Journal of Nuclear and Radiochemical Sciences, Vol. 16, pp. 15-21, 2016

ArticlesRapid isolation method for radioactive strontium using EmporeTM Strontium Rad Disk
Zijian Zhanga,*, Kazuhiko Ninomiyaa, Naruto Takahashia, Takashi Saitob, Kazuyuki Kitac,
Yoshiaki Yamaguchid, Atsushi ShinoharaaaGraduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Shokei Gakuin University, 4-10-1 Yurigaoka, Natori, Miyagi, 981-1295, Japanc
College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, 310-8512, JapandRadioisotope Research Center, Osaka University, 2-4 Yamadaoka, Suita, Osaka, 565-0871, Japanb

Received October 3, 2016; Accepted November 3, 2016, Published online
A rapid and convenient chemical separation method for determining 90Sr activity in aerosol samples using
EmporeTM Strontium Rad Disks was established. The method employed solid-phase extraction and ion-exchange
chromatography, and achieved a Sr yield of > 80%. Furthermore, it completely eliminated 210Pb, which interfereswith 90Sr radiation counting. Sample pretreatment, chemical separation and Cherenkov light measurement
required approximately 4 h, 2.5 h, and 14 days, respectively, to achieve a detection limit of 0.004 Bq. Using thedeveloped method, the 90Sr concentration and 90Sr/137Cs activity ratio in aerosol samples collected after the
Fukushima nuclear accident were determined.

1. Introduction
A serious accident occurred at the Fukushima Daiichi
nuclear power plants (FDNPP) in March 2011, resulting in the
discharge of large amounts of radionuclides into the environment. Of these nuclides, artificial 90Sr has a particularly long
half-life (T1/2 = 28.6 years) and high radiotoxicity. Therefore,
its measurement is a priority in environmental assessment andresponse.
As a fission product from nuclear weapons testing, 90Sr had
already been widely distributed over the Northern hemisphere
before the FDNPP accident. It has been measured in atmospheric fallouts worldwide [1-4], and it was also detected inmilk following the Chernobyl accident [5]. Several studies
have indicated that the mobility of 90Sr in the environment ishigher than that of 137Cs [6-7]. Thus, 90Sr is one of the mostimportant radionuclides in nuclear plant accidents in terms ofimpact on human health.
Following the FDNPP accident, many environmental samples from eastern Japan have been examined to investigate thecircumstances of the accident and to assess the resultant contamination distribution. In a previous study, we used gammaray spectroscopy to determine the radioactivities of 131I, 134Cs,
and 137Cs in such samples. Aerosol samples were collected byhigh-volume air samplers and the time variation of radiocesium activity concentrations was observed [8]. In contrast,
only a few studies have investigated 90Sr in soils and seawater[9-10]. Consequently, data on the 90Sr activity concentration in
the atmosphere and its time variation are needed to fully evaluate the internal radiation exposure and evaluate the amountof radionuclides discharged during the accident.
Because 90Sr and its daughter nuclide 90Y are pure β emitters, the activity determination of 90Sr in environmental samples requires chemical separation. Classical separation
methods including multi-stage operations are time consuming
and produce large amounts of chemical waste[11]. In recent
years, new chemical separation techniques for strontium using
solid-phase extraction have been developed [12-13]. However,*Corresponding author. E-mail: chous11@chem.sci.osaka-u.ac.jp,
TEL: +81-668505417

