RADIATION RESEARCH 174, 137–145 (2010)0033-7587/10 $15.00
g 2010 by Radiation Research Society.
All rights of reproduction in any form reserved.
DOI: 10.1667/RR2043.1

41

Ca in Tooth Enamel. Part I: A Biological Signature of Neutron Exposure in
Atomic Bomb Survivors

A. Wallner,a,b,c,1 W. Rühm,a,d G. Rugel,a,b N. Nakamura,e A. Arazi,b T. Faestermann,b K. Knie,a,b H. J. Maierf and
G. Korschinekb
Radiobiological Institute, University of Munich, 80336 Munich, Germany; b Faculty of Physics, Technical University of Munich, 85747 Garching,
Germany; c VERA Laboratory, Faculty of Physics, University of Vienna, 1090 Vienna, Austria; d GSF – National Research Center for Environmentand Health, 85764 Neuherberg, Germany; e Department of Genetics, Radiation Effects Research Foundation, 5-2 Hijiyama Park, Minami-ku,
Hiroshima City, 732-0815 Japan; and f Faculty of Physics, University of Munich, Am Coulombwall 1, 85748 Garching, Germanyaradiation (1–3). Since the beginning of the Life Span
Study (LSS) at the Atomic Bomb Casualty Commission(ABCC) and subsequently at the Radiation Effects
Research Foundation (RERF), approximately 93,000
survivors exposed to various doses of radiation as well
as 27,000 individuals who were not in Hiroshima or
Nagasaki at the time of the bombings have beenmonitored continuously to document cancer mortality,
cancer incidence and cause of death to assess the impactsof radiation on human health. To verify a correlation
between any suspected health effect associated withradiation exposure, however, individual dose assessments for each of the survivors are essential. These
efforts are currently conducted through the use ofsophisticated computer calculations based on interview
information regarding exposure conditions at the time of
the bombings. However, dose estimates include various
degrees of uncertainties because the radiation fields in
Hiroshima and Nagasaki were complicated and characterized by two components—neutron and c radiation. In
addition, various parameters such as the degree ofshielding and distance from the point of explosion makethe quantification of individual doses a rather complicated issue.
In the past, several dosimetry systems have beendeveloped to estimate individual doses as well as organ
doses [T57D (4), T65D (5), DS86 (6) and DS02 (7)].
However, these calculations and their associated uncertainties need to be verified experimentally, and verification of neutron exposure becomes increasingly difficult
over the years for several reasons, most notably due tothe spontaneous decay of neutron-activated radionuclides.
To illustrate this decay problem, one project started a
few days after the explosions and involved the collectionof samples containing sulfur that contained short-lived32
P (t1/2 5 14.3 days) produced via the fast-neutroninduced reaction 32S(n,p)32P. This was used to reconstruct the fast-neutron fluence close to the hypocenter in

Wallner, A., Rühm, W., Rugel, G., Nakamura, N., Arazi, A.,
Faestermann, T., Knie, K., Maier, H. J. and Korschinek, G.
41
Ca in Tooth Enamel. Part I: A Biological Signature of
Neutron Exposure in Atomic Bomb Survivors. Radiat. Res. 174,
137–145 (2010).
The detection of 41Ca atoms in tooth enamel using accelerator
mass spectrometry is suggested as a method capable ofreconstructing thermal neutron exposures from atomic bomb
survivors in Hiroshima and Nagasaki. In general, 41Ca atoms
are produced via thermal neutron capture by stable 40Ca. Thus
any 41Ca atoms present in the tooth enamel of the survivors
would be due to neutron exposure from both natural sources andradiation from the bomb. Tooth samples from five survivors in acontrol group with negligible neutron exposure were used toinvestigate the natural 41Ca content in tooth enamel, and 16
tooth samples from 13 survivors were used to estimate bombrelated neutron exposure. The results showed that the mean
41
Ca/Ca isotope ratio was (0.17 ± 0.05) 3 10214 in the control
samples and increased to 2 3 10214 for survivors who were
proximally exposed to the bomb. The 41Ca/Ca ratios showed aninverse correlation with distance from the hypocenter at the timeof the bombing, similar to values that have been derived fromtheoretical free-in-air thermal-neutron transport calculations.
Given that c-ray doses were determined earlier for the same
tooth samples by means of electron spin resonance (ESR, orelectron paramagnetic resonance, EPR), these results can serve
to validate neutron exposures that were calculated individuallyfor the survivors but that had to incorporate a number ofassumptions (e.g. shielding conditions for the survivors). g 2010 byRadiation Research Society

INTRODUCTION

The cohort of atomic bomb (A-bomb) survivors in
Hiroshima and Nagasaki provides a major source of
information on health effects after exposure to ionizing
1 Address for correspondence: VERA Laboratory, Faculty ofPhysics, University of Vienna, Währinger Strasse 17, A-1090 Wien,
Austria; e-mail: anton.wallner@univie.ac.at.

