Effective Dose Measured with a Life Size Human Phantom in a Low Earth Orbit Mission

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1 J. Radiat. Res., 50, (2009) Award Review Article # Effective Dose Measured with a Life Size Human Phantom in a Low Earth Orbit Mission Hiroshi YASUDA* Space radiation/effective dose/life size human phantom/astronaut/low earth orbit. The biggest concern about the health risk to astronauts is how large the stochastic effects (cancers and hereditary effects) of space radiation could be. The practical goal is to determine the effective dose precisely, which is difficult for each crew because of the complex transport processes of energetic secondary particles. The author and his colleagues thus attempted to measure an effective dose in space using a life-size human phantom torso in the STS-91 Shuttle-Mir mission, which flew at nearly the same orbit as that of the International Space Station (ISS). The effective dose for about 10-days flight was 4.1 msv, which is about 90% of the dose equivalent (H) at the skin; the lowest H values were seen in deep, radiation-sensitive organs/tissues such as the bone marrow and colon. Succeeding measurements and model calculations show that the organ dose equivalents and effective dose in the low Earth orbit mission are highly consistent, despite the different dosimetry methodologies used to determine them. INTRODUCTION The health effects of space radiation on astronauts need to be precisely quantified and controlled. Astronauts are exposed to a complex radiation field consisting of protons, heavy ions and secondary particles including neutrons with a broad range of energy. 1) The dose rate in space is much higher than that of natural radiation on the ground, and the accumulated dose during a long space mission could present a non-negligible cancer risk. 2) This risk and other biological effects in such a special environment are uncertain and need to be quantified through strategic, programmatic studies. The major concern about the space radiation health risk is stochastic effects, that is, cancers and hereditary effects. This effect is generally quantified with the effective dose (E). 3,4) The definition of E takes into account the different relative radiosensitivities of the various organs and tissues in the human body with respect to radiation detriment from stochastic effects. The E is given as the tissue-weighted sum of equivalent doses in critical organs/tissues; the equivalent dose is defined as the radiation-weighted sum of absorbed dose at each organ/tissue. For applications, however, a single *Corresponding author: Phone: +81-(0) , Fax: +81-(0) , h_yasuda@nirs.go.jp National Institute of Radiological Sciences Anagawa, Inage-ku, Chiba , Japan. doi: /jrr # JRRS Incentive Award w R value of 20 recommended for all types and energies of heavy charged particles is too conservative and, according to ICRP, 4) more realistic approach based on the concept of quality factor (Q), i.e., dose equivalents, may have to be employed. The Q characterizes the biological effectiveness of a radiation based on the ionization density along the tracks of charged particles in tissue and generally given as a function of the unrestricted linear energy transfer (LET). The effective dose has only been measured once, in the STS-91 phantom torso experiment. 5) Most other studies have been devoted to describing transport processes of space radiation components using computer codes 6 9) with anatomical models. 10,11) However, the interactions of energetic space radiation, particularly high-charge and highenergy (HZE) particles of galactic cosmic radiation, are extremely complicated and uncertain. The accuracy of model prediction is desirably to be verified by measurements using a life-size human phantom onboard a spacecraft. Therefore, in this paper, the experimental procedure of the effective dose measurement in the low Earth orbit mission is briefly introduced and the results are compared with those of succeeding studies including numerical simulations. OUTLINE OF EFFECTIVE DOSE MEASURE- MENT IN SPACE Considering the importance of determining the effective dose, the author and his colleagues designed and performed an experiment to measure directly the organ doses of a human phantom torso in the STS-91 Shuttle-Mir mission,

