radiation detection

RADIATION DETECTORS AND THEIR USES

Proceedings of the 26th Workshop on Radiation Detectors and Their Uses

January 24-26, 2012 High Energy Accelerator Research Organization (KEK),

Tsukuba, Ibaraki JAPAN

Edited by S. Sasaki, Y. Kishimoto, M. Hagiwara, T. Sanami

K. Saito, K. Iijima and H. Tawara High Energy Accelerator Research Organization

FOREWORD

The 26th workshop on "Radiation Detectors and Their Uses" was held on

January 24, 25 and 26, 2012 at the High Energy Accelerator Research Organization

(KEK) in Tsukuba, Ibaraki, Japan. The workshop was hosted by the Radiation Science

Center, KEK under the cooperation of the Society of Radiation Science, the affiliate of

Japan Society of Applied Physics. The workshop offers an outstanding opportunity for

scientists interested in the fields of radiation physics, radiation detector, radiation

measurement, nuclear science, high energy physics and their application to meet and

discuss with colleagues from all over the country.

The number of the participants who registered to the workshop was 102. 48

presentations were given at the workshop. This workshop was the first one after the

accident of Tokai Daiichi Nuclear Plants and there were many presentations related to

measurements of radio-activities released from the nuclear plants as well as general

themes. As the fruits of the workshop, this report was published as the proceedings of

"the 26th Workshop on Radiation Detectors and Their Uses". All papers submitted for

publication in the proceedings received the peer review process by the independent

reviewers. Finally, the 18 original papers were published in the proceedings after the

review process.

The editors would like to express our great appreciation to the authors who

prepared the manuscripts in good earnest and the reviewers who spent their precious

time to check the papers.

November, 2012

Shinichi Sasaki

Workshop Program Chair

High energy Accelerator Research Organization (KEK)

Editors of the proceedings;

Yuji Kishimoto (KEK)

Masayuki Hagiwara (KEK)

Toshiya Sanami (KEK)

Kiwamu Saito (KEK)

Kazuhiko Iijima (KEK)

Hiroko Tawara (KEK)

Shinichi Sasaki (KEK)

TABLE OF CONTENTS RESPONSE OF PS-TEPC FILLED WITH PROPANE BASED TISSUE-EQUIVALENT GAS TO PROTON BEAM WITH ENERGY OF 230 MEV 1

Y. Kishimoto, S. Sasaki, K. Saito, K. Takahashi, H. Tawara, K. Terasawa, T. Fuse, A. Nagamatsu, Y. Ito, H. Matsumoto, K. Mori, K. Miuchi, T. Tanimori, Y. Uchihori, T. Doke

MEASUREMENT OF AIRBORNE RADIOACTIVITY FROM THE FUKUSHIMA REACTOR ACCIDENT IN TOKUSHIMA, JAPAN 8

K. Fushimi, S. Nakayama, M. Sakama, Y. Sakaguchi DEVELOPMENT OF A SIMPLE RADIOACTIVITY MEASUREMENT METHOD USING A PORTABLE DOSE RATE METER 17

E. Matsumoto, H. Ito, K. Tominaga, T. Saze, S. Nakayama, M. Sakama, Y. Kuwahara FIRST DEMONSTRATION OF COSMIC RAY MUON RADIOGRAPHY OF REACTOR CORES WITH NUCLEAR EMULSIONS BASED ON AN AUTOMATED HIGH-SPEED SCANNING TECHNOLOGY

27 K. Morishima, N. Naganawa, T. Nakano, M. Nakamura, J. Kawarabayashi, H. Tomita, T. Iguchi, S. Maeda

MEASUREMENTS OF GASEOUS TRITIUM USING MAGNETICALLY-LEVITATED ELECTRODE IONIZATION CHAMBER 37

H. Ichiki, H. Higaki, M. Haruyama, K. Ishibashi, N. Ikeda, S. Fukada, K. Futagami, T. Kawaguchi DESIGN OF A NON-CONTACTING SYSTEM TO MEASURE THE RADIOTRACER CONCENTRATION IN BLOOD FOR NUCLEAR MEDICINE APPLICATIONS 43

A. H. Malik, Y. Nakamura, K. Shimazoe, H. Takahashi STUDY ON BACKGROUND GAMMA-RAY REDUCTION USING A COMPTON BACKSCATTERING COINCIDENCE TECHNIQUE 49

T. Takagi, K. Watanabe, A. Yamazaki, A. Uritani STUDY OF A CdTe PIXEL SENSOR FOR SYNCHROTRON RADIATION EXPERIMENTS 59

H. Toyokawa, T. Hirono, S. Wu, M. Kawase, Y. Furukawa, T. Ohata RESPONSE CHARACTERISTICS OF TES MICROCLORIMETERS WITH DIFFERENT ABSORBER STRUCTURE 66

M. Maeda, T. Yasumune, N. Iyomoto, K. Maehata, T. Hara, K. Mitsuda, N. Yamasaki, K. Tanaka, Y. Yamanaka, T. Ito

DEVELOPMENT OF A TIME OVER THRESHOLD BASED PET DETECTOR 74

K. Shimazoe, H. Takahashi, K. Kamada, Y. Usuki, M. Yoshino, J. Kataoka, T. Yanagida, A. Yoshikawa, K. Kumagai

i

DEVELOPMENT OF LOW TEMPERATURE OPERATING INTEGRATED ELECTRONIC CIRCUITS FOR LIQUID XENON TPC 82

Y. Takagi, T. Tauchi, T. Haruyama, S. Tanaka, S. Mihara, K. Kasami, S. Nakamura, T. Nagoshi, A. Maki, S. Suzuki, T. Chiba, M. Tanaka, H. Ikeda, T. Husayasu, M. Kumada

TRANSMISSION CHARGED-PARTICLE DETECTORS USING CVD DIAMOND THIN FILM FOR SINGLE ION HIT MEASUREMENT 89

W. Kada, T. Satoh, T. Makino, S. Onoda, M. Koka, T. Ohshima, T. Kamiya,,V. Grilj, N. Skukan, M. Jaksic

CHARACTERISTICS OF CHARGE AMPLIFICATION IN A PROPORTIONAL COUNTER FILLED WITH HIGH-PRESSURE XENON 99

H. Kusano, M. Miyajima, N. Hasebe DEVELOPMENT OF SIMPLE NEUTRON COUNTERS 109

S. Satoh, K. Hirota DEVELOPMENT OF FUSION NEUTRON IMAGING BASED ON EMULISON CLOUD CHAMBER 117

H. Minato, J. Kawarabayashi, T. Iguchi, H. Tomita, K. Morishima, T. Nakano, M. Nakamura, M. Isobe

THE PERFORMANCE OF THE THIN NaI(Tl) DETECTOR PICO-LON FOR DARK MATTER SEARCH 122

K. Harada, K. Fushimi, S. Nakayama, R. Orito, S. Iida, S. Ito, H. Ejiri, T. Shima, R. Hazama, E. Matsumoto, H. Ito, K. Imagawa, K. Yasuda

SCINTILLATION RESPONSE OF YAP:Yb CRYSTAL TO RAYS IN CRYOGENIC TEMPERATURES 128

T. Yasumune, K. Maehata, M. Kurihara, N. Iyomoto, K. Ishibashi, A. Yoshikawa A PROBLEM OF ENERGY RESOLUTION IN LIQUID RARE-GAS FILLED IONIZATION CHAMBERS - WHY THE THEORETICAL ENERGY RESOLUTION WAS NOT ACHIEVED - 134

E. Shibamura

ii

RESPONSE OF PS-TEPC FILLED WITH PROPANE BASED

TISSUE-EQUIVALENT GAS TO PROTON BEAM WITH

ENERGY OF 230 MEV

Y. Kishimoto, S. Sasaki, K. Saito, K. Takahashi and H. Tawara High-Energy Accelerator Research Organization (KEK)

Oho 1-1, Tsukuba, Ibaraki 305-0801 Japan

K. Terasawa

Keio University Hiyoshi 4-1-1, Kohoku-ku Yokohama, Kanagawa,

223-8521, Japan Japan Aerospace Exploration Agency (JAXA)

Sengen 2-1-1, Tsukuba, Ibaraki, 305-8505, Japan

T.Fuse, A. Nagamatsu, Y. Ito, H. Matsumoto and K. Mori

Japan Aerospace Exploration Agency (JAXA) Sengen 2-1-1, Tsukuba, Ibaraki, 305-8505, Japan

K. Miuchi

Kobe University Rokkodai 1-1, Nada-ku, Kobe 657-8501, Japan

T. Tanimori

Kyoto University Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

Y. Uchihori

National Institute of Radiological Sciences Anagawa 4-9-1, Inage-ku Chiba, Chiba, 263-8555, Japan

T. Doke

Waseda University Kikui-cho 17, Shinjuku-ku, Tokyo, 162-0044, Japan

Japan Aerospace Exploration Agency (JAXA) Sengen 2-1-1, Tsukuba, Ibaraki, 305-8505, Japan

1

1. INTRODUCTION

Management of radiation exposure is very important for astronauts, because the radiation dose expected in spacecraft is higher by two orders of magnitude than that on the ground. In spacecraft, the main component contributing to the radiation dose is charged particles and neutrons generated by nuclear reactions between charged particles and materials composed of spacecraft (1) (2). The radiation effects on human body in spacecraft are evaluated using dose equivalent H described by the following equation:

)( LQDH ,

where D is the absorbed dose; L is the Linear Energy Transfer (LET); Q is the quality factor given as a function of L. The accuracy of H is important for the dosimeters used in spacecraft and the National Council of Radiological Protection Report No. 142 (NCRP-142) recommends the measurement accuracy of 30% or better (3).