137

138

WALLNER ET AL.

Hiroshima (8–10). Over the following years, various
other longer-lived reaction products such as 60Co, 152Eu
and 36Cl with half-lives ranging from 5 years to
300,000 years were (and still are being) measured [seeref. (11) for a detailed list of references]. These products
are almost exclusively the result of thermal-neutron
exposure. Recently, efforts have been made to measure
fast neutrons using the 63Cu(n,p)63Ni reaction (63Ni: t1/2
5 101 years) (12–17) and the 39K(n,p)39Ar reaction [39Ar:
t1/2 5 269 years; see ref. (18)] at distances from the
hypocenter where people could survive, and these results
were in close agreement with calculated values in theinvestigated samples. In addition, new measurements ofthermal-neutron-induced 36Cl also showed a reasonableagreement with the DS02 calculations (19–24), even at
distances beyond 1,000 m from the hypocenter whereprevious studies found poor agreement with neutrontransport calculations (25, 26).
All these studies were based on inorganic sample
materials taken from locations such as buildings thatsurvived the A-bomb blast and the fires that followed thebombing. Both directly exposed and shielded samples
were used. Because of the limited number of suitablesamples and the complexity of the applied detection
methods, however, neutron fluences could not bereconstructed experimentally for the individual locations
where the majority of the survivors in the epidemiologicalstudy cohort (LSS) were exposed. In addition, most of theneutron measurements were done on radionuclides
produced by thermal neutrons, which provide only anindirect signature of the fast neutrons. Thus all efforts todate to reconstruct the neutron exposure of the survivors
remain indirect and require extrapolation from experimentally reconstructed neutron fluences to the locations
where people survived and to neutron energies and dosesthat were responsible for biological effects.
In the present work, a method is described that allows
a more direct reconstruction of the survivors’ neutron
exposure. Specifically, it is suggested that the long-lived
radionuclide 41Ca (t1/2 5 103,000 years) in the tooth
enamel of the survivors is used for dosimetry purposes.
This radionuclide was produced by thermal neutrons
originating from the A-bomb radiation. In a selected
number of tooth samples, individual 41Ca concentrations
were measured by means of accelerator mass spectrometry (AMS), and these measurements are described indetail below. The results obtained for tooth enamel
samples exposed to A-bomb neutrons are compared tosamples that were not exposed to a significant dose of Abomb neutrons. For the exposed samples, a significant
inverse correlation was found between the distance fromthe hypocenter in Hiroshima and the amount of 41Ca
measured. Such data provide a key quantity in neutron
dosimetry, because these results do not depend on theaccuracy of survivor recall, e.g. distance and shielding
conditions.

TABLE 1
Production of 41Ca in Hiroshima, Based on DS02
Calculations 1 m above the Ground and without
Intervening Shielding (5)Ground range (m)
41

800

900

1000

1100

1200

1300

55

23

12

5.3

2.5

1.2

215

Ca/Ca ratio (in 10)

MATERIALS AND METHODSNatural Production of 41Ca
41
Ca is produced almost exclusively via the capture of neutrons bystable 40Ca, and these neutrons originate from the interaction of primarycosmic radiation with atmospheric components. Typically, 41Ca/Ca
isotope ratios of about 6 3 10215 are expected on the surface of the
Earth if saturation was achieved and Earth surface erosion processes are
neglected [calculated according to ref. (27)]. If erosion occurs, a sample
that had been shielded against cosmic radiation for most of the time
now appears on the Earth’s surface and would have somewhat lower
41
Ca/Ca isotope ratios. Near the geomagnetic poles, however, theshielding of the cosmic rays by the magnetic field of the Earth is lesseffective, and higher ratios are expected. Thus the actual 41Ca/Ca ratio
found in the lithosphere depends on the geographic area (28, 29).
Through erosion processes, 41Ca is distributed into different compartments of the global ecosystem and finally enters the food chain.
Therefore, 41Ca/Ca isotope ratios will also be found in the human body.
As a result, 41Ca/Ca isotope ratios in inorganic or biological materials
are expected to vary somewhat depending on the region where thespecific sample was found. In fact, values as low as 6 3 10216 and as highas 2 3 10214 have been reported [see ref. (28) for a review].

