M. Hajek

Cosmic Radiation Exposure of Biological Test Systems During the EXPOSE-E Mission

In the frame of the EXPOSE-E mission on the Columbus external payload facility EuTEF on board the International Space Station, passive thermoluminescence dosimeters were applied to measure the radiation exposure of biological samples. The detectors were located either as stacks next to biological specimens to determine the depth dose distribution or beneath the sample carriers to determine the dose levels for maximum shielding. The maximum mission dose measured in the upper layer of the depth dose part of the experiment amounted to 238±10 mGy, which relates to an average dose rate of 408±16 μGy/d. In these stacks of about 8 mm height, the dose decreased by 5-12% with depth. The maximum dose measured beneath the sample carriers was 215±16 mGy, which amounts to an average dose rate of 368±27 μGy/d. These values are close to those assessed for the interior of the Columbus module and demonstrate the high shielding of the biological experiments within the EXPOSE-E facility. Besides the shielding by the EXPOSE-E hardware itself, additional shielding was experienced by the external structures adjacent to EXPOSE-E, such as EuTEF and Columbus. This led to a dose gradient over the entire exposure area, from 215±16 mGy for the lowest to 121±6 mGy for maximum shielding. Hence, the doses perceived by the biological samples inside EXPOSE-E varied by 70% (from lowest to highest dose). As a consequence of the high shielding, the biological samples were predominantly exposed to galactic cosmic heavy ions, while electrons and a significant fraction of protons of the radiation belts and solar wind did not reach the samples.

Analysis of the neutron component at high altitude mountains using active and passive measurement devices

The European Council directive 96/29/Euratom requires dosimetric precautions if the effective dose exceeds 1 mSv/a. On an average, this value is exceeded by aircrew members. Roughly half of the radiation exposure at flight altitudes is caused by cosmic ray-induced neutrons. Active (6LiI(Eu)-scintillator) and passive (TLDs) Bonner sphere spectrometers were used to determine the neutron energy spectra atop Mt. Sonnblick (3105 m) and Mt. Kitzsteinhorn (3029 m). Further measurements in a mixed radiation field at CERN as well as in a proton beam of 62 MeV at Paul Scherrer Institute, Switzerland, confirmed that not only neutrons but also charged particles contribute to the readings of active detectors, whereas TLD-600 and TLD-700 in pair allow the determination of the thermal neutron flux. Unfolding of the detector data obtained atop both mountains shows two relative maxima around 1 MeV and 85 MeV, which have to be considered for the assessment of the biologically relevant dose equivalent. By convoluting the spectra with appropriate conversion functions the neutron dose equivalent rate was determined to be 150 +/- 15 nSv/h. The total dose equivalent rate determined by the HTR-method was 210 +/- 15 nSv/h. The results are in good agreement with LET-spectrometer and Sievert counter measurements carried out simultaneously.

The MATROSHKA Experiment: Results and Comparison from Extravehicular Activity (MTR-1) and Intravehicular Activity (MTR-2A/2B) Exposure

Astronauts working and living in space are exposed to considerably higher doses and different qualities of ionizing radiation than people on Earth. The multilateral MATROSHKA (MTR) experiment, coordinated by the German Aerospace Center, represents the most comprehensive effort to date in radiation protection dosimetry in space using an anthropomorphic upper-torso phantom used for radiotherapy treatment planning. The anthropomorphic upper-torso phantom maps the radiation distribution as a simulated human body installed outside (MTR-1) and inside different compartments (MTR-2A: Pirs; MTR-2B: Zvezda) of the Russian Segment of the International Space Station. Thermoluminescence dosimeters arranged in a 2.54 cm orthogonal grid, at the site of vital organs and on the surface of the phantom allow for visualization of the absorbed dose distribution with superior spatial resolution. These results should help improve the estimation of radiation risks for long-term human space exploration and support benchmarking of radiation transport codes.

MATSIM: Development of a Voxel Model of the MATROSHKA Astronaut Dosimetric Phantom

The AIT Austrian Institute of Technology coordinates the project MATSIM (MATROSHKA Simulation) in collaboration with the Vienna University of Technology and the German Aerospace Center, to perform FLUKA Monte Carlo simulations of the MATROSHKA numerical phantom irradiated under reference radiation field conditions as well as for the radiation environment at the International Space Station (ISS). MATSIM is carried out as co-investigation of the ESA ELIPS projects SORD and RADIS (commonly known as MATROSHKA), an international collaboration of more than 18 research institutes and space agencies from all over the world, under the science and project lead of the German Aerospace Center. During MATSIM a computer tomography scan of the MATROSHKA phantom has been converted into a high resolution 3-dimensional voxel model. The energy imparted and absorbed dose distribution inside the model is determined for various radiation fields. The major goal of the MATSIM project is the validation of the numerical model under reference radiation conditions and further investigations under the radiation environment at ISS. In this report we compare depth dose distributions inside the phantom measured with thermoluminescence detectors (TLDs) and an ionization chamber with FLUKA Monte Carlo particle transport simulations due to 60Co photon exposure. Further reference irradiations with neutrons, protons and heavy ions are planned. The fully validated numerical model MATSIM will provide a perfect tool to assess the radiation exposure to humans during current and future space missions to ISS, Moon, Mars and beyond.