2 90 H. Yasuda which flew at nearly the same orbit as that of the International Space Station (ISS) (inclination: 51.6, altitude: ~400 km). 5) Organ dose equivalents were measured with combined dosimeter packages consisting of thermoluminescence dosimeters (TLD) of Mg 2 SiO 4 :Tb (MSO, Kasei Optonics, Inc.) and plastic nuclear track detectors (PNTD) of highly sensitive CR-39 (HARZLAS TD-1, Fukuvi Chemical Industry). Such small-scale dosimeters are much beneficial because they avoid disturbing the surrounding radiation field in the phantom. In general, the efficiency of TLD changes to a large extent depending on the ionization density along the track of a particle which is often denoted as LET ) This phenomenon has successfully been explained a priori for LiF TLDs by means of target-hit models based on the track structure theory ) TLD values obtained as γ-ray equivalent absorbed doses have some errors for space radiation, and the errors have to be corrected based on experimental data obtained in ground-based studies. We thus examined responses of various TLDs and other luminescence dosimeters using protons at the Cyclotron and heavy ions at the Heavy Ion Medical Accelerator in Chiba (HIMAC) of National Institute of Radiological Sciences (NIRS). The relative efficiencies in reference to 137 Cs γ-rays are derived on the absorbed dose basis. The smoothed LET-dependent efficiency curves that were obtained from the data plots of selected dosimeters irradiated with relativistic ion beams are shown in Fig ) From these results, we found that MSO is the most useful TLD substance, because its efficiency is almost unity up to 10 kev μm 1 and decreases in the larger LET range that can be detected with PNTD (Fig. 3). The track formation sensitivities of PNTD, including their incident angle dependence, were quantified as a function of LET using heavy ion beams of NIRS-HIMAC, as shown in Fig. 3. The strong dependence of PNTD sensitivity on incident angles was incorporated into a conservative regression curve that would not underestimate radiation doses (Fig. 3b), adhering to radiological protection practices. A set of three TLD and two PNTD chips was put into a case of tissue-equivalent resin. Fifty-nine detector cases were placed into or on critical organ/tissue positions of a life-size human phantom (RANDO Phantom, Alderson Research Laboratories) composed of polyurethane and a human skeleton, as indicated in Fig. 4. The phantom was covered with a suit of heat-resistant fibers (Nomex, DuPont, Inc.), and four detector cases were put into the suit pockets both at the chest and abdomen. The phantom was fixed with Fig. 1. Regression curves of relative efficiencies as a function of LET for selected integrating dosimeters 15 23) : TLD-Mg 2SiO 4:Tb (MSO), TLD-BeO:Na, TLD- 7 LiF:Mg,Ti, the radiophotoluminescence dosimeter (RPLD) of a phosphate glass, the optically stimulated luminescence dosimeter (OSLD) of Al 2O 3:C, and the direct ion storage dosimeter (DIS-1). Most of the data were obtained using heavy ion beams at NIRS-HIMAC. Fig. 2. Plots of relative TL efficiency of TLD-Mg 2SiO 4:Tb (MSO) as the 137 Cs-γ equivalent versus the unrestricted LET in water. 5) Efficiency less than unity (0.95) was uniformly given for the particles with LET 10 kev μm 1. The efficiency curve in the range of LET > 10 kev μm 1 was given for the plots (black circles) obtained by linear extrapolation of the plots of C, Ne, and Fe in the logarithmic scales to the energy of 100 MeV amu 1.