these new methods have only been applied to soil, milk, andsurface and ground water samples. A simple and rapid solidphase extraction method for 90 Sr in aerosol samples would
offer the advantage of exchange capacity, as no matrix interference (e.g, from calcium or strontium) would be present.
However, such an approach has not been reported to date.
In this study, we developed a new chemical separation strategy for measuring 90Sr in aerosol samples based on solid-phaseextraction. The method consists of three steps: a selective Sr
extraction using a Strontium Rad DiskTM (3M Inc.), an ionexchange separation to eliminate naturally occurring 210Pb,
and finally Cherenkov light measurements for the determination of 90 Sr by detecting 90Y growth. The Strontium Rad
DiskTM can extract Sr selectively but it also extracts Ba, Ra,
and Pb simultaneously [14]. We used cation-exchange chromatography with ethylenediaminetetraacetic acid (EDTA) elution after the extraction to eliminate the naturally occurring
radionuclide 210 Pb, which is found in aerosol samples.
However, Ba and Ra could not be eliminated. For low-background measurements, 90Sr radioactivity was determined bycounting the Cherenkov light emitted from 90Y with a liquid
scintillation counter [15]. Performance indicators of this technique, including Sr yields, Pb elimination rates, total operation
time, and detection efficiency of Cherenkov light measurement, were determined using standard radioactive and/or stable isotope samples. Finally, the developed method wasapplied to aerosol samples collected in Mito City, Japan in
April 2011. Thereby, we determined the 90Sr activity concentration and 90Sr/137Cs activity ratio in the atmosphere following
the FDNPP accident.
2. Materials and Methods
2.1. Reagents. EmporeTM Strontium Rad Disks (47 mm indiameter) were employed to extract Sr from aerosol samples.
These disks contain a proprietary multi-cyclic macromolecule
covalently bound to a solid silica support embedded in a stable, inert matrix of polytetrafluoroethylene fibrils. When 2–4
M HNO3 or HCl solution is passed through the disk, up to 3
mg of Sr in the solution could be selectively extracted [14].
Dowex 50W-X8 cation-exchange resin (100-200 mesh, Wako

© 2016 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/17/2016

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J. Nucl. Radiochem. Sci., Vol. 16, 2015

Zhang

Inc.) was used for cation-exchange chromatography to eliminate 210Pb.
Standard solutions of 89Sr (serial number: ST010, radioactivity: 100 kBq) and 90Sr (serial number: SR010, radioactivity:
100 k Bq) were obtained from the Japan Radioisotope
Association. Lead-210 solution was separated from the 226Ra
standard solution (serial number; RAB65, radioactivity: 185
kBq) obtained from the Czech Metrology Institute. An Ultima
GoldTM uLLT scintillation cocktail (Perkin Elmer Inc.) wasused for beta-ray measurement of these nuclidesin test experimental steps for developing new chemical separation method.
Strontium-85 was generated by proton irradiation of a Rb target using the Azimuthally Varying Field (AVF) cyclotron of at
the Research Center for Nuclear Physics at Osaka University.
The 85Sr radioactivity was 1.4 MBq in October, 2011. A standard stable Sr solutions of Sr(NO3)2 in 0.5 M HNO3 (1000 mg
L-1) was used as a Sr carrier and for external calibration of theinductively coupled plasma-mass spectrometry (ICP-MS)
measurements.
Concentrated solutions of ultrapure analytical grade HNO3
and HCl were purchased from Kishida Inc. Diluted acid solutions were obtained by mixing the concentrated regents with
Milli-Q water. All water used in this study was obtained froma Milli-QTM water purification system. All other chemicals
were of Japanese Industrial Standards special grade.
2.2. Instruments. All gamma-ray measurement was performed with a coaxial-type high-purity germanium detector(GEM40, ORTEC). The detector was connected to a multichannel analysis system (MCA7600, Seiko EG&G).
In the experiments to develop new chemical separation
methods, an LS 6500 liquid scintillation counter (BeckmanInc.) was used to measure beta-rays. For the determination of90
Sr activities in aerosol samples by the Cherenkov lights measurement of 90Y, we used a 1220 QUANTULUSTM Ultra Low
Level Liquid Scintillation Spectrometer (PerkinElmer Inc.)
with 20 mL TeflonTM vials (PerkinElmer Inc.) The detection
efficiency for 90Sr counting was determined by measuring of
the 90Sr standard solution.
An Agilent/HP 4500 ICP-MS (Agilent Technologies Inc.)was used for determining chemical yields of Sr, after thechemical separation and sequential Cherenkov light measurement.
2.3. Aerosol samples. A series of aerosol samples werecollected in April 2011 at Mito City (N36.40, E140.44) in
Ibaraki Prefecture of eastern Japan. In each sample, aerosols
were collected on PaLL TissuquartzTM filters, 2500 QAT-UP(Sigma-Aldrich Inc.), with a Kimoto-121FT high-volume air