Developments and trends in bioequivalent dosimetry.

Significant progress in radiobiology has refined the understanding of radiation-induced biological response at the cellular level and challenged the conventional application of a macroscopic description of radiation action to dosimetry in favour of a microscopic approach. Pioneering experiments, which investigated the stochastics of energy deposition from ionising radiations in volumes of cellular dimensions, contributed to the recognition of microdosimetry as a new scientific discipline. The first quantitative applications of Monte Carlo track structure simulations in radiobiology, however, supported evidence for target sizes of particular biological importance being in the nanometre regime. Bioequivalent dosimetry attempts to link particular features of the response of physical detectors with biological endpoints, exploiting clusters of multiple ionisations within nanometre scales in solid-state, gas- and water-filled devices. This approach supports the continued development of new concepts and quantities in radiation protection to permit evaluation of the biological effectiveness of radiations of different quality independently of dose and dose rate.

Neutron dosimetry onboard aircraft using superheated emulsions

Publisher Summary In terms of the biologically relevant dose equivalent, neutron radiation constitutes the dominant component of the radiation environment at aviation altitudes. The conducted experiments confirmed that superheated emulsions and thermoluminescent dosemeters of the types TLD- 600 and TLD-700 arranged in pair are reliable monitoring instruments for the neutron dose equivalent onboard aircraft. The extension of the so-called pair-method represents a novel approach in neutron dosimetry. Both systems are passive devices—that is, they consume no power and do not interfere with aircraft electronics by the emission of electromagnetic radiation. They are, furthermore, comparably easy-to-handle with detection limits sufficiently low to match the requirements of routine applications. However, with certain expenditure statistical uncertainties of about 15% and below are achievable.

MATSIM: A voxel model for the astronaut dosimetric phantom MATROSHKA

The AIT Austrian Institute of Technology coordinated the project MATSIM (MATROSHKA Simulation) in collaboration with the Vienna University of Technology and the German Aerospace Center, to perform FLUKA Monte Carlo simulations of the MATROSHKA numerical phantom for the radiation environment at the International Space Station (ISS). MATSIM, a voxel model of the MATROSHKA phantom was developed during the project MATSIM Phase-A. In this paper we describe results of the project Phase-B of MATSIM, Monte Carlo simulation of the absorbed dose and the neutron fluence assessed inside the whole model phantom. The simulations are verified by reference measurements using thermoluminescence dosimeters, an ionisation chamber and a tissue equivalent proportional counter (TEPC). Further investigations are carried out for ISS cosmic radiation conditions. MATSIM provides a comprehensive risk assessment of radiation hazard to humans working in space onboard the ISS, for missions to the Moon, Mars and beyond, as well as for terrestrial mixed radiation fields comprising ionizing high-energy particle radiation.

MATSIM: Development of a voxel model of the MATROSHKA astronaut dosimetric phantom exposed onboard ISS

The Austrian Institute of Technology coordinates the project MATSIM (MATROSHKA Simulation) in collaboration with the Vienna University of Technology and the German Aerospace Center, to perform FLUKA Monte Carlo simulations of the MATROSHKA numerical phantom under the radiation environment at the International Space Station (ISS). MATSIM is carried as co-investigation of the ESA ELIPS project MATROSHKA, an international collaboration of more than 18 research institutes and space agencies from all over the world, under the science and project lead of the German Aerospace Center. MATROSHKA is an ESA facility designed to determine the radiation exposure of an astronaut during an extravehicular activity at the ISS. During the project MATSIM a computer tomography scan of the MATROSHKA phantom has been converted into a high resolution 3-dimensional voxel model. The imparted energy and dose inside the model is determined. Part of the project is the phantom validation under reference radiation conditions. Investigations are carried out under ISS cosmic radiation conditions. The aim of the MATSIM project is the provision of comprehensive risk assessment of radiation hazard to humans in space due to ionising high energy particle radiation

Measurements and calculations of the radiation exposure of aircrew personnel on different flight routes

Publisher Summary The conducted experiments demonstrated the influence of the geomagnetic shielding and the 11-year solar activity cycle on the radiation exposure at aviation altitudes. The experimental data were compared with calculated dose values. The recent development of the calculation models succeeded in revealing results that are in general agreement with the measurements. However, the most important insufficiency common to all computational approaches concerns the effects of major solar flares that present a serious danger primarily for future high-altitude and polar orbital flights in causing severe biological hazards. The probability of the occurrence of these irregular events corresponds to the solar activity cycle. This fact is taken into account in the codes by semi-empirical models that certainly have to fail in forecasting accurate dose values for a specific flight. Therefore, dosimetric surveillance of aircrew members is essential and cannot be completely replaced by calculations. However, it shall be remarked that high-precision measurements require considerable expertise. TLDs as a survey instrument for the radiation exposure onboard aircraft are passive and extremely compact devices, permitting their application as a routine monitoring device for every aircrew member and passenger.