3 Effective Dose in LEO 91 (a) (b) Fig. 3. (a) Plots of the track-formation sensitivity (S) of PNTD as a function of beam incident angle; and (b) plots of the unrestricted LET in water as a function of S for vertically incident beams. 5) Using these data, we determined the detection threshold for vertical beams of 5 kev μm 1 and the effective threshold of 12.5 kev μm 1 for isotropic space radiation. The regression curve for calibration was conservatively given as shown in the figure (b). Fig. 4. Illustration of the human phantom torso with indications of the detector case positions. The cross view of each section is seen from above, except the shoulder-bone surface (sec-11). The phantom was covered by a Nomex suit, and the detector cases (No ) for the breast and skin were put into the pockets on the suit. bungee cords onto a rack at the starboard side in the Spacehab Module of the Space Shuttle Discovery (Fig. 5). The shuttle Discovery was launched from NASA Kennedy Space Center (KSC), Florida, at 18:10 June 2, as the STS- 91/9th Shuttle-Mir Mission. The Shuttle docked to the Russian Space Station Mir two days later (13:02, June 4) and, after orbiting the earth for about 4 days, undocked from Mir at 12:05, June 8. The mission continued with lowering altitude for additional 4 days until landing at KSC at 14:03 June 12. Total flight duration was 9 days and 20 hours (9.8 days). All the detector cases were removed from the phantom at the NASA Johnson Space Center (JSC), Texas, and sent to NIRS, Japan, for dosimeter analyses. Absorbed doses were evaluated based on the efficiencycorrected values of TLD as γ-ray equivalent absorbed doses (Fig. 2), and the spectra of LET greater than 10 kev μm 1

4 92 H. Yasuda were estimated from the PNTD track data. Organ dose equivalents were calculated from the absorbed doses and the LET spectra according to the Q (L) function in the ICRP 1990 recommendations 3) ; details of dose determination procedures were explained in the paper 5) of the author and his colleagues. Table 1 shows the values of absorbed dose (D T ), dose equivalent (H T ), and effective quality factor (Q e ) in critical organs/tissues of the phantom torso flown in the STS-91 experiment. The absorbed doses varied by a factor of 1.6 and the dose equivalents by a factor of 1.5 among the organs and tissues. Q e ranged from 1.7 to 2.4, varying by a factor of 1.4. Summing up these organ and tissue doses weighted by the tissue weighting factors of the 1990 ICRP recommendations, 3) we calculated the effective dose (E) as 4.1 msv for this 9.8-day mission. This E value was about 90% of the skin dose equivalent (H skin) measured on the abdomen. About 70% of E came from five internal organs and tissues, the lung, stomach, bone marrow, colon, and gonad (testis). Fig. 5. Illustration of Space Shuttle Discovery (above) and a perspective view of the Spacehab Single Module (below) indicating the location of the phantom torso. The Phantom was fixed with bungee cords to the rack at the starboard side. COMPARISONS WITH THE RESULTS OF SUCCEEDING STUDIES During the STS-91 mission, organ doses of the phantom torso were measured also by Badhwar et al., NASA Johnson Table 1. The values of absorbed dose, dose equivalent, and effective quality factor for organs/tissues and effective dose obtained for the phantom torso experiment in the 9.8-day STS-91 mission (51.6 ~400 km). 5) Measurements were made with combined CR-39/TLD methodology. The tissue weighting factors (w T) and the w T-weighted dose equivalents are also shown. Organ/ Tissue a Absorbed dose (D T) [mgy (H 2O)] b Organ/tissue dose equivalent (H T) [msv] b Effective quality factor (Q c) Tissue weighting factor (w T) H T w T b Skin 2.2 ± ± ± ± Thyroid 2.2 ± ± ± ± Bone surface 2.7 ± ± ± ± Esophagus 2.1 ± ± ± ± Lung 2.1 ± ± ± ± Stomach 2.4 ± ± ± ± Liver 2.3 ± ± ± ± Bone marrow 1.8 ± ± ± ± Colon 1.7 ± ± ± ± Bladder 1.8 ± ± ± ± Gonad (Testis) 2.0 ± ± ± ± Breast (Chest) 2.3 ± ± ± ± Remainder 2.1 ± ± ± ± Effective dose [msv]: 4.1 ± 0.22 a Bone surface is at the shoulder. The dose at the breast was measured in a Nomex-suit pocket on the chest; the skin dose was measured in another pocket on the abdomen. b The value shows a mean (m) ± one standard deviation (σ); the σ indicates a statistical error (type-a) only. Systematic errors (type-b) of the detector system were conservatively incorporated into the values, in keeping with radiological protection practices, by using conservative calibration curves in both the correction of TLD efficiency (Fig. 2) and the determination of LET using PNTD (Fig. 3). c The Q-LET relationship and the w T values were adopted from the 1990 recommendation of ICRP, although the concept of dose equivalent was introduced in 1977 recommendations.

