Kazuyoshi Masumoto

Anthropogenic radionuclides in the atmosphere observed at Tsukuba: characteristics of the radionuclides derived from Fukushima

During a serious accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), a huge quantity of radionuclides was released into the atmosphere and ocean. We measured anthropogenic radionuclides in surface air at Tsukuba, about 170 km from the FDNPP. On March 15, 2011, we detected the radioactivity released from the Fukushima accident in air samples at Tsukuba. The major radionuclides that we observed were radioiodine ((131)I, (132)I, (133)I) and radiocesium ((134)Cs, (136)Cs, (137)Cs). This radioiodine consisted of gaseous and particulate forms; the percentage of particulate (131)I in the total (131)I ranged from 0 to 86%. The percentage of the particulate (131)I to the total (131)I increased on the arrival of the plumes from major emissions of the FDNPP. After activities of the radionuclides attained the maximum on March 15, 2011, the FDNPP-derived radionuclides decreased rapidly in surface air. The activity median aerodynamic diameter of (131)I-bearing particles was 0.7 μm, while those of (134)Cs- and (137)Cs-bearing particles were larger than 1 μm. Large variations of ratios of (131)I/(137)Cs, (132)Te/(137)Cs, and (99)Mo ((99m)Tc)/(137)Cs (all involving different elements) suggest that the behaviors of these radionuclides in the atmosphere, including the processes of their emission, differed each other.

Evaluation of radioactivity induced in the accelerator building and its application to decontamination work

For decommissioning of the cyclotron and electron synchrotron facilities, the residual radioactivity in surface and core samples of concrete, collected from various parts of buildings, was determined by gamma-ray spectrometry. It was found that the concrete samples were activated mainly by neutrons and that the major radioisotopes were 152Eu, 60Co, 134Cs, 22Na and 54Mn. The maximum activity induced by thermal neutron capture was observed at the depth of 10 cm in the concrete wall near the deflector of the cyclotron. Tritium was also produced by the neutron reaction, because its concentration was proportional to the activities of 152Eu and 60Co. The surface dose rates inside the accelerator room were also monitored to define the decontamination area. The surface dose rate was proportional to the residual radioactivity, such as 60Co. A careful evaluation was very useful in order to minimize the radioactive waste during decontamination.

Observation of radioactive hetero-fullerenes using radiochemical techniques

Fullerenes, C60 and C70, were irradiated by 8 and 10 MeV deuterons. The irradiated samples were dissolved in CS2 and filtered to remove insoluble by-products. Finally, radioactive fullerenes and products, such as fullerene dimers, trimers labeled with13N, were isolated and detected in the liquid phase by radiochromatography. This suggests that the energy rich13N atoms successfully incorporated into the fullerene cages and that the disrupted cages reacted with neighboring cages.

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.

Distribution of thermal neutron flux around a PET cyclotron.

The number of positron emission tomography (PET) examinations has greatly increased world-wide. Since positron emission nuclides for the PET examinations have short half-lives, they are mainly produced using on-site cyclotrons. During the production of the nuclides, significant quantities of neutrons are generated from the cyclotrons. Neutrons have potential to activate the materials around the cyclotrons and cause exposure to the staff. To investigate quantities and distribution of the thermal neutrons, thermal neutron fluxes were measured around a PET cyclotron in a laboratory associating with a hospital. The cyclotron accelerates protons up to 18 MeV, and the mean particle current is 20 &mgr;A. The neutron fluxes were measured during both 18F production and 11C production. Gold foils and thermoluminescent dosimeter (TLD) badges were used to measure the neutron fluxes. The neutron fluxes in the target box averaged 9.3 × 106 cm−2 s−1 and 1.7 × 106 cm−2 s−1 during 18F and 11C production, respectively. Those in the cyclotron room averaged 4.1 × 105 cm−2 s−1 and 1.2 × 105 cm−2 s−1, respectively. Those outside the concrete wall shielding were estimated as being equal to or less than ∼3 cm−2 s−1, which corresponded to 0.1 &mgr;Sv h−1 in effective dose. The neutron fluxes outside the concrete shielding were confirmed to be quite low compared to the legal limit.

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.

Direct preparation of radioactive fullerenes as a tracer for applications

The C60 and C70 fullerenes were irradiated by high-energy γ-rays and charged particles. The irradiated samples were dissolved in CS2 and/or toluene and filtered to remove insoluble by-products. Finally, radioactive fullerenes and products labeled with11C or13N were isolated and detected in the liquid phase by radiochromatography. It was found that (1) not only11C or13N radioactive monomer fullerenes but also their dimers (trimers and, possibly, tetramers) were produced by recoil implantation process following nuclear reaction and (2) the radioactive fullerene labeled with11C yields has led to high yields.

Production of Radionuclides in the Cooling Water for Electromagnetic Horns at the J-PARC Neutrino Experimental Facility

The cooling water for the electromagnetic horns is exposed to intense high-energy protons, neutrons and charged pions at the J-PARC neutrino experimental facility. These high-energy particles produce various radionuclides in the cooling water; dominant radionuclides were H and Be, and various -emitting nuclides were also detected by a Ge detector. Specific activities of individual nuclides in the circulating cooling-water were determined and compared with the results obtained by Monte Carlo calculations. The results demonstrated that H and Be were directly produced in water by nuclear spallation of oxygen in water molecules, and that other nuclides were produced in metal components and transferred into water, in accordance with the observations at other high-energy accelerator facilities.

Depth Profile of Radioactivity Induced in the Thick Concrete Shield in EP1 Beam Line at the KEK 12-GeV Proton Synchrotron Facility

Abstract The evaluation of radioactivity induced in the concrete shield is important for the decommissioning of accelerator facilities. Specific activities of gamma-ray emitters of nuclear spallation products and thermal neutron capture products and beta-ray emitters such as tritium and 14C, and 36Cl in the concrete shield along the 12-GeV proton beam line (EP1 beam line, High Energy Accelerator Research Organization) were determined. The depth profiles of the radioactivity of each nuclide in the 6-m-thick concrete shield of the beam lines were compared, and the secondary particles and induced nuclear reactions were discussed.

Target dependence of beryllium fragment production in neutron- and alpha-induced nuclear reactions at intermediate energies

Summary The yields of 7Be from natC, 27Al, natCu, natAg and 197Au targets and 10Be from natCu, natAg and 197Au targets in alpha- (400 MeV) and neutron- (maximum end-point energy, E0 = 500 MeV) induced reactions were measured in order to compare them with the photonuclear-reaction yields at E0 = 1000 MeV. The target-mass dependences of the yields of 7Be and 10Be showed very similar trends among alpha-, neutron-, and photon-induced reactions. The fragmentation yields of 7Be decreased more slowly, and those of 10Be increased gradually with an increase in the target mass; both yields intersected at an approximate target mass of 125. The exponential increase in the fragmentation-yield ratios of 10Be to 7Be with an increase in the neutron-to-proton ratio of the targets suggests that pre-formation of the fragments inside the excited nuclei is strongly influenced by the target properties, such as N/Z. Furthermore, the effect of the incident particle was clearly observed in the yield ratios of 10Be to 7Be from the Cu and Ag targets.