A Simple Measurement Technique of the Equilibrium. Equivalent Thoron Concentration with a CR-39 Detector

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1 Jpn. J. Health Phys., 37 (1), (2002) Technical Paper A Simple Measurement Technique of the Equilibrium Equivalent Thoron Concentration with a CR-39 Detector Shinji TOKONAMI*1, Quanfu SUN*1, Hidenori YONEHARA*1 and Yuji YAMADA*1 (Received on July 25, 2001) (Accepted on December 28, 2001) A simple alpha-track measurement technique for the equilibrium equivalent thoron concentration (hereafter called EETC) using a CR-39 detector was discussed. Detection properties of the CR-39 detector were examined with regard to detection efficiencies for incident energy and angles, respectively. Since more than 90% of the EETC is dominated by the 212Pb concentration, its practical evaluation can be achieved even though the 212Bi concentration is ignored. In this technique, the CR-39 detector is directly placed on a filter after an air sample is taken and an adequate time elapses (radon progeny completely decay). In order to confirm the reliability of measured values, the technique was compared with an alpha spectroscopic method. There was a relatively good agreement between the two. KEY WORDS: equilibrium equivalent thoron concentration, solid-state nuclear track detector, CR-39 detector, alpha track, 212Pb concentration, air sampling I INTRODUCTION Although a lot of studies on radon were conducted all over the world, there are a few studies on thoron. It is considered that doses from thoron progeny are generally smaller than those from radon progeny in the natural environment. Since recent studies have shown that high thoron concentrations were observed in some areas though specifically, however, the dose from thoron progeny is significant in such areas'-4. According to some recent study5, occupational exposure to thoron progeny became significant and the protection against thoron progeny inhalation should be considered in the near future. In the case of radon exposure, the dose can be estimated by the radon concentration under several assumptions. The equilibrium equivalent radon concentration (hereafter called EERC) can be obtained by the product of the radon concentration and equilibrium factor. On the other hand, it is impossible to follow the same *1 Radon Research Group, Research Center for Radiation Safety, National Institute of Radiological Sciences; Anagawa, Inage-ku, Chiba , Japan. procedure as the case of thoron exposure. Since the thoron concentration observed is exponentially decreased with a distance from the source for its short half-life (55. 6s)6), use of the thoron concentration will be meaningless unless the position of measurement is known. Therefore, the equilibrium equivalent thoron concentration (hereafter called EETC) should be directly measured from the viewpoint of the dose assessment. A simple measurement technique for determining thoron progeny concentrations is proposed in the present study though several techniques have been reported'-9. II MATERIALS AND METHODS In order to easily measure the EETC any, the followings should be taken into account: 1. Sampling and measurement are completely separated. 2. Structure of the measuring system and its procedure are simple. 3. There is no limitation on electric condition when taking air samples and making measurements. In order to satisfy the above three conditions, a solid-state nuclear alpha track detector (SSNATD) is used as the

2 60 S. TOKONAMI, Q. SUN, H. YONEHARA and Y. YAMADA Fig. 1 Experimental procedure for alpha-track registration in the proposed measurement technique. detector. CR-39 detector is the most stable and reliable among SSNATDs. The CR-39 detectors named "BAR- YOTRAK#" are commercially available in Japan. When air samples are taken with a DC driving pump, a glass microfiber filter (Whatman GF/F*) is used as the collecting medium. In order to determine thoron progeny concentrations, the information on thoron progeny has to be distinguished from that on radon progeny. Therefore, the filter is left until radon progeny completely decay (more than 6 h). An Al foil with the thickness of 15,um (4. 0 mg cm-2) is then placed on the filter so as to reduce alpha particle energy emitted from 212Po for effective detection. The reason why the energy absorber has to be used is mentioned later. By placing a CR-39 detector directly on the filter covered with the Al foil, subsequently, registration of alpha tracks is made on the CR-39 detector. Fig. 1 illustrates the experimental procedure for alpha-track registration in the present study. Following the manner of etching and reading on the SSNATD, the thoron progeny concentration is eventually determined. III RESULTS AND DISCUSSION 1. Characteristics of CR-39 detector The alpha particle energy dependence of the CR-39 detector was first investigated. An alpha standard source of 241Am was used for evaluating the energy dependence in the range below 5. 5 MeV. As for the range above 5. 5 MeV, a filter radon or thoron progeny were collected was used. The CR-39 detector was irradiated with alpha particles emitted from 214Po (7. 7 MeV) for radon # Fukuvi Chemical Industry Co., Ltd., Fukui, Japan. *Whatman International Ltd., Maidstone, England. Fig. 2 Incident alpha particle energy dependence of progeny, 212Bi (6. 0 MeV) and 212Po (8. 8 MeV) for thoron progeny. When 7. 7 MeV alpha particles are exposed to the detector, a filter is left for about 30 min after sampling until 6. 0 MeV alpha particles emitted from 218Po completely vanish. In order to obtain alpha particles emitted from thoron progeny, a filter is left for around 6 h after sampling until radon progeny completely decay as mentioned above. Although a single energy particle emission is desirable for precise evaluation on energy dependence, it is impossible to separate two alpha particles from 212Bi and 212Po. In the experimental work, therefore, simultaneous exposure of two different alpha particles was used. When evaluating detection efficiencies of alpha particles emitted from these progeny, a part of the filter was used for alpha registration with the CR-39 detector after air samples were taken. The rest was used for the alpha spectroscopic10 measurement so as to obtain an original alpha emission rate. All the detectors exposed were chemically etched for 24 h in a 6 M NaOH solution at 60C. the CR-39 detector. Fig. 2 illustrates the incident alpha particle energy dependence of the CR-39 detector. The minimum detectable energy was estimated to be 0. 6 MeV and the detectable region ranged up to 5. 5 MeV with the 241Am source. Since alpha particles emitted from 214Po were detected, detection of 6. 0 MeV particles could be estimated. When two kinds of alpha particles from thoron progeny were exposed to the CR-39 detector, on the other hand, track density obtained was lower than expected. Reanalyzing the number of alpha tracks and the alpha emission rate, it was reasonable that alpha particles from 212Bi were detected but those from 212Po were not. Based on these energy dependence results, the incident angle dependence corresponding to alpha particle energy

