Journal of Nuclear and Radiochemical Sciences, Vol. 11, No. 2, pp. A5-A9, 2010

Accounts

QA/QC Activities and Estimation of Uncertainty for Ultra-trace Analysis of Uranium andPlutonium in Safeguards Environmental Samples
Shigekazu Usuda,*, a Masaaki Magara,a Fumitaka Esaka,a Kenichiro Yasuda,a Yoko Saito-Kokubu,aChi-Gyu Lee,a Yutaka Miyamoto,a Daisuke Suzuki,a Jun Inagawa,a Satoshi Sakurai,a and Fujio MuratabaJapan Atomic Energy Agency, 2-4, Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japanb
Radiation Application Development Association, 2-4, Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki-ken
319-1195, Japan
Received: July 7, 2010; In Final Form: August 19, 2010
Japan Atomic Energy Agency (JAEA) has been conducting the environmental sample analysis, which is one of the
IAEA’s safeguards measures. In the analysis, isotope ratios of ultra-trace amounts of uranium and plutonium insamples taken from nuclear facilities are determined to detect undeclared nuclear materials and activities. In orderto keep and enhance the reliability of the measurement results, validation of analytical methods and estimation of
measurement uncertainty in such ultra-trace analysis should be properly done in accordance with worldwide standards. In this paper, the current activities on achievement of QA/QC (quality assurance and quality control) andestimation of measurement uncertainty in the ultra-trace analysis at a clean chemistry laboratory (CLEAR) of
JAEA are reported.

1. Introduction

2. Achievement of QA/QC

The International Atomic Energy Agency (IAEA) introduced the environmental sample analysis, which is to detect‘environmental signature’ of undeclared nuclear materials (Uand Pu) and activities, as one of safeguards measures based on‘Programme 93+2’.1,2 In the environmental sample analysis,
isotope ratios of U and Pu in samples taken from nuclear facilities and the related ones are analyzed at a clean chemistry laboratory. Main analytical methods are composed of 1) the bulk
analysis providing information about the quantity and the average isotopic composition of each sample and 2) the particle
analysis for U isotopic compositions of individual particles.
In Japan, the environmental sample analysis has been performed at Clean Laboratory for Environmental Analysis andResearch (CLEAR) of Japan Atomic Energy Agency (JAEA).3-7
Our aims of the analysis are 1) to maintain the independent
verification scheme of the domestic safeguards system, 2) tocontribute the IAEA as a member of NWAL (Network ofAnalytical Laboratories), and 3) to apply the developed techniques to other fields such as environmental science.
In order to find undeclared nuclear activities, ultra-trace
analysis of U, Pu, minor actinides and long-lived fission products is necessary. Especially, the isotope ratios of U and Pu
suggest their origin and related nuclear activities, e.g., nuclear
fuel, weapon, enrichment, reprocessing, natural grade or global
fallout.
To keep and enhance the reliability of the measurement
results, validation of analytical methods under cleanlinessenvironment conditions at the level of worldwide standards andestimation of measurement uncertainty in such ultra-trace
analysis should be reasonably done. This paper deals with thecurrent activities and perspectives on achievement of QA/QC(quality assurance and quality control) and estimation of measurement uncertainty in the ultra-trace analysis of the safeguards environmental samples at CLEAR of JAEA.

