In the case of a nuclear accident with a large release of the radionuclides into the environment, the main concern is exposure to the residents living in the vicinity of the accident site. This paper presents the development of a new thyroid monitor for such measurements and the results from phantom-based experiments.
Trang 1Available online 19 November 2021
1350-4487/© 2021 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
Development of a new hand-held type thyroid monitor using multiple
GAGG detectors for young children following a nuclear accident
Kazuaki Yajimaa, Eunjoo Kima,*, Kotaro Tania, Masumi Ogawaa,b, Yu Igarashia,
Munehiko Kowataria, Osamu Kuriharaa
aNational Institutes of Quantum and Radiological Science and Technology, National Institute of Radiological Sciences (QST− NIRS), 4-9-1 Anagawa, Inage-ku, Chiba,
263-8555, Japan
bSelf-Defense Forces, Central Hospital, 1-2-24 Ikejiri, Setagaya-ku, Tokyo, 154-0001, Japan
A R T I C L E I N F O
Keywords:
Thyroid exposure
Radioiodine
Thyroid monitor
GAGG detector
Nuclear accident
A B S T R A C T One of the most important lessons learned from the Fukushima Daiichi Nuclear Power Plant accident is that
direct (in vivo) measurements of thyroid exposure to radioiodine (mainly, 131I) in the affected populations should
be initiated in a timely manner Furthermore, the existing commercial detectors are not necessarily suitable for the measurement of young children, who are especially vulnerable to radiation exposure and therefore important
to screen This paper presents the development of a new thyroid monitor for such measurements and the results from phantom-based experiments The monitor has two unique probes having multiple high-energy-resolution type Gd3(Al,Ga)5O12(Ce) (GAGG) detectors that can be placed directly on the young subject’s anterior neck The crystal size of the GAGG detector is 1 cm3, and a probe consisting of a 4 × 1 or 4 × 2 detector array can be selected depending on the subject’s body size The thickness of the 4 × 1 array probe is 24 mm, which is less than that for a conventional NaI(Tl) survey meter having a crystal 1 inch in diameter and 1 inch long (38 mm) Experimental and computational calibrations of the new monitor using existing and virtual phantoms allowed us
to determine the full energy peak efficiency for the 131I thyroid contents of different age groups from 3-mo-old to 10-yr-old and minimum detectable activity (MDA) values under various conditions As a result, the attainable MDA for subjects age ≤5 years under a normal background level (~0.05 μSv h− 1) was found to be ~30 Bq, which was low enough to identify children with thyroid-equivalent doses over 10 mSv up to about 25 days after the 131I intake Our new monitor would be useful in direct thyroid measurements for vulnerable young children following a large nuclear accident
1 Introduction
In the case of a nuclear accident with a large release of the
radio-nuclides into the environment, the main concern is exposure to the
residents living in the vicinity of the accident site In particular,
atten-tion should be paid to the internal thyroid exposure to children due to
intake of radioiodine, as demonstrated in the Chernobyl nuclear
acci-dent in 1986 (Cardis et al., 2011) The largest contributor to the internal
thyroid dose is 131I, with a physical half-life of about 8 days The dose
delivered following intake of radioiodine is localized in the thyroid, a
relatively small organ in the human body The thyroid dose per unit of
131I intake (Sv Bq− 1) is much higher in young children than in adults, in
accord with the thyroid mass defined in each age group’s anatomical
model (e.g., 1.78 g for 1-yr-olds, 20.0 g for adults) (ICRP 1995,
International Commission on Radiological Protection, 1998) The actual thyroid doses would be less age-dependent in the same exposure con-dition because inhalation or ingestion intake amounts are smaller for younger people; however, it should be noted that the radiosensitivity of the thyroid per unit dose (e.g., the excess relative risk: ERR) was shown
to be high in young children by epidemiological studies related to the Chernobyl accident (Brenner at al., 2011)
In the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in March 2011, the number of direct thyroid measurements to determine individuals’ 131I thyroid contents was very limited in contrast
to that in the Chernobyl accident (Kim and Kurihara 2020) Therefore, the assessment of thyroid doses to most of Fukushima prefecture’s res-idents had to be performed using other data, such as whole-body counter measurements targeting 134Cs and 137Cs, radiation monitoring results of
* Corresponding author
E-mail address: kim.eunjoo@qst.go.jp (E Kim)
Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
https://doi.org/10.1016/j.radmeas.2021.106683
Received 12 July 2021; Received in revised form 1 November 2021; Accepted 17 November 2021
Trang 2and need to be verified by results of direct measurements This is one of
the critical lessons learned from the FDNPP accident: namely, individual
photon detection to assess thyroid exposure to the possibly affected
populations must be performed in a timely manner so as not to miss the
window for evidence of 131I intake in the case of a major nuclear
acci-dent event
One technical problem in direct thyroid measurements is that
existing photon detectors are not necessarily appropriate for
