Chapter 9
ACTIVATION MEASUREMENTS FOR FAST NEUTRONS
Part C. 63Ni Measurements by Liquid Scintillation Method
Seiichi Shibata, Koichi Takamiya, Yoshiyuki Ota, Norio Nogawa,
Yutaka Ito, Tokushi Shibata
Introduction
Measurement of 63Ni (t1/2 = 100.1 y) produced by fast neutrons induced by the reaction
Cu(n,p)63Ni enables us to evaluate the fast neutron fluence due to the Hiroshima and Nagasaki
atomic bombs even at present. Two methods are considered to be effective to determine the
amount of 63Ni produced in exposed copper samples. One is a method of counting 63Ni atoms by
accelerator mass spectrometry (AMS) (Straume and Marchetti 1994) and the other is that of
measuring beta particles emitted from 63Ni by a liquid scintillation method after extracting 63Ni
chemically (Shibata et al. 1994). We employed the latter method in this work. In a liquid
scintillation method, it is possible to measure beta particles from 63Ni extracted from samples
repeatedly, which is one of the advantages of this method.
For the intercomparison between these two measurements, a part of a copper wire sample,
which was irradiated by fast neutrons emitted from spontaneous fission of 252Cf, was distributed
to each group. The 63Ni produced in the copper sample was estimated to be 2.00 × 108 63Ni/g Cu
from the irradiation condition, and the AMS result was reported to be 2.03 (± 0.12) × 108 63Ni/g
Cu (0.0447 ± 0.0026 Bq 63Ni/g Cu) (Chapter 9, Part B). We determined the 63Ni activity in the
same copper sample to be 0.0429 ± 0.0034 Bq 63Ni/g Cu by the liquid scintillation method. These
results of 63Ni measurements agreed well with each other within the experimental uncertainty,
and therefore the reliabilities of both methods were confirmed.
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Materials and Methods
The copper samples analyzed in this work were two rain gutters collected from a building at
Hiroshima University: the rain gutter 1 (slant range: 1,501 m) was from the directly exposed part
of the building toward the epicenter and the rain gutter 2 (slant range: 1,550 m) from the shadow
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part of the building. The sampling locations for the Hiroshima University building are shown in
Figure 1, and the characteristics of the samples are tabulated in Table 1. The rain gutter samples
consisted of three parts, which were welded together. In Table 1, three parts of the sample are
named as A, B and C. The amount of nickel contained in each copper sample as an impurity was
determined by Kawasaki Steel Techno-Research Corporation.
Figure 1. Location of rain gutter samples 1 and 2 at Hiroshima
University building.
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Since 63Ni emits low-energy beta particles with a maximum energy of 67 keV, a chemical
separation is indispensable for detecting such low-energy beta particles effectively by the liquid
scintillation method. Before chemical separation, each surface of the samples was carefully
polished to remove rust. The outline of the experimental procedure is as follows: (1) a nickel
component was chemically extracted from the copper sample, (2) the chemical yield was
determined by ICP-AES, and (3) 63Ni was measured by a low-background liquid scintillation
counter. The details of the experiment are described below (Ota 2003).
Each copper sample was dissolved in mixed acid solution (HNO3 8% and H2SO4 17%).
Almost all copper in the solution was removed by a constant potential electrolysis using a
platinum wire and a copper plate as anode and cathode, respectively. After the electrolysis, 6 M
(mol/dm3) NaOH was added to the solution in order to precipitate hydroxide of nickel, cobalt and
copper. Then, the solution was filtered. The precipitate on a filter paper was washed with hot
water and dissolved in 6 M HCl. The solution was evaporated, and the residue was dissolved in a
small amount of 9 M HCl. The solution containing nickel, cobalt, copper and other impurities
was poured onto an anion exchange column (DOWEX 1X8, 100-200 mesh). 9 M, 4 M and 0.1 M
HCl were successively passed through the column to isolate nickel, cobalt and copper fractions,
respectively.
The nickel fraction obtained was evaporated and dissolved in dilute HCl. The pH of the
solution was adjusted to be ~8 by adding 25% NH4OH. Dimethylglyoxime in ethanol (1% w/w)
was added to form nickel complexes, which were extracted into chloroform and back-extracted
into 1 M HCl. After the back-extraction, organic matter contained in the aqueous phase was
decomposed by evaporating the solution with conc. HNO3, and the residue was dissolved in 6 M
HCl. The solution was poured onto a cation exchange column (DOWEX 50WX8, 100-200
mesh). 0.5 M HCl was passed through the column to remove impurities, and then nickel was
eluted with 6 M HCl. The nickel fraction thus obtained was evaporated and the residue was
dissolved in dilute HCl. This sample solution was transferred into a vial for a liquid scintillation
counting and an aliquot was taken to determine the chemical yield by ICP-AES. The yields
obtained throughout this chemical treatment are also indicated in Table 1.
