Kiyoshi Shizuma

Measurement of soil contamination by radionuclides due to the Fukushima Dai-ichi Nuclear Power Plant accident and associated estimated cumulative external dose estimation

Soil sampling was carried out at an early stage of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Samples were taken from areas around FDNPP, at four locations northwest of FDNPP, at four schools and in four cities, including Fukushima City. Radioactive contaminants in soil samples were identified and measured by using a Ge detector and included (129 m)Te, (129)Te, (131)I, (132)Te, (132)I, (134)Cs, (136)Cs, (137)Cs, (140)Ba and (140)La. The highest soil depositions were measured to the northwest of FDNPP. From this soil deposition data, variations in dose rates over time and the cumulative external doses at the locations for 3 months and 1y after deposition were estimated. At locations northwest of FDNPP, the external dose rate at 3 months after deposition was 4.8-98 μSv/h and the cumulative dose for 1 y was 51 to 1.0 × 10(3)mSv; the highest values were at Futaba Yamada. At the four schools, which were used as evacuation shelters, and in the four urban cities, the external dose rate at 3 months after deposition ranged from 0.03 to 3.8μSv/h and the cumulative doses for 1 y ranged from 3 to 40 mSv. The cumulative dose at Fukushima Niihama Park was estimated as the highest in the four cities. The estimated external dose rates and cumulative doses show that careful countermeasures and remediation will be needed as a result of the accident, and detailed measurements of radionuclide deposition densities in soil will be important input data to conduct these activities.

Application of neural networks for the analysis of gamma-ray spectra measured with a Ge spectrometer

The analysis of gamma-ray spectra to identify lines and their intensities usually requires expert knowledge and time-consuming calculations with complex fitting functions. A neural network algorithm can be applied to a gamma-ray spectral analysis owing to its excellent pattern recognition characteristics. However, a gamma-ray spectrum typically having 4096 channels is too large as a typical input data size for a neural network. We show that by applying a suitable peak search procedure, gamma-ray data can be reduced to peak energy data, which can be easily managed as input by neural networks. The method was applied to the analysis of gamma-ray spectra composed of mixed radioisotopes and the spectra of uranium ores. Radioisotope identification was successfully achieved.

Residual 152eu and 60co Activities Induced by Neutrons from the Hiroshima Atomic Bomb

Specific activities of 152Eu:Eu in stone samples exposed to the Hiroshima atomic bomb were determined for 70 samples up to a 1,500-m slant range from the epicenter. The specific activities of 60Co:Co were also determined for six samples near the Hiroshima hypocenter. First, the 152Eu data were investigated to find out the directional dependence of neutron activation. Directional anisotropy was not definite; however, there was an indication that the activation in the west-southwest was lower than in other directions. Second, measured 152Eu and 60Co radioactivity data were compared with activation calculations based on DS86 neutrons. It is clearly shown that the measured data are lower than the calculation near the hypocenter and vice versa at long distances beyond 1,000 m. The calculated-to-measured ratios of 152Eu are 1.6 at the hypocenter, 1.0 at approximately 900 m, and 0.05 at a 1,500-m slant range. Present results indicate that systematic errors exist in the DS86 neutrons concerning the source-term spectrum, neutron transport calculations in air, and/or activation measurements.

Paddy-field contamination with 134Cs and 137Cs due to Fukushima Dai-ichi Nuclear Power Plant accident and soil-to-rice transfer coefficients.

The transfer coefficient (TF) from soil to rice plants of (134)Cs and (137)Cs in the form of radioactive deposition from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011 was investigated in three rice paddy fields in Minami-Soma City. Rice crops were planted in the following May and harvested at the end of September. Soil cores of 30-cm depth were sampled from rice-planted paddy fields to measure (134)Cs and (137)Cs radioactivity at 5-cm intervals. (134)Cs and (137)Cs radioactivity was also measured in rice ears (rice with chaff), straws and roots. The rice ears were subdivided into chaff, brown rice, polished rice and rice bran, and the (134)Cs and (137)Cs radioactivity concentration of each plant part was measured to calculate the respective TF from the soil. The TF of roots was highest at 0.48 ± 0.10 in the field where the (40)K concentration in the soil core was relatively low, in comparison with TF values of 0.31 and 0.38 in other fields. Similar trends could be found for the TF of whole rice plants, excluding roots. The TF of rice ears was relatively low at 0.019-0.026. The TF of chaff, rice bran, brown rice and polished rice was estimated to be 0.049, 0.10-0.16, 0.013-0.017 and 0.005-0.013, respectively.