sampler (Kimoto Texh Inc.). The size of the filter was 10 × 8
cm 2. The air-flow of the sampler was approximately 700 L
min-1. Cesium-137 activity was directly determined for eachsample with the germanium semiconductor detector. A standard filter was used to determine the efficiency of the germanium semiconductor detectors. The standard filter spottedradioactive standard solution uniformly was folded into thesame shape of the measurement samples [8].
We chose four filter samples from the samples collected in
April for determining the 90Sr activity concentrations in theaerosols using our new method. The details of the samples areshown in Table 1. These four samples had relatively high 137Cs
activities, and thus we expected that these samples would have90
Sr high activities above that of the liquid scintillation counterdetection limit. We analyzed a half or quarter portion of eachaerosol filter sample.
2.4. Strontium separation procedure.
2.4.1. Sample pretreatment. First, each separated filter
sample was cut into pieces (approximately 5 × 2 cm 2), whichwere then put into a 500-mL Teflon beaker. Next, 100 mL of
12 M HCl, 30 mL of 13 M HNO3, and 1.0 mg of stable Sr carrier were added to the beaker. The acid solution was thenheated on a hot plate to 150 °C for 3 h to extract Sr into thesolution. After cooling to room temperature for 1 h, the solution was filtered with a 0.45-μm membrane cellulose filter(φ47, Advantec no.7) after decantation to roughly separate thesolution and residues. The solution was then placed into a 300
mL-polyethylene container. The residues on the filters were
washed with 30 mL of 3.25 M HCl twice followed by 30 mL
Milli-Q water once. The washing solutions were also added tothe polyethylene container. The mixed sample solution was
weighed, and then 3 mL of the solution was removed to obtain
the initial Sr concentration by ICP-MS for the chemical yield
determination. The remaining solution was transferred to a
500-mL measuring flask. Milli-Q water was added into theacid to adjust the concentration to 3.25 M HCl, which is suitable for Sr extraction by the disk [14].
In this study, the extraction yield of 90Sr from each aerosol
filter was obtained by comparing its activity with those of137
Cs from the same sample before and after chemical treatment. We presumed that the Sr and Cs in the aerosol sample
had similar chemical extractability into the acid solution. The
filter residues after extraction were dried in a desiccator atroom temperature, and then formed into approximately thesame geometric shape as the intact filter for gamma-ray measurement.
2.4.2 Solid-phase extraction. The complete separation procedure, including a solid-phase separation and a cation

TABLE 1: Details of the four FDNPP aerosol samples and chemical analysis results
Aerosol sample name3

Air volume (m )Sampling start date
Measurement date
Effective air-volume (m3)137
Cs yield
Stable Sr yield90
Sr concentration (mBqm-3)
137
Cs concentration (Bqm-3)
90
Sr/137Cs activity ratio (×10 -3)

MIT4

MIT9

MIT12

MIT13

2070
2011/4/9
2015/7/17
497
99±2%
83±6%
1.5±0.1
0.82±0.08
1.9±0.2

1572
2011/4/14
2015/8/1
770
99±5%
87±6%
0.79±0.6
0.057±0.006
14±1

3085
2011/4/19
2015/7/18
771
98±3%
86±6%
0.50±0.04
0.11±0.01
4.6±0.6

2139
2011/4/21
2015/7/13
1069
98±4%
85±6%
0.055±0.005
0.041±0.004
1.3±0.2

Notes: 90Sr and 137Cs activity concentrations were calculated from the equation provided in the “Measurement
instruments” subsection. Effective air-volume was calculated from the mass of aerosol sample analyzed. All
activities were decay collected to 11 March, 2011. The confidence interval was 95%.

Rapid isolation method for radioactive strontium using EmporeTM

J. Nucl. Radiochem. Sci., Vol. 16, 2015

The 90Sr activity concentration (Bq m-3) in air, A, was determined from the 90Y activity as

exchange chromatography, is illustrated in Figure 1. For solidphase extraction, an EmporeTM Strontium Rad Disk was set ina φ47-mm diameter funnel and preconditioned by the additionof 10 mL methanol, waiting 20 min, and then rinsing with 30
mL of 3.25 M HCl by vacuum filtration with a flow rate of
approximately 50 mL min-1. The sample acid solution in the
500-mL flask was then passed through the disk to extract Srions into the solid-phase. The disk was then washed with 15
mL of Milli-Q water to remove residual acids. To elute Sr ion
from the disk, 10 mL of 0.02 M EDTA solution was passed
through at a flow rate of 2 mL min-1 [16]. Another 10 mL ofthis EDTA solution was passed through the disk at 10 mL
min-1 as the final rinse. These two solutions were combined
and used for cation-exchange separation.
2.4.3 Cation-exchange separation. The cation-exchange
process is illustrated in the lower-right portion of Figure 1.
First, 0.25 g of Dowex 50W-X8 (100-200 mesh) ion-exchange
resin was placed into an S-size MuromacTM mini-column (Φ8× 50 mm). Then, 10 mL Milli-Q water and 10 mL of 0.01M
HCl were passed through the column consecutively to condition the ion-exchange resin. To the 20 mL EDTA sample solution, 0.75 mL of 13 M HNO3 was added to adjust its pH, andthe solution was passed through the preconditioned column.
Under these conditions, both Sr and Pb ions were adsorbed
onto the cation-exchange resin. Then, 10 mL of 10 -5 M HCl
solution was passed through the column to wash out the residual EDTA, followed by 15 mL of 2% (w/v) EDTA solution toremove Pb ions from the resin. Finally, 15 mL of 3.25 M HCl
was passed through the column for Sr ion elution. The final
solution was placed into a 20 mL Teflon vial to be measured
by a low-background liquid scintillation counter.

