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Aoki J, Wakaki S, Ishiniwa H, Kawakami T, Miyazaki T, Suzuki K, Takagai Y. Direct Quantification of Attogram Levels of Strontium-90 in Microscale Biosamples Using Isotope Dilution-Thermal Ionization Mass Spectrometry Assisted by Quadrupole Energy Filtering. Anal Chem 2023; 95:4932-4939. [PMID: 36906855 DOI: 10.1021/acs.analchem.2c04844] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/13/2023]
Abstract
Although thermal ionization mass spectrometry (TIMS) has been employed for the high-precision analysis of isotope ratios, direct quantification of artificial mono-nuclide in the environment is difficult by even using isotope dilution (ID) due to the coexistence of the great magnitude of natural stable nuclides or isobars. In traditional TIMS and ID-TIMS, a sufficient amount of stable Sr doped on a filament is required to realize a stable and adequate ion-beam intensity (i.e., thermally ionized beams). However, the background noise (BGN) at m/z 90, detected by an electron multiplier, disturbs 90Sr analysis at low concentration levels due to peak tailing of a significant 88Sr ion beam dependent on the 88Sr-doping amount. Here, TIMS assisted by quadruple energy filtering was successfully employed for the direct quantification of attogram levels of an artificial monoisotopic radionuclide strontium-90 (90Sr) in microscale biosamples. Direct quantification was achieved by integrating the ID quantification of natural Sr and simultaneous 90Sr/86Sr isotope ratio analysis. Additionally, the measurement amount calculated by the combination of the ID and intercalibration was corrected for the net result amount of 90Sr by subtracting dark noise and the detected amount derived from the survived 88Sr, which are equivalent with the BGN intensity at m/z 90. Background correction revealed that the detection limits were in the range of 6.15 × 10-2-3.90 × 10-1 ag (0.31-1.95 μBq), depending on the concentration of natural Sr in a 1 μL sample, and the quantification of 0.98 ag (5.0 μBq) of 90Sr in 0-300 mg/L of natural Sr was successful. This method could analyze small sample quantities (1 μL), and the quantitative results were verified against authorized radiometric analysis techniques. Furthermore, the amount of 90Sr in actual teeth was successfully quantified. This method will be a powerful tool for measuring 90Sr in the measurement of micro-samples, which are required to assess and understand the degree of internal radiation exposure.
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Affiliation(s)
- Jo Aoki
- Faculty of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
| | - Shigeyuki Wakaki
- Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe Otsu, Nankoku, Kochi 783-8502, Japan
| | - Hiroko Ishiniwa
- Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima 960-1296 Japan
| | | | - Takashi Miyazaki
- Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natushima, Yokosuka, Kanagawa 237-0061, Japan
| | - Katsuhiko Suzuki
- Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natushima, Yokosuka, Kanagawa 237-0061, Japan
| | - Yoshitaka Takagai
- Faculty of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
- Institute of Environmental Radioactivity, Fukushima University, 1 Kanayagawa, Fukushima 960-1296 Japan
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Zhou Z, Ren H, Zhou L, Wang P, Lou X, Zou H, Cao Y. Recent Development on Determination of Low-Level 90Sr in Environmental and Biological Samples: A Review. MOLECULES (BASEL, SWITZERLAND) 2022; 28:molecules28010090. [PMID: 36615288 PMCID: PMC9821828 DOI: 10.3390/molecules28010090] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 11/15/2022] [Revised: 12/15/2022] [Accepted: 12/16/2022] [Indexed: 12/24/2022]
Abstract
In the context of the rapid development of the world's nuclear power industry, it is vital to establish reliable and efficient radioanalytical methods to support sound environment and food radioactivity monitoring programs and a cost-effective waste management strategy. As one of the most import fission products generated during human nuclear activities, 90Sr has been widely determined based on different analytical techniques for routine radioactivity monitoring, emergency preparedness and radioactive waste management. Herein, we summarize and critically review analytical methods developed over the last few decades for the determination of 90Sr in environmental and biological samples. Approaches applied in different steps of the analysis including sample preparation, chemical separation and detection are systematically discussed. The recent development of modern materials for 90Sr concentration and advanced instruments for rapid 90Sr measurement are also addressed.