3 A Measurement Technique of the Equilibrium Equivalent Thoron Concentration 61 Fig. 3 Incident angle dependence of the CR-39 detector at the alpha particle energy of 5. 5 MeV. Fig. 5 Pulse height distribution of alpha particles emitted from thoron progeny under the same geometric arrangement as shown in Fig. 1. inserting an Al foil between the two. To be highly sensitive, detection of alpha particles from both 212Bi and 212Po are desirable. Since 8. 8 MeV alpha particles emitted from 212Po cannot be detected as aforementioned, however, a sheet of 15,um thickness Al foil was inserted between the CR-39 detector and fiberglass filter. Fig. 5 illustrates the pulse height distribution of alpha particles emitted from thoron progeny under the same geometric arrangement as shown in Fig. 1. Since the mean energies of 212Bi and 212Po were estimated to be 3. 2 MeV and 6. 8 MeV, respectively, both alpha particles could be detected with the Fig. 4 Relationship between the incident alpha particle energy and the critical angle. was also investigated. Fig. 3 exemplifies the incident angle dependence when the incident energy is 5. 5 MeV. The critical angle is defined as the angle the detection efficiency is 50%. From Fig. 3, the critical angle corresponding to 5. 5 MeV was estimated to be 54%. After following the same procedure on different incident energies, the relationship between the incident energy and the critical angle was obtained as shown in Fig. 4. Those critical angles were distributed from 40% to 65% in the range of 2. 0 to 7. 7 MeV. Consequently the higher the incident energy, the smaller the critical angle as shown in another paper Determination of the EETC 212Pb is the main measuring subject in the measurement system. The reason can be explained as follows: 212Pb can be easily separated from mixed radon progeny because of its long half-life. Since more than 90% of the EETC depends on the 212Pb concentration, the EETC will not be overestimated even if the 212Bi concentration is roughly assumed. As the standard alpha-track registration, after air sample is taken on a glass microfiber filter and 6 h elapse, a CR-39 detector, is directly placed for 24 h after present technique. The geometric detection efficiency is given by the following equation: fi: geometric detection efficiency; O: critical angle. The detection efficiencies of 212Bi and 212Po were derived from the relationship between the incident angle and critical angle in Fig. 4. These detection efficiencies were estimated to be 28. 9% and 17. 2%, respectively, The number of alpha tracks registered results from 212Pb decay. With a unit concentration and unit flow rate, the number of alpha disintegrations of 212Pb decay registered after sampling is stopped (IbC[Bqs])) can be given by eqn (2): IbC21e-AbTw(1e-AbTS)(1-e-AbTm) C 21ae-AcTw(1e-AcTs)(1-e-AcTm) TS: sampling period [s]; Tw: elapsed time after sampling [s]; Tm: measurement period [s]; 2b, AC: decay constants of 212Pb and 212Bi, respectively. (2)

