Characterization of brown rice as a certified reference materialfor Fukushima accident-related radioactivity measurements

Yasuhiro Unno a,b,n, Mayumi Hachinohe c, Shioka Hamamatsu c, Setsuko Todoriki c,Akira Yunoki a, Tsutomu Miura a

a National Metrology Institute of Japan, Japanb The Graduate University for Advanced Studies (Sokendai), Japanc National Food Research Institute, Japan

H I G H L I G H T S

� We developed a brown rice certified reference material (CRM) for post-Fukushima assessments.� Gamma-ray spectrometry and Monte Carlo simulation were employed.� Characterization of the CRM was reported in detail.� The CRM can be used to check the validity of radioactivity measurements.

a r t i c l e i n f o

Available online 25 November 2013

Keywords:Certified reference materialBrown riceCs-134Cs-137Gamma-ray spectrometryFukushima Daiichi Nuclear Power Plantaccident

a b s t r a c t

We developed a certified reference material of brown rice to measure radioactivity from the FukushimaDaiichi Nuclear Power Plant accident. The rice was planted in the spring of 2011, just after the Fukushimaaccident occurred, and it was harvested in the autumn of 2011. The certified value of radioactivityconcentration in the rice was 33.6 Bq kg�1 of Cs-134 and 51.8 Bq kg�1 of Cs-137 on August 1, 2012. Thereference material is being widely distributed by the National Metrology Institute of Japan. To determinethe radioactivity and its uncertainties in the brown rice, we employed gamma-ray spectrometry with ahigh-purity germanium detector and Monte Carlo simulation.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

We formulated a strategy of developing and distributing acertified reference material (CRM) to support the general mea-surement of radioactivity, especially for food products. A CRM canbe used to check the validity of measurements regardless of thetype of radioactivity measurement instrument used and a person'sability to analyze radioactivity from output data. The value of thisCRM is widely acknowledged by public and private measurementinstitutions, and more than a hundred orders have been receivedas of March 2013. Through development and distribution, we offerthe opportunity for interested people and institutions to assess thevalidity of their measurements and to undergo proficiency testing.This is particularly beneficial right now in Japan, where there

is a significant need for professional personnel to investigateradioactivity.

According to a 2011 report by the Japanese government(Government of Japan, 2011), a large amount of radioactivematerial was discharged into the atmosphere in March 2011, whenproduction of this CRM began. From a relatively long-term view,radioactive xenon and iodine have decayed. To protect the publicfrom exposure, it is necessary to investigate the levels of Cs-134and Cs-137. According to a report on a wide-area investigation(MEXT, 2012), other radionuclides were scarcely detected.

Brown rice was planted at fields in Fukushima prefecture in thespring of 2011 and harvested the following autumn, so theabundance ratio between Cs-134 and Cs-137 is same as in samplesinvestigated by other institutions. Brown rice was chosen because(a) the matrix of the rice grain was expected to have sufficientuniformity, (b) the same measurement situation can be replicatedfor rice investigation at the measuring laboratory, and (c) thestability of properly prepared rice grain was expected to last forseveral years. A batch of whole brown rice was carefully mixed by

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0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apradiso.2013.11.036

n Corresponding author at: National Metrology Institute of Japan, Japan.Tel: þ81 29 861 3428; fax: þ81 29 861 5673.

E-mail address: y.unno@aist.go.jp (Y. Unno).

Applied Radiation and Isotopes 87 (2014) 485–488

hand and filled into a U8 container (Yamada and Nakamura, 2005)because, according to an investigation into the distribution ofcesium in rice grain (Tsukada et al., 2002), the concentration ofcesium in rice bran is higher than that in polished rice. The U8container, which is generally used in the measuring institution toprepare samples, is a nominal 100 cm3 cylindrical plastic cup witha diameter and height of 50 mm and 55 mm, respectively; thecontainer can hold approximately 80 g of rice.

To determine the certified value of the CRM, a high-puritygermanium (HPGe) detector was calibrated using the same geo-metry. The measurement arrangement was simulated by theMonte Carlo calculation method using the EGS5 code (Hirayamaet al., 2005) to estimate uncertainties. The calculation can bechanged by varying the material component, the density, and thegeometrical condition. To validate the calculation, a comparisonwith actual measurement was conducted. In this paper, wedescribe these procedures and results to characterize the CRM.

