Physical Radiation Monitoring

Knowledge of the radiation situation inside of a space vehicle is mandatory for each mission under consideration and shall be based on inflight dosimetry data. Such measurements of radiation exposures were performed during manned spaceflights at various altitudes, orbital inclinations, durations, periods during the solar cycle, and mass shielding (Figure 7-16) (McCormack et al. 1988, Swenberg et al. 1993, Reitz 1994).

The deposition of energy by radiation strongly depends on the type of radiation under consideration, both macroscopically and microscopically (see Figure 7-06). Because of the complex mixture of radiations occurring in space, comprising sparsely ionizing components (photons, electrons, pions, muons and protons) and densely ionizing components (heavy ions, neutrons and nuclear disintegration stars) (see Section 2), different dosimetry systems have been applied, that specifically respond to the quality of the radiation under consideration. The contribution of the sparsely ionizing component of the radiation in space has been mostly determined by lithium fluoride Thermoluminescence Dosimeters (TLD). A TLD is an (usually doped) inorganic crystal. It "absorbs" radiation dose by its valence electrons being excited to a higher energy state. The number of electrons at the higher energy state is directly proportional to the amount of ionizing radiation the crystal is exposed to. When the crystal is heated, these electrons fall back to their resting energy and emit photons, causing the crystal to glow. The emitted light intensity as a function of the temperature is called the glow curve. In a heating cycle the amount of emitted light, i.e., the integral of the resulting glow curve, is proportional to the total dose received by the crystal since the last time it was heated ("annealed"). The sensitivity of TLDs is nearly constant in the energy range of interest (Apathy et al. 2002).

Figure 7-17. Integral LET spectra of heavy charged particles measured inside the spacecraft with plastic track detectors during space missions outside of the geomagnetic (Apollo-16 and -17), in low Earth orbit (Spacelab-1 and Dl), and calculated for outer space without shielding and behind a shielding of 70 g/cm2. The nomenclature of the spectra refers to the mission and the location of the detectors inside the spacecraft.

For densely ionizing radiation, the spatial pattern of energy deposition at the microscopic level is important. For example, lesions in the sensitive structures, such as biomolecules or chromosomes, are induced with higher efficiency than by X-rays. The fluence of densely ionizing radiation has been mainly determined by use of plastic track detectors or nuclear emulsions. Plastic detector systems are diallylglycol carbonate (CR39), cellulose nitrate (CN), or polycarbonate (Lexan), which cover different ranges of LET. The tracks of heavy ions are developed by etching in caustic solutions, e.g., in sodium hydroxide 6 N NaOH. The track etching rate grows as a function of the LET. Plastic detectors allow to determine the fluence, charge, and LET spectrum of the heavy ions. Generally different plastic detector systems are arranged in a stack, and the combination of their spectra is used to generate a LET spectrum adequate for dosimetry calculations (Figure 7-17). The density of nuclear disintegration stars has been determined by nuclear emulsions. The absorbed dose deposited by neutrons can be estimated from TLDs differing in their relative contents of the isotopes 6Li and 7Li.

Figure 7-18. Absorbed dose rate and dose equivalent rate (in ¡uGy/d or juS\'/d, respectively) of the sparsely ionizing and the three densely ionizing components of the radiation field measured on board the Mir-92 mission (51.5 deg, 400 km) causing a total dose equivalent of 640 ¡.iSv/d, and during the Spacelab-D2 mission (28.5 deg, 296 km) with a total dose equivalent of 192 ¡.iSv/d. For comparison, the dose equivalent rates are about 60 mSv/3months on board the ISS (51.5 deg, 400 km) and 3 mSv/year at the surface of the Earth.

Figure 7-18. Absorbed dose rate and dose equivalent rate (in ¡uGy/d or juS\'/d, respectively) of the sparsely ionizing and the three densely ionizing components of the radiation field measured on board the Mir-92 mission (51.5 deg, 400 km) causing a total dose equivalent of 640 ¡.iSv/d, and during the Spacelab-D2 mission (28.5 deg, 296 km) with a total dose equivalent of 192 ¡.iSv/d. For comparison, the dose equivalent rates are about 60 mSv/3months on board the ISS (51.5 deg, 400 km) and 3 mSv/year at the surface of the Earth.

Figure 7-18 illustrates the contribution of the different types of radiation measured during two different space flights in LEO and estimations for the ISS.

It is important to note that these passive8 dosimetry systems integrate over the time of exposure. Their advantages are their independence of power supply, small dimensions, high sensitivity, good stability, wide measuring range, resistance to environmental stressors, and relatively low cost. However, long duration space missions, such as on board the ISS or future interplanetary missions, require time-resolved measurements, especially for radiation protection purposes. This requirement has been met by the "Pille" device, a small, portable and space-qualified TLD reader suitable for reading out TLD repeatedly on board (Apathy et al. 2002).

In addition to passive dosimeters, active dosimeters have been developed to provide real-time dosimetry data. The measurement principle is based either on ionizations (e.g., ionization chamber, proportional counter, Geiger-Miiller Counter, semiconductors, charged coupled devices CCD) or on scintillations (e.g., organic or inorganic crystals). A combination of two silicon detectors, the Dosimetry Telescope (DOSTEL), has been flown on board the Space Shuttle, Mir, and the ISS. Particle count rates, dose rates, and LET-spectra were measured separately for GCR, the radiation belt particles in the South Atlantic Anomaly, and solar particle events (Beaujean et al. 2002).

During human spaceflight an individual dosimetry is required for each astronaut. Dosimetry varies for Intra- and Extra-Vehicular Activities (IVA and EVA, respectively), where the astronauts are only shielded by the material of the space suit. A number of active devices such as small silicon detectors or small ionization chambers may be used, but they need power and are difficult to design in sufficiently small dimensions. In most cases, passive integrating detector systems have been used, such as TLDs, also in combination with the "Pille" device (Apathy et al. 2002).

However, these personal dosimetry systems provide only data on the "surface" or skin dose. In order to assess the depth dose distribution within the human body and especially at the most radiation sensitive organs, such as the brain, the blood forming organs and the gonads, human phantoms are required equipped with different dosimetry systems at the sites of sensitive organs. The anthropomorphic phantom "Matroshka" was exposed for one year to the radiation in space outside of the ISS (Figures 7-01 and 7-19) in order to determine the depth dose distribution of radiations within the human body during EVA (Reitz and Berger 2005).

8 The dosimeters are "passive" in the sense that they do not need power during the mission.

Figure 7-19. ISS Cosmonaut Sergei K. Krikalev holds the anthropomorphic phantom Matroshka, a human-torso-like device, after its retrieval from the exterior of the ISS during a spacewalk, for return to Earth. The experiment is designed to better understand the exposure of astronauts, including those making spacewalks, to radiation. Photo courtesy of NASA.

Figure 7-19. ISS Cosmonaut Sergei K. Krikalev holds the anthropomorphic phantom Matroshka, a human-torso-like device, after its retrieval from the exterior of the ISS during a spacewalk, for return to Earth. The experiment is designed to better understand the exposure of astronauts, including those making spacewalks, to radiation. Photo courtesy of NASA.

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