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Direct-reading personal dosemeters

How high is the current radiation exposure for the crew of the ISS? In the future, direct-reading dosemeters might answer this question clearly. To date the astronauts' exposure has only been measured using passive dosemeters. Evaluation takes place subsequently on Earth. The personal dosemeters developed at the PTB can, however, directly display radiation exposure. One of these dosemeters was patented and a prototype has been transferred to industry. A new prototype for possible use in space is being tested at present.

A "phantom" (called Matroshka) similar to a person on the outside of the ISS wearing a waistcoat with various dosemeters for test purposes. (Photos: ESA)

The radiation dose amounts to around 0.5 mSv per day on the International Space Station (ISS), in Low Earth Orbit at a height of approx. 400 km. The astronauts receive half of this dose in the few minutes during which the ISS crosses the radiation belt of the South Atlantic Anomaly several times a day. In view of this high dose received in a very shorttime, carrying a direct-reading dosemeter with an alarm function is desirable. The astronauts could then retreat to better shielded areas of the space station when the dose increases.

The prototype dosemeter developed in the PTB for mixed radiation fields (neutrons and photons) is a handy device with a display which can be read directly and an alarm function. Whereas up till now several semiconductor detectors have always been used for constructing a neutron dosemeter, this dosemeter manages with a single detector (40 µm effective layer thickness and 1 cm2 area), which is surrounded by several thin absorption layers. An industrially manufactured dosemeter based on the PTB's technology is already in use in the nuclear industry. For radiation fields with high neutron energies between 1 MeV and 100 MeV, as they appear at high-energy accelerators and also in space, the dose equivalent response, however, varies by more than one factor of ten.

In the meantime, a new neutron dosemeter has been developed at the PTB which is better suited for radiation fields with high energies. The new prototype essentially differs from the earlier one through the use of a thinner and smaller semiconductor detector (5.6 µm effective layer thickness, 0.25 cm2 area) and the integration of pulses with a smaller pulse height (200 keV to 1 MeV). The dose equivalent response for neutrons only varies by ± 40 % in the neutron energy range between 1 MeV and 100 MeV.

Apart from measurements, Monte-Carlo calculations for the response were also carried out. The results agree well with the measurements in view of the large energy range, the complex computation models and the construction of the dosemeter which has been simplified for the calculation. The dosemeter also seems suitable for use on the ISS. It is being tested for possible use in space in the scope of a project supported by ESA.

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