The TEPC, DOSTEL, RRMD and BBND are known as active dosimeters developed to measure radiation dose in spacecraft (4) (5) (6) (7). However, there is no dosimeter which has enough measurement accuracy for both of charged particles and neutrons. Thus, it is required for spacecraft to equip two or more kinds of dosimeters for complete measurement of radiation dose. This requirement possibly becomes great disadvantage because smaller and lighter instruments are more important for on-board equipment of spacecraft. Therefore, we have been developing a dosimeter called Position-Sensitive Tissue Equivalent Proportional Chamber (PS-TEPC) to realize a dosimeter which has enough accuracy of measurement and sensitivity for the both of charged particles and neutrons.

2. INSTRUMENT

The PS-TEPC is a position sensitive gaseous proportional chamber utilizing a -PIC. The schematic view of the detector is shown as Figure 1. The PS-TEPC consists of a drift plane, shaping frames and a -PIC. The drift plane and shaping frames form an electric field to drift the electrons generated by interactions between the gas and incident particles toward the -PIC. The -PIC acts as an electron multiplier and the schematic view is shown in Figure 2 (8). We use a compact size -PIC with 64 strips

64 strips developed for the PS-TEPC. Owing to the anode strips and the cathode strips, which are perpendicular to the anode strip, of -PIC, two-dimensional position can be acquired. Meanwhile, the Corresponding author; (tel)029-864-5489, (e-mail)[email protected]

2

position along to the drift direction can be obtained by measuring the duration for which primary electrons reach the -PIC. Thus, the PS-TEPC has an ability to reconstruct three-dimensional tracks and can acquire the path length of incident particles. Because each pixel of -PIC functions like a proportional counter, the energy deposit due to incident particles also can be obtained from the amount of collected charge. Therefore, LET of incident particle can be acquired by one event. The shaping frames are made of tissue equivalent plastic A-150. Measuring the protons generated from interactions between neutrons and the plastic, the dose equivalent for neutrons can be estimated.

The basic demonstration of the measurement principle has been done using the prototype version of PS-TEPC filled with P10 gas (9). Meanwhile, we intend to apply Tissue-Equivalent gas (TE gas) for the PS-TEPC finally. Therefore, we are now investigating the specification of the PS-TEPC filled with propane-based TE gas.

3. MEASUREMENT

The particles with the LET from 0.1 to 500 keV/m-water mainly contribute dose equivalent in the spacecraft (10). Thus, this LET range is the target range of PS-TEPC to measure. At the lowest LET of the target range, the signal from the detector is faint because the number of primary electrons is small. Therefore, we performed a beam experiment to irradiate the PS-TEPC with a beam of proton with the energy of 230 MeV. Its LET corresponds to 0.2 keV/m-water and the beam is suitable to demonstrate the ability to detect the charged particles with approximately lowest LET in our target range. The experiment was performed at the Heavy Ion Medical Accelerators in Chiba (HIMAC) and we used the PS-TEPC filled with propane-based TE gas (C3H8:54.8%, CO2:39.7%, N2:5.5%) at 1atm in this experiment. The effective size of the -PIC used in this experiment was 2.6 cm 2.6 cm and the gap between the drift

Fig. 1 The Schematic view of the PS-TEPC Fig. 2 The schematic view of the -PIC

3

plane and the -PIC was 5.0 cm. A plastic scintillator with 0.2 cm thickness and another one with 1 cm thickness were placed backward and forward to the PS-TEPC, respectively. The distance from the beam window to the front scintillator, that from the front scintillator to the detector and that from the detector to the backward scintillator were about 0.2, 1 and 1.5 m, respectively. The atmosphere was air at 1 atm and the beam window was 0.1 mm thickness of Al. The data were acquired by the coincidence trigger signal of the plastic scintillators. The applied voltage to the -PIC and the drift plane were 1010 V and -3300 V, respectively. In this case, the drift velocity corresponds to 0.77 cm/sec. The diameter of the beam was about 1 cm and the incident direction was parallel to the cathode strips. This direction seems to minimize the amount of the charge collected by an anode strip and it is the most difficult situation for detection of the proton signal. The signal of each strip was summed with every 4 strips and amplified by charge sensitive pre-amplifier CP3578. Then the 32 output signals (16 anode channels and 16 cathode channels) from it were fed to the multi-channel digitizer CAEN V1720 and the digitized data were read out by a PC. The beam irradiation was performed for about 4 minutes and the data of 3000 events were obtained.

4. RESULTS and DISCUSSIONS

At first, we analyzed the pulse height of each anode strip to demonstrate the ability to detect the signals originated by protons. Figure 3 shows a signal waveform of an anode strip obtained in this experiment. We defined the pulse height as the difference between the maximum ADC channel and the baseline. Figure 4 shows the distribution of the pulse height of a typical anode strip. In this figure, the horizontal axis shows the pulse height and the vertical axis shows the counts. It is recognized that the noise component distributed narrowly around 0 channel is clearly distinguished from the signal component distributed around several hundred channel. In the same manner, the distributions of all anode strips were analyzed as shown in Figure 5. It shows that clear discrimination has been successfully done for all strips. It is also shown that the distributions of ch00 and ch15, which are located at the edges of the -PIC, are different from those of other channels. The reason seems that these strips also collect the charge which was generated outside the fiducial volume due to the electric field formed by the anode strips.

Fig. 3 A signal waveform of an anode strip. Fig. 4 The distribution of the pulse height of a typical anode strip.

4

Secondary, we reconstructed the track image using the same data and the result is shown in Figure 6.

The tracks of 20 events are superimposed in this figure. Here, the points show the track points calculated from the drift duration and the position of the hit strips, which the pulse heights of them are higher than the threshold. The arrow shows the incident position and direction of the beam determined by the geometry of the experimental setup. The directions of reconstructed tracks align along the beam direction and the

positions of them well correspond to the incident position. Then, we analyzed the length of each track and

Fig. 5 The distributions of the pulse height of all anode strips.

Fig. 6 The track image of the

proton beam.

Fig. 7 The distribution of the track length.

5

investigated its distribution. The result is shown as Figure 7. A very sharp peak around about 2.5 cm is shown in this histogram and it is consistent with the horizontal size of the fiducial volume determined by the size of the -PIC.

Finally, we analyzed the amount of collected charge to evaluate the energy resolution. We converted the pulse height of each anode strip into collected charge using the calibration data obtained from pre-amplifier calibration and then calculated the total charge by summing them. Figure 8 shows the distribution of the total charge. We fitted the distribution with a Gaussian and the solid line shows the best-fit curve. From the best-fit parameters, we obtained the charge resolution of 25.9% (1). Because amount of charge generated in the gas is proportional to the energy deposition, the resolution of charge can be considered as energy resolution. Considering energy resolution of proportional counter becomes better for higher energy deposition and the LET of the irradiating beam approximately corresponds to the lowest LET in our target range, it is suspected that the PS-TEPC has better energy resolution than that of 30% inoverall target range.

5. SUMMARY

We have been developing a position-sensitive tissue equivalent proportional chamber PS-TEPC as a dosimeter in spacecraft. We performed a beam experiment at HIMAC. Irradiating the PS-TEPC with proton beam with the energy of 230 MeV, it is recognized that the signal due to the incident proton has enough pulse height to be distinguished from that of noise. From the position information obtained by analyzing the same data, the track image which is consistent with the beam profile was obtained. Additionally, the distribution of the track length shows a sharp peak corresponding to the distance limited by the size of -PIC. From the distribution of the collected charge, the energy resolution was estimated to be 25.9% (1).

It is reported that collected charge in filling gas distorts due to the presence of impurities (11). The same effect might be occurred for our detector when the beam experiment performed. In this case, better

Fig. 8 The distribution of the total collected charge.

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energy resolution may be obtained if purity of the TE gas in our detector improves. Therefore, we plan to optimize our vacuum system and baking condition to improve the performance of our detector.

ACKNOWLEDGMENT

This study was carried out as a part of the Research Project with Heavy Ions at NIRS-HIMAC.

