インドの高自然放射線地域におけるモナザイトに含まれる天然放射性核種と希土類元素の分布に関する研究

東京都立大

Precise measurement of uranium isotope ratios in Fukushima soils using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Precise measurement of 234U/238U and 235U/238U isotope ratios was performed using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in this study. The 234U ions were measured using a Daly detector, the 235U ions were measured using a Faraday cup with a 1012 U resistor, and the 238U ions were measured using a Faraday cup with the usual 1011 U resistor. Chemical separation of uranium was carried out by three different methods using either combinations of anion exchange and UTEVA resins or UTEVA resin three times. Chromatography combining three UTEVA resin columns provided the highest recovery (%) of uranium and biggest improvement in precise measurements ofuranium isotope ratios. The certificate reference material (CRM) JLk-1 was used as an in-house standard for the uranium isotope ratio measurements. The long-term reproducibility of the in-house standard was 0.60% for 234U/238U and 0.05% for 235U/238U. The measured ratios were compared with the natural ratios,and the relative deviation obtained for uranium isotope ratios was less than 0.005% for 234U/238U and 235U/238U. Finally, the present method was applied to measure uranium isotope ratios in Fukushima microwave-digested soil samples

Geochemical behavior of uraniumand thoriumin sand and sandy soil samples from a natural high background radiation area of the Odisha coast, India

Owing to their natural radioactivity, uranium (U) and thorium (Th) play significant roles in environmental sciences for monitoring radiation dose and in geological sciences for understanding sedimentary processes. The Odisha coastal area, ineastern India, is a well-known high background radiation area that is rich in monazites and rutile. This area was selected to study geochemical characteristics of U and Th in sand and sandy soil samples. The concentrations of U and Th weremeasured using inductively coupled plasma mass spectrometry (ICP-MS). The median, geometric mean, and standard deviation for U were determined to be 6, 4.5, and 2.5 μg/g and for Th were 186, 123.3, and 3.1 μg/g, respectively. Major element concentrations were evaluated using X-ray fluorescence spectroscopy to get the mineralogical composition and state of chemical weathering. The ratios of Th/U and Th/K varied from 4 to 37 and from 13 to 1058, respectively. These resultsclearly indicate that the samples from the coastal region were formed in an oxidizing and intense chemical weathering terrestrial environment with an enrichment of radiogenic heavy minerals (monazites and zircon) and clay mineral association. Since the majority of the samples have undergone moderate to intense weathering in the oxidizing environment, U is leached from the soil and sand matrix. Eventually, Th resides in the matrix and becomes a major source for radiation exposure in the environment. The high ratios of Th/U, along with the strong positive correlation between Th and P2O5, evidence the enrichment of the Th-bearing radioactive mineral, monazite, in these samples

ICP-MS Measurement of Trace and Rare Earth Elements in Beach Placer-Deposit Soils of Odisha, East Coast of India, to Estimate Natural Enhancement of Elements in the Environment

Inductively coupled plasma mass spectrometry (ICP-MS) has been used to measure the concentration of trace and rare earth elements (REEs) in soils. Geochemical certified reference materials such as JLk-1, JB-1, and JB-3 were used for the validation of the analytical method. The measured values were in good agreement with the certified values for all the elements and were within 10% analytical error. Beach placer deposits of soils mainly from Odisha, on the east coast of India, have been selected to study selected trace and rare earth elements (REEs), to estimate enrichment factor (EF) and geoaccumulation index (Igeo) in the natural environment. Enrichment factor (EF) and geoaccumulation index (Igeo) results showed that Cr, Mn, Fe, Co, Zn, Y, Zr, Cd and U were significantly enriched, and Th was extremely enriched. The total content of REEs (ƩREEs) ranged from 101.3 to 12,911.3 µg g−1, with an average 2431.1 µg g−1 which was higher than the average crustal value of ΣREEs. A high concentration of Th and light REEs were strongly correlated, which confirmed soil enrichment with monazite minerals. High ratios of light REEs (LREEs)/heavy REEs (HREEs) with a strong negative Eu anomaly revealed a felsic origin. The comparison of the chondrite normalized REE patterns of soil with hinterland rocks such as granite, charnockite, khondalite and migmatite suggested that enhancement of trace and REEs are of natural origin

Geochemical characterization of monazite sands in placer deposit from Kanyakumari southern coast of Tamil Nadu, India: Implication of Uranium isotope ratios and high content of rare earth elements