5 Effective Dose in LEO 93 Space Center (NASA-JSC), using small active detectors of Si PIN diodes. 28) Table 2 shows a comparison of the absorbed doses in critical internal organs/tissues measured by Badhwar et al. to our measurements. Both sets of data agreed well, and the error is within 20%. Also, comparisons of calculations by Cucinotta et al. 29) using the HZETRN/QMSFRG transport model 30) to the measured values are shown in Table 3. Very good agreement with less than 25% error was seen for all organs, and, regarding the effective dose, both data were in excellent agreement with only 5% difference. Another phantom torso experiment was performed by the scientists of NASA-JSC for the ISS Increment-2 mission in In this experiment, small active Si detectors were located in critical organs of a phantom torso, as described by Badhwar et al. 28) ; in addition, a tissue equivalent proportional counter (TEPC) and a charged particle directional spectrometer (CPDS) were included on the flight. 30) Comparisons of the measurements to the calculations using the HZETRN/QMSFRG model resulted in very good agreement (Table 4). 29) Considering the large uncertainties of biological effects, predictions of effective dose using transport models have a high level of accuracy at this time, at least for low Earth orbit missions. We found that the dose rate obtained in the STS-91 experiment (about 0.4 msv d 1 ) is smaller than that estimated by previous observations (0.6 to 0.8 msv d 1 ) using a multilayer Si detector and combined CR-39/TLD methodology. 32,33) Relatively high dose rates were observed in previous experiments using TEPC for the Space Shuttle mission. 34,35) The dose rate, however, markedly decreased with increasing polyethylene cover thickness (Table 5). 35) The polyethylene Table 2. Comparison of absorbed doses measured using combined CR-39/TLD methodology 5) with those using small Si diode detectors 28) for the STS-91 phantom torso experiment. Organ/ Tissue Measured absorbed dose for 9.8 days [mgy (H 2O)] Yasuda et al. Badhwar et al. with CR-39/TLD with Si diode Ratio (Badhwar/Yasuda) Brain 2.4 ± ± ± 0.09 Bone surface 2.7 ± ± ± 0.08 Esophagus 2.1 ± ± ± 0.06 Lung 2.1 ± ± ± 0.15 Stomach 2.4 ± ± ± 0.14 Liver 2.3 ± ± ± 0.14 Bone marrow 1.8 ± ± ± 0.09 Colon 1.7 ± ± ± 0.19 Bladder 1.8 ± ± ± 0.09 Gonad (Testis) 2.0 ± ± ± 0.07 Note: The value shows a mean (m) ± one standard deviation (σ). with a thickness of 13 g cm 2 reduced the dose equivalent to about half of the bare TEPC s value. This fact suggests that a human body works as an efficient shielding material against space radiation exposure. A t-test between H skin and H T values (Table 1) obtained in STS-91 showed that H skin was higher than H T in almost all the organs and tissues tested. H T values greater than H skin were observed at the shoulder-bone surface only. This Table 3. Comparison of organ dose equivalents measured using combined CR-39/TLD methodology 5) with calculations using the HZETRN/QMSFRG transport model 29) for the STS- 91 phantom torso experiment. Organ/ Tissue Organ dose equivalent for 9.8 days [msv] Measured by Yasuda et al. Calculated by Cucinotta et al. Ratio (Cucinotta/Yasuda) Skin 4.5 ± Thyroid 4.0 ± Bone surface 5.2 ± Esophagus 3.4 ± Lung 4.4 ± Stomach 4.3 ± Liver 4.0 ± Bone marrow 3.4 ± Colon 3.6 ± Bladder 3.6 ± Gonad (Testis) 4.7 ± Breast (Chest) 4.5 ± Remainder 4.0 ± Effective dose 4.1 ± Note: The value shows a mean (m) ± one standard deviation (σ). Table 4. Comparison of the organ dose rates measured using small Si detectors to calculations using the HZETRN/ QMSFRG model for the ISS Increment-2 phantom torso experiment in ) Organ Absorbed dose rate [mgy d 1 ] Trapped radiation GCR Total Expt. Model Expt. Model Expt. Model Ratio (Model/ Expt.) Brain Thyroid Heart Stomach Colon