The QA/QC system of the IAEA’s NWAL should be basedon the principles of worldwide standards. Main related international and national standards (or guidelines) in this field are
as follows: 1) ISO/IEC 17025: 2005 [JIS Q 17025: 2005](General requirements of the competence for testing and calibration laboratories), 2) ISO 14644 Series (Cleanrooms andassociated controlled environments), and 3) ISO/IEC Guide 98
Part 3 (Guide to the expression of uncertainty in measurement[GUM: 1995]).
Based on the above standards, the clean chemistry laboratory, CLEAR, was designed, constructed and is being operated,
and the environmental samples have been analyzed. 3-7 The
samples are composed of non-radioactive ones and radioactive
ones taken from a hot-cell facility and are, therefore, screenedby radiometric techniques. On the other hand, the samples
with less than 1 Bq order of U and Pu are acceptable for theultra-trace analysis at CLEAR. The samples which containedmuch more radioactivity as hot-cell ones are analyzed at theradiochemistry facility, NUCEF (Nuclear Fuel Cycle Safety
Engineering Research Facility) of JAEA.
The QA/QC and analytical procedures are systematized as‘Manuals on safeguards environmental sample analysis andquality assurance’ as shown below:
1) Outline of analysis and quality control,2) Sample receipt, storage and screening technique,3) Bulk analysis for non-radioactive samples by ICP-MS(inductively coupled plasma mass spectrometry),4) Hot-cell sample analysis for radioactive samples by TIMS
(thermal ionization mass spectrometry),5) Particle analysis by SIMS (secondary ion mass spectrometry) method,6) Particle analysis by FT (fission track)-TIMS method, and
7) Cleanliness control.
In the QA/QC system of such a clean chemistry laboratory,
in addition to the documentation and document control, a number of important requirements such as organization, management system, accommodation and environmental conditions,analytical methods and method validation, measurement traceability, etc. are contained as management and technical require-*Corresponding author. E-mail: usuda@cyric.tohoku.ac.jp†Present address: Cyclotron and Radioisotope Center, Tohoku
University, Aoba 6-3, Aramaki, Aoba-ku, Sendai-shi, Miyagi-ken
980-8578, Japan© 2010 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 01/14/2011

UsudaJ. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

Pass box
Window

Figure 1. Layout of the analytical building of CLEAR.

TABLE 1: Cleanliness control activities in the analyticalbuilding of CLEAR
Daily inspection• Cleanliness check
1. Hoods (ISO Class 4) and benches (ISO Class 4–5): 1/month
2. Cleanrooms (ISO Class 5–7): 1/month
3. Air showers and pass boxes (ISO Class 5–6): if necessary• Area blank measurement: 1/month• Pressure difference: 1/day• Temperature (23±1 °C [21±1 °C in winter]): 1/day• Humidity (50± 5 % [45±5 % in winter]): 1/day
Annual inspection• Air flow rate in hoods and benches: 1/year, FFU: 1/ 2
years
• Air flow direction: whenever necessary• Air flow visualization: 1/year• Filter leakage in hoods and benches: 1/year, FFU: 1/ 2
years
• Cleanliness recovery characteristics: 1/year
Cleaning• Cleanrooms (ISO Class 5–7): 1/year• Plenum chambers (ISO Class 5–7): 1/year

3. Estimation of uncertainty
The IAEA recommends the typical accuracies in the analysis for the environmental samples with expected characteristics.
The accuracies for each instrument of mass spectrometry forthe bulk analysis and the particle analysis at JAEA were summarized in Table 2, together with the IAEA-expected values inthe table foodnote.
The accuracies obtained for the respective instruments weresatisfactory, compared with those recommended by IAEA

����
�����
�����
�����
�����
�����
�����

����
����
����
����
���
�

������

Air shower������

ISO class 7������

(Radiation
detectors)������

(SEM-EDX)

ISO class 6������

General laboratory

Entrance toclean area

ISO class 5������(ICP-MS)(TIMS)

������

EPMA,
TXRF)

������

Instrumental
analysis area (SIMS,

Clean room area
(positive pressure)

Entrance tocontrolledarea

Chemicaltreatment area

Samplestorage������

Service area

Electricity
room������

Machine room(cold)

������

Machine room(hot)

Hot area(negative pressure)