measure-ments of young children or newborns (Broggio et al., 2019) We found
that a NaI(Tl) survey meter with a probe, in which a crystal of 1 inch
diameter and 1 inch length is installed, was too bulky to be placed in
close proximity to the anterior neck of preschool children (age <4
years), possibly resulting in decreased sensitivity and reproducibility of
the measurements This device (TCS-171/172, Hitachi, Ltd., Japan) has
been widely used in Japan for environmental monitoring and was also
intended to be used for the screening of thyroid exposure to radioiodine
in case of nuclear emergencies (JAERI 1993) Indeed, it was used in the
FDNPP accident (Kim et al., 2012) To solve this problem, we proposed
the concept of a new thyroid monitor suited for young children making
use of multiple small photon detectors (Yajima et al., 2020) We selected
high-energy-resolution type Gd3(Al,Ga)5O12(Ce) (hereinafter, GAGG)
detectors, each equipped with a 1-cm cube-shaped crystal; the resulting
monitor performed spectrometric measurements in contrast to the
count/dose rate measurements made with the NaI(Tl) survey meter Our
previous study used age-specific thyroid phantoms developed by the
Institut de Radioprotection et de Sûret´e Nucl´eaire (IRSN) (Beaumont
et al., 2017) to compare the minimum detectable activity (MDA) values
for 131I of two arrangements of 8 GAGG detectors with those of NaI(Tl)
spectrometers with larger crystal sizes, and found equal or better
per-formance by the former This paper introduces our newly developed
thyroid monitor for young children and its performance
2 Development of the new thyroid monitor
The system for the new thyroid monitor has been described in our
previous paper (Yajima et al., 2020) It uses commercially available
GAGG detectors (Model: HR-GAGGG1C1C1C-type, Clear-Pulse Co.,
Tokyo Japan) enclosed in originally designed and manufactured
the variation of temperature allows us to minimize the need for frequent energy calibration
Based on the results of the previous study and mock-up experiments using manikins or child volunteers and temporal probes (Fig 1), we decided upon two detector arrangements for the anterior neck probe: a 4 (row) × 1 (column) array and a 4 × 2 array We also interviewed two pediatricians about how to safely examine young children using the probe As a result, we obtained the following valuable comments
• Two subject postures, sitting and supine, are possible For the use of the seated position, the subject sits on a small chair or on his/her mother’s legs while being hugged (although mothers may not always
be the person in charge) The measurer sits face-to-face to the subject and places the probe on their anterior neck When the supine position
is used, the subject lies on a bed or is held by his/her mother, who can place the probe on the subject’s neck herself
• It will be challenging for young children, in particular those aged 2–5 years old who are likely to move actively, to remain still for the duration of the measurement time, a few minutes at least In the sitting posture, the mother would be able to hold the subject’s head gently if necessary Using a toy or something to attract the child’s attention may also allow the subject to be measured with greater ease In any case, the mother’s presence and participation are the best way for the child to be reassured in such an unusual situation Considering the above comments, we developed the new hand-held thyroid monitor as demonstrated in Fig 2 This thyroid monitor can select one of the two detector arrangements described above, and its weight is about 1 kg (without the laptop), which is lighter than the NaI (Tl) survey meter (1.5 kg for TCS-171/172) The thickness of the probe is
24 mm in the case of the 4 × 1 detector array with a plastic cover, which
is less than the thickness of the NaI(Tl) survey meter’s probe (38 mm) The structures of the two probes of the developed thyroid monitor are shown in Fig 3 In this illustration, the detectors are arrayed along the outer surface of the IRSN 10-yr-old phantom with a diameter of 88 mm The two probes are currently meant to be used separately depending on the subject’s age; the 4 × 1 probe for subjects aged ≤5 years old and the
4 × 2 probe for subjects aged >5 years old
Fig 1 Scenes of mock-up experiments performed to identify the optimal detector arrangements for the new thyroid monitor
Trang 33 Calibration and minimum detectable activity
3.1 Counting efficiency
The new thyroid monitor was calibrated using the age-specific
thy-roid phantoms developed by IRSN (Beaumont et al., 2017) Two
phan-toms imitating a 5-yr-old and a 10-yr-old were used in this study Pulse
height spectra were obtained by the thyroid monitor with a setting of
about 1 keV per channel (1024 channels in total) In the calibration
using the IRSN phantoms, we installed portions of 131I standard solution
(Code: IO010, Japan Radioisotope Association, Tokyo Japan) into the
thyroid-shaped container in each phantom and determined the net peak
area at the primary peak line (365 keV, 81.7%) using a software package
for spectrum analysis (Prime Advanced Fusion Technology Co Ltd.,
Tokyo Japan) The radioactivity in the phantom was adjusted to avoid
problems related to the counting rate (e.g., the dead time above a few
percentages and the energy shift due to the pile-up) We then calculated
the full energy peak (FEP) efficiency values for the 131I thyroid contents