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Affiliation(s)
| | | | | | | | | | | | - Yiyao Cao
- Correspondence: ; Tel.: +86-(0571)-87115089
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3
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Evaluation of sperm fertilization capacity of large Japanese field mice (Apodemus speciosus) exposed to chronic low dose-rate radiation after the Fukushima accident. JOURNAL OF RADIATION RESEARCH AND APPLIED SCIENCES 2022. [DOI: 10.1016/j.jrras.2022.07.001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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4
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Cao Y, Zhou L, Ren H, Zou H. Determination, Separation and Application of 137Cs: A Review. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH 2022; 19:ijerph191610183. [PMID: 36011815 PMCID: PMC9408292 DOI: 10.3390/ijerph191610183] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2022] [Revised: 08/13/2022] [Accepted: 08/14/2022] [Indexed: 05/21/2023]
Abstract
In the context of the rapid development of the world's nuclear power industry, it is necessary to establish background data on radionuclides of different samples from different regions, and the premise of obtaining such basic data is to have a series of good sample processing and detection methods. The radiochemical analysis methods of low-level radionuclides 137Cs (Cesium) in environmental and biological samples are introduced and reviewed in detail. The latest research progress is reviewed from the five aspects of sample pretreatment, determination, separation, calculation, application of radioactive cesium and the future is proposed.
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Oka T, Takahashi A, Koarai K, Kino Y, Sekine T, Shimizu Y, Chiba M, Suzuki T, Osaka K, Sasaki K, Shinoda H. Detection limit of electron spin resonance for Japanese deciduous tooth enamel and density separation method for enamel-dentine separation. JOURNAL OF RADIATION RESEARCH 2022; 63:609-614. [PMID: 35780302 PMCID: PMC9303628 DOI: 10.1093/jrr/rrac033] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/07/2021] [Revised: 08/27/2021] [Indexed: 06/15/2023]
Abstract
Electron spin resonance (ESR) dosimetry is one of the most powerful tools for radiation dose reconstruction. The detection limit of this technique using human teeth is reported to be 56 mGy or 67 mGy; however, the absorbed dose of Fukushima residents after the Fukushima Daiichi Nuclear Power Plant (FNPP) accident was estimated to be lower than this detection limit. Our aim is to assess the absorbed radiation dose of children in Fukushima Prefecture after the accident; therefore, it is important to estimate the detection limit for their teeth. The detection limit for enamel of deciduous teeth of Japanese children separated by the mechanical method is estimated to be 115.0 mGy. The density separation method can effectively separate enamel from third molars of Japanese people. As we have collected thousands of teeth from children in Fukushima, the present technique may be useful to examine their external absorbed dose after the FNPP accident.
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Affiliation(s)
- Toshitaka Oka
- Corresponding author. Research Group for Nuclear Chemistry, Chemistry, Environment and Radiation Division, Nuclear Science and Engineering Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan.