6 94 H. Yasuda finding is favorable for controlling space radiation exposure, because it is expected that the individual dose of an astronaut could be measured conservatively on the skin surface at the breast/abdomen. It should be noted, however, that the H T values in selected radiation-sensitive organs such as the lung, stomach, gonad, and breast were not significantly different from the H skin value. A similar flat distribution of dose equivalents in a life-size phantom was observed in the ISS phantom torso experiment. 29) The data are shown in Table 4. Organ dose equivalents at the brain, thyroid, heart, stomach, and colon were at the same level, despite the varying depths of these organs from the skin. Such a flat distribution of internal dose was not expected based on previous model calculations 34 36) ; Table 5. Absorbed doses and dose equivalent rates measured during a Shuttle-Mir mission (51.6 ~400 km) using TEPC under different thicknesses of polyethylene shielding. 35) Sphere thickness [g cm 2 ] Dose equivalent rates [msv d 1 ] GCR Trapped particles Total dose equivalents in deep organs had been predicted to be much lower than that at the skin. Although previous predictions may be true for a simplified structure like a polyethylene sphere, 35) the human body is surely different from such an idealized shape. The difference in findings between earlier studies and the present study suggests that operational quantities based on the ICRU sphere concept, such as the 1-cm ambient dose equivalent, may be inappropriate to apply for astronauts. In 2007, ICRP published new recommendations 4) with new values of tissue weighting factor (w T ); the w T value of breast increased from 0.05 to 0.12; that of gonad decreased from 0.20 to 0.8; those of bladder, esophagus, liver and thyroid decreased from 0.05 to 0.04; and the value of 0.12 is given in place of 0.05 for remainder tissues, including adrenals, extrathoracic region, gall bladder, heart, kidneys, lymphatic nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus and uterus/cervix. In order to examine the effect of these changes, w T-weighted dose equivalents for selected organs/tissues and effective dose were calculated using 2007 w T values as shown in Table 6, compared to those obtained using 1990 w T values. It should be noted that contribution of the gonad remarkably decreased from 23% to 10%, whereas that of breast increased from 6% to 14%. As results, the effective dose slightly (about 3%) reduced from 4.1 msv to 4.0 msv. So far the update of w T values in 2007 does not seem to affect so Table 6. Organ dose equivalents weighted using the 2007 tissue weighting factors (w T) 4) and effective dose for the STS-91 phantom torso experiment; the results using the 1990 tissue weighting factors are also shown for comparison. Organ/Tissue Organ dose equivalent (H T) [msv] b 2007 tissue weighting factor (w T) H T w T using 2007 w T H T w T using 1990 w T Skin 4.5 ± ± ± Thyroid 4.0 ± ± ± Bone surface 5.2 ± ± ± Esophagus 3.4 ± ± ± Lung 4.4 ± ± ± Stomach 4.3 ± ± ± Liver 4.0 ± ± ± Bone marrow 3.4 ± ± ± Colon 3.6 ± ± ± Bladder 3.6 ± ± ± Gonad (Testis) 4.7 ± ± ± Breast (Chest) 4.5 ± ± ± Remainder 4.0 ± ± ± Effective dose [msv] 4.0 ± ± 0.22 Note: The value shows a mean (m) ± one standard deviation (σ).