Estimation of 41Ca Production by Neutrons from the Bomb
To calculate the neutron exposure of A-bomb survivors, the DS02
model considers the propagation of the neutrons produced by the Abomb explosion through the atmosphere to the ground. Based onthese calculations and the ENDF/B-VI production cross-section (30),
the 41Ca content in samples containing stable 40Ca can be estimated.
To provide a first estimate, Table 1 shows 41Ca/Ca isotope ratios thatwere calculated based on DS02 neutron fluences at a height of 1 m
and free-in-air (i.e. without shielding) for different distances from the
hypocenter in Hiroshima (7). Because the cross-section is largest at
low energies, thermal neutrons dominate in the production of 41Ca. In
fact, based on DS86 neutron spectra from Hiroshima, neutrons withenergies below 10 keV account for more than 99% of the 41Ca
production at a distance of 1,000 m from the hypocenter.
Calcium in the Human Body
Based on the reference man defined by the International
Commission on Radiological Protection, calcium accounts for 1.6%
for males and 1.4% for females, respectively, of the total body weight(31). In absolute terms, this corresponds to about 1,000 g of calcium,
most of which is bound to the skeleton (99%). Specifically, 13 g ofcalcium is concentrated in human teeth, and the concentration ishighest in the tooth enamel (36%). In addition, enamel shows thehighest inorganic fraction within the body. Approximately 1 g ofcalcium is incorporated daily via ingestion, and about 30% is
absorbed by the body. After dentition is finished, however, tooth
enamel represents a nearly closed system, tooth calcium metabolism isstrongly reduced, and exchanges with natural calcium are negligible[see e.g. 90Sr investigations in enamel in refs. (32, 33)].
Tooth Enamel
In the present study, 16 tooth samples from 13 survivors who were
proximally exposed to A-bomb neutrons (,1,200 m from the41

Ca IN TOOTH ENAMEL. PART I

hypocenter) were examined. From three of the survivors, twodifferent teeth were investigated to test the variability of 41Ca/Ca
ratios in different teeth from the same donor. The results werecompared to those obtained from six survivors in a control group(estimated total c-ray plus neutron doses of ,5 mGy were calculatedusing the DS02). All tooth samples were extracted for medical reasons
and were donated by the survivors.
The research plan for this study was approved by the Human
Investigation Committee and the Ethics Committee of the Radiation
Effects Research Foundation.
AMS Sample Preparation
The AMS technique used in the present study requires the use ofnegative ion sputter sources: Sample material, inserted into an ion
source, is sputtered, and negatively charged ions are extracted andanalyzed for their mass and energy. Therefore, the original sample
material must be converted into a chemical form that readily forms
negative ions.
Generally, isobars (i.e., isotopes having the same mass as theisotope of interest) represent a major contribution to backgroundsignals when mass spectrometry techniques are applied. In the case of
41
Ca, the interfering isobar is stable 41K having a mass difference from41
Ca of only 0.007%, and such a small difference is difficult to resolveby mass spectrometry. Unfortunately, potassium forms negative ionsmore effectively than calcium, and the 41K signal would be farstronger then that of 41Ca if elemental calcium with some potassium
impurities were used. Because of the very low isotope ratios expected
to be present in the exposed samples (see Table 1), an effective isobar
suppression method is essential for this type of study. Therefore, it
was decided to convert the enamel material to calcium hydride (CaH2)
as described previously (34). From this material, high currents ofnegative CaH32 can be extracted from the ion source and thecorresponding KH3 molecules do not form negative ions (35, 28).
Despite the complicated procedures required for sample preparation,
this method was adopted because of its effectiveness in suppressing
the isobar signal already in the ion source.
The conversion of enamel into AMS samples was performed as
described earlier (36). Briefly, enamel was mechanically separated
from dentin and crushed into small particle sizes for ESR
measurements (37). The samples were then dissolved in nitric acid.
Under controlled pH conditions, calcium precipitated as Ca-oxalate(CaC2O4) in the presence of ammonia and oxalic acid. CaC2O4 was
ignited at 900uC and fully converted to oxides (CaO). After reduction
to metallic calcium at a temperature of 1,400uC, the addition ofhydrogen at a temperature of 630uC led to the production of calciumhydride (CaH2). The chemical procedures of reduction and hydrogenation used have been described (34, 38, 39). This procedure resulted
in an overall efficiency of calcium recovery of about 90%. Finally,
CaH2 material was mixed with a silver powder for better thermal andelectrical conductivity, and a few milligrams of this material was thenpressed into copper sample holders. This last step was done in an
argon atmosphere because CaH2 is strongly hygroscopic.
AMS Measurements
The 41Ca/Ca isotopic ratios in 22 tooth enamel samples were
measured at the AMS facility in Munich, Germany, with a few
samples also measured at the VERA facility in Vienna, Austria. Thesesystems allow detection of 41Ca/Ca levels below 1 3 10215. In thefollowing, the measurement procedure is demonstrated by the
Munich facility: Negative 41CaH32 ions are produced in a cesium
sputter source and are pre-accelerated and pass through a low-energy
mass spectrometer that is adjusted for a mass of 44. The ions are thenfurther accelerated and injected into a tandem accelerator (typicaltandem voltage: 12.7 MV). Any molecules that might contribute tomolecular interference (with mass 44) are completely destroyed in the139

