J. Radiat. Res., 50, 89–96 (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)43-206-3233,
Fax: +81-(0)43-206-3542,
E-mail: [email protected]
National Institute of Radiological Sciences Anagawa, Inage-ku, Chiba 2638555, Japan.
doi:10.1269/jrr.08105
#
JRRS Incentive Award
wR 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 codes6–9) 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 MEASUREMENT 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,
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp
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 Mg2SiO4: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.13–23) This phenomenon
has successfully been explained a priori for LiF TLDs by
means of target-hit models based on the track structure theory.24–27) 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 137Cs 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.
1.15–23) 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 dosimeters15–23): TLD-Mg2SiO4:Tb
(MSO), TLD-BeO:Na, TLD-7LiF:Mg,Ti, the radiophotoluminescence dosimeter (RPLD) of a phosphate glass, the optically stimulated luminescence dosimeter (OSLD) of Al2O3: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-Mg2SiO4:Tb
(MSO) as the 137Cs-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.
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp
Effective Dose in LEO
(a)
91
(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. 52–59) 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 STS91/9th 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
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp
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
recommendations3); details of dose determination procedures
were explained in the paper5) of the author and his colleagues.
Table 1 shows the values of absorbed dose (DT), dose
equivalent (HT), and effective quality factor (Qe) 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. Qe 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 (Hskin) 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 (wT) and the wT-weighted dose equivalents are also shown.
Organ/
Tissuea
Absorbed dose
(DT)
[mGy (H2O)]b
Organ/tissue dose
equivalent (HT)
[mSv]b
Effective
quality factor
(Qc)
Tissue
weighting
factor (wT)
HT × wTb
Skin
2.2 ± 0.17
4.5 ± 0.05
2.0 ± 0.16
0.01
0.05 ± 0.001
Thyroid
2.2 ± 0.12
4.0 ± 0.21
1.9 ± 0.16
0.05
0.20 ± 0.011
Bone surface
2.7 ± 0.24
5.2 ± 0.22
1.9 ± 0.12
0.01
0.05 ± 0.002
Esophagus
2.1 ± 0.13
3.4 ± 0.49
1.7 ± 0.17
0.05
0.17 ± 0.024
Lung
2.1 ± 0.31
4.4 ± 0.76
2.1 ± 0.20
0.12
0.53 ± 0.091
Stomach
2.4 ± 0.30
4.3 ± 0.94
1.8 ± 0.50
0.12
0.52 ± 0.113
Liver
2.3 ± 0.33
4.0 ± 0.51
1.7 ± 0.33
0.05
0.20 ± 0.026
Bone marrow
1.8 ± 0.10
3.4 ± 0.40
1.9 ± 0.14
0.12
0.41 ± 0.048
Colon
1.7 ± 0.22
3.6 ± 0.42
2.2 ± 0.44
0.12
0.43 ± 0.050
Bladder
1.8 ± 0.16
3.6 ± 0.24
2.0 ± 0.25
0.05
0.18 ± 0.012
Gonad (Testis)
2.0 ± 0.05
4.7 ± 0.71
2.4 ± 0.37
0.20
0.94 ± 0.142
Breast (Chest)
2.3 ± 0.16
4.5 ± 0.11
1.9 ± 0.13
0.05
0.23 ± 0.006
Remainder
2.1 ± 0.15
4.0 ± 0.57
1.9 ± 0.22
0.05
0.20 ± 0.029
Effective dose [mSv]:
a
4.1 ± 0.22
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 wT values were adopted from the 1990 recommendation of ICRP, although
the concept of dose equivalent was introduced in 1977 recommendations.
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp
Effective Dose in LEO
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
model30) 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
2001. 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 methodology5) with those using small Si
diode detectors28) for the STS-91 phantom torso experiment.
Organ/
Tissue
93
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 Hskin and HT values (Table 1) obtained in
STS-91 showed that Hskin was higher than HT in almost all
the organs and tissues tested. HT values greater than Hskin
were observed at the shoulder-bone surface only. This
Table 3. Comparison of organ dose equivalents measured
using combined CR-39/TLD methodology5) with calculations
using the HZETRN/QMSFRG transport model29) for the STS91 phantom torso experiment.
Organ dose equivalent
for 9.8 days [mSv]
Organ/
Tissue
Ratio
(Cucinotta/Yasuda)
Measured by Calculated by
Yasuda et al. Cucinotta et al.
Skin
4.5 ± 0.05
4.7
1.04
Thyroid
4.0 ± 0.21
4.0
1.00
Bone surface
5.2 ± 0.22
4.0
0.77
Esophagus
3.4 ± 0.49
3.7
1.09
Lung
4.4 ± 0.76
3.8
0.86
Stomach
4.3 ± 0.94
3.6
0.84
Liver
4.0 ± 0.51
3.7
0.93
Bone marrow
3.4 ± 0.40
3.9
1.15
Colon
3.6 ± 0.42
3.9
1.08
Bladder
3.6 ± 0.24
3.5
0.97
Gonad (Testis)
4.7 ± 0.71
3.9
0.83
Breast (Chest)
4.5 ± 0.11
4.5
1.00
Remainder
4.0 ± 0.57
4.0
1.00
Effective dose
4.1 ± 0.22
3.9
0.95
Note: The value shows a mean (m) ± one standard deviation (σ).