7 Effective Dose in LEO 95 much the risk estimates of stochastic effects for astronauts. CONCLUDING REMARKS According to the results obtained in the past phantom torso experiments, 5,28,29) one would expect that the individual dose for an astronaut could be properly measured using a small personal dosimeter on the skin. To determine whether this is so, further investigation is needed on the effects of potential modifying factors such as variations of spacecraft shielding and/or orientation of a human body. Since a human body itself can work as a shielding material, the direction and posture of an astronaut should affect organ doses and could change effective dose, even at the same location in a spacecraft. Also, the balance of effective dose and specific organ doses might change depending on the solar cycle and flare events. Monitoring of solar activity changes is especially important to protect astronauts from flare particles during extravehicular activities. 36) One challenge is that we cannot know the true value of the effective dose for each astronaut. Although the author tried to develop a simple method using combined small dosimeters to determine the dose equivalent at any position in a spacecraft, 19,37,38) conversion from a dosimeter value to the effective dose is associated inevitably with certain uncertainties due to the complex transport phenomena of space radiation, particularly the HZE particles of galactic cosmic rays. The uncertainties need to be conservatively incorporated into the dose value in view of radiological protection, which leads to unnecessary restriction of human activities in space. Future longer missions such as moon base construction and Mars exploration will require more precise determination of career doses. Intensive efforts should be devoted to establishing a more reliable and practical method for risk estimation, such as probabilistic prediction in accordance with possible scenarios of astronaut behavior. Also, active dosimeter that can tell an astronaut the unexpected exposure to high dose radiation caused by solar particle event is desirable to be developed. Finally, ongoing efforts in the fields of radiation biology and medicine 39 42) are no doubt important to reduce the large uncertainties associated with the biological effects of the HZE particles and their secondary radiations encountered in space ) ACKNOWLEDGEMENTS Our studies have been supported and encouraged by many colleagues and collaborators. Special thanks are given to Dr. Kazunobu Fujitaka (NIRS) for his continuous guidance and encouragement. The author appreciate the excellent management of Mr. Tatsuto Komiyama (Japan Aerospace Exploration Agency). The late Dr. Gautam D. Badhwar (NASA Johnson Space Center) modeled excellence in science, and his guidance is gratefully acknowledged. The STS-91 phantom torso experiment was perfectly operated by NASA and National Space Development Agency of Japan (NASDA). Most of the data for detector calibration were obtained as part of the Research Project using Heavy Ions at NIRS- HIMAC. A part of this study was supported by the Groundbased Research Program for Space Utilization promoted by the Japan Space Forum. REFERENCES 1. National Council on Radiation Protection and Measurements (2000) Radiation Protection Guidance for Activities in Low- Earth-Orbit. NCRP Report 132, NCRP, Bethesda, MD. 2. Sinclair, W. K. (1997) History of development of radiation protection standards for space activities. In NCRP Symposium Proc. 3 - Acceptability of Risk From Radiation -Application to Human Space Flight. pp , NCRP, Bethesda. 3. International Commission on Radiological Protection (1991) 1990 Recommendations of the International Commission on Radiological Protection. Publication 60, Annals of ICRP 21, Pergamon Press, Oxford. 