terminal stripper foil of the accelerator. The stripper is responsible forcharge exchange and produces positively charged ions. After exiting
the tandem accelerator, 41Ca10z ions are selected by a 90u analyzing
magnet for further transport to the detector. The particles have anenergy of about 140 MeV. Rejection of isotopic interference from40
Ca and 42Ca atoms was achieved with a Wien filter and a time-offlight system. Further reductions of any isobaric and isotopic
interference were achieved by means of a GAMS (gas-filled analyzing
magnet system) detection system that consists of a gas-filled magnet(GFM) and a position sensitive multi-delta-E ionization chamber. In
the GFM, the incoming ions undergo collisions with the gas
molecules and as a result reach an equilibrium charge state that
depends on the nuclear charge of the incoming ions. Due to themagnetic field, ions from different isobars follow different trajectories
and can be separated in the position-sensitive ionization chamber. In
the detector, besides the position, the total energy and five energy-loss
signals from the incoming ions are also measured. This system is
described in refs. (40) and (41), and its application for the detection of41
Ca is described in detail in ref. (36); the second AMS facility used,
VERA, is based on a 3-MV tandem accelerator and is equippedwith a dedicated ionization chamber with high energy resolution
for 41Ca detection (42, 43). This setup provides isotopic and isobaric
suppression also for medium-mass isotopes: For 41Ca detection,
mainly due to use of 41CaH32 ions, a substantially reduced background of 41K is observed. Residual particles entering the detector are
separated in energy due to their different energy loss in the detector
entrance foil and the detector gas (DE/E detector). Energy loss andresidual energy are measured via a segmented anode [for more details,
see ref. (43)].
To illustrate the entire detection process, Fig. 1 shows typical
spectra obtained from a standard with a known 41Ca/Ca ratio of 10211
and from a sample with an isotope ratio of 4 3 10215. Figure 1a
depicts all the accumulated events during a measurement run. The
area of expected 41Ca signals is indicated by the solid contour line.
The dense area below the region of interest stems from 42Ca-generated
events that are still able to find their way to the detector. Figure 1b
shows the final events generating these data when combined with theinformation from all the other signals. The 42Ca events are completely
rejected while most of the 41Ca events are still accepted. 40Ca or 42Ca
currents are measured with Faraday cups positioned along thebeamline. The ratio of the 41Ca count rate measured with the detector,
and the stable calcium particle rates deduced from the current
measurement gives the 41Ca/40Ca and 41Ca/42Ca isotope ratios.
To monitor particle transmission through the overall system,
standards with known 41Ca/Ca isotope ratios are measured at certain
intervals (36). Typically, a transmission of several percent can beachieved; i.e., out of 100 41Ca ions injected into the tandem
accelerator, depending on the measurement setup used, between 3
to 20 41Ca nuclei are finally detected. Losses are due to the transportof the ions from the entrance of the accelerator to the detector, thestripping yield to the positive charge state 10z when passing theterminal foil of the accelerator, and also software data reduction
when analyzing the detector spectra. If normalized to the ion current
after leaving the tandem accelerator, the corresponding detection
efficiency was about 40%.
The 41Ca/Ca ratio in a sample of interest is calculated from the 41Ca
events in the detector and the number of stable calcium ions injected
into the system that were measured by means of a Faraday cup. If asample was measured several times during independent runs, the final
41
Ca/Ca ratio can be obtained by calculating the weighted average ofthe individual results.
In addition, although it is rather unlikely in the experimental setups
used, another ion may mimic a true 41Ca event in the detector, and thepossibility that some contamination during the chemical preparationor present in the ion source could affect the low measured 41Ca/Ca
ratio cannot be excluded. To quantify or exclude such possible
background contributions, blank or control samples were also

140

WALLNER ET AL.