REFERENCES (1) J. E. Keith, et al., Nucl. Tracks Radiat. Meas., 20 (1992) 41. (2) V. E. Dudkin, et al., Nucl. Tracks Radiat. Meas., 17 (1990) 87. (3) NCRP Report No.142; National Council on Radiation Protection and Measurements (2002) 56. (4) P. Kliauga, et al., Raidat. Protec. Dosim., 61 (1995) 309. (5) R. Beaujean, et al., Radiat. Protec. Dosim., 85 (1999) 223. (6) T. Doke, et al., Radiat. Meas., 33 (2001) 373. (7) H. Matsumoto, et al., Radiat. Meas., 33 (2001) 321. (8) H. Kubo, et al., Nucl. Instr. and Meth., A513 (2003) 94. (9) T. Nagayoshi, et al., Nucl. Instr. and Meth., A581 (2007) 110. (10) T. Sakaguchi, et al., Nucl. Instr. and Meth., A437 (1999) 75. (11) T. Kohno, et al., Nucl. Instr. and Meth., A450 (2000) 456.

7

Measurement of airborne radioactivity from the Fukushima reactoraccident in Tokushima, Japan

K.Fushimi, S.Nakayama, M.Sakama2 and Y.Sakaguchi3

Institute of Socio Arts and Sciences , The University of Tokushima, 1-1 MinamiJosanjimacho Tokushima city, 770-8502 Tokushima, JAPAN2 Department of Radiological Science, Division of Biomedical Sciences, Institute ofHealth Biosciences, The University of Tokushima, 3-18-15 Kuramotocho Tokushima city,770-8509 Tokushima, JAPAN3 Faculty of Integrated Arts and Sciences, The University of Tokushima, 1-1 MinamiJosanjimacho Tokushima city, 770-8502 Tokushima, JAPAN

Abstract

The airborne radioactive isotopes from the Fukushima Daiichi nuclear plant wasmeasured in Tokushima, western Japan. The continuous monitoring has been carriedout in Tokushima. From March 23, 2011 the ssion product 131I was observed. Theradioisotopes 134Cs and 137Cs were also observed in the beginning of April. Howeverthe densities were extremely smaller than the Japanese regulation of radioisotopes.

1 Introduction

Serious damage to the Fukushima Daiichi nuclear power plant (14127E, 3745N) hasbeen caused by huge tsunami followed by the huge earthquake on 11 March 2011. Theplants 1,2 and 3 were operating and the plant 4 was stopped before the earthquake. Theplants made emergency stop just after the earthquake, however, all the power plants inFukushima Daiichi were seriously damaged by the following big tsunami. All the electricpower got fault and the cooling system was collapsed. From 12 March 2012, a largeamount of radioactive materials was vented to avoid more serious damages. Total amountof vented radioactive isotopes were estimated as 1.51017Bq for 131I and 1.21016Bq for137Cs [1].

The largest ejection of radioactivity from the plants occurred on 15th March and theamount of ejected radioactivity decreased after 17 March[1]. During 15 and 16 March,the wind direction changed from north and south, the wind direction raised the seriouspollution in Iidate village and north Kanto district. After 16 March, the wind directionchanged to west and continued for a few days. The westly wind brought the radioactiveisotopes to the Northern Hemisphere.

In the present paper, the measurement of airborne radioactive isotopes in Tokushimawhich is placed in western district of Japan is reported. The arrival date of radioactivity inthe world was analyzed to investigate the behavior of plume exhausted from the reactor.The precise information for the detection eciency of gamma ray was determined toanalyze the radioactive isotopes. The detection eciency of gamma rays were preciselyestimated by Monte Carlo simulation. The coincidence eect of the detection eciency forgamma rays which are emitted through cascade transition was appropriately simulated.

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Measurement of airborne radioactivity from the Fukushima reactor accident in Tokushima, Japan

2 Sampling and measurement

The sampling of airborne radioactivity was started on 18 March, 2011, seven days afterthe Great East Japan Earthquake. The sampling site was placed at the top of the buildingof the University of Tokushima, placed at 13433E longitude, 344N latitude and 15maltitude, about 700km southwest of Fukushima. The airborne radioactive isotopes werecollected by a high volume air sampler HVC-1000N provided by SIBATA whose samplingrate was 1m3/min. The lter for sampling was a commercial glass lter GB-100R providedby ADVANTEC with the dimension of 203mm254mm. The eciency for retaining par-ticles with a size of 0.3m is 99.88%. The sampling was started at 12:00 and continued23 hours.

The lter was striped into 1cm width bands and contained into a sample containermade of polycarbonate. A sample container was placed in front of the end cap of a HPGedetector. The distance between the end cap and the sample container was 0.3cm. Fig.1shows the Ge detector system, whose shield was opened. The Ge detector and the sample

Figure 1: Ge detector system. The front shield was opened in this picture. A samplecontainer was put on a foam polystyrene mount.

container was covered with 1cm thick OFHC (Oxygen Free High Conductive) copper platesand 10cm thick lead bricks. The total gamma ray background was reduced three ordersof magnitude by the shield. The signal from pre-amplier was shaped by the shapingamplier ORTEC 571. The pulse height was digitized by a multichannel analyzer. Theenergy spectrum was stored into a hard disk every two hours and the data taking wascontinued for 24 hours after the end of sampling. Since about 12 hours from the end ofsampling, the background events is dominated by the ones from the progeny of Rn, thedata for the present work were taken between 16 hours and 24 hours after the end ofsampling.

The Rn density was used for the check of the sampling eciency of aerosol. The annualmodulation of Rn density in Tokushima has been measured for 17 years. The density ofRn does not change every year in the same season[2].

The signicant ssion products and activation products were measured from 23 March,

9

2011. The energy spectrum was shown in Fig.2. After 23 March, clear peaks due to 131I,134Cs and 137Cs were observed.

ENERGY(keV)300 400 500 600 700 800

CO

UN

TS

0

100

200

300

400

500

600

700

800

900

I-131

Cs-134

Cs-137

Cs-134

Figure 2: The energy spectra of gamma rays which was emitted by radioactive isotopesattached to a lter. The red line is the energy spectrum taken on 6 April 2011. The blackline is the one taken before the Great East Japan Earthquake.

3 Monte Carlo simulation to determine the eciency

The detection eciency for gamma rays from 134Cs and 131I must be carefully estimated.The detection eciency is distorted by coincidence of some gamma rays which are emittedby cascade decay. For example, the 604keV gamma ray from the excited state of 134Ba isaccompanied by the other gamma rays. Consequently, the detection eciency is distortedby the coincidence with other gamma rays. The eciency distortion depends on thegeometrical distribution of the source and the detector. However, the correction of thedistortion is rather dicult because the geometrical arrangement may change by eachmeasurement.

The Monte Carlo simulation is the good tool to determine the detection eciency fora complex geometrical arrangement. In the present work, Geant4.9.4.p02 was used todetermine the eciency. Geant4 [3] is the simulation tool kit to simulate the trans-portation of gamma ray, beta ray and other ionizing radiation particles. The classG4RadioactiveDecay in Geant4.9.4.p02 generates the radioactive decays of almost allthe nuclei. The properties of unstable nuclei, half life, decay mode, excited state, branch-ing ratio are listed in the class. The cascade emission of gamma rays is properly simulatedby the G4RadioactiveDecay class.

To verify the simulation, a simulation and a practical measurement were performed.The measurement was performed by using the IAEA-444 standard source [4]. The stan-dard source was contained in a U-8 pack, whose dimension was 50.4mm60.2mm. Theradioactive sources were uniformly mixed into soil. The U-8 pack was put in front of theend cap of a HPGe detector. The simulation was performed with the same dimension.The energy dependence of the detection eciency which were derived by experiment and

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simulation was plotted in Fig.3. The Monte Carlo simulation well agreed with the ex-

Figure 3: The detection eciency of gamma rays. Box: Experimental result. Circle:Simulated result. Experimental result for 569keV and 563keV could not dissolved so thatthe eciency was calculated as 569keV gamma ray.

perimental result. Especially, the detection eciency of 604keV gamma ray was lowerthan the energy dependence of eciency which was tted by polynomial function. Wedetermined the detection eciency for the various shapes of sample containers by MonteCarlo simulation.

4 Analysis

4.1 Correction of radioactive decay

From the peak yields measured by Ge detector, the density of radioactivity was calculated.The eect of the radioactive decay is signicant for calculating the proper radioactivity ofshort lived nuclei, for example, 131I. The half life of 131I is so short as 8.04 day that thedecay between the sampling was not negligible. The amount of attached nuclei on a lterA is corrected by means of following procedure[6].

First, the total number of attached nuclei AT is expressed as

AT = V TSg, (1)

where , V , TS and g are the density of the nucleus in air, the volume of sampled air perunit time, the sampling time and the collection eciency of lter, respectively. Duringthe sampling, the number of nuclei on the lter A(t) increases as P = V g. The numberof the nuclei decreases by radioactive decay, thus the equation is

dA(t)

dt= P A(t). (2)

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Solving the equation, one gets the actual number of nuclei attached on the lter at theend of sampling, say AT as,

AT = A(TS)TS

1 exp(TS) , (3)

where, = ln 2/T1/2 is decay constant and T1/2 is half-life.After sampling shed, the nuclei decays with the decay constant , thus the number

of the nuclei when the beginning of measurement is

A(TC) = A(TS) exp(TC), (4)where TC is the time interval between the ending of sampling and the starting of measure-ment.