4. International Commission on Radiological Protection (2007) 2007 Recommendations of the International Commission on Radiological Protection. Publication 103, Annals of ICRP Vol. 37 (2/4), Pergamon Press, Oxford. 5. Yasuda, H., Badhwar, G. D., Komiyama, T. and Fujitaka, K. (2000) Effective dose equivalent in the ninth Shuttle-Mir Mission (STS-91). Radiat. Res. 154: Wilson, J. W., Townsend, L. W., Schimmerling, W., Khandelwal, G. S., Khan, F., Nealy, J. E., Cucinotta, F. A., Simonsen, L. C., Shinn, J. L. and Norbury, J. W. (1991) Transport methods and interactions for space radiations. NASA RP-1257, NASA, Washington, D.C. 7. Cucinotta, F. A., Wilson, J. W., Katz, R., Atwell, W. and Badhwar, G. D. (1995) Track structure and radiation transport models for space radiobiology studies. Adv. Space Res. 18: Hoff, J. L., Townsend, L. W. and Zapp, E. N. (2002) Space radiation protection: comparison of effective dose to bone marrow dose equivalent. J. Radiat. Res. 43: S125 S Wilson, J. W., Tripathi, R. K., Qualls, G. D., Cucinotta, F. A., Prael, R. E., Norbury, J. W., Heinbockel, J. H., Tweed, J. and Angelis, G. De. (2002) Advances in space radiation shielding codes. J. Radiat. Res. 43: S87 S Gudowska, I., Andreo, P. and Sobolevsky, N. (2002) Secondary particle production in tissue-like and shielding materials for light and heavy ions calculated with the Monte-Carlo code SHIELD-HIT. J. Radiat. Res. 43: S93 S Atwell, W. (1994) Anatomical models for space radiation applications: an overview. Adv. Space Res. 14: Saganti, P. B., Cucinotta, F. A., Wilson, J. W. and Schimmerling, W. (2002) Visualization of particle flux in the human body on the surface of Mars. J. Radiat. Res. 43: S119 S Hoffman, W. and Prediger, B. (1984) Heavy particle dosimetry with high temperature peaks of CaF 2:Tm and 7 LiF phosphors. Radiat. Prot. Dosim. 6: Geiß, O. B., Krämer, M. and Kraft, G. (1998) Efficiency of

8 96 H. Yasuda thermoluminescent detectors to heavy charged particles. Nucl. Instrum. Meth., B142: Yasuda, H. and Fujitaka, K. (2000) Responses of TLD- Mg 2SiO 4:Tb and Radiophotoluminescent glass to heavy charged particles and space radiation. Radiat. Prot. Dosim. 87: Yasuda, H. and Fujitaka, K. (2000) Glow curves from beryllium oxide exposed to high-energy heavy ions. Radiat. Prot. Dosim. 87: Yasuda, H. and Fujitaka, K. (2000) Non-linearity of hightemperature peak area ratio of 6 LiF:Mg,Ti (TLD-600). Radiat. Meas. 32: Yasuda, H. and Fujitaka, K. (2001) Responses of TLD- BeO:Na (UD-170A) to heavy ions and space radiation. Radiat. Prot. Dosim. 94: Yasuda, H. (2001) Conservative evaluation of space radiation dose equivalent using the glow curve of 7 LiF:Mg, Ti (TLD700). Health Phys. 80: Yasuda, H. and Kobayashi, I. (2001) Optically stimulated luminescence from Al 2O 3:C irradiated with relativistic heavy ions. Radiat. Prot. Dosim. 95: Yasuda, H. (2001) Responses of Direct Ion Storage dosimeter (DIS-1) to heavy charged particles. Radiat. Res. 156: Yasuda, H. (2001) Glow-peak stability in 6 LiF:Mg,Ti (TLD- 600) exposed to a Fe-ion beam. J. Radiat. Res. 42: Yasuda, H., Kobayashi, I. and Morishima, H. (2002) Decay patterns of optically stimulated luminescence from Al 2O 3:C for different quality radiations. J. Nucl. Sci. Technol. 