FIG. 1. Panels a and b: Typical spectra of a standard with a ratio of 41Ca/Ca <10211 in terms of one energy loss signal from the detector(ordinate) relative to the position in the detector (abscissa); for details see text. Panels c and d: Typical spectra of a background sample with aratio of 41Ca/Ca ,4 3 10215 in terms of one energy loss signal, DE4, from the detector (ordinate) relative to the position in the detector (abscissa).
measured. Pure blank samples (commercially available CaH2 with an
unknown but low natural 41Ca/Ca ratio) as well as so-called chemicalblanks were used. The latter had to undergo the same chemical
preparation steps as the experimental samples and were also measured
repeatedly to obtain information on possible machine or chemistry
background events. Importantly, the tooth samples from thesurvivors in the control group were used to evaluate the natural
background ratio. Table 2 summarizes parameters that are typical forthe preparation of the enamel samples and the 41Ca AMS
measurements.
Estimation of Uncertainty
Uncertainties (in this paper, these are expressed at the 1s level) are
given in a summary table for the individual samples (Table 3) andinclude all sources. Because of the limited number of counts detected(between 17 and 64 counts of 41Ca per sample), the uncertainty due toPoisson counting statistics was between 13% and 24%. Measurementof the transmission through the system (which was performed byusing standard samples) represented an additional source of
uncertainty. While the uncertainty from Poisson statistics for thesemeasurements was usually negligible, uncertainties from fluctuations

TABLE 2
Typical Parameters Used for the 41Ca AMS
Measurements of the Tooth Samples
Typical parameters in 41Ca-AMS
Total mass of enamel sample
Total mass of calcium in enamel
Sample mass per cathode
Sample material
Ion currents (CaH32)
Terminal voltage
Charge state after tandem
Transmission
Particle energy

60–200 mg
22–70 mga few mg
CaH2
0.3–5 mA
MunichVienna12.5–13 MV
10z
3–5%
140 MeV

3 MV
4z
25%
15 MeV

Note. Transmission is defined as the fraction of particles, which is
identified by the detector from the particles injected into theaccelerator.

41

TABLE 3
Sources of Uncertainties Considered toContribute to the Total Uncertainty of the Final
Isotope Ratios
Uncertainty contribution

Type of error

Poisson statistics
Particle current
Reproducibility of standards
Absolute value of standards
Uncertainty of background ratio

R
R
R
S
S

Total uncertainty for exposed samples

141

Ca IN TOOTH ENAMEL. PART I

13–24%
6%
5%
10%
5–40%
20–49%

Note. R indicates random types of uncertainty contributions, and S
indicates systematic types of uncertainty contribution.
in the particle current due to instabilities in the machine as well as
long-term drifts in the overall transmission current contributed moresignificantly. This uncertainty is considered random and was typically
4–6%. Besides random errors, systematic uncertainties were also
considered. For example, for the determination of the absoluteisotope ratio of the unknown samples, the uncertainty of the 41Ca/Ca
ratio of the standard sample material has to be taken into account,and this was estimated to be 10%. In addition, the uncertainty of the
background ratio (0.5 3 10215) contributes a systematic uncertainty to
the net absolute value of the unknown ratios.
Table 3 summarizes these contributions. Because they are considered uncorrelated, the total uncertainty given in Table 5 for the
exposed samples was calculated by applying the Gaussian law of error
propagation. Although considerable efforts were made to quantify
the background, the uncertainty contribution still dominates theuncertainty of the lower measured 41Ca/Ca ratios. For samples withhigher measured 41Ca/Ca ratios, both systematic and random
contributions are similar.