Area blank at several points in the cleanroom area is measured to check contamination of U, Th, Pb and other elements
of interests. No significant contamination has ever been
observed. Cleanliness recovery characteristics of the cleanrooms have been yearly examined. After restarting the fan-filter units (FFU), the cleanliness was immediately recovered
within a few minutes.4 Due to not only excellent designing of aclean chemistry laboratory but also the continuous activities ofcleanliness control, the cleanliness environment at CLEAR has
been successfully maintained.
2.2. Validation of methods. The analytical methods andprocedures suitable for various types of the environmental
samples have been developed.8-27 The methods and procedures
were validated, for example, by the use of reference materials
certified by NBL (New Brunswick Laboratory) such as
CRM111A ( 233 U), CRM130 ( 2 42 Pu), CRM U015 (U) andSRM947 (Pu) as spikes and isotopic standards.
JAEA has been participating in inter-laboratory comparisonsoperated by IRMM (Institute for Reference Materials andMeasurements), 28 such as NUSIMEP (Nuclear Signatures
Interlaboratory Measurement Evaluation Programme) and
REIMEP (Regular European Interlaboratory Measurement
Evaluation Programme),29 for validation of the developed bulk
and particle analytical methods. The JAEA’s measurement
results and capabilities were proved to be excellent in the comparisons.
The IAEA has crosschecked the analytical results and done
blind tests using QC samples. The methods and results havebeen discussed and evaluated every year at the IAEA technicalmeetings, which are composed of the IAEA and NWAL members. The JAEA’s methods and results have been evaluated tobe reliable and accurate until now.
In addition, process blank of U and Pu must be exactly analyzed to correct the measured values. The blank amounts inthe bulk analysis at CLEAR were approximately 10 pg of natural U and 0 fg of Pu. It was found to be maintained at theextremely low level even now. Consequently, the effect ofblank on isotope ratio measurement at CLEAR was negligible.

������

ments. Here, activities on control of cleanliness environment
and validation of methods are illustrated as examples of QA/
QC achievement.
2.1. Control of cleanliness environment. The CLEAR is a
key laboratory in Japan to accomplish the environmental sample analysis,7 and consists of administration building and analytical building. Figure 1 shows the layout of analyticalbuilding and the classification of air cleanliness for each cleanroom area.
The cleanroom area is controlled as cleanliness environment
at positive pressure mainly according to ISO 14644 Series andthe manual of cleanliness control, while hot area except thecleanroom area is maintained at negative pressure according to
the domestic regulation and JAEA’s rules. Table 1 summarizes
periodic cleanliness control activities in the analytical building
prescribed in the manual of cleanliness control.
Usually no particle were detected in every hood and benchwith ISO class 4, the upper limit of which is 1,020 particle/m3
for the particles of more than 0.3 µm in diameter. As an example of cleanliness checks, Figure 2 shows monitoring results ofairborne particle concentration in some cleanrooms. The
cleanliness was unfailingly maintained at the respectiveclasses.

Particles / m3

A6

Figure 2. Example of cleanliness check results of airborne particle
concentration in some cleanrooms of the chemistry area (A 01-05:
ISO class 5 [upper limit of 0.3 µm sized particles; 10,200/m 3] andinstrument area (B 01, 02: ISO class 6 [upper limit; 102,000/m3]).

QA/QC Activities and Estimation of Uncertainty for Ultra-trace

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

A7

TABLE 2: Accuracies obtained for each instrument of mass spectrometry at JAEA
Instruments (Techniques)

Measured values
U amount
Pu amount
U amount
Pu amount235
U/238U ratio
240
Pu/239Pu ratio
235
U/238U ratio
240
Pu/239Pu ratio
235
U/238U ratio
240
Pu/239Pu ratio
235
U/238U ratio
240
Pu/239Pu ratio

ICP-MS [ELEMENT](CLEAR/bulk by IDM)
TIMS [MAT262](NUCEF/ hot-cell sample by IDM)
ICP-MS [ELEMENT](CLEAR/bulk)
TIMS [MAT262](NUCEF/hot-cell sample)
SIMS [Cameca 6f](CLEAR/particle by SIMS)TIMS [TRITON](CLEAR/particle by FT-TIMS)

Range
-11
10 –10 -6 g
10 -14 –10 -10 g
10 -9–10 -4 g
10 -13–10 -7 g
2×10 -6 –102
10 -3–102
10 -6 –102
10 -4 –102
10 -5–102−-6
10 –102
10 -4 –102

Relative accuracy (2σ) a
0.5 % (at 5 ng natural U) b
5 % (at 1 pg) c
1 % (at 4 ng natural U) b
1 % (at 1 ng) c
0.6 % (at 500 pg/mL natural U) d
1 % (at 5 pg/ml SRM947) e≤1 % (at 10 ng natural U) d
≤1 % (at 100 pg Pu) e≤10 % (at 10 pg natural U) f−
≤5 % (at 10 pg natural U) g
≤5 % (at 1 pg SRM947)

a: Corresponding to confidence level ≈ 95%, b: typical accuracy expected by IAEA; ≤20 % for 10 ng of U, c: ≤10 % for 5 pg ofPu, d: ≤1 % for 10 ng of U, e: ≤10 % for 5 pg of Pu, f: ≤10 % for 1 µm particle of UO2 (~5 pg of U ), g: ≤1 % for 1 µm particle ofUO2.
TABLE 3: Accuracies in radioactivity measurements
Methods
γ-ray spectrometry
α-ray spectrometry