39: Kalef-Ezra, J. and Horowitz, Y. S. (1982) Heavy charged particle thermoluminescence dosimetry: track structure theory and experiments. Int. J. Appl. Radiat. Isot. 33: Moscovitch, M. and Horowitz, Y. S. (1988) A microdosimetric track interaction model applied to alpha particle induced supralinearity and linearity in thermoluminescent LiF:Mg,Ti. J. Phys. D: Appl. Phys. 21: Geiß, O. B., Krämer, M. and Kraft, G. (1998) Verification of heavy ion dose distributions using thermoluminescent detectors. Nucl. Instrum. Meth. B148: Olko, P., Bilski, P., budzanowski, M., Waligorski, M. P. R. and Reitz, G. (2002) Modeling the response of thermoluminescence detectors exposed to low-and high-let radiation fields. J. Radiat. Res. 43: S59 S Badhwar, G. D., Atwell, W., Badavi, F. F., Yang, T. C. and Cleghorn, T. F. (2002) Space radiation absorbed dose distribution in a human phantom. Radiat. Res. 157: Cucinotta, F. A., Kim, Myung-Hee. Y., Willingham, V. and George, K. A. (2008) Physical and biological organ dosimetry analysis for International Space Station astronauts. Radiat. Res. 170: Wilson, J. W., Townsend, L. W., Schimmerling, W. G., Khandelwal, S., Khan, F., Nealy, J. E., Cucinotta, F. A., Simonsen, L. C., Shinn, J. L. and Norbury, J. W. (1991) Transport methods and interactions for space radiations. NASA TP Badhwar, G. D. (2001) Radiation measurements on the International Space Station (2001) Physica Medica 17 (Suppl. 1): Doke, T., Hayashi, T., Nagaoka, S., Ogura, K. and Takeuchi, R. (1995) Estimation of dose equivalent in STS-47 by combination of TLDs and CR-39. Radiat. Meas. 24: Hayashi, T., Doke, T., Kikuchi, J., Hasebe, J., Badhwar, G. D., Nagaoka, S. and Kato, M. (1996) Measurement of LET distribution and dose equivalent on board the Space Shuttle STS- 65. Radiat. Meas. 26: Badhwar, G. D., O Neill, P. M. and Cucinotta, F. A. (1993) Depth-dose equivalent relationship for cosmic rays at various solar minima, Radiat. Res. 134: Badhwar, G. D. and Cucinotta, F. A. (1998) Depth dependence of absorbed dose, dose equivalent and linear energy transfer spectra of galactic and trapped particles in polyethylene and comparison with calculations of models. Radiat. Res. 149: Dudkin, V. E. and Potapov, Yu. V. (1992) Doses from galactic cosmic ray particles under spacecraft shielding, Nucl. Tracks. Radiat. Meas., 20: Yasuda, H. (2002) Application of solid-state integrating dosemeters to biological experiments in space. Radiat. Prot. Dosim. 100: National Council on Radiation Protection and Measurements (2002) Operational Radiation Safety Program for Astronauts in Low-Earth Orbit: A Basic Framework. NCRP Report No.142, pp. 1 83, NCRP, Bethesda. 39. Yamaguchi, H. and Waker, A. J. (2007) A model for the induction of DNA damages by fast neutrons and their evolution into cell clonogenic inactivation. J. Radiat. Res. 48: Endo, S., Takada, M., Onizuka, Y., Tanaka, K., Maeda, N., Ishikawa, M., Miyahara, N., Hayabuchi, N., Shizuma, K. and Hoshi, M. (2007) Microdosimetric evaluation of secondary particles in a phantom produced by carbon 290 MeV/nucleon ions at HIMAC. J. Radiat. Res. 48: Satoh, D., Takahashi, F., Endo, A., Ohmachi, Y. and Miyahara, N. (2008) Calculation of dose contributions of electron and charged heavy particles inside phantoms irradiated by Monoenergetic Neutron. J. Radiat. Res. 49: Hada, M. and Georgakilas, A. G. (2008) Formation of clustered DNA damage after high-let irradiation. J. Radiat. Res. 49: Fry, R. J. M. (1997) Biology relevant to space radiation. In NCRP Symposium Proc. 3 - Acceptability of Risk From Radiation -Application to Human Space Flight. pp. 5 32, NCRP, Bethesda. 44. Ohnishi, T., Takahashi, A. and Ohnishi, K. (2002) Studies about space radiation promote new fields in radiation biology. J. Radiat. Res. 43: S7 S National Council on Radiation Protection and Measurements (2006) Radiation Protection Research Recommendations for Missions Beyond LEO. NCRP Report 153, NCRP, Bethesda, MD. Received on October 16, 2008 Revision received on December 23, 2008 Accepted on December 27, 2008 J-STAGE Advance Publication Date: February 7, 2009