WANG Yiming, ZHANG Yingying, WU Ning, WU Bingwei, LIU Yan, CAO Xuan, and WANG Qian

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Monte Carlo Simulation ofGamma-Spectra Recorded by NaI (Tl) Detector in the Marine Environment

WANG Yiming1), ZHANG Yingying2), *, WU Ning2), WU Bingwei2), LIU Yan2), CAO Xuan2), and WANG Qian2)

1),,266061,..2),,266001,..

To develop a NaI (Tl) detector for in situ radioactivity monitoring in the marine environment and enhance the confidence of the probability of the gamma-spectrum analysis, Monte Carlo simulations using the Monte Carlo N-Particle ( MNCP ) code were performed to provide the response spectra of some interested radionuclides and the background spectra originating from the natural radionuclides in seawater recorded by a NaI (Tl) detector. A newly developed 75mm×75mm NaI (Tl) detector was calibrated using four reference radioactive sources137Cs,60Co,40K and54Mn in the laboratory before the field measurements in seawater. A simulation model was established for the detector immersed in seawater. The simulated spectra were all broadened with Gaussian pulses to reflect the statistical fluctuations and electrical noise in the real measurement. The simulated spectra show that the single-energy photons into the detector are mostly scattering low-energy photons and the high background in the low energy region mainly originates from the Compton effect of the high energy γ-rays of natural radionuclides in seawater. The simulated background spectrum was compared with the experimental one recorded in field measurement and they seem to be in good agreement. The simulation method and spectra can be used for the accurate analysis of the filed measurement results of low concentration radioactivity in seawater.

Monte Carlo simulation; marine radioactivity monitoring; NaI (Tl) detector; response spectra; background spectra

1 Introduction

The radioactive monitoring in the marine environment has been using the method of in situ sampling, subsequent processing and analyzing in the laboratory in China. This complicated handling procedure always takes two or three days to get the quantitative results. It is impossible to timely and effectively monitor the radioactivity in the marine environment, not to mention providing the pollution warning. Thus there is a need to develop an autonomous underwater detector for the continuous and long-term radioactive monitoring in the marine environment.

NaI (Tl) scintillation crystal detectors are the most common forradioactivity measurements in seawater due to their good efficiency, high reliability, low power consumption and low cost. There are a few NaI (Tl) detectors and even marine radioactive monitoring networks abroad (Tsabaris,2008;Wedekind,1999;Osvath, 2005;Caffrey, 2012). In China, NaI (Tl) detector has been widely used for the rapid measurements of natural and artificial radionuclides on land. The research on NaI (Tl) detector for in situ radioactive monitoring in the marine environment just gets started (Pan., 2014; Su., 2010 ).

The γ-ray spectra, recorded by NaI (Tl) detector, are complicated because of the interactions between γ-photos and substances in seawater. Especially, there is a high background in the low energy region mainly originating from the Compton effect of natural radionuclides such as40K with high concentrations in seawater. These make it difficult to discriminate the few artificial radionuclides such as137Cs and further to quantitatively calculate them. Thus, to improve the spectrum analysis, it is important to study the radioactive spectra in the marine environment, including the background and certain artificial radionuclides. But it is very difficult to carry out the test experiment in the laboratory, since there are a rather limited number of single-energy, gamma-emitting radionuclides soluble in seawater, and there is no seawater tank big enough to imitate the open sea environment and put some artificial radionuclides into it. The big volume of the contaminated seawater is very difficult to handle.

For the development and application of in situ NaI (Tl) detector for the marine environment in China, Monte Carlo simulations were performed in the present work using the MNCP code to study the γ-ray spectra of in situ radioactive measurement. The energy and efficiency calibrations of the detector developed were described. The background and response spectra were simulated for the radionuclide discrimination and calculation. In order to test the reliability of these simulations, experimental spectra of the detector were deduced from the field measurements and used to compare with the simulation results.

2 Simulation Model and Method

Forradioactive monitoring in the marine environment, an underwater NaI (Tl) detector was developed. The detector, consisting of a 75mm×75mm NaI (Tl) crystal connected with a photomultiplier and integrated electronic circuits with a density of 3.667×10?3gmm?3, was packaged in a watertight cylindrical enclosure made from stainless steel. A simulation model was established for the NaI (Tl) detector. The photomultiplier as well as the integrated electronic circuits and cables of the detector were discarded, and so the detector was simplified to be merely a scintillator crystal and placed in the centre of the seawater tank in the model. The model geometries are shown in Fig.1. The attenuation of γ-ray due to stainless steel enclosure around the crystal is considered in the model for better simulation. To simulate the measurement of the detector immersed in seawater, the seawater with standard composition and density of 1.025gcm?3is even- ly distributed in the tank modeled with radius of=100, 200,…,1500mm.

Fig.1 Measurement simulation model of NaI (Tl) detector immersed in seawater.

To reflect the statistical fluctuations and electrical noise in the measurement, the γ-ray spectra recorded by the NaI (Tl) detector were broadened with Gaussian pulses in the simulation. The broadening parameters were deduced from the experimental tests of the detector developed.