2. Methods

We formulated a schematic procedure to develop the certifiedreference material, as shown in Fig. 1. The radioactivity concentra-tion of different batches of brown rice was measured preliminarily.Previously, the Japanese government had established a criterion(100 Bq kg�1) to determine when to abort shipments of food.Therefore, we chose a batch with slightly less than 100 Bq kg�1

total of Cs-134 and Cs-137. The batch was approximately 90 kg,which was comparable to 600 U8 container samples and a surplusof rice. A development schedule was established, and distributionof the CRM to the interested public was set to begin by autumn of2012, which was the first season for harvesting rice after regrowthfrom the Fukushima accident. The procedure used to characterizethe CRM was taken from the ISO Guide 35 (ISO, 2006).

2.1. Sample preparation

First, candidate batches of brown rice were mixed so that therewould be a uniform distribution of radioactivity. The mixing wasdone by a repeated operation of building and dividing a conicalmound of the brown rice by hand. To dispense the rice into a U8container, a portion was divided from the batch, weighed, and anyanomalous material was removed (e.g., bad grain and dirt). The net

mass of brown rice that was filled in the U8 container was within0.1% of 81 g. This was realized by a very careful treatment using asingle grain of rice as the unit. The height of the filled sample, asdetermined by a visual check, was around 48 mm from the innerbottom of the U8 container. Vibration was used so that the ricegrains were tightly packed in the U8 container. As shown in Fig. 2,an acrylic circular plate and an expanded polystyrene board wereplaced on the brown rice to ensure that the rice did not moveduring transport. Sealing tape was put around the outside of thescrew cap of the container, and the filled container was irradiatedfor sterilization by 25 kGy of γ-ray. The sealing was never openedduring the procedure to prevent the aseptic sample from becom-ing contaminated after sterilization. Prepared samples were keptunder room-temperature conditions, ideally in a desiccator underan atmosphere of saturated saline.

To calibrate counting efficiency, the geometrical condition ofthe U8 container was ascertained. In Fig. 3, the relationshipbetween the height and weight of a water sample in the measure-ment geometry is compared to the work of Yamada and Nakamura(2005). The comparison was done by using non-radioactive watercolored with India ink to facilitate the visual check. The relation-ship indicates that the U8 container has a slightly tapered innerdiameter due to a misalignment of the simple cylindrical model.We decided to control the geometrical condition by regulating theweight of the radioactive water solution in the calibration using apolynomial fitting of the relationship (relative humidity 65–70%).

2.2. Measuring instrument

We used an HPGe detector (Model No. GEM-130225) todetermine the radioactivity of the brown rice in the U8 containerusing γ-ray spectrometry. A crystal of the HPGe detector is 90 mmin diameter and 100 mm in length, and the relative efficiency at1.33 MeV of Co-60 was measured as 147.3%. The detector was set

Mixing

Removal of anomaly

Dispensing to U8 container

Weighing

Batches of brown rice

Sealing

γ-ray sterilization

HPGe detector

Calibration Modeling

Measurement

Estimation of uncertainties

Certified value

Determination of radioactivity

Feedback

Fig. 1. Block diagram of the total process to develop the certified referencematerial.

Brown rice

Acrylic circular plate

Expanded polystyrene board

Screw cap of container

Fig. 2. Prepared brown rice in the U8 container in an immobile state due to theplacement of an acrylic circular plate and an expanded polystyrene board.

0 20 40 60 80 1000

10

20

30

40

50

Sam

ple

heig

ht [m

m]

Water weight [g]

Cylindrical Polynomial Present Yamada (2005)

Fig. 3. Relationship between water sample height and water weight in the U8container (solid line: polynomial fitting of presently obtained data, dotted line:simple cylindrical model).

Y. Unno et al. / Applied Radiation and Isotopes 87 (2014) 485–488486

in a heavy lead shield on the ground floor. The backgroundcounting rate of the full energy peak of 661.7 keV of Cs-137was around 0.004 cps in March of 2013 after the Fukushimaaccident despite repeated decontamination of the inner part ofthe shield.

The HPGe detector was calibrated by a γ-ray volume source ofa radioactive water solution in a U8 container. The radioactivewater solution was prepared separately for Cs-137 and Cs-134 ataround 30 Bq g�1 in a CsCl carrier solution (0.05 mg g�1 in 0.1 NHCl). The container was placed on the top center of thealuminum end cap of the HPGe detector with an immovablejig to ensure repeatability of the sample position within 0.2 mmin the lateral direction and less than 0.1 mm in the verticaldirection. A layer of liquid paraffin over the solution controlledvariation in the amount and concentration of the radioactivewater. Because of this arrangement, the volume sourcesremained stable to within less than 0.01 g during at least severalmonths. The water-filled U8 container was bonded with struc-tural plastic adhesive (DP8005) at the thread part to preventleakage of the liquid paraffin.