C – CbεRVA=where C is the count rate after 90Sr-90Y radioactive equilibration, Cb is the background count rate, and ε is the counting
efficiency of Cherenkov light for 90Y. In our measurement
system, the background counting rate was approximately
0.028 count per second (CPS), and the detection efficiency of
Cherenkov light from 90Y was 68.7 ± 0.1 %. R is the yield of
Sr chemical separation determined by ICP-MS, and V is thecollected air volume (m 3) of the aerosol samples. In the
ICP-MS measurement, the chemical yield was determined bymeasuring the amount of stable Sr carrier ions in the measurement sample after a sequential Cherenkov light measurement.
A 1 mL aliquot of each measurement sample was diluted 1000
times with water and HNO3, and adjusted to a final concentration of 5% w/v HNO3. Four external calibration solutions wereprepared with the 5% w/v HNO3 to 0, 10, 20, 50, and 100 ng
mL -1. The calibration curve for the Sr was found to be linear(R 2 ≥ 0.9999). In each measurement, we performed at least six
analytical runs and used the averaged value. The chemicalyields were determined from the ratio of the Sr mass in themeasurement sample to that of the Sr added initially.
The uncertainties of the 137Cs radioactivity concentrations
were the combined uncertainties of the efficiency and counting statistics in the gamma-ray measurement. The uncertainties of the stable Sr yield resulted from errors in the ICP-MS
measurement. The total uncertainties in the 90Sr radioactivities were the combined uncertainties of 137Cs yields, stable Sr
yield, counting statistics in the 90Sr measurement, and uncertainty in the efficiency of the Cherenkov light measurement.
When reporting the results, 95% confidence intervals wereused in this study.

2.5. Radioactivity calculation. Beta-rays from 90Y, adaughter nuclide of 90 Sr, were measured to determine theamount of 90Sr in the aerosol samples. The beta rays emitted
from 90Y have an endpoint energy of 2.2 MeV and can effectively emit Cherenkov light in water. The Cherenkov lightcounting efficiency for 90Sr is much lower than that for 90Y,
therefore we can measure 90Y activity selectively. We canidentify the activity of 90Sr from the growth of 90Y over time.
Owing to the large difference between the half-lives of 90Sr
and 90Y, radioactive equilibrium was reached within 2 weeks.
The 90Sr activity approximately coincides with the 90Y activity
after 2 weeks from 90Sr isolation.

3. Results and Discussion
3.1. Optimization of 210Pb cation-exchange separation
conditions. Cation exchange was applied after extraction by
the Rad Disk method to eliminate 210Pb, because contamination by naturally occurring 210Pb causes considerable background signals during 90Sr detection. Pb ions in solution areusually separated by precipitation in a chromate form at pH

Sample + Sr carrier 1 mg
H2O 10 mL
0.02M EDTA 20 mL
Solid-phase extraction+ HNO3 0.8 mL
Waste A Waste B

17

pH5 HCl 10 mL
pH5 2%EDTA 15 mL
3M HCl 15 mL
Cation-exchange resin column

Waste C Waste D

Waste E
Measurement Sample

Figure 1. Schematic of the chemical separation procedure for the isolation of 90Sr from aerosol samples by solid-phase extraction