The radioactive decay should be considered if the half-life is short. The number ofdecay N between the measurement time TM is expressed as

N = N(TC)N(TC + TM )= N(TC) {1 exp(TM )} . (5)

The present measurement, the peak yield Y acquired by Ge detector is expressed as ,

Y = aN, (6)

where and a are the detection eciency of gamma ray and the emitting ratio of thegamma ray.

The important parameters for the present measurement are listed in Table 1.

Table 1: The parameters of the present measurement.

Parameters Value

Sampling time TS [hour] 23Time interval TC [hour] 16Measurement time TM [hour] 8Decay constant of 131I [hour1] 3.592 103

4.2 Eciency of lter

The commercial lter GB-100R provided by ADVANTEC was used for sampling the air-borne radioactivity. The lter eciency of aerosol whose diameter is larger than 300nmis as large as 0.9999 [7]. The capture eciency of airborne radioactive isotope depends onthe chemical property.

4.2.1 Cesium

137Cs and 134Cs are exhausted from a reactor attached on aerosol. The size of aerosolwhich attaches cesium was measured as 830860nm [8]. The lter eciency for cesiumwas conrmed by measuring the density of 7Be. The size of aerosol containing 7Be is thesame as the one containing cesium. The density of 7Be in Tokushima has been measuredsince 2005. The density of 7Be in Tokushima was 26mBq/m3, which agrees the resultsmeasured at another sites [9]. The lter eciency of GB-100R for cesium isotopes isconsidered as g = 0.999.

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4.2.2 Iodine

The lter eciency g for iodine depends on the chemical structure of iodine. The gaseousiodine such as I2, HOI and CH3I cannot be collected by normal glass lter. Only a smallfraction of iodine ion and chemical compounds of iodine which are attached on aerosol iscached by our glass lter. The size of aerosol particle which iodine is attached is rathersmaller than the size of 137Cs, the average size is 590613nm [8]. The lter eciency foriodine attached on aerosol is treated as 0.999, however, the fraction of the particle iodinemust be considered.

The fraction of iodine forms was investigated by Noguchi [10]. The detail of the fractionis shown in technical note on the monitoring of airborne radioactivity[11]. The averagefraction of iodine attached on aerosol is 0.200.1. The total lter eciency is the productof 0.999 and the average fraction of particle iodine, resulting g = 0.2 0.1.

5 Results and Discussion

The daily variation of 131I densities is shown in Fig. 4. The error of each data is dominated

Date

3D

ensi

ty m

Bq/

m

0

2

4

6

8

10

12

14

16

18

20

Mar19Mar24

Mar29Apr3

Apr8Apr13

Apr18Apr23

Apr28

Figure 4: The daily variation in the density of 131I in air.

by the error of correction eciency g. The signicant radioactivity was observed after 23March. The behavior of cesium isotopes was the same as iodine, however, the concentrationof cesium isotopes was one order smaller than the one of iodine.

The maximum of the density was observed on 6 April, about three weeks after the largevent. This behavior cannot be explained that the radioactive plume came directly fromFukushima Daiichi Nuclear Plant. The dates which the maximum density was measuredin other sites have a strong correlation. The speed of the transportation of radioactiveplume is explained that westerly wind brought the radioactive plume. Fig.5 shows therelationship between the dates which the maximum radioactivity was observed in eachcites. The dates of each cites and their longitude have a strong correlation. The speed ofthe plume was calculated from the linear tting. The speed was 40km/sec which agreedthe speed of westerly wind.

In western side of American Continent (Seattle), the rst signicant observation wason 17 March and reached the maximum on 19 March[12]. In eastern side of American

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DATE Mar 1 =110 15 20 25 30 35 40 45

Long

itude

(Fuk

ushi

ma=

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Figure 5: The dates which the maximum density of radioactivity was observed. A:Seattle (USA)[12] B:Chapel Hill (USA)[13] C:Huebla (Spain)[14] D:Orsay (France)[15]E:Thessaloniki (Greece)[16] F:Tokushima (Japan, This work)

Continent (Chapel Hill), the rst observation was on 18 March, however, the maximumwas observed on 29 March. This discrepancy came from the rainfall on 20 March [13].They observed three peaks of density. The rst peak of density was observed on 25 March.After the rst peak, two peaks were observed on 30 March and 2 April. The density of therst peak was reduced by rainfall, so we considered that the plume arrived on 25 March.

The radioactive plume was brought to Europe by westerly wind. In western Spain(Huevla), the maximum was reported on 28 March[14]. However, the sampling was madeonly a few times, 15-17, 21-23 and 28-29 March. After 28-29, they continuously measuredtill 15-17 April. In France (Orsay), the continuous measurement was reported by IRSN(Institut de Radioprotection et de Surete Nucleaire)[15]. The rst signicant observationwas on 25 March and the maximum was observed on 31 March. In Greece (Thessaloniki),the rst signicant observation was on 26 March and the maximum was observed on 29[16].The relationship between the dates of maximum density and the longitude was well ttedby linear function.

To conrm the hypothesis that the radioactive plume which arrived on 6 April wentaround the Northern Hemisphere, further analysis was carry out. The isotopic componentin a plume from nuclear plant was dominated by cesium so that the ratio R 131 I/137Csis larger than 10 [11]. The ratio becomes smaller during the long travel by the decay of131I and by dissolving into rainwater. The ratio in Seattle was reported as large as 31,while, the ratios in Europe was between 10 to 4. The decrease in R is more rapid thanthe radioactive decay of the isotopes.

The eect on the isotopic ratio was clearly observed by Asian sites. In Taiwan, thevalue of R was dropped from 1 to 0.3 between the end of March and the beginning ofApril[17]. In Vietnam, the value R decreased exponentially[18] and the ratio was small.

The temporary decrease of R was clearly observed in Tokushima. The daily variationin the ratio R in Tokushima was analyzed as shown in Fig.6. In the beginning of April(1st April 2nd April), the value of R was large, which suggests the radioactive plumecame directly from Fukushima. During 3 to 7 in April, the ratio R temporarily decreased

14

Figure 6: The daily variation in the ratio of R 131 I/137Cs (Upper) and R 131 I/134Cs(Lower).

as R 4. From the isotopic component, the biggest peak around 6 April was caused bythe radioactive plume exhausted on 1215 in March and the plume traveled around theNorthern Hemisphere. In Vietnam, the altitude is so small that the radioactive plume didnot pass through there.

The concentration of measured radioactivity was about ve orders of magnitude smallerthan the regulation in Japan. The estimated dose was negligibly low expecting no healtheect in western Japan.

6 Acknowledgements

The authors thank the University of Tokushima for supporting the continuous measure-ment.

References

[1] TEPCO Web site. (http://www.tepco.co.jp/nu/fukushima-np/index-e.html)

15

[2] S.Miyamoto et al., Natural Science Research Faculty of Integrated Arts and Sciences,the University of Tokushima 13 (2000) 1.

[3] Geant4 Web site. (http://geant4.web.cern.ch/geant4/)

[4] Reference sheet of IAEA-444, Gamma emitting radionuclides in soil.

[5] National Nuclear Data Center (http://www.nndc.bnl.gov)

[6] JAEA-Technology 2010-039 (2010). (In Japanese)

[7] ADVANTEC Web site (http://www.advantec.co.jp/english/)

[8] A.Reineking et al., Radiation Protection Dosimetry 19 (1987) 159.

[9] M.Yamamoto et al., J.Env.Rad. 86 (2006) 110.

[10] H.Noguchi, The study on the change of property of radioactive iodine and tritiumin the environment, Thesis, Nagoya University (1991).

[11] Technical sheet on the monitoring of radioactive iodine.,(2003) Ministry of Education,Culture, Sports, Science and Technology-JAPAN. (In Japanese)

[12] J. Diaz et al., J.Env.Rad. 102 (2011) 1032.

[13] S.MacMullin et al., J.Env.Rad., in press.

[14] R.L.Lozano et al., Environment International, 37 (2011) 1259.

[15] Institut de Radioprotection et de Surete Nucl eaire Web site.(http://www.irsn.fr/EN/news/Pages/201103_seism-in-japan.aspx)

[16] M.Manolopoulou et al., J.Env.Rad. 102 (2011) 796.

[17] Chin-An Huh et al., Earth & Planet. Sci. Lett. 319-320 (2012) 9.

[18] N.Q.Long et al., J.Env.Rad., in press.

16

Development of a simple radioactivity measurement method

using a portable dose rate meter

Erika Matsumoto*, Hiroshi Ito, Koji Tominaga, HORIBA, Ltd.