by dividing the net peak areas by the counting time and the loaded
radioactivity
We also performed a computational calibration using age-specific mathematical phantoms (3-mo-old, 1-yr-old, 5-yr-old, and 10-yr-old)
to cover broader age groups The mathematical phantoms have been described elsewhere (Ulanovsky and Eckerman, 1998) and were kindly supplied by Dr A.V Ulanovsky at our request It is noted that the thyroid-shaped containers in the IRSN phantoms were designed with reference to the configurations of these mathematical phantoms The computational calibration was performed only for the detector arrangement of the 4 × 1 array through a series of simulations using the Monte Carlo N-Particle® code ver 6 (MCNP6) (Werner, 2017) In the simulations, the internal structure of the GAGG detectors was modeled for the thyroid monitor, and the number of photons emitted was set to be
large enough to reduce the relative standard deviation to <1% We also
evaluated the FEP values in a manner similar to that used for the experimental calibration, by using the pulse height tally of the MCNP6
A reference location of the probe against each age-specific phantom was determined in a manner similar to that used in the experiments (described later) except for the 3-mo-old phantom, for which it was necessary to model the head tilting backward for probe placement Fig 4 shows the simulation models for the probe (4 × 1 detector array) together with the 3-mo-old, 1-yr-old, and 5-yr-old mathematical phantoms
3.2 Efficiency variation with the probe’s displacement
The probe of the new thyroid monitor is designed to be placed on the subject’s anterior neck during measurements However, some gap could
be generated between the probe and the neck in children with excessive movement Some deviation of the probe from the location at the beginning of the measurement is also possible, although the exact probe location cannot be identified in practice because of the wide interindi-vidual difference in the thyroid volume (or shape) and a possible inhomogeneous radioactivity distribution in the thyroid Thus, we experimentally obtained FEP values for different locations of the probe against the IRSN phantoms A reference location of the probe on each of these phantoms was decided upon so that the center of the probe was aligned with the top of the isthmus of the thyroid-shaped container in the phantom The displacement of the probe from the reference location for each phantom was arranged as follows: 0–3.0 cm for the distance between the probe and the phantom, − 1.0 to 1.0 cm in the vertical di-rection, and 0◦–20◦in the rotational angle (the last two displacements were the setting on the phantom) We assume that measurements with the monitor would be reattempted when the displacement of the probe exceeds these ranges
3.3 MDA
The minimum detectable activity (MDA) values of the thyroid
Fig 2 Exterior views of the new thyroid monitor (Panel A: the probe for 4 × 1 detector array, Panel B: the probe for 4 × 2 detector array) The dimensions are the
same in the two pictures
Fig 3 Structures of the two probes of the new thyroid monitor (left: the probe
for 4 × 1 detector array, right: the probe for 4 × 2 detector array) The lower
figures are probes with thicknesses of 24 mm and 44 mm, respectively
Trang 4monitor for the 131I thyroid content were calculated by the following
equation (Currie 1968; Gilmore 2008):
MDA = 2.71 + 4.65
̅̅̅
B
√
ε ⋅t
where ϵ is the FEP efficiency at 365 keV of 131I, B is the background
count for the corresponding energy interval, and t is the measurement
time (which was the same between the sample and the background in
this study) We experimentally determined MDA values of the thyroid
monitor with different locations of the probe and under elevated
back-ground radiation levels (up to 2.5 μSv h− 1) by using several 137Cs-
shielded sources placed around the thyroid monitor This upper
back-ground radiation level was due to the limitation of our experimental
setup However, this level would be within a reasonable range for the
practical use of the monitor, based on the experiences from the FDNPP
accident where the ambient dose rate was elevated to a few μSv h− 1 or
more at various places in Fukushima Prefecture (including the
munici-palities where prompt evacuation orders were not issued) (Kim et al.,
2020)
yr-old), whereas the simulations were performed for the four phan-toms aged ≤10-yr-old
The simulation results agreed within 10% with the experimental results using the 4 × 1 detector array for both the 5-yr-old phantom and the 10-yr-old phantom, suggesting that our simulations were likely reasonable for the other phantoms as well The detector model in the simulations was validated through benchmark experiments using a133Ba point source placed at different locations close to the center of the probe for the 4 × 1 detector array; the discrepancy in the FEP efficiency at 356 keV (62.1%) between the experiments and the summations was observed to be within 5% One notable finding in the simulation results was that the FEP efficiency value for the 4 × 1 detector array was less influenced by the differences in the phantoms for ages ≤5 yr This im-plies that the correction of the FEP efficiency value against the body size
or the subject’s age can be simplified when measuring young children
On the other hand, the experiments demonstrated that the FEP effi-ciency of the 4 × 2 detector array was almost double that of the 4 × 1 detector array in the case of the 5-yr-old phantom and the 10-yr-old phantom