The variations of the counting efficiency of the HPGe detectoralong with the sample height are shown in Fig. 4. This is obtainedby varying the volume of the radioactive water solution; the datapoints indicate the counting efficiency as determined by actualmeasurements of the water-filled U8 container. The lines indicatethe calculated counting efficiency as determined by the EGS5 codeand, for Cs-134, the author's programming method (Unno et al.,2012). The EGS5 code has an interface with a user code, which isacceptable for primary parameters to treat transports and inter-actions of beta and gamma rays in component materials. For theEGS5 code, we installed a program that enables a user to simulatesimultaneous emission of gamma rays along with input of nucleardecay data [for this work, data from Table of Isotope, 8th edition(Firestone et al., 1996)]. This simultaneous emission program wasapplied to Cs-134 in the present study. The calculated resultsof the simulation agreed with the actual measurement results atall the sample heights within 3%. This result also reveals acontinuous dependency between the sample height and thecounting efficiency. The calculation was validated by a comparisonwith the measurement, which can be applied to estimate thedependency on the sample height. The calculation technique wasalso used to estimate the self-absorption correction and wasperformed using a different density and composition within thesame parameters for all other conditions. The self-absorptiondifferences between water (1.00 g cm�3) and rice (0.94 g cm�3)

were estimated as 0.987 for 661.7 keV photons of Cs-137, and0.979 and 0.978 for 604.7 keV and 795.9þ802.0 keV of Cs-134,respectively.

Uncertainties related to the calibration of the HPGe detectorwere estimated as 1.82% and 1.57% for Cs-137 and Cs-134,respectively (coverage factor k¼1). This includes the relevantuncertainties of (a) the visual check of the height of the radioactivewater solution; (b) calibration of radioactivity using the metrolo-gical standard from NMIJ (ion chamber); and (c) variability in thepreparation of the water-filled U8 container. Uncertainty related tothe stability of the HPGe was 0.1% during the estimation derivedfrom a check by another source of the U8 container that was filledwith uniform radioactive alumina powder.

We measured a randomly chosen sample to estimate thecertified value of the parent population of the brown rice in theU8 container and the dispersion among the filled U8 containers.The measurement took into account the non-uniformity of theparent population of the brown rice. Statistical uncertainty cannotbe ruled out, especially in the case of the present work. Thisuncertainty is due to the following: (a) the period of measurementin which the certification had to be completed was limited toaround 2 months before the end of August 2012, when the peak ofrice harvesting began; and (b) the radioactivity concentration ofthe brown rice was at a slightly low level, in compliance with ajudgment criterion for shipment as dictated by the governmentof Japan.

3. Results and discussion

The results of measurements of 12 bottles of brown rice in U8containers indicated that the radioactivity concentration could becertified as 33.672.4 Bq kg�1 of Cs-134 and 51.874.3 Bq kg�1 ofCs-137 at 0:00 UTC on August 1, 2012 (coverage factor k¼2).Supplemental information revealed a concentration of 72 Bq kg�1

of K-40, even though with poor counting statistics. The efficiencyfor K-40 was obtained by Monte Carlo simulation with the samemodel. It took 50,000 s per bottle to acquire approximately 7000detected net counts for a full energy peak of 661.7 keV of Cs-137. InFig. 5, we show one of the resulting gamma-ray spectra. Fullenergy peaks of Cs-134, Cs-137, K-40, and other naturally occur-ring radioisotopes were found. In Table 1, we show the uncertaintybudget of the certified value. The total uncertainties were domi-nated mainly by (a) the statistics of the net count; (b) thecalibration of the HPGe detector; (c) the sample height in the U8container; and (d) the standard deviation of the between-bottleinhomogeneity.

10 20 30 40 50

2

4

6

8 661.7 keV 604.7 keV 795.9+802.0 keV

Cou

ntin

g ef

ficie

ncy

[%]

Sample height [mm]

Fig. 4. Variation of the counting efficiency of full energy peaks with the height ofthe radioactive water solution (dotted line: calculated for 661.7 keV, dashed line:for 604.7 keV, solid line: for 795.9þ802 keV).

40002000010-4

10-3

10-2

10-1

K-40

Cs-134 (795.9+802.0 keV)

Cs-134 (604.7 keV)

Cou

nt p

er se

cond

ch

Cs-137

Fig. 5. Gamma-ray spectrum obtained by the HPGe detector for a U8 containerfilled with brown rice (acquisition time: 50,000 s).