2 Miyanohigashi, Kisshoin, Minami-ku, Kyoto 601-8510, JAPAN

Takuya Saze, Research Center of Radioisotope, the University of Tokushima,

3-18-15 Kuramoto-cyo, Tokushima 770-8503, JAPAN

Shintaro Nakayama, Faculty of Integrated Arts and Sciences, the University of Tokushima,

1-1 Minami Josanjima-cyo, Tokushima 770-8502, JAPAN

Minoru Sakama, Yoshinori Kuwahara, Department of Radiological Science, Division of Biomedical Information Sciences, Institute of Health

Biosciences, the University of Tokushima, 3-18-15 Kuramoto-cho, Tokushima 770-8509, JAPAN

1. INTRODUCTION

After the accident at Tokyo Electric Power Companys Fukushima Dai-ichi Nuclear Power Plant, there has been increasing demand for measurements of radioactivity contamination of food, soil, straw, etc. The germanium semiconductor detectors that are generally used for the measurement are expensive, few in number and require comparatively long measurement time. There is another method that uses a NaI (Tl) detector for the measurement, but the operation of the instrument is not simple. In this paper, we propose a new method for the estimation of the radioactivity concentration [Bq/kg] using a portable dose rate meter (HORIBA PA-1000 Radi) and a dedicated specimen container for this monitor (HORIBA PA-K). We have evaluated the performance of the method by using various specimens. Erika Matsumoto; (tel)075-313-8121, (fax)075-321-5648, (e-mail)[email protected]

17

2. EQUIPMENT AND EXPERIMENTS

2.1 Principle As long as the energy of gamma-rays and the geometrical configuration between a gamma-ray detector

and a specimen is same, the counting rate of the detector is proportional to the radioactivity in the specimen. Therefore the radioactivity of the specimen held in a fixed geometrical configuration to the detector can be obtained from the dose equivalent rate measured by the dose rate meter; gamma-ray dose rate meters generally converts the counting rate of gamma-rays to the dose equivalent rate. We have developed plastic holders to keep our dose rate meter and a specimen container to measure the radioactivity as simply as possible. If the contaminating nuclide is only 137Cs, it is possible to make a correlation line between the radioactivity and the dose rate by preparing several standard contamination specimens with a standard liquid solution of 137Cs. In the case of the contamination by the accidents at Fukushima, main contaminants are 131I, 134Cs and 137Cs. As the life time of 131I is short and has decayed already, we can limit 134Cs and 137Cs as main contamination sources in soil or food produced on land. The response of dose rate meters to the gamma-rays from 134Cs and 137Cs of the same radio activity are different. But because the difference of the dose equivalent rate from the same radioactivity of 134Cs and 137Cs can be calculated and the abundance of those nuclides are easily estimated, it is relatively easy to convert the correlation line for 137Cs to any combination of 134Cs and 137Cs.

2.2 Equipment 2.2.1 Portable dose rate meter: PA-1000 The detector used in this equipment is a CsI (Tl) scintillator coupled with a silicon PIN photo-diode. It

has a sensitivity of over 2,500 cpm per 1 Sv/h with the standard 137Cs source. The dose rate [Sv/h] is converted from the moving average of count rate [cpm] for 60 seconds and updated every 10 seconds. PA-1000 is calibrated using the standard 137Cs radiation source. The specifications are shown in Table 1.

Table 1: Specifications of PA-1000.

2.2.2 Container: Simple radioactivity measurement kit PA-K This kit consists of a stand which holds the portable dose rate meter PA-1000 upright, and a bowl

designed to contain the specimen. The bottom of the bowl has a concavity to fit PA-1000 in and to maintain the same geometry between the detector and the sample. The capacity of this bowl is 1 liter. The

CsI(Tl) crystal size 111119 mmEnergy Response 0.53.0 (1501250 keV)Sensitivity of gammer ray 2500 cpm/Sv/h (with Cs-137)Energy Resolution 8.5% (at 662 keV of Cs-137)Photodiode Silicon PIN Photodiode

18

specifications are listed in Table 2. The portable dose rate meter PA-1000 and the simple radioactivity measurement kit PA-K are shown in Fig. 1.

Table 2: Specifications of PA-K.

Fig. 1 Portable dose rate meter PA-1000 set in the simple radioactivity measurement kit PA-K.

2.2.3 Preparation of standard radioactive specimens When creating the correlation diagram of radioactive Cesium concentrations and measured values of the

portable dose rate meter, we used 137Cs and 134Cs standard nuclide solutions obtained from Japan Radioisotope Association and prepared standard radioactive specimens. The specifications of the standard nuclide solution are shown in the Table 3.

Table 3. Cesium standard nuclide solution.

We prepared standard radioactive specimens by adding these standard nuclide solutions to water, brown

rice, meat, and shiitake mushroom. The sample mass in 1 liter bowl and the concentration ranges of the specimens are shown in Table 4. We prepared the specimens of three to five different Cesium concentrations within these radioactive ranges by mixing certain amounts of solutions and specimens.

Table. 4. Specimen, sample mass in 1 liter bowl and concentration ranges of the standard radioactive test specimens.

Generally the counts of photoelectron absorption peak of the gamma ray in the spectrum are used to obtain the radioactivity. But PA-1000 counts all the signals including Compton scattering background. As long as the incident gamma ray spectrum is same, this count including Compton scattering constituent is

Code Number ConcentrationCs-134 CZ005 7.42x103 Bq/g2%Cs-137 CS010 9.51x104 Bq/g2%

Specimen Sample mass in1L bowl[g]Cs-137 concentration

range [Bq/kg]Cs-134+Cs-137

concentration range [Bq/kg] Notes

7810000 0.01%KCl solution 43580 0.8%KCl solution

Brown rice 930 921010 100500 Uncooked meat 930 1071071 Processed in blenderShiitake mushroom 860 86860 Processed in blender

1000Water

Material Vinyl ChlorideSize 200200 mmDetection Efficiency 0.9%Distance from CsI(Tl) Crystalcenter to Sample(Minimum) 20mm

Distance from CsI(Tl) Crystalcenter to Sample(Maximum) 93mm

Capacity of bowl 1 liter

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proportional to the count of photoelectron peak.

2.3 Experiments The measuring method is as described below.

1. After pouring water (1 liter) into the container, the measurement values were recorded five times at one-minute intervals (5 minutes in total). The average of these five values is considered to be the background value. Ideally, background measurement should be performed using the same material as the specimens. But we used water for background measurement because realistically it is difficult to find food which is confirmed not to be contaminated at all. 2. One liter of the specimen was put into the container, and the measurement values were recorded five times at one-minute intervals (5 minutes in total). The average value obtained from the procedure is considered to be the specimen value. It is expected that deviation becomes smaller as it takes longer to measure. But, the deviation does not significantly differ between 10 times measurements (600 data) and 5 times measurements (300 data). Therefore we chose 5 times measurements to improve time efficiency. 3. Net value was obtained by subtracting the background value from the specimen average value. This procedure was repeated for all the specimens. We made correlation lines between the net value and the concentration of the radioactive Cesium. The measuring time required for this method is about 15 minutes, including specimen preparation time,

which is much shorter than the time needed for the germanium semiconductor detector measurement, i.e. usually one to three hours.

3. Results

3.1 137Cs results The result for the standard water specimen is shown in Fig. 2 and the result for the brown rice specimen

is shown in Fig. 3. In both of these results, we obtained a linear relationship between the net value in Sv/h and concentration of radioactive 137Cs in Bq/kg. We repeated measurements 5 times using the same brown rice specimen to evaluate the measurement

error. The result shows about up to 30% measurement error. The measurement results of the standard radioactive specimens of water, brown rice, meat and shiitake

mushroom are shown in Fig. 4. We obtained a linear relationship with the same gradient for all the specimens we measured. Therefore, it can be said that ingredients of the specimen do not have an significant effect on the linear relationship.

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Fig. 2 Results for measurement of standard radioactive water specimen. X-axis shows the concentration of the standard solution of 137Cs, while Y-axis shows the net value after subtracting the background. The right graph is with 10 times expanded scale of low concentration area of the left graph.

Fig. 3 Results for measurement of standard radioactive brown rice specimen. X-axis shows the concentration of the standard solution of 137Cs, while Y-axis shows the net value after subtracting the background.

Fig. 4 Results of measurement of standard radioactive water, brown rice, uncooked meat and shiitake mushroom specimen. X-axis shows the concentration of the standard solution of 137Cs, while Y-axis shows the net value after subtracting the background.

Standard radioactive brown rice specimen

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Here, we should discuss the effect of 40K with 1461 keV on the measurement. Many foods contain 40K, but the energy response of PA-1000 in over 1250 keV is very low. We examined the influence of 40K by measuring dried kelp and dried shiitake mushroom, which are rich in 40K. Table 5 shows the net value measured using PA-K, and radioactive concentration of Cesium and 40K measured by the germanium semiconductor detector. From the result, it can be said that the gamma rays of 1461 keV emitted by 40K have little effect on the measurement in the condition that 40K concentration is less than about 1600 Bq/kg.

Table. 5 Measurement results of dried kelp and dried shiitake mushroom. Concentration of 134Cs, 137Cs, and 40K is measured by a germanium semiconductor detector.

3.2 134Cs+137Cs results Since both 134Cs and 137Cs are contained in the radioactive Cesium emitted from the nuclear power plant

disaster, specimens with these two Cesium types need to be examined. We show here the measurement results of the standard specimens created using a 1:1 ratio of 134Cs and 137Cs concentration. The results for the standard water specimen are shown in Fig. 5, and the results for the brown rice specimen are shown in Fig. 6. They show that the dose rate from the specimens containing both 134Cs and 137Cs is larger than that from the specimens containing 137Cs only, even if they have same radioactivity concentration.