4.2 Efficiency variation with the probe’s displacement
Figs 6–8 present the experimental results for the IRSN phantoms (5- yr-old and 10-yr-old) to confirm the variation of the FEP efficiency with the positional displacement of the probe from its reference location on the phantom The error bars in these figures have the same meaning as described above Fig 6 displays the relative FEP efficiency (to that in the case of the zero distance) as a function of the distance between the probe and the phantom The FEP efficiency was decreased with increasing distance, and its relative values were 0.75–0.80 at the distance of 5 mm Fig 7 displays the variation of the relative FEP efficiency with the ver-tical displacement of the probe Here, a positive (negative) displacement denotes that the probe moves upward (downward) from the reference location As shown, the FEP efficiency was found to be an asymmetric
Fig 4 Simulation models of the probe for a 4 × 1 detector array together with
mathematical phantoms for 3-mo-old (the upper), 1-yr-old (the middle) and 5-
yr-old (the bottom) in the same scale The right figures are the horizontal views
of each phantom at the level of the dotted lines in the left figures
Fig 5 Comparison of the FEP efficiency for the 131I thyroid contents in different phantoms
Trang 5profile in the vertical direction This variation was smaller in the 4 × 2
detector array than in the 4 × 1 detector array Fig 8 displays the
variation of the relative FEP efficiency with the rotation angle of the
phantom The results are provided only for one direction because the
IRSN phantoms have bilateral symmetry As shown, the relative FEP
efficiency decreased to 0.9 when the phantom was rotated by 20◦
4.3 MDA
Fig 9 presents the MDA values of the 4 × 1 and 4 × 2 detector arrays
for the 131I thyroid content in two IRSN phantoms (5-yr-old and 10-yr-
old) as a function of the measurement time under a normal
back-ground condition (~0.05 μSv h− 1) The MDA values in the case of a
counting time of 180 s were around 30 Bq Fig 10 presents the MDA
values of the same arrays for two IRSN phantoms (5-yr-old and 10-yr-
old) as a function of the surrounding ambient dose rate, H*(10), in the
case of a measurement time of 180 s As a result, the MDA value (1
standard deviation of three-times measurements) increased from 31(6)
Bq at the normal background level of ~0.05 μSv h− 1 to 229(3) Bq at 2.5
μSv h− 1 These MDA values can be applied to the younger age groups (3-
mo-old and 1-yr-old) because the FEP efficiency values of these groups
were comparable or slightly higher than that of the IRSN 5-yr-old
phantom (Fig 5)
5 Discussion
It is of great importance to perform direct thyroid measurements of
individuals without delay in the case of a major nuclear accident This
task would be a great challenge for most of the countries where
com-mercial nuclear power plants are in operation The significance of the
measurements seems to have been reinforced by the FDNPP accident as
seen in recent publications (IAEA 2013; Youngman 2013) A series of papers on the Child and Adult Thyroid Monitoring After Reactor Acci-dent (CAThyMARA) project (Broggio et al., 2019) described critical reviews and recommendations for direct thyroid measurement of the public They also address the necessity of having suitable spectrometric devices for young children (less than 5-yr-old or around) with a contact geometry to the neck Our new thyroid monitor appears to meet the requirement on this technical issue The thickness of the probe for the 4
×1 detector array (24 mm) was less than the length of the neck modeled
in the mathematical phantom for a newborn with a tilted head (31 mm) (Khrutchinsky et al., 2012) As a result, this probe could be placed on the anterior neck in the simulation model (Fig 3) Fig 11 displays scenes of mockup measurements by the new monitor with the use of a manikin imitating a newborn infant, suggesting that the contact geometry would
be achievable for real subjects However, this needs to be confirmed in a future study through experiments on human volunteers The manikin shown in the figure was a commercial product for a nursing practice; its height is about 50 cm
In the selection of the detector for the new thyroid monitor, we prioritized the practicability as a system rather than the performance In terms of the energy resolution, cadmium zinc telluride (CdZnTe) tectors have shown better performance compared to scintillation de-tectors including those developed in recent years; e.g., cerium bromide (CeBr3), strontium iodide (SrI2(Eu)), and lanthanum bromide
(LaB-r3(Ce)) scintillation detectors (Nishino et al., 2020) However, CdZnTe detectors are the most expensive when one compares these detectors with the same sensitive volume, and such a high cost would be a major obstacle for the mass production of a new thyroid monitor CeBr3 and SrI2(Eu) scintillation detectors have better energy resolution compared
to conventional NaI(Tl) detectors (Nishino et al., 2020; Hosoda et al., 2019) The energy resolution of our newly developed thyroid monitor is
Fig 6 Relative FEP efficiency of the two probes for the 131I thyroid contents in
two IRSN phantoms (5-yr-old and 10-yr-old) as a function of the distance
be-tween the phantom and the probe
Fig 7 Variation of the relative FEP efficiency due to the vertical displacement