TABLE 4
Ca/Ca Isotope Ratios (in units of 10215)
Obtained from Samples from Survivors in theControl Group
41

41

Sample number(ID)

Counts TM (h)1 (R408)
2 (R196)
3 (R556)
4 (R621)5 (R556-2)
6 (R577)
All controls

3
10
1
3
0
3
20

11
17
10
10
6
4
59

Ca/Ca ratio(in 10215)

z1s

21s

1.5
2.6
1.0
4.7
0.0
2.1
1.7

1.2
1.0
1.7
3.6
1.4
1.6
0.5

1.0
0.8
0.6
3.0
0.0
1.4
0.5

Notes. Listed are the total 41Ca counts detected, total measuring
time TM, and the measured isotope ratios. Given errors include only
uncertainties due to Poisson statistics (44).

RESULTS AND DISCUSSION

Control Samples
Five tooth samples from six survivors in the control
group in Hiroshima were measured repeatedly. In allcases, the number of 41Ca events detected was low, andthe total number of counts per sample was between 0
and 10 counts (Table 4).
Figure 2 shows the individual ratios obtained fromthe AMS measurements. Due to the low number ofcounts obtained from these samples, the error bars are
dominated by the uncertainty due to Poisson counting
statistics. Also shown are the weighted mean back-

TABLE 5
Summary of the Exposed Tooth Samples
41

Ca/Ca ratio (in 10215)

Sample IDa

Tooth positionb

Gamma-raydose in toothc

Groundrange (m)f

Counts 41Ca

TM (h)

Measured value

Backgroundcorrected data

F04
H43
E90
G91
G65
A72
A34
M93
K67
D20
I29
B51
B53
L80
C47
J66

7
6
1
7(RU)d6(LU)d2(LU)d3(RU)d86784(RU)d4(LL)d7/8
8
7

3.1 Gy2.5 Gy2.3 Gy3.1 Gy2.7 Gy3.6 Gy3.9 Gy1.2 Gy1.2 Gy2.7 Gy1.8 Gy2.4 Gye
2.4 Gye
1.7 Gy2.8 Gy1.2 Gy

827
884
894
952
952
956
956
1000
1018
1057
1108
1111
1111
1116
1124
1163

64
40
26
53
55
44
37
44
29
17
24
24
17
53
17
23

12
14
9
8
11
11
7
16
12
7
21
9
11
10
10
14

21 ± 315 ± 119 ± 413 ± 211 ± 220 ± 320 ± 39±2
7±1
10 ± 3
6±2
9±2
9±2
9±2
8±2
5±2

19.5 ± 4.0
13.5 ± 2.0
16.9 ± 4.1
11.3 ± 2.3
9.1 ± 2.0
18.2 ± 4.0
18.5 ± 4.0
7.2 ± 2.5
5.0 ± 1.5
8.6 ± 2.7
3.9 ± 2.0
7.4 ± 2.1
7.5 ± 2.4
7.6 ± 2.0
6.0 ± 2.1
2.8 ± 3.0

Notes. Uncertainties in background corrected data include all uncertainty contributions (see text). The isotope ratios are given in units of10214.
a
The survivor IDs are the same as described in the previous report in which the ESR measurements of c-ray doses were described (37).
b
1 and 2 5 incisor, 3 5 canine, 4 5 bicuspid, 6–8 5 molar.
c
The doses were estimated by ESR from lingual part of the samples, otherwise noted.
d
LU 5 left upper jaw, RU 5 right upper jaw, LL 5 left lower jaw.
e
Molar dose from the same donor.
fGround ranges were individually deduced by means of a customized approach described in ref. (51).

142

WALLNER ET AL.

exposed to the bomb and stayed within 1,200 m(between 827 m and 1163 m) of the hypocenter. These
results showed significantly higher ratios than thoseobtained from the control cases (Table 4). Table 5 also
includes information on tooth position, distance fromthe hypocenter, number of 41Ca events identified in thedetector, and total time of measurement (in hours). The
41
Ca/Ca ratio given for individual samples was calculated from results obtained during several runs using Eq.
((2)). Finally, for each tooth sample, the net result was
obtained by subtracting the background value of (1.7 ±0.5) 3 10215 (Table 4, last line).
Advantage of AMS for Studies on Neutron Exposure
from A-Bomb
FIG. 2. AMS results for the six samples that were not significantly
exposed to A-bomb neutrons (symbols). The error bars depict thestatistical errors only. The solid horizontal line gives the weighted
mean ratio, the dashed lines the 1s-standard deviation of the meanvalue. Samples 3 and 5 were from the same donor (see Table 4).

