Kimikazu Sasa

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.

Possible ferroelectricity in perovskite oxynitride SrTaO2N epitaxial thin films

Compressively strained SrTaO2N thin films were epitaxially grown on SrTiO3 substrates using nitrogen plasma-assisted pulsed laser deposition. Piezoresponse force microscopy measurements revealed small domains (101–102 nm) that exhibited classical ferroelectricity, a behaviour not previously observed in perovskite oxynitrides. The surrounding matrix region exhibited relaxor ferroelectric-like behaviour, with remanent polarisation invoked by domain poling. First-principles calculations suggested that the small domains and the surrounding matrix had trans-type and a cis-type anion arrangements, respectively. These experiments demonstrate the promise of tailoring the functionality of perovskite oxynitrides by modifying the anion arrangements by using epitaxial strain.

Ultra-trace analysis of 36 Cl by accelerator mass spectrometry: an interlaboratory study

A first international 36Cl interlaboratory comparison has been initiated. Evaluation of the final results of the eight participating accelerator mass spectrometry (AMS) laboratories on three synthetic AgCl samples with 36Cl/Cl ratios at the 10−11, 10−12, and 10−13 level shows no difference in the sense of simple statistical significance. However, more detailed statistical analyses demonstrate certain interlaboratory bias and underestimation of uncertainties by some laboratories. Following subsequent remeasurement and reanalysis of the data from some AMS facilities, the round-robin data indicate that 36Cl/Cl data from two individual AMS laboratories can differ by up to 17%. Thus, the demand for further work on harmonising the 36Cl-system on a worldwide scale and enlarging the improvement of measurements is obvious.

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling

Global cooling in intermediate glacial climate with northern ice sheets preconditions climatic instability with bipolar seesaw. Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.

Estimation of Groundwater Residence Time Using the 36Cl Bomb Pulse

We propose a methodology for estimating the residence time of groundwater based on bomb-produced (36)Cl. Water samples were collected from 28 springs and 2 flowing wells located around Mt. Fuji, Central Japan. (36)Cl/Cl ratios in the water samples, determined by accelerator mass spectrometry (AMS), were between 43 × 10(-15) and 412 × 10(-15). A reference time series of the above-background (i.e., bomb-derived) (36)Cl concentration was constructed by linearly scaling the background-corrected Dye-3 data according to the estimated total bomb-produced (36)Cl fallout in the Mt. Fuji area. Assuming piston flow transport, estimates of residence time were obtained by comparing the measured bomb-derived (36)Cl concentrations in spring water with the reference curve. The distribution of (36)Cl-based residence times is basically consistent with that of tritium-based estimates calculated from data presented in previous studies, although the estimated residence times differ between the two tracers. This discrepancy may reflect chlorine recycling via vegetation or the relatively small change in fallout rate, approximately since 1975, which would give rise to large uncertainties in (36)Cl-based estimates of recharge for the period, approximately since 1975. Given the estimated ages for groundwater from flowing wells, dating based on a (36)Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s.

Evolution of solution dolines inferred from cosmogenic 36Cl in calcite

Quantification of the development of solution dolines provides important information for understanding the long-term evolution of karst landscapes. This study reports the initial results of an investigation of the long-term denudation rates along the side slopes of a solution doline based on analyses of cosmogenic 36 Cl in calcite. The denudation rates increase in proportion with increasing size of the topographic contributing area, thereby supporting the hypothesis that the rate of surface lowering in carbonate terrains is controlled by water convergence in the epikarst. A simple model based on the empirical correlation between denudation rate and contributing area is successful in explaining the form of several solution dolines located close to the analyzed doline. The model reveals that these solution dolines, which have varying diameters, developed over similar time scales of the order of 10 5 yr.

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.

AMS analysis of 36Cl induced in concrete of accelerator facilities

Accelerator mass spectrometry (AMS) was applied to the analysis of 36Cl induced in concrete samples obtained from accelerator facilities. In order to use a small amount of concrete sample and to separate chlorine as pure as possible, an improvement of separation process was developped. Chlorine was extracted from 1–5 g of concrete into 0.01 M nitric acid in a pressurized decomposition vessel. After determining chlorine using ion chromatography, a certain amount of NaCl solution was added to obtain sufficient amounts of AgCl precipitate and to dilute to a suitable isotope ratio of 36Cl to 35Cl (36Cl/35Cl) for the AMS (10-12–10-10). A careful purification procedure was applied to reduce 36S interference in AMS. Good reproducibility and small error throughout the chemical process for sample preparation was attained. Depth profiles of 36Cl/35Cl in concrete of a medium-energy cyclotron were measured by the developed method and compared with the results of γ-emitters induced by thermal neutrons. Since it was confirmed that 36Cl was produced by thermal neutron capture of 35Cl, the thermal neutron fluence irradiated during accelerator operation could be obtained using 36Cl/35Cl. In order to estimate the neutron fluences, the 36Cl/35Cl measurement by AMS is more useful than radioactivity measurements of other isotopes such as γ-emitters because AMS directly provides the isotope ratio and the half-life of 36Cl is very long.

Energy Spectra of Electrons Induced by MeV Atom Clusters

The first observation of the energy distribution of electrons emitted from solids bombarded by MeV atom clusters is reported. In the backward direction, using graphite and Si bombarded by Cn+ and Aln+ (n ≤8), an appreciable suppression of electron emission has been observed at electron energies lower than ~10 eV. Electron yield per atom decreases with increasing n, and becomes less than 50% at n ≥3, relative to the case of n=1. The experimental results cannot be explained in terms of projectile stopping cross sections nor by the clearing-the-way effect. It is probable that the suppressed electron emission is a result of the suppression of the transport or surface transmission of the produced low-energy electrons, rather than of the suppression of ionization.

Hydrogen analysis of mineral samples at University of Tsukuba

This study reports the present status of our work on the hydrogen analysis of mineral and rock samples. The preparation of a standard material by means of ion implantation, a method of its calibration and the application of the method are described. The number of hydrogen atoms per unit volume in the standard material can be determined from the simultaneous observation of α-particles, γ rays and α–γ coincidence events for the 1H(19F,αγ) reaction at the 16.44 MeV resonance energy. The hydrogen content in a natural obsidian determined with the method mentioned above is in agreement with that obtained by FTIR. A heavy-ion microbeam system under construction, which consists of a Russian-type quadrupole magnet for beam focusing, a beam defining slit system and a γ-ray detector, is also described.