| | - Atsushi Takahashi
- Tohoku University Hospital, Tohoku University, 1-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8574, Japan
| | - Kazuma Koarai
- Collaborative Laboratories for Advanced Decommissioning Science, Sector of Fukushima Research and Development, Japan Atomic Energy Agency, 790-1 Ohtsuka, Motooka, Tomioka Town, Futaba County, Fukushima 979-1151, Japan
| | - Yasushi Kino
- Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai City, Miyagi 980-8576, Japan
| | - Tsutomu Sekine
- Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-aoba, Aoba-ku, Sendai City, Miyagi 980-8578, Japan
- Institute for Excellence in Higher Education, Tohoku University, 41 Kawauchi, Aoba-ku, Sendai City, Miyagi 980-8576, Japan
| | - Yoshinaka Shimizu
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
| | - Mirei Chiba
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
| | - Toshihiko Suzuki
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
| | - Ken Osaka
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
| | - Keiichi Sasaki
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
| | - Hisashi Shinoda
- Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai City, Miyagi 980-8575, Japan
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Zhang F, Wang J, Bi Q, Du J. 90Sr in seawater of the East China Sea: Inventory, new potential source, and environmental implications. THE SCIENCE OF THE TOTAL ENVIRONMENT 2021; 764:144266. [PMID: 33401046 DOI: 10.1016/j.scitotenv.2020.144266] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2020] [Revised: 11/30/2020] [Accepted: 11/30/2020] [Indexed: 06/12/2023]
Abstract
90Sr is useful for tracing water mass movement in oceans. We collected a suit of seawater samples from the East China Sea (ECS) in the May 2011 to investigate the spatial and vertical distribution of 90Sr and to understand its transportation and fate. To understand the sources and transportation of 90Sr more clearly, published 137Cs data from the same cruise were used to obtain the 90Sr/137Cs activity ratios. The results showed that 90Sr activities were controlled by the circulation system of the ECS, with high values in coastal regions and low values in oceanic waters. The plume with a high 90Sr/137Cs ratio showed that in late spring, the Changjiang Diluted Water could flow southeastward and extend to 126-127° E, which is farther than the previously known value of 124° E. The high 90Sr/137Cs ratios (1.35 ± 0.62) and a long effective half-life of 90Sr (20.0 ± 0.3 y) in the ECS surface water revealed that 90Sr is surplus in comparison with 137Cs. However, historical variations in the 90Sr/137Cs ratio seem to preclude the simple explanation that riverine input causes a 90Sr surplus in the ECS. Groundwater discharge with a high 90Sr but very low 137Cs may be a new potential source. However, it is difficult to quantify the contribution of groundwater discharge at present, and more detailed studies are required in this regard. Additionally, we compiled 90Sr and 137Cs water column inventory data in the western North Pacific and found that the cumulative fallout onto the ocean was different from that on land in the 20-40° N band.
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Affiliation(s)
- Fule Zhang
- State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200241, China
| | - Jinlong Wang
- State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200241, China.
| | - Qianqian Bi
- State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200241, China
| | - Jinzhou Du
- State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200241, China
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Komatsu K, Iwasaki T, Murata K, Yamashiro H, Goh VST, Nakayama R, Fujishima Y, Ono T, Kino Y, Simizu Y, Takahashi A, Shinoda H, Ariyoshi K, Kasai K, Suzuki M, Palmerini MG, Belli M, Macchiarelli G, Oka T, Fukumoto M, Yoshida MA, Nakata A, Miura T. Morphological reproductive characteristics of testes and fertilization capacity of cryopreserved sperm after the Fukushima accident in raccoon (Procyon lotor). Reprod Domest Anim 2021; 56:484-497. [PMID: 33372327 DOI: 10.1111/rda.13887] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2020] [Accepted: 12/27/2020] [Indexed: 12/17/2022]
Abstract
Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, we have established an archive system of livestock and wild animals from the surrounding ex-evacuation zone. Wildlife within the alert zone have been exposed to low-dose-rate (LDR) radiation for a long continuous time. In this study, we analysed the morphological characteristics of the testes and in vitro fertilization (IVF) capacity of cryopreserved sperm of racoons from the ex-evacuation zone of the FDNPP accident. The radioactivity of caesium-137 (137 Cs) was measured by gamma-ray spectrometry, and the measured radioactivity concentration was 300-6,630 Bq/kg in the Fukushima raccoons. Notably, normal spermatogenesis was observed in the seminiferous tubules of the testes, with the germinal epithelium composed of a spermatogenic cell lineage with no evident ultrastructural alterations; freeze-thawing sperm penetration ability was confirmed using the interspecific zona pellucida-free mouse oocytes IVF assays. This study revealed that the chronic and LDR radiation exposure associated with the FDNPP accident had no adverse effect on the reproductive characteristics and functions of male raccoons.