Fig. 5 Measurement results of standard radioactive water specimen. X-axis shows the concentration of the standard solution of 134Cs+137Cs, while Y-axis shows the net value after subtracting the background.

Dried kelp 0.005 1 1659Dried shiitake mushroom 0.002 0.1 73

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Fig. 6 Measurement results of standard radioactive brown rice specimen. X-axis shows the concentration of the standard solution of 134Cs+137Cs, while Y-axis shows the net value after subtracting the background.

4. DISCUSSIONS

4.1 Influence of 134Cs Here, the reason why the gradient of 134Cs+137Cs specimens was larger than that of 137Cs specimens will be discussed. The difference in the contribution to the ambient dose by 134Cs and 137Cs can be estimated from the conversion factors to the 1cm dose equivalent from a unit radioactivity at 1m distance. These factors are expressed as the values of 1cm dose rates in Sv/h measured at 1m distance from 1 MBq radioactive sources of 134Cs and 137Cs. The conversion factors are shown in Table 6. This table shows the conversion factor of 134Cs is 2.69 times higher than that of 137Cs. The energy response of PA-1000 to 134Cs is 1.07 times higher than to 137Cs. We can estimate that the contribution of 134Cs to the dose rate is 2.88 times higher than that of 137Cs, by multiplying the ratio 2.69 of these conversion factors and 1.07 of PA-1000 energy response. Fig. 7 shows the results of calculating the value for a mixture of 134Cs and 137Cs based on the actual measurement of 137Cs, using these ratio. This figure shows the synthesized line (dot) agrees very well with the measured dose rates of the water specimens containing both 134Cs and 137Cs.

Using this estimation, we synthesized a correlation line for 134Cs+137Cs using the regression line obtained from the measurements of the specimens containing 137Cs only. Although the ratio of 134Cs and 137Cs changes with time due to difference in half life, the correlation line can be converted to apply to an appropriated mixture of 134Cs and 137Cs, from the correlation line made at a different ratio.


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Fig. 7 The conversion graph including 134Cs. The dotted line shows the results of calculating the value for a mixture of 134Cs and 137Cs based on the actual measurement of 137Cs, using the ratio of a 1-cm dose equivalent rate coefficient and the energy response of PA-1000. X-axis shows the concentration of the standard solution of 134Cs+137Cs, while Y-axis shows the value difference after subtracting the average background radiation.

4.2 Detection limit The detection limit of this method can be estimated from the standard deviation of background. For example, in Fig. 8 the background during the measurement of the standard water specimens was 0.07 Sv/h and the standard deviation was 0.006. This measurement contains 300 data, each measured for five times at one-minute intervals. Based on this, the detection limit is calculated to be approximately 220 Bq/kg in 99% of reliability (3), approximately 140 Bq/kg in 97% of reliability (2), and approximately 70 Bq/kg in 68% of reliability (1). The detection limit depends on the variation of the background. And the variation becomes larger when the background is higher. Therefore to improve the detection limit, it is necessary to reduce the background by shielding the instruments by lead or other material.

Conversion factors to the 1cm dose equivalentCs-134 0.249Cs-137 0.0927

Theoretical calculation forstandard radioactive water specimen

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Theoretical calculation forstandard radioactive water specimen

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Fig. 8 Detection limit of this method. X-axis shows the concentration of the standard solution of 134Cs+137Cs, while Y-axis shows the net value after subtracting the background.

4.3 Comparison with a germanium semiconductor detector We applied this measuring method to six kinds of food and cattle feed from Fukushima. And we also

compared the results with the measurement results by a germanium semiconductor detector. The correlation line used for this measurement was for the brown rice. The measurement place was in a small building at Fukushima Prefecture. Background radiation was 0.04 Sv/h there. The results of the comparison are shown in Fig. 9. The X-axis is the measurement result using the

germanium semiconductor detector, and the Y-axis is the measurement result using this method. This figure shows that the results of this method agree with the results by the germanium semiconductor detector.


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Fig. 9 Comparison of measurement values between germanium semiconductor detector and the method we are proposing. X-axis is the result of the measurement using the germanium semiconductor detector, and the Y-axis is the result of the measurement using this method. The results show that the values measured by this method agree very with the values measured by the germanium semiconductor detector.

5. SUMMARY We propose a simple radioactivity measurement method using a portable dose rate meter (HORIBA

Radi: PA-1000) and its dedicated container (HORIBA easy radioactivity measurement kit: PA-K). With this method we obtained a linear relationship between the net value of dose rate and the concentration of the radioactive Cesium isotopes. The same linear relationship can be applied for various specimens. We can estimate the radioactivity

concentration [Bq/kg] from the dose rate [Sv/h] by using a linear relationship. We will verify that this can be applied to any specimens. Furthermore, even if the ratio of concentration of 134Cs and 137Cs changes after making a correlation line,

the conversion is possible as long as the ratio is known. In addition, it is also possible to estimate a correlation line for 134Cs+137Cs based on measurements of 137Cs standard solutions only. The detection limit was approximately 140 Bq/kg in 97% of reliability at 0.07 Sv/h background. We compared the results of this method with the results of a germanium semiconductor detector, using

food from Fukushima. Both results agreed very well.

REFERENCES

(1) T. Ido et al, the Isotope Notebook the 11th edition (Isotope TECHOU, Japanese text only)

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First demonstration of cosmic ray muon radiography of reactor

cores with nuclear emulsions based on an automated high-speed

scanning technology

Kunihiro Morishima*, Naotaka Naganawa, Toshiyuki Nakano, Mitsuhiro Nakamura F-lab., Department of Physics, Graduate School of Science, Nagoya University,

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan

Jun Kawarabayashi, Hideki Tomita, Tetsuo Iguchi

Department of Quantum Engineering, Graduate School of Engineering, Nagoya University,

Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, JAPAN

Shigetaka Maeda Oarai Research and Development Center, Japan Atomic Energy Agency,

4002 Narita-cho, Oarai-machi, Ibaraki 311-1393, Japan

1. INTRODUCTION

One of the most important issues on Fukushima Dai-ichi nuclear power plant is the clarification of the distribution of fuel debris, especially after melting and solidification, in the reactor containment vessel after the probable meltdown of core, for its dismantling and after treatment.

Muon radiography using cosmic ray muons is the most powerful means for it. It is done by detecting tracks of muons produced in the atmosphere and analyzing their angular distributions. It has been applied to the analysis of inner structure of pyramid by Alvarez 's group with spark chambers(1), to volcanoes by Nagamine's with plastic scintillator hodoscope(2) and so on.

In the case of Fukushima Dai-ichi, high radioactivity and lack of electronic power supplies should be taken into account. Electronic detectors seriously suffer from these factors. They require power supplies and huge shielding material to reduce count rate from the background radioactivity to the practical level.

We propose "nuclear emulsion(3)(4)(5)" as the best muon detector for probing the inner-structure of Fukushima Dai-ichi. It is high-resolution photographic film for detecting 3-demensional tracks of charged particles in its volume with the position resolution of sub-m, which gives us the angular resolution of a few mrad. As it does not require electronic power, the unit of the detector is compact. Because nuclear emulsion is accumulative detector, the observation period is limited by integrated background. In the case of observation under 10 mSv per hour in radioactivity, the maximum of the period with keeping the practical background level by using a shielding material of 10cm thick lead is about 10 days. Under this environment and the following conditions: 1, uranium dioxide block, which size is 1m cubic, is placed behind the 10 m thick concrete wall; 2, the uranium dioxide block is located in the direction of angle of 45 degrees from vertical on the ground and 20 m away from the detector; we need 1 m2 detector area to detect the uranium dioxide block beyond statistical error of 3 sigma with a position resolution of 40cm, which is

27

equal to an angular resolution of 20 mrad. Moreover, we need larger area to measure in higher radioactive environment or to perform the tomographic analysis using multiple detectors placed around the target. Thus, scanning speed of nuclear emulsion is the essential point to realize muon tomography with nuclear emulsion.

We succeeded in developing a high-speed 3-demensional nuclear emulsion read-out system S-UTS(6)(7) based on optical microscope with the speed of 72 cm2 per hour through a neutrino oscillation experiment, OPERA(8). This scanning speed was achieved by developing the new contraption of realizing the scanning speed of 50 views per second with the following conditions of image acquiring: the size of field of view is about 230x230 m2, imaging resolution is about 0.45 m per pixel, and 16 tomographic images acquired per view. The technical detail was written in ref (6). The system was also applied to and provided the result in muon radiography using nuclear emulsion for volcanoes done by Tanaka's group(9).

We have conducted the experimental test of muon radiography with nuclear emulsion at the experimental fast reactor Joyo,(10) which belongs to Japan Atomic Energy Agency, in order to demonstrate an imaging method of reactor cores. The result of the demonstration is reported here.