J. Nucl. Radiochem. Sci., Vol. 16, 2015

3.2. Estimation of Sr losses in each chemical separation
step. The Sr yield in each step of the optimized method wasinvestigated by measuring the gamma-rays from 85Sr (0.514
MeV) using a germanium semiconductor detector, and/or theamount of stable Sr carrier in the sample solution by ICP-MS.
The Sr yield by the solid-phase extraction method is determined to be more than 95%, which is consistent with the disk
extraction performance (Waste A) [14]. Cations other than Sr
do not cause serious interference in this step. In our method,
to remove any residual acid from the disk that may disrupt
EDTA complex formation, the disk was washed with 10 mL
water before Sr ion elution. With this washing step, the loss of
Sr ions in Waste B (see Figure 1) is suppressed to within 2–3%.
In the cation-exchange operation, Sr-containing EDTA eluate
from the disk was mixed with 0.75 mL of 13 M HNO3, andthen passed through the cation-exchange resin. The Sr ion
losses during passage through the column, pH conditioning,and elution of 210Pb from the column are contained in Wastes
C, D, and E, respectively. We measured the activity of 85Sr inthese wastes by gamma-ray counting and the amount of stable
Sr in these wastes by ICP-MS. The Sr losses in these wastes
are less than 1%. Hence, we conclude that no significant lossof Sr occurs in these steps.
3.3. Extraction yi elds of 90 Sr from aerosol samples.
Cesium-137 yields in acid pretreatment step were measured foralternative evaluating 90Sr leaching yields from aerosol samples. Stable Sr yields of the chemical separation were measured by ICP-MS. From the Cherenkov light counting result,the 137Cs yields and the stable Sr yields, the activities of 90Sr in
the aerosol samples were determined and are summarized inTable 2.
The activities of 90Sr in the atmosphere are in the range of
0.055–1.5 mBq m -3, which is consistent with previous mea-

100%
Radioactivity Ratio

5.5 [17]. However, this method does not eliminate Pb in thepresence of EDTA solution, which forms a stable complex
with Pb ions at this pH. Instead, we employed cationexchange chromatography for Pb ion elimination. Thus, optimization of the conditions for the separation of 90Sr from 210Pb
was necessary.
In our separation method (illustrated in Figure 1), Sr ions in
0.02 M EDTA solution eluted from the solid-phase extraction
disk were adsorbed onto the cation-exchange resin and separated from the Pb ions. To estimate the separation efficiency
and determine the elution curves of Pb and Sr ions, a 210Pb
solution was passed through the cation-exchange column
under conditions comparable to those in Figure 1. After passing 10 -5 M HCl solution through the column to adjust the pH
conditions, 2% (w/v) EDTA solution at pH = 5 was passed
through the column and each 1 mL fraction of the elute wasstored in a plastic vial. Then, 6 mL scintillation cocktail wasadded to each vial and the activities were measured by a liquid
scintillation counter. The resulting 210 Pb elution curve isshown in Figure 2. Elution of 210Pb from the column starts
after 7 mL EDTA solution is eluted, and 210Pb elimination iscompleted with 12 mL of EDTA solution. From this result, weconclude that 210Pb separation is achieved by 15 mL of EDTA
solution (Figure 1).
The elution behavior of Sr ions from the column was investigated using the 89Sr isotope. To obtain an elution curve, 3.25
M HCl was passed through the column and collected in 2 mL
fractions. We added 6 mL scintillation cocktail to each fraction and the 89Sr activities were measured with a liquid scintillation counter. The resulting Sr elution curve (Figure 3) showsthat 95% Sr is recovered from the column with 12 mL HCl
solution. We adopted 15 mL as the optimized volume of the
HCl solution for the elution of Sr ions from the resin, as shownin Figure 1.

Zhang

80%
60%
40%
20%
0%
0

2

4
6
8
10
Volume of 2% EDTA solution (mL)

12

14

Figure 2. Elution curve of 210Pb f rom the cation-exchange column
by 2% EDTA solution at pH = 5. The elution profile was determinedusing 1 mL aliquots. The y-axis is the activity ratio of eluted 210Pb toinitially added 210Pb.

100%

Radioactivity Ratio

18

80%
60%
40%
20%
0%
0

5

10

15

Volume of 2% EDTA solution (mL)

Figure 3. Elution curve of 89Sr from the cation-exchange column by
3.25 M HCl. The elution profile was determined using 2 mL aliquots. The y-axis is the activity ratio of eluted 89Sr to initially added
89
Sr.

