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Dosimeter for astronauts


The astronauts on board the international space station (ISS) are exposed to high levels of radiation. The dose equivalent - measured by means of a tissue-equivalent proportional counter (TEPC) within the space station - is of approx. 0.5 mSv per day. In the case of several months spent in the station, the yearly limit value of 20 mSv valid for personnel working on Earth can be considerably exceeded. This requires a particularly intensive surveillance. Measurements realised by means of spectrometers show that approx. one third of the dose equivalent originates from sparsely ionising radiation (mainly caused by high-energy protons, LET < 10 keV/µm) and two thirds from high-density ionising radiation (neutrons, heavy ions, LET > 10 eV/µm). Thereby, especially the fraction of the dose caused by neutrons can only be determined with high uncertainties. Presently, data stating the fraction of the total dose due to neutrons vary between 10% and 30%.

Up to now, PTB has been involved in two missions to ISS with regard to a personal neutron dose equivalent meter. A neutron dosimeter with nuclear track detectors was used in which the neutron dose was determined by analysing the traces of recoil protons and those of alpha particles. The latter are generated in the dosimeter by thermalised neutrons which are scattered back by the phantom via a 10B(n,α) reaction and can be more clearly attributed to neutron radiation.

Two dosimeters were on board the ISS for 126 days in summer 2001. The measurement of recoil protons yielded a dose rate of (28 ± 5) µSv/d; with the procedure based on reaction on 10B, (23 ± 8) µSv/d were determined [1]. The absolute values include additional calibration uncertainties which are not mentioned here since reference fields on Earth were used for calibration, which are different from neutron radiation fields in outer space.

Four other neutron dosimeters of PTB were located on the outside of the ISS in space on the Matroshka's jacket from February 2004 until August 2006. The Matroshka is a human-shaped phantom which was mounted on the outside of the ISS and equipped with radiation protection devices (see figure). The dosimeters are presently being analysed and compared with calibration dosimeters which were irradiated at the high-energy reference neutron radiation fields of CERN (Switzerland) and TLABS (South Africa) last year.

Up to now, the astronauts of the ISS have only been equipped with passive personal dose equivalent meters based on thermo-luminescence and nuclear track detectors which will be analysed after the mission. In the longer term, the development of direct reading, electronic personal dose equivalent meters for astronauts is envisaged. Due to its experience in the development of electronic personal dose equivalent meters for mixed neutron/photon fields (see Annual Report 2006), PTB has been involved in a project which was tendered by ESA.

Figure : Mounting of the Matroshka on the ISS


  1. Reitz, R. Beaujean, E. Benton, S. Burmeister, Ts. Dachev, S. Deme, M. Luszik-Bhadra, P. Olko:
    Space radiation measurements on-board ISS - the DOSMAP experiment.
    Radiat. Prot. Dosim. 116 (2005), pages 374-379.