Norikazu Kinoshita

Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan

A tremendous amount of radioactivity was discharged because of the damage to cooling systems of nuclear reactors in the Fukushima No. 1 nuclear power plant in March 2011. Fukushima and its adjacent prefectures were contaminated with fission products from the accident. Here, we show a geographical distribution of radioactive iodine, tellurium, and cesium in the surface soils of central-east Japan as determined by gamma-ray spectrometry. Especially in Fukushima prefecture, contaminated area spreads around Iitate and Naka-Dori for all the radionuclides we measured. Distributions of the radionuclides were affected by the physical state of each nuclide as well as geographical features. Considering meteorological conditions, it is concluded that the radioactive material transported on March 15 was the major contributor to contamination in Fukushima prefecture, whereas the radioactive material transported on March 21 was the major source in Ibaraki, Tochigi, Saitama, and Chiba prefectures and in Tokyo.

A Shorter 146Sm Half-Life Measured and Implications for 146Sm-142Nd Chronology in the Solar System

A New Lease on Half-Life Radiometric dating relies on measuring the abundance of long-lived radionuclides relative to the abundance of their radiogenic decay products—a process determined by the original radionuclides half-life. For primordial radionuclides that decay slowly, such as 146Sm decaying to 142Nd, this method provides the timing of some of the earliest processes in solar system history. Using accelerator mass spectrometry, Kinoshita et al. (p. 1614) provide a revised estimate for the 146Sm half-life of ∼68.7 million years, which is 30% shorter than the previously accepted value. This shorter half-life suggests that reductions need to be made in the estimated ages for differentiation of Earths mantle and the solidification of the Moons magma ocean and for other more recent processes. Mantle differentiation of Earth, the Moon, and Mars occurred earlier and over a shorter time scale than previously estimated. The extinct p-process nuclide 146Sm serves as an astrophysical and geochemical chronometer through measurements of isotopic anomalies of its α-decay daughter 142Nd. Based on analyses of 146Sm/147Sm α-activity and atom ratios, we determined the half-life of 146Sm to be 68 ± 7 (1σ) million years, which is shorter than the currently used value of 103 ± 5 million years. This half-life value implies a higher initial 146Sm abundance in the early solar system, (146Sm/144Sm)0 = 0.0094 ± 0.0005 (2σ), than previously estimated. Terrestrial, lunar, and martian planetary silicate mantle differentiation events dated with 146Sm-142Nd converge to a shorter time span and in general to earlier times, due to the combined effect of the new 146Sm half-life and (146Sm/144Sm)0 values.

Adsorption of Db and its homologues Nb and Ta, and the pseudo-homologue Pa on anion-exchange resin in HF solution

Abstract Anion-exchange chromatography of element 105, dubnium (Db), produced in the 248Cm( 19F, 5n) 262Db reaction is investigated together with the homologues Nb and Ta, and the pseudo-homologue Pa in 13.9 M hydrofluoric acid (HF) solution. The distribution coefficient (Kd) of Db on an anion-exchange resin is successfully determined by running cycles of 1702 chromatographic column separations. The result clearly indicates that the adsorption of Db on the resin is significantly different from that of the homologues and that the adsorption of anionic fluoro complexes of these elements decreases in the sequence of Ta≈ Nb>Db≥Pa.

Experimental studies of shielding and irradiation effects at high-energy accelerator facilities

Abstract Experimental studies of shielding and radiation effects are carried out at Fermi National Accelerator Laboratory (FNAL) under collaboration between FNAL and Japan, aiming at benchmarking simulation codes and studying irradiation effects for the upgrade and design of new high-energy accelerator facilities. The purposes of this collaboration are (a) acquisition of shielding data in a proton beam energy region above 100 GeV, (b) further evaluation of predictive accuracy of the PHITS and MARS codes, (c) modification of physics models and data in these codes if needed, (d) characterization of radiation fields for studies of radiation effects, and (e) development of a code module for an improved description of radiation effects. The first campaign of the experiment was carried out at the Pbar target station and NuMI experimental station at FNAL, which use irradiation of targets with 120-GeV protons for antiproton and neutrino production, respectively. The generated secondary particles passing through steel, concrete, and rock were measured by activation methods as well as by other detectors such as a scintillator with a veto counter, phoswich detector, and a Bonner ball counter on trial. Preliminary experimental and calculated results are presented.

Ultra-sensitive detection of p-process nuclide 146Sm produced by (γ, n), (p, 2nε) and (n, 2n) reactions

We are developing a technique of ultra-sensitive detection of the p-process 146Sm nuclide by accelerator mass spectrometry. 146Sm was produced via the reactions (γ,n), (p,2ne) and (n,2n) on 147Sm. Preliminary results demonstrate for the first time the capability of identifying unambiguously 146Sm through separation and discrimination of its stable 146Nd isobar and other background ions. We consider applying the detection method to an independent determination of 146Sm half-life and to the measurements of cross sections of low-energy nuclear reactions producing long-lived nuclides in the vicinity of 146Sm.