2. HIGH-SPEED NUCLEAR EMULSION SCANNING TECHNOLOGY

2.1 Nuclear emulsion OPERA film Nuclear emulsion is one of the earliest truly three-dimensional radiation track detectors(3). They are

made of a thick (10-1000 m) gel with interspersed silver halide (AgBr) crystals. When a charged particle passes through the detector it produces sequential latent images in the crystals along the trajectory. These latent images appear as metallic silver grains after development. The size of the single grain is about 1 m. Nuclear emulsion is analyzed by high-resolution optical microscope.

The OPERA film(4)(5) is a nuclear emulsion developed under the collaboration between Nagoya University and Fuji Photo Film Co., Ltd for the largest scale nuclear emulsion experiment OPERA(8), which intends to detect tau neutrino appearance from neutrino oscillation. The photographs of OPERA film are shown in Fig. 1. The OPERA experiment required about 120000 m2 nuclear emulsions in total but it was impossible to produce this amount and ensure uniform quality by hand. To overcome these problems, the OPERA film was developed as the first nuclear emulsion produced by the industrial mass production system. The structure of the film is a pair of 44 m-thick emulsion layers on either side of a 205 m-thick plastic base as shown in Fig. 1 (b). This structure was especially optimized for automated scanning. A photograph of tracks of a muon and electrons, in a developed emulsion layer, is shown in Fig. 2.

The sensitivity of emulsion is evaluated from the linear density of grains constituting tracks of minimum ionizing particles. Typically the number of grains in 100 m is used as the sensitivity, which is called Grain Density. The OPERA film has 35 grains / 100 m, which value is optimized to achieve high cost performance and meet the request of the OPERA experiment at the point of efficiency of automated scanning. Thanks to machine production, the OPERA film has high uniformity in thickness and sensitivity. Owing to such features, the OPERA film had been applied to some experiments which needed large statistics with automated scanning, for example, basic study of radiation therapy(11), a gamma ray telescope(12), muon radiography(9) and a fast neutron detector(13).

28

Fig. 1 Photographs of the nuclear emulsion OPERA film; (a) before development, (b) a cross sectional photograph by an electric microscope.

Fig. 2 A photograph, which shows inside of the emulsion layer after development taken by a high-resolution optical microscope. A signal of the present experiment is a straight track of muon. Two scattered tracks of electrons and randomly distributing silver grains are background noises of nuclear emulsion.

2.2 An automated nuclear emulsion scanning system

We have developed an automated nuclear emulsion scanning system S-UTS with scanning

speed of 72 cm2h-1 to realize the OPERA experiment with a large amount of nuclear emulsion(6)(7). The

photograph of S-UTS is shown in Fig. 3. This system with an high-resolution optical microscope takes 16

tomographic images of an emulsion layer by a CMOS image sensor and searches the grains on the straight

line in the tomographic images to recognize them as micro tracks by an online digital processor. The

micro tracks are defined by their positions, angles and pulse heights. The pulse height is the number of

tomographic images, in which contain grains forming the micro track. Pulse height distributions of muons

originated from cosmic ray and fake tracks originated from background noises (see Fig. 2), which pulse

heights are 6 or more, is shown in Fig. 4 (a). On this scanning condition, the micro track recognition

29

efficiency of muon tracks is about 95 %. Although, the recognition efficiency is limited by the fake track

density, because the number of tracks is able to be recognized by the system is limited by the processing

power. Thus, we made the lower threshold of pulse height for scanning, which determines the efficiency.

After recognition of two micro tracks of the top and bottom layer, they are connected across the plastic

base using information of their positions and angles in order to reduce fake tracks produced by background

in emulsion itself, which are randomly distributed silver grains called fog and tracks of very low energy

electron recoiled by environmental gamma rays. The fake tracks due to electrons are not able to cross the

plastic base linearly because the electrons do not have enough energy. Connected tracks are called base

tracks, as shown in Fig. 5. The angular resolution of base tracks of this system is a few mrad in sigma

fitted by Gaussian function, as shown in Fig. 4 (b). Five S-UTS systems are operating now at the nuclear

emulsion scanning laboratory in Nagoya University, Japan.

Fig. 3 A photograph of automated nuclear emulsion scanning system S-UTS based on an optical

microscope.

Fig. 4 The performance of S-UTS; (a) A pulse height distribution of micro tracks. Solid line shows

signal tracks and dashed line shows fake tracks. (b) Angular resolution of base tracks and fitted by

Gaussian function. Data from minimum ionizing particles penetrating OPERA film in the vertical

direction on an emulsion surface are plotted on them.

30

Fig. 5 Definition of the micro track and the base track.

3. DEMONSTRATION IN JOYO

3.1 Experimental overview

We have conducted the demonstration to take the muon tomographic image of the reactor core at

the experimental fast reactor Joyo(10). Joyo is the first liquid sodium fast reactor in Japan. Emulsion

detectors were placed in the room located just under the vessel of Joyo plant, as shown in Fig. 6. In this

demonstration, 3 OPERA films were assembled into an emulsion detector, as shown in Fig. 7, at Nagoya

University. This structure was designed in order to reject background of base tracks by connecting fake

tracks to each other, as described in the next section. Fig. 8 shows a chamber consisted of 10 emulsion

detectors. The emulsion detector had a very simple structure and was lightweight enough in order to be

carried through narrow passages into the room under the reactor vessel and to be assembled easily and

quickly on the job site. Two chambers were placed with keeping the interval of about 1m. In such

installation condition as shown in Fig. 9, the reactor core was viewed from different directions, which

differed by about 0.1 rad. The observing period was 21 days.

31

Fig. 6 The left figure shows the overall layout of the reactor building. The reactor vessel and the

room are under ground level. The right shows the close-up layout of the structure focused on the

reactor vessel and its surroundings. A is the reactor core, which is a complex structure made of

uranium, plutonium, stainless steel and liquid sodium. B is inside of the reactor vessel filled with

liquid sodium and argon cover gas. C is radiation and thermal shields made of graphite. D is a wall

made of reinforced concrete. Scales are shown at the left bottom of each layout.

Fig. 7 The emulsion detector using OPERA films; (a) a photograph of the detector and (b) a cross

sectional view of the detector. The three OPERA films could be thoroughly bonded each other by

vacuum packaging with an aluminum-coated film. A muon passing thorough three OPERA films

leaves latent tracks in all emulsion layers, as shown in the right figure.

32

Fig. 8 The chamber assembled with 10 nuclear emulsion detectors; (a) a photograph of the chamber

without a rubber plate and lead plates and (b) a side view figure of the structure of the chamber. An

aluminum honeycomb plate gives the flatness of the chamber. The rubber plates are used to

distribute the pressure of the weight of the lead plates, which were used not to shield from radiation

but to fix the detectors on the aluminum honeycomb plate. This chamber was able to be assembled

without screws.

Fig. 9 A photograph (left) and a schematic view (right) of the observation setup. Chamber A and B

were placed at different positions under the reactor vessel. A distance between detectors and the

reactor vessel is about 10 m. The left photograph shows that the nuclear emulsion detectors are very

compact and are able to be taken data without a power supply.

33

3.2 Analysis of the nuclear emulsion detector

The development and readout of films were carried out at the nuclear emulsion scanning laboratory

in Nagoya University, Japan. The S-UTS system read out tracks with pulse height of 7 or more from

emulsion layers. The angular acceptance of track recognition was |tan| = 0.6. means the angle from a

perpendicular line on an emulsion film surface. In small angle, tan is almost equal rad. The recognition

efficiency was estimated to be about 80 %. The effective detection area of about 100 cm2 was scanned

detector by detector.

A track density of micro tracks was about ~ 2 x 105 cm-2 in this scanning. Base tracks were built

with the maximum angular difference between two micro tracks of |tan| = 0.07. The density of base

tracks was ~ 2 x 104 cm-2, which was reduced to 1/10 of micro tracks. The base tracks included not only

muon tracks, as signals for the present muon radiography, but also fake base tracks produced by accidental

coincidence of fake micro tracks as described in section 2.2. Thus, fake base tracks cannot penetrate more

than two emulsion plates. In order to reject such fake tracks and select signal tracks recorded in the period

of the alignment between films as detectors, we connected base tracks penetrating more than two emulsion

plates with the maximum angular difference of |tan| = 0.04 and the threshold of sum of pulse height, for

example, more than 36 for vertical tracks less than 0.1rad. After this processing, track density was reduced

to about ~ 2 x 103 cm-2, which was 1/10 of base tracks. Although, these tracks include fake base tracks not

rejected. We have estimated the density of fake tracks not rejected by simulation using real data. The

estimated density of fake base track was ~ 10 cm-2, which is less than the order of 10-2 of real tracks.

3.3 Result

In this paper, we describe the result of two nuclear emulsion detectors. One was placed on the

chamber A and the other on the chamber B. The interval between them was about 1 m in the x-direction,

as already shown in Fig 9. The 2-dimensional histogram of angular distribution of muon tracks recognized

by the scanning system was plotted with the bin of tan = 0.04 as shown in Fig. 10 (a). The histogram was

made using tracks found in more than 2 plates out of 3 plates to recover track recognition inefficiency and

reduce background tracks.