After the chemical separation, the radioactivities of 63Ni were measured by a low-background
liquid scintillation counter (Packard, TRI-CARB-2770 TR/SL) at Radioisotope Center,
University of Tokyo. The nickel sample in a vial was evaporated slowly by infrared lamp. The
sample in vial was dissolved in dilute HNO3, and 25% NH4OH was added in order to prepare a
weak alkali solution, which was effective to measure 63Ni with several tens of mg of nickel
carrier (Kojima and Furukawa 1985). Then, the scintillator (clear-sol I, Nakarai Tesque, Inc.) was
added to make a cocktail for a liquid scintillation counting.
In order to examine the quenching effect in the liquid scintillation counting, standard
solutions of 9 Bq of 63Ni containing 0-100 mg nickel were prepared by the same condition as the
samples mentioned above. The rain gutters 1 and 2, blank and 63Ni standard samples were set in
the scintillation counter and measured. The measurement times for the rain gutter 1 and the blank
sample were more than 20,000 min. The measurement of the rain gutter 2 is still being continued.
Fluctuation of the counting was obtained to be less than 0.3% throughout the measurements for
the 63Ni standard samples. Also, the change of spectrum shape caused by gain shift was not found
in all the samples. The variation of detection efficiencies with concentration of NiCl2 caused by
quenching effect was measured using the 63Ni standard samples. The detection efficiency for the
rain gutter was estimated to be 24% in the energy region of 10-30 keV used for the analysis.
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Results and Discussion
The obtained beta-particle spectra for the rain gutters 1 and 2 are shown in Figures 2(a) and
2(b), respectively. The solid line, dashed line and closed circles in Figure 2 show the energy
spectra obtained from the rain gutter, the blank and the 63Ni standard samples, respectively. In the
figure, the spectra obtained by the subtraction of the blank spectra from the rain gutter ones are
also shown by the symbols of open circles. From Figure 2(a), it is apparent that the spectrum
shape obtained by the subtraction is almost identical with that of the 63Ni standard sample in the
energy region of 10-30 keV. The 63Ni produced in a copper sample exposed by the Hiroshima
atomic bomb was clearly detected by the liquid scintillation method for the first time. On the
other hand, in Figure 2(b) the spectrum shape of the rain gutter 2, which was collected from the
shielded place for the neutron irradiation, is the same as that of the blank sample.
From the spectrum obtained by the subtraction in Figure 2(a), we estimated the number of
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Ni atoms in the rain gutter 1 to be 1.26 (±0.31) × 105 63Ni/g Cu. At present, we do not have any
data measured by the liquid scintillation method for the background correction of 63Ni production
by cosmic rays, etc. Therefore, the background data of 7.3 × 104 63Ni/g Cu measured by AMS
Figure 2. Beta-particle spectra measured by liquid scintillation
method: rain gutters 1 and 2 (solid line), blank (dashed line),
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Ni standard (closed circles), and spectra obtained by the
subtraction of the blank from the rain gutter (open circles).
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was used for the correction (Chapter 9, Parts B and D), although it might well be that this
correction underestimates somewhat the background that is actually typical for the liquid
scintillation method. Future blank runs by means of liquid scintillation counting will provide the
basis for a more accurate background estimate. Additionally, our result for rain gutter 1 was
corrected for 63Ni production by thermal neutron-induced reaction 62Ni(n,γ)63Ni (Chapter 3) and
decay of 63Ni after August 1945. Finally, we obtained 7.97 (±3.58) × 104 63Ni/g Cu as the 63Ni
produced by the Hiroshima atomic bomb. The results are summarized in Table 2. The obtained
result is plotted as a function of slant distance with the AMS results in Figure 3. In this figure, it
is shown that the result obtained by the liquid scintillation method is consistent with the AMS
ones (Chapter 9, Part B), although there still remained the problem such as background
production of 63Ni to be elucidated.
Figure 3. Measured 63Ni from fast neutrons in rain gutter 1 from Hiroshima
University. The closed circle shows the result of this work with background
subtraction, correction for decay of 63Ni, and correction for the yield of
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Ni(n,γ)63Ni by thermal neutron capture. The open triangles show the AMS
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Ni measurements (Chapter 9, Part B) and the crosses show the DS02
calculated values for the AMS 63Ni measurements (Chapter 9, Parts B and E).
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References
Kojima, S.; Furukawa, M. “Liquid scintillation counting of low activity 63Ni.” J. Radioanal Nucl. Chem.,
Letters 95: 323; 1985.
Ota, Y. Study on Estimation of Fast-neutron Fluence of the Hiroshima Atomic Bomb by 63Cu(n,p)63Ni (in
Japanese). Kyoto, Japan: Kyoto University; Graduate School of Engineering; MS Thesis; 2003.
Shibata, T.; Imamura, M.; Shibata, S.; Uwamino, Y.; Ohkubo, T.; Satoh, S.; Nogawa, N.; Hasai., H.;
Shizuma, K.; Iwatani, K.; Hoshi, M.; Oka, T. “A method to estimate the fast-neutron fluence for the
Hiroshima atomic bomb.” J. Phys. Soc. Japan 63: 3546; 1994.
Straume, T.; Marchetti, A. A. A Plan for 63Cu(n,p)63Ni Measurements in Hiroshima using AMS.
Presented at the Meeting of the Japan and U. S. Dosimetry Measurement Groups, Radiation Effects
Research Foundation, Hiroshima, Japan, August 2-3, 1994.
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