will be examined on 12.5.2023 at 12:00 in lecture hall FYS1 of the Department of Physics. Opponent is Dr. Philippe Paillet (CEA, France) and custos is Staff Scientist, PhD Heikki Kettunen (Ģ��ֱ��). The language of the dissertation is English.

It will also be possible to follow the dissertation online: