ATOMIC BOMB INDUCED152Eu: RECONCILIATION OF
DISCREPANCY BETWEEN MEASUREMENTS AND
K. Komura,* M. Hoshi,†S. Endo,†T. Imanaka,‡S. D. Egbert,§W. Ru ¨hm,**
H. Fukushima,††and S. Fujita‡‡
Abstract—In order to resolve the discrepancy between the
measured and calculated152Eu activity induced by the atomic
bomb at Hiroshima, extremely low background gamma-ray
spectrometry was performed for 17 granite samples collected
from 134 m to more than 3 km from the hypocenter. Measure-
ments agreed well with theoretical calculations based on DS02
up to 1.4 km from hypocenter.
Health Phys. 92(4):366–370; 2007
Key words: atomic bomb;
152Eu; neutrons; spectrometry,
IN 1976, residual
recognized by in situ gamma-ray spectrometry per-
formed inside of the “Genbaku Dome” (“A-bomb
Dome”) in Hiroshima (Sakanoue and Komura 1977).
152Eu was a very important nuclide for evaluating the
thermal neutron fluences from the atomic bombs in
Hiroshima and Nagasaki. A great number of
measurements have since been made from samples ex-
posed to either the Hiroshima or Nagasaki atomic bomb
152Eu (half-life ? 13.3 y) was first
(Hasai et al. 1987; Hoshi et al. 1989; Nakanishi et al.
1983, 1987, 1991; Okajima and Miyajima 1987; Saka-
noue et al. 1987; Shizuma et al. 1992, 1993, 2003). The
“Dosimetry System 1986” (DS86) was formulated for
the evaluation of individual radiation dose to the survi-
vors based on a computational model that was verified by
the use of experimental data from Hiroshima and Na-
gasaki (Roesch 1987). When DS86 was being developed,
152Eu data were not sufficient to support or contradict the
DS86 calculated values because152Eu data beyond 1 km
from the hypocenter were not available at that time.
Therefore, some problems remained unresolved when the
DS86 Final Report was published. One of the important
problems was the discrepancy between thermal neutron
measurements and calculations, i.e., measured
agreed rather well with the calculations between 0.5 and
1 km; however, closer than 0.5 km the measured values
were lower than the calculations and beyond 1 km
measured values became higher than the calculations: at
around 1.5 km the discrepancy amounted to 10–15 times
higher (Loewe et al. 1987).
In 2000, a U.S.–Japan Working Group was orga-
nized to reassess DS86 (Young and Kerr 2005). One of
the main objects was to find a solution to explain the
discrepancy between measured and calculated activities
both for60Co and152Eu. The U.S. group took the lead role
in the theoretical (calculations) approach, and the Japa-
nese group took the lead in the experimental (measure-
ments) approach. Besides critical examinations of past
152Eu data, Accelerator Mass Spectrometry (AMS) of
36Cl (half-life ? 3.0 ? 105y) of atomic bomb-exposed
samples had already been performed in the U.S.
(Straume et al. 1992) and Germany (Kato et al. 1990;
Ru ¨hm et al. 1992). The63Ni induced in copper samples
by the fast neutron reaction
discussed in workshops (Shibata et al. 1994; Straume et
al. 1994; McAninch et al. 1997; Ru ¨hm et al. 2000).
During the last years, joint efforts to detect36Cl by means
of AMS included groups from the University of Tsukuba
63Cu(n,p)63Ni was also
* Low Level Radioactivity Laboratory, Institute of Nature and
Tatsunokuchi-machi, Ishikawa 923-1224, Japan;†Research Institute
for Radiation Biology and Medicine, Hiroshima University, Kasumi
1-2-3, Minami-ku, Hiroshima 734-8553, Japan;
Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan;§Sci-
ence Applications International Corporation, 10260 Campus Point
Drive, San Diego, CA 92121; ** Technical University of Munich,
Physics Department, Garching, Germany; Ludwig Maximilians Uni-
versity, Radiobiological Institute, Munich, now GSF Federal Center
for Environment and Health, Institute for Radiation Protection, 85764
Neuherberg, German;††Japan Chemical Analysis Center, Sannocho,
Inage-ku, Chiba 263-0002, Japan;
Foundation, Hijiyama Koen 5-2, Minami-ku, Hiroshima 732-0815,
For correspondence contact: K. Komura, Low Level Radioactiv-
ity Laboratory, Institute of Nature and Environmental Technology,
Kanazawa University, Wake, Tatsunokuchi-machi, Ishikawa 923-
1224, Japan, or email at email@example.com.
(Manuscript accepted 19 October 2006)
Copyright © 2007 Health Physics Society
‡‡Radiation Effects Research
(Nagashima et al. 2005), the Los Alamos National
Laboratory (LLNL; Straume et al. 2005), and the Maier-
Leibnitz-Laboratory (MLL; Huber et al. 2003, 2005). An
intercomparison between152Eu and36Cl results obtained
from the same samples was one of the most important
studies to confirm the reliability of the experimental
aspect of the Dosimetry System 2002 (DS02).
Reassessment of152Eu began by using the measure-
ments of152Eu activity in 17 Hiroshima and 7 Nagasaki
samples, which had already been measured and reported
by Shizuma et al. (1993). For this study, ultra low-
background Ge detectors equipped in the Ogoya Under-
ground Laboratory (Komura 1998; Hamajima and
Komura 2004) were used to detect
ground level and detection efficiency of the Ge detectors
are about 40 and 3 times, respectively, better than
previous measurements by Shizuma et al. (2003). The
344.3-keV gamma ray from152Eu could not be detected
except for the samples collected within 1 km and 0.6 km
from the hypocenter of the Hiroshima and Nagasaki
atomic bombs, respectively. This was due mainly to
insufficient sample mass taken for the original
measurements and partly to severe interference by the
342.7-keV gamma ray from naturally-occurring
that remained in the counting source. In order to detect
152Eu beyond 1 km from the hypocenter, we proposed
samples compared with previous measurements. This
idea was adopted in the U.S.–Japan workshop held in
November 2001 as an intercomparison study between
152Eu and36Cl measurements. AMS measurements of36Cl
using the same samples were adopted to compare the
results with those of152Eu. Details of this intercompari-
son study are given in DS02 (Hoshi et al.§§).
Among the samples collected in Hiroshima, 13
granite rocks, some of which had already been measured
by Shizuma et al. (1993), were chosen for152Eu and36Cl
intercomparison measurements. Four granite rocks col-
lected far from the hypocenter (? 3 km) were used as
reference samples to evaluate background152Eu and36Cl
activities induced by environmental neutrons. Sampling
locations are shown in Fig. 1. The surface (5 cm thick) of
each granite rock was cut out and taken for an intercom-
was used for the152Eu measurement and the residual for
36Cl measurements at the three AMS facilities mentioned
152Eu. The back-
152Eu measurements using at least 10 times larger
36Cl. Most of the sample material
above. Detailed descriptions of the samples are given in
the report of DS02 (Young and Kerr 2005).
In order to measure extremely low-level
chemical enrichment and purification of europium were
performed at the Japan Chemical Analysis Center
(JCAC). The trace amount of europium in each granite
sample was enriched and purified. With this chemical
treatment, europium was enriched to about 40 times the
value in raw sample, while contributions of uranium and
thorium series nuclides and40K were generally reduced
by about 1/2000, 1/100, and 1/10000, respectively. How-
interferes with the detection of trace amounts of
could be reduced only to 1/50 in the best case and only
1/5 in the worst case because chemical properties of
europium and actinium are quite similar. Gamma ray
counting sources with 19 mm diameter were prepared by
using a hydraulic press, and then
measured for 1–3 wk per sample by large-volume well-
type Ge detectors in the Ogoya Underground Laboratory.
Examples of gamma-ray spectra are shown in Fig. 2.
As described above, some interference from the
342.7-keV gamma ray of227Ac still remained; therefore,
the peak area of the 344.3 keV gamma ray from
was evaluated by a least squares fitting method to resolve
the 342.7 ? 344.3 keV doublet. Then decay-corrected
152Eu activity, on 6 August 1945, was calculated after the
subtraction of natural152Eu activity induced by environ-
mental neutrons. Practically no correction was required
because contribution of natural
mated to be only 7.9 mBq g?1Eu (Endo et al. 2005).
227Ac, which emits a 342.7 keV gamma ray and
152Eu activity was
152Eu activity was esti-
§§Hoshi M, Endo S, Ishikawa M, Straume T, Komura K, Ru ¨hm
W, Nolte E, Huber T, Nagashima Y, Fukushima H, Imanaka T.152Eu
and36Cl inter comparison study for Hiroshima and Nagasaki Atomic
Bomb Dosimetry System 2002. Submitted to Health Physics.
Fig. 1. Sampling points of granite.
367 Atomic bomb induced152Eu●K. KOMURA ET AL.
The152Eu activities (Bq per mg Eu) obtained by this
study are summarized in Table 1. Errors shown in the
table represent 1-? (68% confidence level) uncertainty,
including counting statistics, systematic errors in the
calibration of the Ge detectors, and chemical yield. The
152Eu activities obtained by the present study are plotted
in Fig. 3 together with previous measurements and
sample-specific calculated values based on DS02 neutron
fluences. Our152Eu activities are in good agreement with
those of DS02 calculations in the full range from 134 m
to 1,424 m, although the measured value is a little lower
at 134 m and slightly higher at 1,424 m.
The discrepancy at distances less than 0.5 km was
resolved by the new calculations of DS02, in which the
height of the Hiroshima atomic bomb was revised up-
ward to 600 m, which is 20 m higher than for DS86. The
large discrepancy of the
previous measurements and DS86 and DS02 calculations
beyond 1 km may be explained by problems in the
evaluation of peak areas in the spectral region of the
344.3 keV gamma ray. This gamma ray suffers from
strong interference by the 342.7 keV gamma ray from
227Ac, which naturally exists in the sample and was not
removed well due to the similarity of chemical properties
between actinium and europium. Furthermore, poorer
counting statistics of the 344.3 keV peak in earlier work,
in addition to the higher background of the Ge detector
and coarse energy setting (gain ? 0.5 keV/channel
compared to 0.25 keV/channel in the present work),
made it difficult to evaluate the extremely low-level
152Eu, particularly in the samples collected at a long
distance (? 1 km) from the hypocenter.
Thus, the discrepancy between measurements and
calculation of152Eu activity was reconciled successfully
by the present study. Results of
validate the calculated thermal fluences in the new dose
system DS02 and provide much greater confidence in the
DS02 neutron doses.
152Eu activity between the
Acknowledgments—Authors are indebted to the members of the U.S.–
Japan Working Group, particularly K. Shizuma, who kindly provided us
the opportunity to measure the152Eu and60Co samples measured previously
by him, B. Bennett of the Radiation Effects Research Foundation (RERF),
and R. Young and H. Hasai, who coordinated the U.S. and Japanese Group
of the Reassessment of A-Bomb Dosimetry and provided fruitful discus-
sions and encouragement during the152Eu measurement. Thanks are due to
S. Taira of RERF, who financially supported the chemical enrichment of
europium in large granite rock samples. The present work was supported
Fig. 2. Gamma-ray spectra of the granite samples collected at
Motoyasu Bridge (134 m), Kozenji Temple (1,177 m), Kikkawa
Ryokan (1,424 m), and Senzoji Temple (8791 m).
Table 1.152Eu activity in granite samples in Hiroshima.
152Eu on 6 August
1945 (Bq mg?1Eu)
Motoyasu Brg. Railing
Shirakami Shrine Fence
Old Prefectural office
City Office pavement
18A Hiroshima Univ. “E” Bldg.-1
18B Hiroshima Univ. “E” Bldg.-2
17 Kikkawa Ryokan
14 Senzoubo local stone
15 Myokenji Temple
99.4 ? 5.5
15.2 ? 0.9
6.2 ? 0.4
1.57 ? 0.10
0.99 ? 0.07
1.06 ? 0.09
0.78 ? 0.10
0.27 ? 0.09
0.15 ? 0.03
0.04 ? 0.02
Fig. 3. Comparison of present and previous measurements and
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