ground ratio (solid horizontal line) and the 1s-standard
deviation of the mean (dashed lines).
Overall, for background control values, 20 counts wererecorded in 59 h of measuring time. The weighted mean
41
Ca/Ca ratio of the six individual results is (1.72 ± 0.46) 310215. The error is the standard deviation of the weighted
mean. If it is assumed that all samples showed an identicalisotope ratio, i.e., if the scattering of the data reflects only
statistical measurement uncertainties, then the resultssummarized in Table 4 can be interpreted as being
obtained from one single measurement with 20 counts.
In this case a mean background value of (2.0 z 0.5/20.4)
3 10215 is obtained, which is consistent with the result of(1.72 ± 0.46) 3 10215 given above. The given individual
uncertainty includes only statistical uncertainty and was
calculated by applying the Poisson uncertainty determination (44). The comparison of external (standard
deviation of the mean) and internal uncertainties (Poisson
statistics) shows that at this level, no significant additional
uncertainty contributions to the obtained backgroundvalue were observed.
To summarize, the 41Ca/Ca isotope ratios obtained inthese samples were consistently low and <1.7 3 10215(Table 4). Because some background contributions fromchemistry and measurement techniques cannot be
excluded, this ratio represents an upper limit for the
41
Ca/Ca ratio occurring naturally in the Hiroshima area.
In the following sections, a background value of (1.7 ±0.5) 3 10215 is used for the correction of the samples
exposed to A-bomb neutrons.
Exposed Samples
Table 5 summarizes the 41Ca/Ca isotope ratios obtained from tooth samples from the survivors who wereAs mentioned earlier, 41Ca/Ca isotope ratios expectedin samples exposed to A-bomb neutrons or to secondary
neutrons from cosmic radiation are of the order of 10215
to 10214. Because about 50 mg of calcium was available
on average for each tooth sample, a typical sample
contains about 106 to 107 41Ca atoms. Given a half-life of
103,000 years (45, 46), this corresponds to radioactivities
of about 0.1 to a few mBq, which is far below thedetection limit of decay-counting techniques. Therefore,
only atom-counting methods can be applied for this typeof study, and it is for this reason that the AMS technique
was used in the present study for the detection of 41Ca.
The application of the Munich MP tandem accelerator to detect 41Ca produced by A-bomb neutrons has
already been demonstrated in an earlier study [a generaldescription of 41Ca-AMS is given elsewhere (28, 47, 48)].
In this study, a 41Ca profile was measured in a granite
gravestone exposed to neutrons from the bomb at close
distances from the hypocenter in Hiroshima (49, 50).
More recently, an improved AMS setup was installedfeaturing a GFM (40). This setup is characterized by amuch lower 41Ca detection limit and therefore allows
41
Ca measurements to be made in survivors who were
exposed at longer distances from the hypocenter in
Hiroshima where they could survive the explosion. In
addition, recent improvements allow the detection of41
Ca in environmental samples with 41Ca/Ca levels evenbelow 10215 (36, 43).
Specific Characteristics of AMS Analysis of Tooth
EnamelThe overall detection efficiency at the AMS laboratories, which includes the machine transmission, theefficiency for producing negative ions in the ion source[<0.2% (28)], and the chemistry yield, is of the order of afew 31024. If a typical enamel sample is considered witha mass of 100 mg, i.e. 36 mg calcium, and a 41Ca/Ca
isotope ratio of 5 3 10215, about 3 3 106 41Ca atoms
would be present. The value of 1–3 3 1024 for the overall
efficiency would then correspond to a total number of41

Ca IN TOOTH ENAMEL. PART I

FIG. 3. Results of 41Ca/Ca AMS measurements obtained fromsurvivors who donated two teeth. The numbers in the boxes represent
the sample-ID numbers in Table 5.

about 300 to 1000 41Ca atom counts, which in principle
can be detected with AMS if all of the sample material iscompletely sputtered (this would correspond to ameasuring period of about 25 h).
It should be noted that, due to the observed
background, the applicability of the proposed method
is limited to a ground range of less than about 1,200 m
from the bomb hypocenter. For those who survived at
greater distances, the 41Ca/Ca ratio induced by A-bomb
neutrons is expected to be smaller than the backgroundratio measured (Table 1).
Intertooth Variation in the Same Survivors
To assess the reproducibility of the AMS measurements, intertooth variation of the induced 41Ca/Ca ratio
was measured by examining two teeth from the same
survivor (performed for a total of three survivors andindicated by letter descriptors in the first column ofTable 5). The results of the two measured ratios for eachsurvivor were in close agreement (Fig. 3), which wasbetter than expected from the uncertainty estimated forindividual measurements. Although the number ofsamples tested was limited, the results indicate that the
AMS method produces consistent results and that the41
Ca signals induced by thermal neutrons from the bomb
do not differ extensively among the different teeth in themouth (cf. two molars from donor G; one was on theright side and the other on the left side of the donor’s
mouth).
41