from the reference location (Panel A for 4 × 1 detector array and Panel B for 4
×2 detector array)
Trang 6almost the same as that demonstrated in our previous study (Yajima
et al., 2020) (data not shown here) and is comparable to that of the NaI
(Tl) spectrometers, i.e., 7%–8% for the full width at half maximum
(FWHM) at 662 kev According to the manufacturer’s information, the
energy resolution of the GAGG detector used is 5.1% at 662 keV We thus
speculate that the energy resolution was degraded because of the use of
the original detector casing and multiple detector arrays However, this
would not be a significant problem when measuring 131I The main
ra-dionuclides that were identified soon after the FDNPP accident were
131I, 132Te–132I, 134Cs and 137Cs (Kurihara et al., 2012), and the adjacent
peak lines (with emission yields >10%, photon energy >100 keV) to the
primary peak line of 131I (365 keV, 81.7%) were from 132Te (228 keV,
88.0%) and 132I (523 keV, 16.0%) Our new thyroid monitor thus has
sufficient energy resolution to observe each of these three peak lines on a
pulse height spectrum without interference from the other two peak
lines The advantages of the GAGG detectors are described above in
Section 2 In particular, the significant gain-shift due to the variation of
temperature at a measurement place can be a common problem in
scintillation detectors The GAGG detectors used in the new monitor
solve this problem by fine gain adjustments based on characteristic tests,
and the detectors are also reliable for the use of new thyroid monitor
Our new monitor is intended to be used with a contact geometry
against the neck; however, it is important to examine the measurement
uncertainty due to the probe’s likely displacement, especially as the
target subjects are young children This study examined one of the
pri-mary uncertainty factors, the variation of the FEP efficiency due to the
probe’s displacement As demonstrated in Fig 6− 8, this variation was
found to be relatively small for the vertical/lateral displacements, but
was expected to be rather significant if there is a continuous gap
between the probe and the neck during measurements of subjects We estimated that the uncertainty due to the probe’s displacement is within
~20% in practical use (i.e., assuming a gap of a few millimeters between the probe and the subject’s neck and some other minor displacements) Larger displacements of the probe would appear to be abnormal by eye and were not assumed here However, actual measurements will also be accompanied with the counting statistics, the measurement uncertainty due to the inter-individual differences in the thyroid volume and thyroid-overlying tissue thickness (Likhtarev et al., 1995; Ulanovsky
et al., 1997), and so on To reduce the measurement uncertainty, the counting distance needs to be maximized, as suggested by the previous studies (Kramer and Crowley, 2000; Beaumont et al., 2018); however, there is a trade-off relationship with the sensitivity A long counting distance is beneficial if the subject’s body has been heavily exposed to radionuclides (IAEA, 1988), in particular in the case of direct mea-surements at the thyroid, in which the exact location of this small organ
is not visible from outside the body The experiences gained from the FDNPP accident suggest that situations in which members of the public are overexposed in a nuclear accident are unlikely to happen, as long as appropriate radiation protection measures are taken in a timely manner; however, all conceivable situations should be considered for the response to a future nuclear disaster
The evaluated MDA values (Figs 9 and 10) are useful to estimate the period of time during which direct thyroid measurements targeting 131I are feasible, given its relatively short physical half-life and biological half-life in particular for young children (ICRP 1989) Our new thyroid monitor achieved an MDA value of ~30 Bq for a counting time of 180 s under normal background conditions, which was comparable to that by
Fig 8 Variation of the relative FEP efficiency due to the lateral displacement
of the reference location (Panel A for 4 × 1 detector array, Panel B for 4 × 2
(Panel A for 4 × 1 detector array, Panel B for 4 × 2 detector array)
Trang 7a NaI(Tl) spectrometer with a 1-inch-dia., 1-inch thick crystal in similar
experiments (Yajima et al., 2020) It is notable that the total volume that
is sensitive to radiation is much smaller in the new thyroid monitor (4 or
8 cm3) compared to this NaI(Tl) spectrometer (12.9 cm3) On the other
hand, the MDA was ~40 Bq for a stationary-type thyroid monitor
equipped with a high-purity germanium (HPGe) semiconductor detector
mounted in a 50-mm-thick annular-shaped lead shield at our institute
(Kunishima et al., 2019; NIRS, 2016), although this monitor was difficult
to be applied to children
Fig 12 illustrates the TEDs corresponding to the MDA value for
different age groups as a function of the time after intake: Panel A for the
normal background and Panel B for the elevated background The MDA
values are described in Section 4.3 Here we assumed that the
physi-ochemical form was elemental iodine and used the datasets of the age-
specific thyroid 131I retention rates (Bq per Bq Intake) taken from the
MONDAL code (Ishigure et al., 2004) to prepare the figure The
MON-DAL code has the database of retention/excretion rates for 42