The number of tracks per bin was calculated by the number of reconstructed tracks minus the

number of fake tracks estimated as described in the previous section. The bin is larger than the angular

resolution of base track. It corresponds to the spatial resolution of 40 cm at the position of the reactor core,

which exists 10 m above the detector. Fig. 10 (b) shows the cross sectional view of the histogram on the

X-axis within the range of tan from -0.02 to 0.02 on the Y-axis. The error bars of 1 sigma were made

from statistics. The resolution of muon radiographic image is limited by these errors, but can be improved

by longer observation time and/or a larger area detector in principle.

A comparison between A and B shows us that the angle of turning point of intensity is different, as

shown in Fig. 10 (b) with arrows. The difference of angle is about tan = 0.1. It corresponds to the distance

of about 1m, which is almost the same with the distance between the two chambers. The width of the

34

valley pointed by the arrows in Fig. 10 (b) is about 4 bins, equal to tan = 0.16. This value is almost

consistent with the diameter of the reactor core, which is about 1.5 m, equal to about tan = 0.15 in angle.

On the other hand, two distributions of a region outside of valley give smaller difference in direction. This

is because the region was behind the core. In order to conclude the core effect clearly by reconstructing

three-dimensional distribution of matter density, we need to perform the tomographic analysis using more

detectors placed around the vessel. These radiographs suggest that such compact emulsion detectors

enable us to detect the reactor core and the plant structure.

Fig. 10 Histograms of raw data obtained by track reconstruction processing of nuclear emulsion.

The unit of these histograms is the number of tracks.

4. SUMMARY

An automated high-speed nuclear emulsion scanning technology realized the cosmic ray muon

radiography with nuclear emulsion, which has the advantage of compact, lightweight and no need of a

power supply. We have conducted the basic study of muon radiography of reactor core at Joyo. In this

paper, we described the analysis of two nuclear emulsion detectors placed at different positions. As the

result, we have observed the decrease of muon flux at the direction of the reactor core and the effect of the

35

other plant structure. Also the difference of decreasing by angle of two detectors was consistent with the

difference of the positions of detectors. The result validates the possibility of the imaging of the reactor

core with nuclear emulsion.

REFERENCES

(1) L.W. Alvarez, et al., Science 167 (1970) 832. (2) K. Nagamine, et al., Nucl. Instr. and Meth. A 356 (1995) 585.

(3) R. Brown, et al., Nature 163 (1949) 47.

(4) T. Nakamura, et al., Nucl. Instrum. Meth. A (2006) 55680.

(5) N. Naganawa and K. Kuwabara, Journal of Instrumentation 5 (2010) P02006 (6) K. Morishima and T. Nakano, Journal of Instrumentation 5 (2010) P04011

(7) K. Morishima and T. Nakano, 2011, Ionizing Radiation, vol.37-1 (2011) -.

(8) M. Guler, et al., An appearance experiment to search for oscillations in the CNGS beam:

experimental proposal , 2000, CERN-SPSC-2000-028.

(9) H.K.M. Tanaka, et al., Earth Planet. Sci. Lett. 263 (2007) 104.

(10) Y. Maeda et al., Nuclear Technology, 150[1], 16-36 (2005).

(11) T. Toshito, et al., Phys. Rev. C 75 (2007) pp.054606-1~8,.

(12) S. Aoki, et al., Nuclear Physics B Volume 196 (2009) 50-53.

(13) K. Morishima, Ionizing Radiation, vol.37, No.3, (2011) 173-178.

36

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42

DESIGN OF A NON-CONTACTING SYSTEM TO MEASURE THE RADIOTRACER CONCENTRATION IN BLOOD FOR NUCLEAR

MEDICINE APPLICATIONS

Azhar H. Malik*, Y. Nakamura, K. Shimazoe, H. Takahashi Department of Nuclear Engineering. & Management, Graduate School of Engineering., University of Tokyo,

7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan

1. INTRODUCTION

The concentration of radiotracer in blood is of primary concern in nuclear medicine and it is measured conventionally by taking blood samples and measuring their activity in laboratory. The method not only takes time but also becomes very inconvenient and time consuming if sampling is required for large number of times. In this study, a method is proposed to online measure the radiotracer concentration. The injected radiotracer 18F-FDG is mixed in blood and the concentration of radiotracer in blood is measured quantitatively by online system. The method is simple as well as convenient and immediate results will be very helpful in reducing the diagnosis and treatment time. As a first step, the conventional detection system used for dose measurements in nuclear power plant systems (NPPS) is assumed (1),(2). The reason for selection of system is the availability of published results for comparison and the concentration of radioactive fission fragments in coolant is small, analogous to our blood vascular source, with difference of the larger dimensions as compared to present case. Electron Gamma Shower 5 (EGS5) code is used as simulation tool to obtain the detector response along with considering the detector and collimator dimensions, various geometrical parameters which affect the detector response. The obtained absolute full energy efficiency curves may be used to estimate the predicted number of counts from a typical gamma source. These predicted counts may be helpful to decide whether a particular system is suitable for the measurement of activity, and hence concentration, of given source or not.

2. RADIOACTIVITY MEASUREMENT

The concentration of radiotracer is intended to measure by radioactivity measurement. The blood vascular is assumed to contain the radionuclei 18F which emits positron whose annihilation with electron results in emission of two 511keV photons in opposite directions. The detector, single or in form of ring around the blood vascular, determine the activity which corresponds to the concentration of radiotracer in the blood. Following equation is used to measure the activity of a point source (3)

A = SC/( Tl abs kc ) (1) where, abs is detector absolute efficiency, S is net peak area, C is correction factor, Tl is measurement time, A is source activity, is emission probability of gamma, and kc is decay during measurement. In our case, the source is not a point source but instead it is cylindrical source which will be modeled by considering the various parameters which include the blood vascular dimensions, its density, the blood ______________________________________

Corresponding author; (tel)03-5841-6974, (fax)03-5841-2932, (e-mail)[email protected]

43

velocity, the medium and distance between the source and detector etc. As a first case, we discretize a cylindrical source to many point sources and try to determine the suitability of system for the measurement of activity of a weak source.

3. MONTE CARLO SIMULATIONS AND SYSTEM MODEL

The geometry used in simulation is shown in Fig. 1 and various parameters are mentioned. The

detector is High Purity Germanium (HPGe) semiconductor detector of 10mm thickness and 18mm diameter shielded by lead collimator of two diameters 10mm and 20mm. In order to obtain a good statistical certainty, large number of particles was generated and, hence by using source biasing technique, reasonable counts were obtained for all cases. The particles were transported through the material and were discarded if they either left the area of interest or their energy was below a certain cut off energy. In first case, a point source was assumed at distance of 1 m and 106 particles were made incident on detector front face. The intrinsic full-energy efficiency, which was determined from simulation, was used to find the absolute full-energy efficiency by using analytical relation. In second case, the results are reported by varying the radius of disk source at same distance. The solid angle needed to find absolute efficiency from intrinsic efficiency is calculated by using various approximated or series expansion formulas derived from Ruby's formula as described by Pomme et al. (4). EGS5, used as simulation tool, is a powerful system which can be used to produce very complex Monte-Carlo simulations in various geometries (5). Fig. 1. Simulation geometry

4. ABSOLUTE FULL-ENERGY EFFICIENCY

The absolute full-energy efficiency, very important parameter in the measurement of source strength, is the ratio of number of detected photoelectron photons of energy E in a full-energy peak Ndet to the total number of photons emitted Nemit by source with same energy (6).

abs = Ndet / Nemit (2)

4.1 Point Source The absolute efficiency for point source can be found by using the relation

Nemit = Ndet.(4/if ) (3) where, if is detector full-energy intrinsic efficiency and is the solid angle (6). In first case a point source is assumed and the effect of source to detector distance and radial displacement of source is studied for various values of gamma-ray energies of practical interest. The simulation results for

44

source to detector distance are compared with those obtained by Rodenas et al. by using MCNP (2). The normalized efficiency is obtained at various distances from 1 to 2m with step of 25cm and a good agreement was found. The values of absolute full energy efficiency along with normalized efficiency at different source to detector distance are given, and hence compared, in Table I. The point source is then displaced in transversal direction from the axis of detector and absolute full-energy efficiency is calculated at different radial distance and shown in Fig. 2. The obtained results are compared with already published results by using GEANT4 (1) and MCNP (2) as well as experimental results (2), and are summarized in Table II.

TABLE I Effect of source-to-detector distance on absolute full-energy efficiency for point source

(a) (b) Fig. 2. Variation of absolute full-energy efficiency with radial distance of point source for collimator

of size (a) 10mm and (b) 20mm

4.2 Disk Source In second case a disk source is considered at origin as a source detector parallel system. The source

radius is varied from zero (point source) to 10cm and variation in absolute full-energy efficiency with source radius is shown in Fig. 3. The results are compared with those obtained by using Monte-Carlo code Geant4 (1) for disc source of diameter 5cm and are shown in Table III. For the disk source, solid angle is used to convert the intrinsic efficiency to the absolute efficiency. The solid angle is calculated

45

by using approximated and series expansion formulas of Rubys formula as given by Pomme et al. (

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radiation detection