Facility
CLEAR
NUCEF
CLEAR
NUCEF

Range
10 –105 Bq
101–109 Bq
10 -4 –104 Bq
10 -3–106 Bq-3

Relative accuracy
5–10 % (at 1 kBq 137Cs)
5–10 % (at 10 kBq 137Cs)
2–5 % (at 3 kBq 244Cm)
2–5 % (at 10 kBq 241Am)

Expected γ-activity range by IAEA: 0.01 to 108 Bq/swipe, expected relative accuracy: ≤ 10 %.

TABLE 4: Components of standard uncertainty in bulk
analysis at CLEAR
Type A (The statistical analysis)
Standard deviation of the mean of each isotope ratio measured value
Correction of mass bias (235U/238U ≈ 0.5 %)Correction of blank (quantity: ≤1%, ratio: negligible)
Correction of tailing (236U/238U ≈ 2×10 -6)
236
U correction by UH evaluation (≈ 4×10 -5)Type B (Means other than the statistical analysis)
Data sheet of reference materials (spike and mass bias)[<1%]
Weighting (negligible)
Evaporating (spike solution ≈ -0.03% per month)
Adsorption and leaching out (negligible)
except for the particle analysis by TIMS, however, it is difficult
to simply compare between them because of different analytical conditions.
The accuracies in radioactivity measurements for screening
and/or analyzing minor actinides, fission and activation products by α- and γ-ray spectrometry were also roughly obtained(see Table 3). They mainly depended on accuracy in the data
sheet of radioisotope reference sources such as 137Cs, 241Am and244
Cm.
3.1. Uncertainty in bulk analysis. Concentrations of U and
Pu and their isotope ratios are determined with IDM (isotope
dilution method) by mass spectrometry. The traceability of theresults to the SI (Systeme Internationale) units of measurement
is established mainly by utilizing the certified reference materials and the measured values are properly corrected. Eachstandard uncertainty is evaluated by separating Type A and B
in accordance with the GUM. Table 4 summarizes components of the standard uncertainty in bulk analysis at CLEAR.
Usually, the standard deviation of the mean of each isotope

ratio measured value in Type A is main component and theothers are minor. Uncertainty in weighing was negligible, even
if it was overevalued. Change in the concentration of the spikesolution due to evaporation was corrected by weighting it
before use. Adsorption of Pu into various vessels and leachingof U from the vassels were also examined to be negligible
because the vessels of high quality were selected and used.
Table 5 shows a representative example of the uncertainty
evaluation in U isotope ratio measurement of a blank swipe
sample (swipe cotton cloth: TexWipe-304), which currently
used in the environmental analysis of the IAEA and contains a
few ng of natural U.30 In every case, the standard deviation of
the mean of each isotope ratio measured value was main component. In 236U/ 238U ratio analysis, correction of tailing from238
U peak to 236 U peak was serious component. The relative
expanded uncertainty of the corrected 236U/ 238U ratio was over
100 %.
3.2. Uncertainty in particle analysis. Isotope ratios of U
in a single particle in each sample are determined by SIMS andFT-TIMS methods. The standard uncertainty in the SIMS particle analysis has been reported according to the IAEA procedures. In this paper, components of the uncertainty is estimatedin the same way as the bulk analysis (see Table 6).
The standard deviation of the mean of each isotope ratio
measured value in Type A is also main component and the others are minor. The blank amounts in the particle analysis were
negligible at CLEAR. In the SIMS method, uncertainty in 236U
correction by UH evaluation is nonnegligible component of theuncertainty, and determination of 240Pu/239Pu ratio was exceedingly difficult due to the 238UH formation.
At present, investigation is focused on minor isotope ratiosof U and/or Pu in sub-micrometer sized particles by the FTTIMS method. In bulk analysis, the interferance of alkali metals as well as polyatomic ions, such as IrAr, PtAr, AuAr, PbO2,
ReO3, WO3, etc., on the isotope ratio measurements of U and
Pu have been well examined, and chemical separation was