Vertical distribution and formation analysis of the 131I, 137Cs, 129mTe, and 110mAg from the Fukushima Dai-ichi Nuclear Power Plant in the beach soil

We have investigated vertical distributions of 110mAg, 131I, 129mTe, and 137Cs and existence in beach soil collected three months after the Great East Japan Earthquake. All the nuclides passed through tsunami deposit and original beach soil, then reached fine particle layer lying beneath them in the three months. Moreover, depth profiles of all the nuclides did not exhibit exponential decrease. Behavior of 131I, 129mTe, and 137Cs were explained with distribution coefficients (Kd) but that of 110mAg was not explained. Particulate form was observed for the 137Cs; number of the particle was proportional to the 137Cs concentration in the soil sample.

Anthropogenic Pu distribution in Tropical East Pacific.

The geographical distribution of the anthropogenic radionuclides (238)Pu and (239+240)Pu in the Tropical East Pacific in 2003 was studied from the viewpoint of material migration. We measured the contents of Pu isotopes in seawater and in sediment from the sea bottom. The distributions of Pu isotopes, together with those of coexisting nitrate and phosphate species and dissolved oxygen, are discussed in relation to the potential temperature and potential density (sigma-θ). The Pu contents in sediment samples were compared with those in the seawater. Horizontal migration across the Equator from north to south was investigated at depths down to ~800m in the eastern Pacific. The Pu distribution at 0-400m correlated well with the distribution of potential temperature. Maximum Pu levels were observed in the subsurface layer at 600-800m, corresponding to the depth where sigma-θ≈27.0. It is suggested that the Pu distribution depends on the structure of the water mass and the particular temperature and salinity. The water column/sediment column inventory ratio and the vertical distribution of Pu may reflect the efficiency of scavenging in the relevant water areas.

ADSORPTION BEHAVIOR OF RADIONUCLIDES ON ION-EXCHANGE RESIN FROM COOLING WATER FOR THE K2K TARGET AND MAGNETIC HORNS

Abstract The beam power in a new project of a long-baseline neutrino oscillation experiment from Tokai to Kamioka (the T2K experiment) will be approximately 100 times higher than that in a substantial long-baseline neutrino oscillation experiment from the High Energy Accelerator Research Organization (KEK) to Kamioka (the K2K experiment). In the T2K experiment, radionuclides at serious activity levels will be produced in a target, magnetic horns, and partially dissolved into the cooling water. Radionuclides at serious activity levels will also be produced in the cooling water. Therefore, we measured the total activity and distribution of 7Be, 22Na, 54Mn, 57Co, 60Co, 88Y, 101gRh, 102mRh, and 110mAg collected on a demineralizer in the K2K water cooling system, in order to consider reducing both exposure to personnel from the demineralizer and radioactivity released by draining the cooling water at regular intervals in the T2K experiment. The total activity of the individual nuclides was estimated to range from 0.9 MBq to 0.7 GBq at the end of the 2-yr K2K operation. When the results are projected to the T2K experiment, 70 GBq of 7Be and 6 GBq of 22Na are particularly high, and shielding from the radiation must be provided for the entire water system. Half of the demineralizer was saturated with the Al, Cu, and Ag ions dissolved from the target and magnetic horns. When the entire column is saturated with the ions, all high activity of 22Na located at the bottom of the demineralizer will be released into the cooling water immediately. Although 88Y, 101gRh, and 102mRh cannot be collected completely because of weak retention by hydrolysis and/or association with colloid, a reduction in the surface area of the Ag metal is possible for new magnetic horns and will result in a decrease of the activities in the cooling water.

Search for α-decay of 229mTh produced from 229Ac β-decay following 232Th(γ, p2n) reaction

Summary We produced 229Th in the nuclear reaction 232Th(γ, p2n) 229Ac, followed by disintegration to 229m,gTh. The 229mTh α-decay was searched for. Alpha-particle events of energy between 4.93 MeV and 5.05 MeV were observed in the separated thorium sample of β-decay products from an actinium source highly purified from the 232Th + γ reaction products. The energies of the α-particles coincide with those expected for 229mTh.

Energy shift of electronic X-rays emitted from pionic atoms

Summary The electronic K X-ray energies of pionic atoms were precisely measured on elements ranging from zinc to lead with a low-energy photon detection system. Some differences between pionic and muonic atoms were found in the atomic-number (Z) dependence of the energy shift. In the pionic atoms, the energy shift gradually decreases with an increase in atomic number, which is opposite to the trend for muonic atoms. In the low-Z region investigated elements, the energy shifts of Kβ X-rays were obviously larger than those of Kα X-rays.