radionu-clides included in ICRP Publication 54 (International Commission on
Radiological Protection, 1989) and 78 (International Commission on
Radiological Protection, 1997) and was validated by comparisons with
the data on these references Based on this result, it is expected that
thyroid exposure corresponding to 10 mSv in TED for the 1-yr-old age
group can be detected until about 25 days post intake under a normal
background level of 0.05 μSv h− 1 (or until 10 days post intake under an
elevated background condition at 2.5 μSv h− 1) The same estimation can
be applied to the other age groups using the figure, with the result that
the older an age group is, the longer the period is These periods could be
regarded as an index for the time limitation of direct thyroid
measure-ments with the new thyroid monitor for each age group, although the
periods vary with the dose level of concern The direct thyroid mea-surements should be initiated in a timely manner as noted in the Introduction; however, some delay would be unavoidable in scenarios in
which a significant release of radionuclides lasts >1 week and residents
living in the affected areas are then ordered to shelter indoors by au-thorities during the release The period available for the measurements would thus be expected to be shortened (especially for young children), and the prioritization of subjects should be considered using the above estimations
It is desirable that direct thyroid measurements are performed at places with as low as possible ambient dose rates The 2013 IAEA guideline recommends that the ambient dose rate at the measurement location be less than 0.2 μSv h− 1 However, this was difficult to imple-ment in the screening campaigns conducted to identify the levels of internal thyroid exposure to children at the end of March 2011, about two weeks after the accident (Kim et al., 2012) Considerable elevations
of background radiation level can occur across vast territories after a large-scale nuclear accident; indeed, this is considered one of the major obstacles for the early initiation of direct thyroid measurements Finally, we would like to address the advantages of our new monitor over existing devices First, the unique probe shape allows stabilization against the curved surface of the neck during measurements of young children Second, the multiple detector system allows identification of irregular deposition of iodine in the thyroid (e.g., a difference between the left and right lobes) if necessary Third, the variation of the FEP
Fig 10 MDA for the 131I thyroid contents as a function of the ambient dose
rate in the case of a measurement time of 180 s (Panel A for 4 × 1 detector
array, Panel B for 4 × 2 detector array)
Fig 11 Scenes of mockup measurements with the new thyroid monitor
Trang 8efficiency is expected to be small for children aged ≤5 yrs (Fig 5) The
experimental FEP efficiency for the 5-yr-old IRSN phantom can be
applied to the FEP efficiency for younger children; the calculated peak
efficiencies for the 3-mo-old age group and the 1-yr-old age group were
about 20% higher (Fig 5), resulting in reasonable overestimations of
the 131I thyroid content
We currently consider that the best use of our monitor is the
addi-tional or detailed measurements in combination with screening using
NaI(Tl) survey meters (e.g., TCS-171/172) at the venue designated on
the regional evacuation plan for each nuclear power plant site Potential
subjects for the monitor are supposed to be persons (in particular, young
children) whose internal thyroid doses are found to be higher than a
certain dose level (e.g., 50 mSv in TED) Comparative measurements of
the same subjects by the two methods would surely be useful to validate
the results of the screening for dealing with large populations Non-
spectrometric devices would offer accurate determinations of the 131I
thyroid content based on appropriate calibration (Isaksson et al., 2019),
although one should evaluate the interference of other radionuclides in
measurements In this regard, a dose rate meter recently developed for
thyroid monitoring is also of interest to improve the early response in a
nuclear disaster (Meisenberg and Gerstmann, 2017) We will describe
further considerations on the use of our new thyroid monitor in the
anterior neck of the 1-yr-old mathematical phantom The full energy peak efficiency (FEP) of the monitor for 131I (at 365 keV) in the thyroid was obtained by experiments using IRSN phantoms and simulations using mathematical phantoms Based on the results, minimum detect-able activity (MDA) was evaluated for various ages The MDA values for subjects aged ≤5-yr-old were estimated as ~30 Bq at a normal back-ground level for a counting time of 180 s This value would be adequate
to detect the 131I thyroid content in 1-yr-old children corresponding to a thyroid-equivalent dose of 10 mSv until about 25 days post intake The possible variation of the FEP values due to the displacement of the probe was estimated to be about 20% Our monitor would be useful for direct thyroid measurements of young children following a major nuclear accident
Funding
This work was financially supported by the Nuclear Regulation Au-thority of Japan under the Radiation Safety Research Promotion Fund (JPJ007057)
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgements
We would like to express our gratitude to Dr Susumu Yokoya of Fukushima Medical University and Dr Nao Nishimura of National Center for Child Health and Development for their valuable comments
on direct thyroid measurements of young children We also appreciate Drs Tiffany Beaumont and David Broggio of IRSN for their support regarding the development and delivery of the phantom sets and Dr Alexander Ulanovsky for providing MCNP’s input decks for his devel-oped mathematical phantoms
References
Beaumont, T., Ideias, P.C., Rimlinger, M., Broggio, D., Frank, D., 2017 Development and test of sets of 3D printed age-specific phantoms for 131 I measurements Phys Med Biol 72, 4673–4693 https://doi.org/10.1088/1361-6560/aa6514 , 2017 Beaumont, T., Rimliger, M., Broggio, D., Ideias, P.C., Frank, D., 2018 A systemic experimental study of parameters influencing 131-iodine in vivo spectrometric measurements using age-specific thyroid phantoms J Radiol Prot 38, 651–655
https://doi.org/10.1088/1361-6498/aab967 Brenner, A.V., Tronko, M.D., Hatch, M., Bogdanova, T.I., Oliynik, V.A., Lubin, J.K., Zablotska, L.B., Tereschenko, V.P., McConnell, R.J., Zamotaeva, G.A., O’Kane, P., Bouville, A.C., Chaykovskaya, L.V., Greenebaum, E., Paster, I.P., Shpak, V.M., Ron, E., 2011 I-131 dose response for incident thyroid cancers in Ukraine related to the Chernobyl accident Environ Health Perspect 119 (7), 933–939 https://doi org/10.1289/ehp.1002674
Broggio, D., Baud´e, D., Belchior, A., Berkovskyy, V., Bonchuck, Y., Dewogh´ela¨ere, J., Etherington, G., Fojtík, P., Franck, D., Gomez-Ros, J.M., Gregoratto, D., Helebrant, J., H´eriard Dubreuil, G., Hůlka, J., Isaksson, M., Kocsonya, A., Lebacq, A.- L., Likhtarev, I., Lombardo, P., Lopez, M.A., Mal´atov´a, I., Marsh, J.W., Mitu, I., Monteiro Gil, O., Moraleda, M., Navarro, J.F., O´sko, J., P´antya, A., P´azm´andi, T., Perez, B., Pospisil, V., Ratia, G., Saizu, M.-A., Sz´ant´o, P., Teles, P., Tymi´nska, K.,
Fig 12 Thyroid-equivalent doses (TEDs) corresponding to the MDA for
different age groups as a function of the time after intake via inhalation of 131I
in the form of elemental iodine (Panel A for the normal background: 0.05 μSv
h− 1, Panel B for the elevated background: 2.5 μSv h− 1) The MDA values for
each condition are described in Section 4.3
Trang 9Vanhavere, F., Vaz, P., Vrba, T., Vu, I., Youngman, M., Zagyvai, P., 2019 Child and
adult thyroid monitoring after a reactor accident (CAThyMARA): technical
recommendations and remaining gaps Radiat Meas 128, 106069 https://doi.org/
10.1016/j.radmeas.2019.02.008
Cardis, E., Hatch, M., 2011 The Chernobyl accident − an epidemiological perspective
Clin Oncol 23 (4), 251–260 https://doi.org/10.1016/j.clon.2011.01.510
Currie, L.A., 1968 Limits for qualitative detection and quantitative determination
Application to radiochemistry Anal Chem 40 (3), 586–593 https://doi.org/
10.1021/ac60259a007
Gilmore, G., 2008 Practical Gamma-Ray Spectrometry, second ed Wiley, New York,
ISBN 9780470861967
Hosoda, M., Iwaoka, K., Tokonami, S., Tamakuma, Y., Shiroma, Y., Fukuhara, T.,
Mayo, Y., Taniguchi, J., Akata, N., Osanai, M., Tsujiguchi, T., Yamaguchi, M.,
Kashiwakura, I., 2019 Comparative study of performance using different gamma-ray
spectrometer for thyroid monitoring under emergency situations Health Phys 116
(1), 81–87 https://doi.org/10.1097/hp.0000000000000954
Hosokawa, Y., Hosoda, M., Nakata, A., Kon, M., Urushizaka, M., Yoshida, M.A., 2013
Thyroid screening survey on children after the Fukushima Daiichi nuclear power
plane accident Rad Emerg Med 2 (1), 82–86 http://crss.hirosaki-u.ac
jp/wp-content/files_mf/1465542161vol2_ rem_13_yoichirohosokawa.pdf
International Atomic Energy Agency, 1988 The Radiological Accident in Goiˆania https
://www-pub.iaea.org/MTCD/Publications/PDF/Pub815_web.pdf
International Atomic Energy Agency, 2013 Actions to Protect the Public in an
Emergency Due to Severe Conditions at a Light Water Reactor EPR-NPP PUBLIC
PROTECTIVE ACTIONS, 2013 https://www-pub.iaea
org/MTCD/Publications/PDF/EPR-NPP_PPA_ web.pdf
International Commission on Radiological Protection, 1989 Individual monitoring for
intakes of radionuclides by workers ICRP Publication 54 Ann ICRP 19 (1− 3)
International Commission on Radiological Protection, 1995 Age-dependent doses to the
members of the public from intake of radionuclides: part 4 inhalation dose
coefficients ICRP Publication 71 Ann ICRP 25 (3− 4)
International Commission on Radiological Protection, 1997 Individual monitoring for
internal exposure of workers ICRP Publication 78 Ann ICRP 27 (3− 4)
International Commission on Radiological Protection, 1998 ICRP Database of Dose
Coefficients: Workers and Members of the Public version 3
Isaksson, M., Broggio, D., Fojtík, P., Lebacq, A.L., Navarro Amaro, J.F., O´sko, J., P´erez
L´opez, B., Vu, I., Battisti, P., B¨orjesson, J., Carlsson, M., Castellani, C.M.,
Gårdestig, M., Hill, P., Krajewska, G., Lünendonk, G., Meisenberg, O., Stenstr¨om, M.,
El Mantani Ordoulidis, S., 2019 Assessing 131 I thyroid by non-spectrometric
instruments ‒ A European intercomparison exercise Radiat Meas 128, 106115
https://doi.org/10.1016/j.radmeas.2019.04.018
Ishigure, N., Matsumoto, M., Nakano, T., Enomoto, H., 2004 Development of software
for internal dose calculation from bioassay measurements Radiat Protect Dosim
109 (3), 235–242 https://doi.org/10.1093/rpd/nch048
Japan Atomic Energy Research Institute, 1993 Health Physics in JAERI JAERI-M-93-
172 https://jopss.jaea.go.jp/pdfdata/JAERI-M-93-172.pdf