3 Calibration and Measurement

The NaI (Tl) detector had to be calibrated before its deployment in the seawater. In the laboratory, the detector was energy-calibrated and efficiency-calibrated using four reference radioactive sources137Cs,60Co,40K and54Mn. In the seawater, there is a certain radionuclidewith the different activities, and the counting rates of its photopeak are also different. A relative counting rate is defined as the photopeak counting rate of the above-mentioned radio- nuclide per unit activity. The relative counting rate can be used to represent the marine detection efficiency.

Some experimental tests were carried out in the laboratory and field measurements were completed near the Eight Gap Qingdao port. In the field measurements, the detector was deployed with a crane into seawater as shown in Fig.2, the water depth being 4 to 8m. The detector was positioned 3m below the water level in order to eliminate the measurement interferences from the seabed and cosmic radiation. It was connected to a notebook for realtime measurement and data collection.

Fig.2 Detector ready to immersed into seawater in field measurement.

4 Response Spectra Simulation

The full width at half maximum (FWHM) of137Cs,60Co and40K were all taken from the experimental tests and that of131I was given by the fitting curve of FWHM. All their photopeaks were taken from the experimental tests in the laboratory. Then the broadening parameters of Gaussian pulses for the radionuclides were deduced for their response spectrum simulations. The curves of the calculated response spectra of NaI (Tl) detector are shown in Fig.3. It can be seen that the single-energy photons into the detector are mostly scattering low-energy photons. The calculation mode used in the simulation is mode P and the particle number is 1e8.

Fig.3 The calculated response spectra of NaI (Tl) detector. a) for131I; b) for137Cs; c) for60Co; d) for40K.

5 Background Spectrum Simulation

Monte Carlo simulations using the MNCP code were performed to produce the background spectrum induced by the natural radionuclides in the marine environment. There are 37 radionuclides from the natural series of232Th,238U and235U in seawater. Among them, the γ-rays of several radionuclides such as214Bi,214Pb,212Bi,208Ti,228Ac,.have considerable influences on in situ radioactive monitoring of NaI (Tl) detector. There are also some long-lived radionuclides existing independently in seawater. Among them,40K is the most important. Then a number of strong γ-rays, weighted by their relative intensity, were selected for the background spectrum simulation. The background spectrum is the sum over all response spectra of strong γ-rays. By adjusting the relative contri- bution of these spectra and getting the best fit to the real spectrum (taken with NaI (Tl) of the detector developed near the Eight Gap Qingdao port), the background spectrum simulated, as shown in Fig.4, is in good agreement with the experimental one. There is a high background in the low energy region, which makes it difficult to discriminate the few artificial radionuclides mainly with low energies and low concentrations.

1.平行避險(xiǎn)線（不應(yīng)從漂浮物體引出，如燈浮，而應(yīng)從固定物，如燈塔，燈樁等）;2.海圖的更換；（兩海圖接圖—接圖點(diǎn)、船位等）;3.定位的方法和時(shí)間間隔（駕駛員最容易疏忽）；4.明顯的導(dǎo)航和雷達(dá)物標(biāo)；5.禁止進(jìn)入?yún)^(qū)（不鼓勵(lì)過多地標(biāo)繪“禁止進(jìn)入?yún)^(qū)”）;6.避險(xiǎn)線和避險(xiǎn)方位；7.疊標(biāo)、導(dǎo)標(biāo)和導(dǎo)航線;8.重要的潮流和海流；9.安全航速和必需的航速變化；10.最小富裕水深；11.應(yīng)開啟回聲測(cè)深儀的船位；12.（距危險(xiǎn)物的）安全距離；13. 錨位寬余量；；14.意外事件計(jì)劃；15.放棄進(jìn)港計(jì)劃的最后位置；16. VTS和報(bào)告點(diǎn)等。

6 Conclusions

There are many differences between in situ radioactive monitoring in the marine environment and that on land. In order to improve the performance of underwater NaI (Tl) detector and produce more reliable results in the concentration of low level radioactivity in seawater, Monte Carlo simulations using the MNCP code were performed in the present work to generate the response spectra of some interested radionuclides and the background spectrum in seawater recorded by NaI (Tl) detector. It can be seen that the single-energy photons into the detector are mostly scattering low-energy photons, and the high background in the low energy region mainly originates from the Compton effect of the high energy γ-rays of natural radionuclides in seawater. The comparison of the simulated background spectrum with the experimental one shows promising results for the simulation code as well as for the spectrum analysis.

The development of in situ radioactive monitoring in the marine environment using NaI (Tl) detector just start- ed in China. We anticipate that sustained research into the Monte Carlo simulation ofmarine radioactivity measurement will be conducted and the results will be used for the detector development and applications. Based on the simulation results, the spectrum analysis algorithm is the next step.

Acknowledgements

We acknowledge the financial support from the International Science & Technology Cooperation Program of China (No. 2013DFR90220), National Natural Science Foundation of China (No. 41206076) and Qingdao Applied Basic Research Project (NO. 14-2-4-94-jch). The discussion with Dr. Christos Tsabaris in Hellenic Center for Marine Research is greatly appreciated. The authors are sincerely grateful for the comments and suggestions of the anonymous reviewers.

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(Edited by Ji Dechun)

10.1007/s11802-015-2841-4

(December 25, 2014; revised February 10, 2015; accepted February 19, 2015)

. E-mail:triciayyz@163.com

ISSN 1672-5182, 2015 14 (3): 471-474

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015