4 62 S. TOKONAMI, Q. SUN, H. YONEHARA and Y. YAMADA Using the above values and timetable, the relationship between 212Pb concentration and alpha track density can be expressed as the following equation: (N-Nbg)A-IbcXXPb212XvXBi-212X IbcXXPb212XVXl7Po-212X (3) N: alpha-track density [mm 2]; Nbg: background alpha-track density [mm-2]; A: effective area of filter [mm2]; XPb212: 212Pb concentration [Bqm-3]; v: flow rate [m3 s-1]; nbi-212, 7Po212: detection efficiency for 212Bi and 212Po alpha particles, respectively. Assuming that these detection efficiencies are equal to geometric detection efficiencies aforementioned with given values, eqn (3) can be rewritten as follows: XPb212-Tvfl)1Za (4) The detection limit of the 212Pb concentration with the proposed technique was evaluated with Currie's formula12. For instance, when the sampling period is 8 h with the flow rate of 1. 0 L min-1, the elapsed time after sampling is 6 h, the measurement period is 24 h and the background track density is mm-2, the lowest detectable concentration was estimated to be Bq m-3. Since more than 90% of the EETC depends on the 212Pb concentration, the EETC can be approximately estimated from 212Pb concentration as below. Even though the concentration ratio of 212Pb to 212Bi changes from 1: 0 to 1:1, the EETC will scarcely change. Since it is reasonable that 212Pb: 212Bi=1: 0. 5, the EETC can be determined with the following equation: EETC=0.957Xpb212 (5) In order to verify the validity of the proposed measurement technique, this method was compared with the alpha spectroscopic method. Fig. 6 shows the comparison between the proposed technique and alpha spectroscopic Fig. 6 Comparison of the EETC with between the proposed technique and the alpha spectroscopy. method. Since there is a relatively good agreement between the two, this technique is applicable to the practical EETC determination Iv CONCLUSION at field measurements. For the purpose of the practical EETC determination, a simply measurement technique with a CR-39 detector was discussed. The technique can provide unlimited use any as long as air sampling is carried out with a DC driving pump. Although the measurement of thoron concentrations is useful for screening of high thoron exposure, thoron progeny concentrations have to be determined in any case for dose assessment. Such data are of great interest all over the world, this simple technique will facilitate their data accumulation. The authors are grateful to Ryuhei Kurosawa (Professor Emeritus at Waseda University) for his useful suggestions throughout the present work. REFERENCES 1) M. Doi and S. KOBAYASHI; Characterization of Japanese wooden houses with enhanced radon and thoron concentrations, Health Phys., 66, (1994). 2) J. MA, H. YONEHARA, T. AOYAMA, M. DOI, S. KOBAYASHI and M. SAKANOUE; Influence of air flow on the behavior of thoron and its progeny in a traditional Japanese house, Health Phys., 72, (1997). 3) W. CHUNG, S. TOKONAMI and M. FURUKAWA; Preliminary survey on radon and thoron concentrations in Korea, Radiat. Prot. Dosim., 80, (1998). 4) L. F. TOUSSAINT, S. TOKONAMI, M. DOI, S. B. SOLOMON and JR. PEGGIE; The measurement of thoron concentrations in Australia using the Japanese passive R-T dosimeter, In Proc. of the 7th Tohwa University International Symposium (ed. by A. KATASE and M. SHIMO), pp (1998), World Scientific, Singapore. 5) A. REICHELT, K.-H. LEHMANN, W. HAUK and FL. LEHR; Unrestricted release of a thorium-contaminated building, In Proc. of the 10th International Congress of the International Radiation Protection Association, May , Hiroshima, Japan (2000). 6) M. DOI, K. FUJIMOTO, S. KOBAYASHI and H. YONEHARA; Spatial distribution of thoron and radon concentrations in the indoor air of a traditional Japanese wooden house, Health Phys., 66, (1994). 7) T. YAMASAKI and T. IIDA; Measurements of thoron progeny concentration using a potential alpha-energy monitor in Japan, Health Phys., 68, (1995). 8) W. ZHUO and T. IIDA; An instrument for measuring

5 A Measurement Technique of the Equilibrium Equivalent Thoron Concentration 63 equilibrium equivalent concentrations of az2rn and 220Rn with etched track detectors, Health Phys., 77, (1999). 9) W. ZHUO and T. IIDA; Estimation of thoron progeny concentrations in dwellings with their deposition rate measurements, J. Health Phys., 35, (2000). 10) D. E. MARTZ, D. F. HOLLEMAN, D. E. McCURDY and K. J. SCHAGER; Analysis of atmospheric concentrations of RaA, RaB and RaC by alpha spectroscopy, Health Phys., 17, (1969). 11) R. BARILLON, M. FROMM and A. CHAMBAUDET; Variation of the critical registration angle of alpha particles in CR-39: Implication for radon dosimetry, Radiat. Meas., 25, (1995). 12) L. A. CURRIE; Limits for qualitative detection and quantitative determination, Analyst. Chem., 40, (1968). Shinji TOKONAMI Senior Researcher at National Institute of Radiological Sciences. Speciality; 1) environmental monitoring on radon and its progeny. 2) Relevant environmental dosimetry. Doctor's degree in applied physics from the Waseda University.

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