Measured absorbed dose
for 9.8 days [mGy (H2O)]
Ratio
(Badhwar/Yasuda)
Yasuda et al. Badhwar et al.
with CR-39/TLD with Si diode
Brain
2.4 ± 0.20
2.23 ± 0.09
0.93 ± 0.09
Bone surface
2.7 ± 0.24
2.16 ± 0.08
0.80 ± 0.08
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 2001.29)
Esophagus
2.1 ± 0.13
1.71 ± 0.06
0.81 ± 0.06
Absorbed dose rate [mGy d–1]
Lung
2.1 ± 0.31
1.92 ± 0.15
0.91 ± 0.15
Stomach
2.4 ± 0.30
2.05 ± 0.23
0.85 ± 0.14
Liver
2.3 ± 0.33
1.88 ± 0.19
0.82 ± 0.14
Bone marrow
1.8 ± 0.10
1.75 ± 0.13
0.97 ± 0.09
Colon
1.7 ± 0.22
1.71 ± 0.24
Bladder
1.8 ± 0.16
Gonad (Testis)
2.0 ± 0.05
Organ Trapped radiation
GCR
Total
Ratio
(Model/
Expt.)
Expt.
Model
Expt. Model Expt. Model
Brain
0.051
0.066
0.076 0.077 0.127 0.143
1.13
1.01 ± 0.19
Thyroid
0.062
0.072
0.074 0.077 0.136 0.148
1.09
1.58 ± 0.07
0.88 ± 0.09
Heart
0.054
0.061
0.075 0.076 0.129 0.137
1.06
1.75 ± 0.13
0.88 ± 0.07
Stomach 0.050
0.057
0.076 0.077 0.126 0.133
1.06
0.056
0.073 0.076 0.128 0.131
1.02
Note: The value shows a mean (m) ± one standard deviation (σ).
Colon
0.055
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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 HT
values in selected radiation-sensitive organs such as the
lung, stomach, gonad, and breast were not significantly different from the Hskin 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 calculations34–36);
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)
–1
Sphere thickness Dose equivalent rates [mSv d ]
[g cm2]
GCR Trapped particles Total
0.00
0.48
0.42
0.90
4.32
0.32
0.25
0.57
12.68
0.29
0.18
0.47
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 recommendations4) with
new values of tissue weighting factor (wT); the wT 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, wT-weighted dose
equivalents for selected organs/tissues and effective dose
were calculated using 2007 wT values as shown in Table 6,
compared to those obtained using 1990 wT 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 wT values in 2007 does not seem to affect so
Table 6. Organ dose equivalents weighted using the 2007 tissue weighting factors (wT)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 dose
2007 tissue
HT × wT
equivalent (HT) weighting factor
using 2007 wT
b
[mSv]
(wT)
Organ/Tissue
HT × wT
using 1990 wT
Skin
4.5 ± 0.05
0.01
0.05 ± 0.001
0.05 ± 0.001
Thyroid
4.0 ± 0.21
0.04
0.16 ± 0.008
0.20 ± 0.011
Bone surface
5.2 ± 0.22
0.01
0.05 ± 0.002
0.05 ± 0.002
Esophagus
3.4 ± 0.49
0.04
0.14 ± 0.020
0.17 ± 0.024
Lung
4.4 ± 0.76
0.12
0.53 ± 0.091
0.53 ± 0.091
Stomach
4.3 ± 0.94
0.12
0.52 ± 0.113
0.52 ± 0.113
Liver
4.0 ± 0.51
0.04
0.16 ± 0.020
0.20 ± 0.026
Bone marrow
3.4 ± 0.40
0.12
0.41 ± 0.048
0.41 ± 0.048
Colon
3.6 ± 0.42
0.12
0.43 ± 0.050
0.43 ± 0.050
Bladder
3.6 ± 0.24
0.04
0.14 ± 0.010
0.18 ± 0.012
Gonad (Testis)
4.7 ± 0.71
0.08
0.38 ± 0.057
0.94 ± 0.142
Breast (Chest)
4.5 ± 0.11
0.12
0.54 ± 0.013
0.23 ± 0.006
Remainder
4.0 ± 0.57
0.12
0.48 ± 0.068
0.20 ± 0.029
4.0 ± 0.19
4.1 ± 0.22
Effective dose [mSv]
Note: The value shows a mean (m) ± one standard deviation (σ).
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp
Effective Dose in LEO
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 medicine39–42)
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.43–45)
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 phan-
95
tom 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 NIRSHIMAC. A part of this study was supported by the “Groundbased Research Program for Space Utilization” promoted by
the Japan Space Forum.
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Received on October 16, 2008
Revision received on December 23, 2008
Accepted on December 27, 2008
J-STAGE Advance Publication Date: February 7, 2009
J. Radiat. Res., Vol. 50, No. 2 (2009); http://jrr.jstage.jst.go.jp