Khrutchinsky, A., Drozdovitch, V., Kutsen, S., Minenko, V., Khrouch, V., Luckyanov, N.,
Voillequ´e, P., Bouville, A., 2012 Mathematical modeling of a survey-meter used to
measure radioactivity in human thyroids: Monte Carlo caluclations of the device
response and uncertainties Appl Radiat Isot 70, 743–751 https://doi.org/
10.1016/j.apradiso.2011.12.032
Kim, E., Kurihara, O., 2020 Thyroid doses in children from radioiodine following the
accident at the Fukushima Daiichi nuclear power plant J Radiat Prot Res 45 (1),
2–10 https://doi.org/10.14407/jrpr.2020.45.1.2
Kim, E., Kurihara, O., Suzuki, T., Matsumoto, M., Fukutsu, K., Yamada, Y., Sugiura, N.,
Akashi, M., 2012 Screening survey on thyroid exposure for children after the
Fukushima Daiichi nuclear power station accident In: Proceedings of the First NIRS
Symposium on the Reconstruction of Early Internal Dose in the TEPCO Fukushima
Dai-Ichi Nuclear Power Station Accident National Institute of Radiological Sciences,
Chiba, Japan, pp 59–66 NIRS-M-252 https://repo.qst.go.jp/?action=pages_vie
w_main&active_action=repository_view_ main_item_detail&item_id=73765&ite
m_no=1&page_id=13&block_id=21
Kim, E., Kurihara, O., Tani, K., Ohmachi, Y., Fukutsu, K., Sakai, K., Akashi, M., 2016 Intake ratio of 131 I to 137 Cs derived from thyroid and whole-body doses to Fukushima residents Radiat Protect Dosim 168 (3), 408–418 https://doi.org/10.1097/ hp.0000000000001345
Kim, E., Yajima, K., Hashimoto, S., Tani, K., Igarashi, Y., Iimoto, T., Ishigure, N., Tatsuzaki, H., Akashi, M., Kurihara, O., 2020 Reassessment of internal thyroid doses
to 1,080 children examined in a screening survey after the 2011 Fukushima nuclear disaster Health Phys 118 (1), 36–52 https://doi.org/10.1097/
hp.0000000000001125 Kramer, G.H., Crowley, P., 2000 The assessment of the effect of thyroid size and shape
on the activity estimate using Monte Carlo simulation Health Phys 78 (6), 727–738
https://doi.org/10.1097/00004032-200006000-00018 Kunishima, N., Tani, K., Kurihara, O., Kim, E., Nakano, T., Kishimoto, R., Tsuchiya, H., Omatsu, T., Tatsuzaki, H., Tominaga, T., Watanabe, S., Ishigure, N., Akashi, M.,
2019 Numerical simulation based on individual voxel phantoms for a sophisticated evaluation of internal doses mainly from 131 I in highly exposed workers involved in the TEPCO Fukushima Daiichi NPP accident Health Phys 116 (5), 647–656 https:// doi.org/10.1097/hp.0000000000000995
Kurihara, O., Kanai, K., Nakagawa, T., Takada, C., Momose, T., Furuta, S., 2012 Direct measurements of employees involved in the Fukushima Daiichi nuclear power station accident for internal dose estimates: JAEA’s experiences In: Proceedings of the First NIRS Symposium on the Reconstruction of Early Internal Dose in the TEPCO Fukushima Daiichi Nuclear Power Station Accident National Institute of Radiological Sciences, Chiba, Japan NIRS-M-252 :13–25 https://repo.qst.go.jp/?act ion=pages_view_main&active_action=repository_view_main _item_detail&item_id
= 73765&item_no=1&page_id=13&block_id=21 Likhtarev, I.A., Grulko, G.M., Sobolev, B.G., Kairo, I.A., Pr¨ohl, G., Roth, P., Henrichs, K.,
1995 Evaluation of the 131I thyroid-monitoring measurements performed in Ukraine during May and June of 1986 Health Phys 69 (1), 6–15 https://doi.org/ 10.1097/00004032-199507000-00002
Meisenberg, O., Gerstmann, U.C., 2017 Thyroid monitoring of adults and children after reactor accident with a new dose rate measurement device Radiat Meas 125, 150–153 https://doi.org/10.1016/j.apradiso.2017.04.006
National Institute of Radiological Sciences, 2016 Activity Records of NIRS Employees in Their Responses to the Fukushima Daiichi Nuclear Power Plant Accident NIRS-M-
286 March 2016 (in Japanese) https://repo.qst.go.jp/?action=reposito ry_uri&item_id=73799&file_id=10& file_no=1
Nishino, S., Tanimura, Y., Yoshitomi, H., Takahashi, M., 2020 Porotype test of a portable thyroid dose monitoring using gamma-ray spectrometers Radiat Meas 134,
106292 https://doi.org/10.1016/j.radmeas.2020.106292 Ohba, T., Ishikawa, T., Nagai, H., Tokonami, S., Hasegawa, A., Suzuki, G., 2020 Reconstruction of residents’ thyroid equivalent doses from internal radionuclides after the Fukushima Daiichi nuclear power station accident Sci Rep 10, 3639
https://doi.org/10.1038/s41598-020-60453-0 Ulanovsky, A.V., Eckerman, K.F., 1998 Modification to the ORNL phantom series in simulation of the responses to thyroid detectors Radiat Protect Dosim 79 (1− 4), 429–431 https://doi.org/10.1093/oxfordjournals.rpd.a032443
Ulanovsky, A.V., Minenko, V.F., Korneev, S.V., 1997 Influence of measurement geometry on the estimate of 131 I activity in the thyroid: Monte Carlo simulation of a detector and a phantom Heath Phys 72 (1), 34–41 https://doi.org/10.1097/ 00004032-199701000-00004
Werner, C.J (Ed.), 2017 MCNP User’s Manual LA-UR-17-29981 https://mcnp.lanl.gov /pdf_files/la-ur-17-29981.pdf
Yajima, K., Kim, E., Tani, K., Kurihara, O., 2020 A new thyroid monitor using multiple high resolution Gd 3 (Al, Ga) 5 O 12 (Ce) detectors for direct thyroid measurements of small children following a nuclear accident Radiat Meas 133, 106272 https://doi org/10.1016/j.radmeas.2020.106272
Youngman, M.J., 2013 Practical Guidance on Thyroid Monitoring for Radioiodine Using Hand-Held Instruments Health Protection Agency HPA-CRCE-044 https://assets publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/f ile/337139/HPA-CRCE-044_for_website.pdf