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Affiliation(s)
- Kazuki Komatsu
- Graduate School of Science and Technology, Niigata University, Niigata, Japan
| | - Tsugumi Iwasaki
- Graduate School of Science and Technology, Niigata University, Niigata, Japan
| | - Kosuke Murata
- Graduate School of Science and Technology, Niigata University, Niigata, Japan
| | - Hideaki Yamashiro
- Graduate School of Science and Technology, Niigata University, Niigata, Japan
| | | | - Ryo Nakayama
- Graduate School of Health Sciences, Hirosaki University, Aomori, Japan
| | - Yohei Fujishima
- Department of Radiation Biology, Tohoku University School of Medicine, Sendai, Japan
| | - Takumi Ono
- Graduate School of Science, Tohoku University, Sendai, Japan
| | - Yasushi Kino
- Graduate School of Science, Tohoku University, Sendai, Japan
| | | | | | - Hisashi Shinoda
- Graduate School of Dentistry, Tohoku University, Sendai, Japan
| | - Kentaro Ariyoshi
- Integrated Center for Science and Humanities, Fukushima Medical University, Fukushima, Japan
| | - Kosuke Kasai
- Graduate School of Health Sciences, Hirosaki University, Aomori, Japan
| | - Masatoshi Suzuki
- International Research Institute of Disaster Science, Tohoku University, Sendai, Japan
| | - Maria Grazia Palmerini
- Department of Life, Health and Environmental Sciences, University of L'Aquila, L'Aquila, Italy
| | - Manuel Belli
- Department of Life, Health and Environmental Sciences, University of L'Aquila, L'Aquila, Italy
| | - Guido Macchiarelli
- Department of Life, Health and Environmental Sciences, University of L'Aquila, L'Aquila, Italy
| | - Toshitaka Oka
- Sector of Nuclear Science Research, Japan Atomic Energy Agency, Ibaraki, Japan
| | - Manabu Fukumoto
- RIKEN Center for Advanced Intelligence Project, Pathology Informatics Team, Saitama, Japan
| | - Mitsuaki A Yoshida
- Department of Risk Analysis and Biodosimetry, Institute of Radiation Emergency Medicine, Hirosaki University, Aomori, Japan
| | - Akifumi Nakata
- Department of Life Science, Faculty of Pharmaceutical Sciences, Hokkaido University of Science, Hokkaido, Japan
| | - Tomisato Miura
- Department of Risk Analysis and Biodosimetry, Institute of Radiation Emergency Medicine, Hirosaki University, Aomori, Japan
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8
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Furukawa M, Takagi K, Matsunami H, Komatsuzaki Y, Kawakami T, Shinano T, Takagai Y. Rapid Quantification of Radioactive Strontium-90 in Fresh Foods via Online Solid-Phase Extraction-Inductively Coupled Plasma-Dynamic Reaction Cell-Mass Spectrometry and Its Comparative Evaluation with Conventional Radiometry. ACS OMEGA 2019; 4:11276-11284. [PMID: 31460230 PMCID: PMC6649156 DOI: 10.1021/acsomega.9b01381] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/13/2019] [Accepted: 06/13/2019] [Indexed: 05/15/2023]
Abstract
This paper describes a rapid quantification method for radioactive strontium (90Sr) in fresh foods (perishable foods) and has been comparatively evaluated with the common classical radiometric quantification method. Inductively coupled plasma-dynamic reaction cell-mass spectrometry with online solid-phase extraction (cascade-ICP-MS) rapidly determines 90Sr in a pure water-based sample. Despite its advantages, its application to fresh foods (perishable foods) has not yet been reported; however, the analytical potential of this method for fresh foods must be evaluated. In this study, 90Sr was determined in 12 fresh foods via improved cascade-ICP-MS (Icas-ICP-MS). Addition and recovery tests were demonstrated using real samples of grape, apple, peach, Japanese pear, rice, buckwheat, soybean, spinach, shiitake mushroom, grass, sea squirt, and flounder. With a decomposed solution of Japanese pear, the measurement value coincided with the amount of spiked 90Sr. The reproducibility of the measurements was represented by relative standard deviations of 14.2 and 5.0% for spiked amounts of 20 and 200 Bq/kg, respectively (n = 10), and the recovery rates were 93.7 ± 7.1%. In this case, the limit of detection (LOD) was 2.2 Bq/kg (=0.43 pg/kg). These results were compared with the data obtained using a common classical radiometric quantification method (nitrate precipitation-low background gas flow counter (LBC) method) in the same samples. Both the methods showed equivalent performances with regard to reproducibility, precision, and LODs but different analysis times. Icas-ICP-MS required ∼22 min for analysis, whereas the nitrate precipitation-LBC method required 20 days, confirming that Icas-ICP-MS is the suitable method for analyzing 90Sr in fresh foods.