Ca Yield in Relation to Distance from the Hypocenter

The present report confirms earlier studies that foundevidence for a significantly higher 41Ca signal in enamel
from one exposed survivor (36). To further demonstratethat the observed 41Ca signal is primarily attributable to143

FIG. 4. Decrease of 41Ca/Ca ratios with distance from the
hypocenter. For comparison, DS02 calculations (dashed line) are
shown, which were calculated without taking individual shielding
conditions into account (free-in-air kerma dose, 1 m above ground; see
Table 1). The error bars include all sources of uncertainties. A detailed
comparison between measured and calculated 41Ca/Ca ratios including
the individual shielding situation of the survivors is given in ref. (51).

neutron exposure from the bomb, the backgroundcorrected 41Ca/Ca isotope ratios (Table 5) were plotted
as a function of distance from the hypocenter (Fig. 4). A
clear trend of decreasing signals with increasing distanceis observed. The measured data indicate a slightly lower
decrease in 41Ca production with distance to the
hypocenter compared to DS02. Because the latter does
not directly calculate the individual shielding conditions,a more quantitative discussion of these results is given in
an accompanying publication with detailed calculationsof the expected 41Ca/Ca ratios, also including modelingof the individual’s shielding conditions (51).
Nonetheless, to provide a view of the quantitative
results obtained in this study, the calculated 41Ca/Ca
ratios using DS02 (at a height of 1 m, free-in-air
conditions; Table 1) and the results obtained here were
included in Fig. 4 for comparison (dashed line). As canbe seen, the calculated ratios decrease with increasing
distance from the hypocenter, from 2.2 3 10214 at 900 m
to about 0.3 3 10214 at 1,163 m, while the measured data
give values of 1.5 3 10214 at 900 m and about 0.5 3 10214
at 1,163 m, respectively. Most of the observation points
are below the curve for the calculated values except forthree points at around 1,115 m that are slightly higher
than the calculated values. The lower observed values
within 1,020 m are most likely due to shielding effects ofbuildings or any other intervening materials, whereas theslightly higher ratios at the remote distance might be dueto the limiting statistics associated with the 41Ca
measurements and/or presence of potential build-up
materials (low-Z material such as water) that moderate
fast neutrons effectively. These issues are discussed indetail in ref. (51).

144

WALLNER ET AL.

Conclusion
In the present study, a direct signal of exposure to Abomb neutrons was detected for the first time by meansof AMS of tooth samples from A-bomb survivors. Suchsignals are independent of survivor recall and provide a
direct biological fingerprint of neutron exposure. In
combination with the neutron energy spectrum duringexposure, the measured 41Ca/Ca ratio provides a direct
measure of neutron dose. Due to the expected low
signals, a dedicated chemical sample preparation method was required to produce calcium hydride from thetooth enamel. Both AMS facilities used, the Munich
tandem accelerator, which provides high particle energies in combination with a sophisticated particledetection setup (GAMS), and the VERA facility
permitted the measurement of 41Ca down to naturally
occurring background levels. In comparison to thenonexposed survivors, those exposed to A-bomb radiation within 1,200 m of the hypocenter showed clearlyhigher 41Ca/Ca isotope ratios. In addition, decreasing
41
Ca/Ca ratios were found in samples with exposuresfrom increasing distances from the hypocenter, which isconsistent with DS02 neutron transport calculations.

ACKNOWLEDGMENTS
The authors are deeply indebted to the A-bomb survivors who
donated their extracted teeth. Without their collaboration, the present
study would not have been possible. The authors greatly appreciate
Dr. H. M. Cullings for his efforts in collecting information concerning
the exposure conditions of the tooth donors and Dr. Leon Kapp forhis careful reading of the manuscript. This publication is partly basedon research performed at the Radiation Effects Research Foundation(RERF), Hiroshima and Nagasaki, Japan. RERF is a private
foundation funded by the Japanese Ministry of Health, Labor andWelfare, and the U.S. Department of Energy through the U.S.
National Academy of Sciences. This publication was supported by
RERF Research Protocol B-42.
Received: October 21, 2009; accepted: March 24, 2010; published online:
May 25, 2010

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