A8

Usuda

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

TABLE 5: A representative example of uncertainty evaluation in U isotope ratio measurement of a blank swipe sample(Instrument: ICP-MS)
MeasuredvaluesType ofuncertainties

Components or sources

ValuesU/238U ratio of sample
U/238U ratio of reference a
Number of 235U atoms of reference b
Number of 238U atoms of reference b
Weighting, evaporating, etc.
Corrected 235U/238U ratio
234
U/238U ratio of sample
235
U/238U ratio of reference a
Number of U atoms of reference b
Weighting, evaporating, etc.
Corrected 234U/238U ratio
236
U/238U ratio of sample
235
U/238U ratio of reference a
Correction of UH evaluation
Tailing from 238U to 236U peak
Number of U atoms of reference b
Weighting, evaporating, etc.
Corrected 236U/238U ratio

7.233×10 -3
1.54327×10 -2
1.53230×10 -2
9.84430×10 -1—
7.295×10 -3
7.76×10 -5
1.54327×10 -2——
7.85×10 -5
2.80×10 -6
1.54327×10 -2
3.33×10 -5
1.80×10 -6——
7.6×10 -7

235

A
235

U/238U ratio

B

A
234

U/238U ratio

B

A
236

U/238U ratio
B

235

Uncertainties(Relative values)
2.3×10 -5 (0.32 %) c
8.6×10 -6 (0.056 %) c
7.5×10 -6 (0.049 %) c
1.0×10 -5 (0.00001%) c—
2.4×10 -5 (0.33 %) d
5.3×10 -6 (6.8 %) c
8.6×10 -6 (0.056 %) c——
5.4×10 -6 (6.9 %) d
5.0×10 -7 (18 %) c
8.6×10 -6 (0.056 %) c
2.5×10 -6 (7.5 %) c
7.0×10 -7 (39 %) c——
-7
8.7×10 (114 %) d

Relative
Contribution
95 %
2.8 %
2.2 %
0.0 %
(Negligible)
—
100 %
0.0 %
0.0 %
(Negligible)
—
34 %
0.0 %
0.0 %
66 %
0.0 %
(Negligible)—

a: Mass bias correction, b: data sheet of reference, c: standard uncertainty, d: expanded uncertainty (coverage factor: 2, confidence
level: 95%).
TABLE 6: Components of standard uncertainty in particle
analysisType A (The statistical analysis)
Standard deviation of the mean of each isotope ratio measured value
Mass bias correction (235U/238U ≈ 1 %)Tailing correction (negligible)[TIMS: 237U/238U ≈ 2×10 -6]
236
U correction by 235UH(≈ 1×10 -4 by SIMS, negligible by TIMS)Type B (Means other than the statistical analysis)
Data sheet of reference materials [< 1%]applied to reduce the interferance.9,10 On the other hand, it isdifficult to complement the isotope ratio measurements in theparticle analysis, because it is performed without chemical separation in principle. In future, besides the UH evaluation,interference of polyatomic ions with the minor isotope ratios
should be investigated.
In the particle analysis, in addition, there is another serious
problem of “Mixing”, which means simultaneous measurementof plural particles of different isotope ratios, resulting in anintermediate value of the measured ratio. In order to overcome
the problem in SIMS method, we developed a method to pickup individual particle containing U or fissile materials by
micro-manipulation in SEM-EDX (scanning electron microscope-energy dispersive X-ray analysis) (see Figure 3 left).14-16
For the FT-TIMS method, it is easy to observe FT by an optical
microscope and to collect the particles separately (see Figure 3
right).21-23 This method enables us to selectively detect highly
enriched U particles, which is critical in the nuclear safeguards.25
4. Conclusions
Cleanliness control of the CLEAR and validation of analytical methods were successfully achieved. The uncertainty inthe environmental bulk and particle analyses has been routinely