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Affiliation(s)
- Makoto Furukawa
- Faculty
of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
- PerkinElmer
Japan Co., Ltd., 134
Godo, Hodogaya, Yokohama, Kanagawa 240-0005, Japan
- Faculty
of Agriculture, University of Tokyo, 1−1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
| | - Kyoko Takagi
- Fukushima
Research Station, Tohoku Agricultural Research Center, National Agriculture and Food Research Organization, 50 Harajuku-minami, Arai, Fukushima 960-2156, Japan
| | - Hisaya Matsunami
- Fukushima
Research Station, Tohoku Agricultural Research Center, National Agriculture and Food Research Organization, 50 Harajuku-minami, Arai, Fukushima 960-2156, Japan
| | | | | | - Takuro Shinano
- Fukushima
Research Station, Tohoku Agricultural Research Center, National Agriculture and Food Research Organization, 50 Harajuku-minami, Arai, Fukushima 960-2156, Japan
| | - Yoshitaka Takagai
- Faculty
of Symbiotic Systems Science, Cluster of Science and Technology, Fukushima University, 1 Kanayagawa, Fukushima 960-1296, Japan
- Institute
of Environmental Radioactivity, Fukushima
University, 1 Kanayagawa, Fukushima 960-1296, Japan
- E-mail: (Y.T.)
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Shao Y, Yang G, Tazoe H, Ma L, Yamada M, Xu D. A review of measurement methodologies and their applications to environmental 90Sr. JOURNAL OF ENVIRONMENTAL RADIOACTIVITY 2018; 192:321-333. [PMID: 30029206 DOI: 10.1016/j.jenvrad.2018.07.013] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2018] [Revised: 07/13/2018] [Accepted: 07/14/2018] [Indexed: 06/08/2023]
Abstract
The high fission yield product 90Sr has been released into the environment in large amounts due to nuclear weapon tests, nuclear power plant accidents, and nuclear fuel reprocessing industries. It is a long half-life radionuclide (28.9 y), with serious consequences to human health; hence, it is desirable to perform routine monitoring of 90Sr in environmental samples. Many 90Sr radiometric methods have been developed in the past decades, which generally require complicated separation and purification steps with a relatively long analytical time. Moreover, some nominally rapid methods usually have high method detection limits, making them unsuitable for the environmental samples with ultra-low 90Sr levels. In this review, some rapid and practical methods for 90Sr routine monitoring are summarized. Different sample pretreatments and major purification procedures for 90Sr developed in recent years, such as variable digestion methods and extraction chromatography using Sr resin or DGA resin, are especially described. Additionally, four conventional and widely used β spectrometric and mass spectrometric methods are demonstrated. Finally, 90Sr evaluations focusing on contaminated soil and seawater samples collected after the Fukushima Daiichi Nuclear Power Plant accident, and 90Sr application as tracers for environmental behavior are also reviewed.
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Affiliation(s)
- Yang Shao
- Beijing Engineering Research Center of Radiographic Techniques and Equipment, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China; Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan; School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China
| | - Guosheng Yang
- Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan
| | - Hirofumi Tazoe
- Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan
| | - Lingling Ma
- Beijing Engineering Research Center of Radiographic Techniques and Equipment, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
| | - Masatoshi Yamada
- Department of Radiation Chemistry, Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan.
| | - Diandou Xu
- Beijing Engineering Research Center of Radiographic Techniques and Equipment, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
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