Y. Unno et al. / Applied Radiation and Isotopes 87 (2014) 485–488 487

4. Conclusion

The aim of this study was to develop a method to check andstrengthen the measurement quality of radioactivity investigation,especially related to the Fukushima accident. The CRM can beutilized to validate any gamma-ray spectrometry measurementmethod and by any personnel. It can also be used to validate othermethods and instruments. It is particularly effective for the currentsituation in Japan, where many types of instruments for radio-activity investigation are available on the market. The CRM canalso be used to help develop a new kind of analysis of radioactivityin the future. In fact, more than 100 of the U8 containers that werefilled with the brown rice have already been distributed during theinitial harvesting and later on after the accident at the FukushimaDaiichi Nuclear Power Plant. The U8 containers were widely usedin many institutions in Japan to determine radioactivity in naturalsamples, including foods. Our work is aimed at supportingexcellent radioactivity measurement and the systematic utilizationof measurement results.

For example, we organized a circular proficiency check for a 2 LMarinelli beaker, which is also widely used for more rapidinvestigation, some of which contain brown rice derived fromthe same batch as the CRM. These beakers were distributed tomore than 40 institutions to confirm the validity of their methodsbased upon their own judgment criteria. We plan to offer aproficiency test this year using another batch of brown rice, which

will be prepared using the same method with the current devel-opment of the CRM, and will have an unknown radioactivityconcentration.

In this work, we describe methods to estimate uniformity andstability of candidate samples for CRM, and certify the radio-activity concentration. The present work shows that it is possibleto realize sufficient uniformity even with rice grains. Brown rice ismore advantageous in terms of long-term stability. If more timewas needed to acquire the net count using the HPGe detector, theuncertainties of inhomogeneity between the bottles might besmaller than is shown in Table 1. The same holds true for thecase of a higher level of radioactivity concentration.

Acknowledgments

We would like to express our sincere thanks to our colleaguesin NMIJ and NFRI. Their professional advice and cooperation wereexcellent and improved the quality of this work. We would alsolike to express our deep respect for those individuals who carriedout work involving social contributions during this emergency. Wepray for recovery from the severe damage caused by this disasteras soon as possible.

References

Firestone, R.B., Shirley, V.S.S., Baglin, C.M., Zipkin, J., Chu, S.Y.F., 1996. Table ofIsotopes, Eighth Edition.

Government of Japan, 2011. Report of the Japanese Government to the IAEAMinisterial Conference on Nuclear Safety – The Accident at TEPCO's FukushimaNuclear Power Stations.

Hirayama, H., Namito, Y., Bielajew, A.F., Wilderman, S.J., Nelson, W.R., 2005. TheEGS5 Code System SLAC-R-730 and KEK Report 2005-8.

ISO, 2006. ISO Guide 35:2006, Reference materials – General and statisticalPrinciples for Certification. International Organization for Standardization,Geneva.

MEXT, 2012. Results of the Research on Distribution of Radioactive SubstancesDischarged by the Accident at TEPCO's Fukushima Dai-ichi NPP, Ministry ofEducation, Culture, Sports, Science and Technology in Japan.

Tsukada, H., Hasegawa, H., Hisamatsu, S., Yamasaki, S., 2002. Rice uptake anddistributions of radioactive 137Cs, stable 133Cs and K from soil. Environ. Pollut.117, 403–409.

Unno, Y., Sanami, T., Hagiwara, M, Sasaki, S., 2012. Calculation of detectionefficiency for multi cascade nuclides using EGS5 code. In: Proceedings of theNineteenth EGS Users' Meeting in Japan 10–15.

Yamada, T., Nakamura, Y., 2005. Examination of the U8 type polypropylenecontainer used for radioactivity standard volume sources. Radioisotopes 54,105–110.

Table 1Uncertainty budget for the radioactivity concentration of the brown rice in the U8container as a certified reference material.

Component Cs-134 Cs-137

U [%] (k¼1) U [%] (k¼1)

Measurement Statistics of net count 1.94 1.27Back ground 0.32 0.06Calibration 1.57 1.82Stability of detector 0.25 0.13Decay 0.0014 0.0004Timer 0.10 0.10Sample height 1.48 1.70Self-absorption 0.52 0.14

Between-bottle inhomogeneity 1.87 3.08Total (k¼2) 7.0 8.3

Y. Unno et al. / Applied Radiation and Isotopes 87 (2014) 485–488488