U

Pb

FT

U
Figure 3. Picking up a particle containing U by observing SEMEDX image (left) and identification of particles containing fissile
materials by observing FT (right).

evaluated. On the other hand, the difficulties in the particle
analysis should be solved in near future. As a result, achievement of QA/QC and estimation of measurement uncertainty inultra-trace analysis of U and Pu are at the level of worldwide
standards, which properly keeps and enhances the reliability ofthe measurement results at CLEAR.
Further pursuit of sensitivity and reliability will be necessary for development of advanced techniques of determining
the isotope ratios of U and Pu. Anytime, efforts are required
to estimate potential sources of the uncertainty and to evaluate
its amount.
Acknowledgements. A portion of this work was performed
under the auspices of the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
References(1) D.L. Donohue, J. Alloy. Compd. 271-273, 11 (1998).
(2) S. Deron, D. Donohue, E. Kuhn, K. Sirisena, and A.
Tsarenko, J. Nucl. Mater. Management 28, 27 (2000).
(3) M. Magara, Y. Hanzawa, F. Esaka, Y. Miyamoto, K.
Yasuda, K. Watanabe, S. Usuda, H. Nishimura, and T.
Adachi, Appl. Radiat. Isot. 53, 87 (2000).
(4) S. Sakurai, S. Usuda, K. Watanabe, M. Magara, Y.
Hanzawa, F. Esaka, K. Yasuda, Y. Saito, M. Takahashi, H.

QA/QC Activities and Estimation of Uncertainty for Ultra-trace

Gunji, T. Sakakibara, S. Kurosawa, Y. Miyamoto, K. Gunji,
and T. Adachi, Proc. 42nd INMM Annual Meeting, INMM,
Indian Wells (2001), CD-ROM.
(5) M. Magara, F. Esaka, S. Sakurai, M. Takahashi, K. Yasuda,
T. Taguchi, H. Gunji, S. Kurosawa, J. Inagawa, K. Takai, Y.
Saito, Y. Miyamoto, D. Suzuki, K. Watanabe, S. Usuda,
and T. Adachi, Proc. 43rd INMM Annual Meeting, INMM,
Orlando (2002), CD-ROM.
(6) S. Usuda, K. Watanabe, S. Sakurai, M. Magara, F. Esaka,
N. Kohno, M. Takahashi, K. Yasuda, C.G. Lee, H. Gunji,
S. Kurosawa, J. Inagawa, K. Esaka, Y. Saito, Y. Miyamoto,D. Suzuki, S. Ichimura, H. Fukuyama, K. Iguchi, T.
Onodera, and Jy Chai, Proc. 44th INMM Annual Meeting,
INMM, Phoenix (2003), CD-ROM.
(7) Y. Hanzawa, M. Magara, K. Watanabe, F. Esaka, Y.
Miyamoto, K. Yasuda, K. Gunji, S. Sakurai, S. Takano, S.
Usuda, and T. Adachi, J. Nucl. Sci. Technol. 40, 49 (2003).
(8) M. Takahashi, M. Magara, T. Sakakibara, S. Kurosawa, S.
Sakurai, Y. Hanzawa, F. Esaka, K. Watanabe, S. Usuda, andT. Adachi, J. Nucl. Sci. Technol., Suppl. 3, 568 (2002).
(9) M. Magara, T. Sakakibara, S. Kurosawa, M. Takahashi, S.
Sakurai, Y. Hanzawa, F. Esaka, K. Watanabe, and S. Usuda,
J. Nucl. Sci. Technol. 39, 308 (2002).
(10) M. Magara, T. Sakakibara, S. Kurosawa, M. Takahashi, S.
Sakurai, Y. Hanzawa, F. Esaka, K. Watanabe, and S. Usuda,
J. Anal. At. Spectrom. 17, 1157 (2002).
(11) F. Esaka, K. Watanabe, M. Magara, Y. Hanzawa, and S.
Usuda, J. Trace and Microprobe Technol. 19, 487 (2001).
(12) F. Esaka, K. Watanabe, T. Onodera, T. Taguchi, M. Magara,
and S. Usuda, Spectrochim. Acta B 58, 2145 (2003).
(13) F. Esaka, K. Watanabe, H. Fukuyama, T. Onodera, K.T.
Esaka, M. Magara, S. Sakurai, and S. Usuda, J. Nucl. Sci.
Technol. 41, 1027 (2004).
(14) F. Esaka, K.T. Esaka, M. Magara, S. Sakurai, S. Usuda,
and K. Watanabe, Nucl. Instrum. Meth. B 251, 218 (2006).
(15) F. Esaka, K.T. Esaka, C.G. Lee, M. Magara, S. Sakurai, S.
Usuda, and K. Watanabe, Talanta 71, 1011 (2007).
(16) F. Esaka, K. Watanabe, T. Onodera, C.G. Lee, M. Magara,
S. Sakurai, and S. Usuda, Appl. Surface Sci. 255, 1512
(2008).
(17) F. Esaka, M. Magara, C.G. Lee, S. Sakurai, S. Usuda, andN. Shinohara, Talanta 78, 290 (2009).