TABLE 2: Yields of stable Sr after chemical separation andloss of Sr in each separation step
Sample name

MIT4

MIT9

MIT12

MIT13

Waste A
Waste B
Waste C
Waste D
Waste E
Stable Sr yield

1%
2%
1%
<1%
<1%
83±6%

3%
3%
1%
<1%
<1%
87±6%

1%
1%
2%
<1%
<1%
86±6%

2%
5%
1%
<1%
<1%
85±6%

Notes: See Figure 1 for detailed separation procedure. The
confidence interval was 95%.

surement results [2].
For the four analyzed aerosol samples, the 137Cs is perfectlyleached during acid pretreatment. Cesium-134 and -137 aretransported through the atmosphere in sulfate aerosol form
[18], and sulfate aerosols dissolve in HCl solution easily.
Strontium is also transported through the atmosphere in soluble aerosol form, as, unlike iodine, Sr does not exist in gaseousform. Therefore, we conclude that the 90Sr in the filter is completely removed by the acAfter the pretreatment, five waste
solutions (A-E) were collected during chemical separation, andthe chemical separation yield and Sr loss were estimated bystable Sr ion measurement using ICP-MS. The Sr yields andlosses in the waste solutions are summarized in Table 2. The
Sr chemical yields in all four samples are more than 80%. The
sum of the Sr yields and losses in each waste are less than

Rapid isolation method for radioactive strontium using EmporeTM

100% because of the margin of error in the ICP-MS measurement, and the retention of Sr ions on the disk and/or cationexchange resin.
3.4. Backgrounds in measurement of 90 Sr for aerosol
samples. There are three major background interferences for90
Sr (90Y) in the four aerosol samples in this study, i.e., 89Srfrom the FDNPP accident, 90Sr from global fallout, 210Pb, and226
Ra and its daughter nuclides. Each of their effects on the90
Sr determination are discussed in details as follows.
Strontium-89 with a half-life of 50.5 days is one of the mostimportant radionuclides from the FDNPP nuclear accident.
The activity of 89Sr can be detected by Cherenkov light counting, owing to its high-energy beta-rays. However, the Day 0
counting rate in Figure 4 is approximately 0.02–0.03 CPS,
which agrees well with that of a blank sample (0.028 CPS).
Thus, we conclude that 89Sr activity is not detected in our measurements. The 89Sr/90Sr activity ratio of the FDNPP fuel was
calculated as 0.43 by ORIGEN 2 code [19]. We began our
measurement in 2015 (approximately 1500 days after the accident), and the 89Sr/90Sr activity ratio was estimated as 1.6 ×
10 -10.
The global background of 90Sr in the atmosphere is negligible in our study. The background 90Sr atmospheric concentration was 1 × 10 -6 Bq m-3 in the Northern hemisphere in 1983
[20]. A sharp increase in the atmospheric concentration of 90Sr
was observed in Europe after the Chernobyl nuclear accident.
However, the 90Sr activity concentration of the global fallout
returned to its pre-accident level [2] and has continued todecrease to date. The atmospheric concentration of 90Sr follows the fallout behavior and its level should be lower than 1 ×
10 -6 Bq m-3. However, in our data, the lowest 90Sr atmosphericconcentration is 55 × 10 -6 Bq m-3. Thus, we can conclude thatthe 90 Sr activity detected in our study originates from the
FDNPP accident release event.
Naturally occurring 210Pb provide severe background signals for 90Sr counting by Cherenkov lights measurements. The
90
Sr activity concentrations obtained in the present study are

