Development of passive dosimeter case using optical fibres for space radiation detection

In the Low Earth Orbit (LEO) lies the International Space Station (ISS), Earth observation satellites and the Chinese Tiangong Space Station. Currently, the National Aeronautics and Space Administration (NASA) is actively creating LEO economy with the production, distribution, and trade of goods and services within the LEO. However, without the protection of the Earth’s atmosphere, any object existing in LEO is subject to the harsh space environment, which affects its performance. When designing a casing to be placed within the LEO, engineer needs to consider the effect of ionising radiation. To monitor this, passive dosimeters can be employed alongside the casing. Advantages of passive dosimeter compared to active dosimeter are its small size and the independency on power source, making the passive dosimeters suitable to be employed for long term radiation monitoring. In this work, a passive dosimeter casing was designed which incorporates the germanium doped optical fibres (OF) as it was found to produce high thermoluminescence (TL), having good reproducibility, dose rate-independent, angular independent, irradiation temperature-independent and low in fading rate. The passive dosimeter casing is designed by incorporating engineering design process. The passive dosimeter casing consisted of polyetheretherketone (PEEK) case where the main body has nine blind holes to place the filters and dosimeter blocks made from PEEK that held the dosimeters. Before exposing the casing to space environment, random vibration tests, static load tests and compressive tests are conducted on the assembled casing. While producing resonance and anti-resonance region starting from approximately 445 Hz in all directions that exceeds/fall below the high/low abort, the casing is not damaged after performing random vibration test and static loads test. Mechanically, the thinnest (1 mm) part of the casing would fail at lowest compressive force of 304 N with a safety factor (SF) of 1.5 or up to 600 N (SF=2.88) meaning that the casing can work even with the possibility of force from the robotic arm being applied during attachment/detachment for exposure. On the other hand, the passive dosimeters were screened using gamma (Caesium 137), irradiated with beta (Strontium 90), gamma-neutron mixed field (Americium-Beryllium), and proton (produced by accelerator) as preliminary tests. Upon assembly, the casing was exposed to space radiation outside of the ISS for 433 days. Upon retrieval, the passive dosimeters were analysed and calibrated using proton (based on linearity, energy dependency and fading analysis) to calculate the absorbed dose received during exposure. Flat fibres doped with 6 mol% Germanium (Ge) produced the highest TL signal normalised by the core volume. All dosimeters had a high coefficient of determination of ≥ 0.98 for all type of irradiations. The dosimeters showed up to 18% reduction in TL signal after 358 days of irradiation. Upon calibration which include linearity and fading correction, the passive dosimeters are exposed to absorbed dose rate up to about 2000 μGy/day outside of ISS. In summary, a dosimetry casing to employ passive dosimeter for space exposure is successfully conducted and found OF to be operational as a passive dosimeter for radiation detector in space.

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