J. Nucl. Radiochem. Sci., Vol. 11, No. 2, 2010

A9(18) K.T. Esaka, F. Esaka, J. Inagawa, K. Iguchi, C.G. Lee, S.
Sakurai, K. Watanabe, and S. Usuda, Jpn. J. Appl. Phys.
43, L915 (2004).
(19) K. Iguchi, K.T. Esaka, C.G. Lee, J. Inagawa, F. Esaka, T.
Onodera, H. Fukuyama, D. Suzuki, S. Sakurai, K.
Watanabe, and S. Usuda, Radiat. Meas. 40, 363 (2005).
(20) C.G. Lee, K. Iguchi, F. Esaka, M. Magara, S. Sakurai, K.
Watanabe, and S. Usuda, Nucl. Instrum. Meth. B 245, 440(2006).
(21) C.G. Lee, K. Iguchi, F. Esaka, M. Magara, S. Sakurai, K.
Watanabe, and S. Usuda, Jpn. J. Appl. Phys. 45, L294
(2006).
(22) C.G. Lee, K. Iguchi, F. Esaka, M. Magara, S. Sakurai, and
S. Usuda, Jpn. J. Appl. Phys. 45, L1121 (2006).
(23) C.G. Lee, K. Iguchi, J. Inagawa, D. Suzuki. F. Esaka, M.
Magara, S. Sakurai, K. Watanabe, and S. Usuda, J.
Radioanal. Nucl. Chem. 272, 299 (2007).
(24) X.Z. Zhang, F. Esaka, K.T. Esaka, M. Magara, S. Sakurai,
S. Usuda, and K. Watanabe, Spectrochim. Acta B 62, 1130
(2007).
(25) C.G. Lee, D. Suzuki, F. Esaka, M. Magara, N. Shinohara,
and S. Usuda, J. Nucl. Sci. Technol. 46, 809 (2009).
(26) S. Sakurai, M. Magara, F. Esaka, F. Hirayama, C.G. Lee,K. Yasuda, J. Inagawa, D. Suzuki, K. Iguchi, Y.S. Kokubu,
Y. Miyamoto, N. Shinohara, and S. Usuda, Proc. Int.
Safeguards Symp. on Addressing Verification Challenges,
IAEA-CN-148/116, IAEA, Vienna (2006), p.791.
(27) Y. Miyamoto, K. Yasuda, M. Magara, T. Kimura, and S.
Usuda, Proc. Int. Workshop on Low-level Measurement ofRadionuclides and Its Application to Earth andEnvironmental Sciences, Eds. M. Yamamoto, S. Nagao, Y.
Hamajima, M. Inoue, and K. Komura, Kanazawa
University, Kanazawa (2009), p.45.
(28) http://irmm.jrc.ec.europa.eu/html/interlaboratory_comparisons/index.htm.
(29) S. Richter, A. Alonso, J. Truyens, H. Kuhn, A. Verbruggen,and R. Wellum, Int. J. Mass Spectrom. 264, 184 (2007).
(30) M. Takahashi, S. Usuda, S. Sakurai, M. Magara, S.
Kurosawa, S. Ichimura, F. Esaka, T. Onodera, K. Watanabe,
B.N. Hoang, and K. Kizu, Proc. 45th INMM Annual
Meeting, INMM, Orlando (2004), CD-ROM.