Kanyakumari region in southern coast of Tamil Nadu, India is a well-known natural high background radiation area due to the abundances of monazite in the beach placer deposits. Natural gamma dose rate measurements were carried out covering an area of 50 km along the coastal belt. The ambient dose rate varied from 65 nGy h-1 to 25.5 Gy h-1. Based on the ambient dose rate 23 locations were selected for sand sample collection. The main aim of the study is to understand radiological as well as geochemical characterization of placer deposit. Monazite is an ore mineral for Th with relatively high concentration of Ce, Nd and U. The activity concentration of 226Ra, 228Ac and 40K were measured by a High purity Ge gamma spectroscopy and associated radiological hazard parameters were estimated. The concentration of major oxides and U, Th and REEs were measured using XRF and ICP-MS respectively. TiO2 and Fe2O3 concentrations are high compared to average upper continental crust. High TiO2 could be attributed to the presence of ilmenite in the sands. Average ƩREE concentration in sand samples is about thirty times higher than the average crustal concentration. The LREE/HREE is greater than 1 with negative Eu anomaly, it could be either felsic or granitic source. On the other hand, Th/U ratio ranged from 8 to 24 which clearly indicates Th is the dominant radionuclide for the enhanced natural radioactivity in the environment. The concentration of U varied from 18 to 230 g g-1. The 234U/238U and 235U/238U isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). However, the activity ratio of U ranged from 0.97 to 1.06 in the samples. The detail information will be discussed during presentation.VII. Terrestrial Radioisotopes in Environment International Conference on Environmental Protection, 10-13 AUGUST, 2020 (TREICEP 2020

Chemical Separation of Uranium and Precise Measurement of 234U/238U and 235U/238U Ratios in Soil Samples Using Multi Collector Inductively Coupled Plasma Mass Spectrometry

A new chemical separation has been developed to isolate uranium (U) using two UTEVA columns to minimize iron and thorium interferences from high background area soil samples containing minerals like monazites and ilmenite. The separation method was successfully verified in some certified reference materials (CRMs), for example, JSd-2, JLk-1, JB-1 and JB-3. The same method was applied for purification of U in Fukushima soil samples affected by the Fukushima dai-ichinuclear power station (FDNPS) accident. Precise and accurate measurement of 234U/238U and 235U/238U isotope ratios in chemically separated U were carried out using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). In this mass spectrometric method, an array of two Faraday cups (10e11 Ω, 10e12 Ω resistor) and a Daly detector were simultaneously employed. The precision of U isotope ratios in an in-house standard was evaluated by replicatemeasurement. Relative standard deviation (RSD) of 234U/238U and 235U/238U were found to be 0.094% (2σ) and 0.590% (2σ), respectively. This method has been validated using a standard reference material SRM 4350B, sediment sample. The replicate measurements of 234U/238U in SRM shows 0.7% (RSD). This developed method is suitable for separation of U and its isotope ratio measurement in environmental samples

Changes of absorbed dose rate in air in metropolitan Tokyo relating to radiocesium released from the Fukushima Daiichi Nuclear Power Plant accident: Results of a five-year study.

Car-borne surveys were carried out in metropolitan Tokyo, Japan, in 2015, 2016, 2017 and 2018 to estimate the transition of absorbed dose rate in air from the Fukushima Daiichi Nuclear Power Plant accident. Additionally, the future transition of absorbed dose rates in air based on this five-year study and including previously reported measurements done in 2014 by the authors was analyzed because central Tokyo has large areas covered with asphalt and concrete. The average absorbed dose rate in air (range) in the whole area of Tokyo measured in 2018 was 59 ± 9 nGy h-1 (28-105 nGy h-1), and it was slightly decreased compared to the previously reported value measured in 2011 (61 nGy h-1; 30-200 nGy h-1). In the detailed dose rate distribution map, while areas of higher dose rates exceeding 70 nGy h-1 had been observed on the eastern and western ends of Tokyo after 2014, the dose rates in these areas have decreased yearly. Especially, the decreasing dose rate from radiocesium (Cs-134 + Cs-137) in the eastern end of Tokyo which is mainly covered by asphalt was higher than that measured in the western end which is mainly covered by forest. The percent reductions for the eastern end in the years 2014-2015, 2015-2016, 2016-2017 and 2017-2018 were 49%, 21%, 18% and 16%, and those percent reductions for western end were 26%, 18%, 6% and 3%, respectively. Additionally, the decrease for dose rate from radiocesium depended on the types of asphalt, and that on porous asphalt was larger than the decrease on standard asphalt