J. Nucl. Radiochem. Sci., Vol. 16, 2015

0.055–1.5 mBq m-3. Conversely, Tanahara et al. reported thatthe 210Pb activity concentrations in Okinawa Islands, Japan,
were 0.06 to 1.98 mBq m-3 from 2004 to 2011 [21]. Lead-210
only emits low-energy end-point beta-rays (0.06 MeV), but its
daughter nuclide 210Bi emits relatively high-energy beta-rays(1.2 MeV). The beta-rays of 210Bi emit Cherenkov light inwater, and its efficiency is approximately 30% that of 90Y. As
we discussed above, our method eliminates the 210Pb contribution completely. In addition, we also confirm the possibility of210
Pb existence owing to radioactive equilibrium. If 210Pb contamination was present, more time would be required toachieve radioactive equilibrium relative to that required for thepure 90Sr-90Y equilibrium, because 210Bi has a longer half-live
than 90Y. However, all of our sequential measurement results
show good agreement with the calculated 90Sr-90Y radioactive
equilibrium. Thus, we conclude that complete 210Pb elimination is achieved and thus does affected our determination of90
Sr in the aerosol samples.
Radium-226 and its daughter nuclides of 222Rn, 214Bi and214
Pb may cause background signals in the beta-ray measurements [22], because 226 Ra is not eliminated by our method.
However, the atmospheric concentration of 226Ra is approximately 1.5 μBq m -3 [23], and the low-activity 226 Ra and its
daughters compared with that of 90Sr allow it to be ignored in
the Cherenkov light measurements.
3.5. Activities of 90 Sr in four aerosol samples collected
after the FDNPP accident. The activities of 90Sr in the aerosol samples collected after the FDNPP accident were determined using the optimized separation method describedabove. The chemical separation method with solid-phaseextraction was performed on each aerosol sample, and 15-mL
3.25 M HCl measurement samples were obtained. Cherenkov
counting was performed over more than 2 weeks for each measurement sample. The 90Y growth curves for the aerosol samples are shown in Figure 4.
As shown in Table 2, the 90Sr activity concentrations areapproximately 1/1000 those of 137Cs. Comparing the 90Sr/137Cs

0.5

0.5

(b)

0.4

0.4

0.3

0.3

CPS

CPS

(a)

0.2
0.1

0.2
0.1

0.0

0.0
0

5

10

15

20

0

5

Day

10

15

20

15

20

Day

0.5

0.10

(c)

(d)

0.4

0.08

0.3

0.06

CPS

CPS

19

0.2
0.1

0.04
0.02

0.0

0.00
0

5

10

Day

15

20

0

5

10

Day

Figure 4. Yittrium-90 growth curves from 90Sr in (a) MIT4, (b) MIT9, (c) MIT12, and (d) MIT13 aerosol samples. The data points were obtainedby the 1220 QUANTULUSTM counter. The solid lines are the 90Y growth, calculated by fitting the counting data to the equation for radioactive
equilibrium.

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J. Nucl. Radiochem. Sci., Vol. 16, 2015

activity ratio in aerosols with those from soil and seawater
samples provides us with a better understanding of the radioactivity discharge conditions in the FDNPP event. Soil samples collected in eastern Japan had the 90 Sr/137Cs ratio of
1/10-1/10000 [10]. The Ministry of Education, Culture,
Sports, Science & Technology in Japan (MEXT) reported that90
Sr/137Cs ratios in soils were approximately 1/1000 at many
survey points in Fukushima Prefecture, including some points
with ratios as high as 1/10-1/100 [24]. The soil 90Sr/137Cs ratios
are comparable to our aerosol values; however, our data
exhibit large variation. The 90Sr/137Cs ratio for seawater samples collected at 30-600 km offshore in June 2011 was reported
to be 0.0265±0.006 [25]. Povinec et al. reported that the90
Sr/137Cs ratio at the coast near the FDNPP varied between
0.006 and 64.5 from April 2011 to February 2012 [26]. The
ratio is higher than that from our aerosol samples, probably
because of the direct discharge of 90Sr from the FDNPP site to
the sea [27]. Because 90 Sr is less volatile than 137Cs, the90
Sr/137Cs ratio in aerosols would be lower. Water can containadditional 90Sr in soluble chemical forms, which may also contribute to a higher 90Sr/137Cs ratio in seawater than in aerosols.
4. Conclusions
A rapid isolation method for 90 Sr in aerosol samples has
been developed using EmporeTM Sr Rad Disk. The problem ofcontamination by 210Pb during solid extraction was resolved bycation exchange. The yield of Sr in this scheme was > 80%
and 210Pb was completely eliminated. Because the separation
method consists of only two steps, the 90Sr chemical separationfrom the pretreated sample solution is achieved much faster(approximately 2.5 h) than in conventional methods. Aerosol
samples collected in Mito City after the FDNPP accident wereanalyzed by the developed method, and the activities of 90Sr
were determined for the first time. The 90Sr/137Cs activity ratio
in aerosol samples collected at Mito about 130 km south fromthe FDNPP site following the accident were found experimentally to be in the order of 1/1000.
Acknowledgements
This work was supported by JSPS K AKENHI Grant
Number JP16K45678 and a Grant-in-Aid for Scientif ic
Research (KAKENHI) on Innovative Areas under the A04-08
research teams (grant number 24110009).
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