Low-background shielding of Ge detectors for the measurement of residual 152Eu radioactivity induced by neutrons from the Hiroshima atomic bomb

Low-background gamma-ray spectrometers were constructed for the measurement of residual 152Eu activity induced by the atomic-bomb neutrons. Optimum thickness of lead shielding, inner linings and background characteristics were investigated for an ordinary coaxial- and a well-type Ge detector. In addition, an anticoincidence shielding was installed for the well-type detector. As a result, the background counting rate due to cosmic rays was greatly reduced. It was also shown that a sample preparation to enrich the objective activity and eliminate background activities was important in the case of the 152Eu measurement.

Residual 60Co activity in steel samples exposed to the Hiroshima atomic-bomb neutrons

Residual 60Co radioactivity produced by the neutrons from the Hiroshima atomic bomb has been measured by means of gamma-ray spectrometry for seven steel samples located up to 1,800 m slant range and one control sample obtained at 4.5 km from the hypocenter. After removing the main constituent of iron ions from the steel sample by the solvent extraction method, nickel and cobalt were chemically separated by the ion exchange process. Gamma-ray measurements were performed with a low background well-type germanium detector. Derived specific activities 60Co:Co were compared with previous measurements and with the calculation based on the neutron fluence of current dosimetry system DS86. It has been shown that a systematic discrepancy exists between the measured and calculated activity. The calculated-to-measured ratios for 60Co are consistent with those of 152Eu and 36Cl activity.

Early radiation survey of Iitate village, which was heavily contaminated by the Fukushima Daiichi accident, conducted on 28 and 29 March 2011.

AbstractFollowing the news that the radiation level in Iitate Village, located 25–45 km from the Fukushima Daiichi Nuclear Power Plant, was seriously increased, an urgent field survey was carried out on 28 and 29 March 2011. Radiation levels at 130 locations were measured inside a van that traveled throughout the village using a CsI pocket survey meter and an ionization chamber. Soil samples were also taken at five locations and submitted to gamma ray analysis using a Ge detector. A radiation exposure rate of more than 20 &mgr;Sv h−1 was observed in the southern part of Iitate Village. Volatile radionuclides such as iodine and cesium were found to be the main components of radioactive contamination. A trace amount of plutonium isotopes originating from the accident was also confirmed in several soil samples, the level of which was less than the global fallout. Based on the measured density of radionuclides at the highest contamination location during the present survey, an exposure rate of about 200 &mgr;Gy h−1 at 1 m above the ground was estimated at the time of the radioactive deposition on March 15. At this location, the cumulative exposure would reach 50 mGy in the middle of May 2011.Health Phys. 102(6):000-000; 2012

Analyses of monochromatic gamma-ray and annihilation radiation spectra obtained with Ge(Li) detectors

Abstract First, analysis of the monochromatic gamma-ray peak is described. The peak center was precisely determined by fitting of a Gaussian function. A response function of the gamma-ray peak is expressed by a Gaussian and a tailing function. Secondly, two kinds of the unfolding methods of the Doppler broadened annihilation spectrum are described. These methods are useful for both the precision energy measurement and the study of the position annihilation in various materials. Annihilation spectra for 23 samples were successfully unfolded by an iterative unfolding method and intrinsic distributions of the annihilation radiations were obtained.

A Method to Estimate the Fast-Neutron Fluence for the Hiroshima Atomic Bomb

A new method to estimate the fast-neutron fluence of the Hiroshima atomic bomb is proposed. 63 Ni produced by the 63 Cu(n, p) 63 Ni reaction provides a unique measure by which to estimate the fast-neutron fluence of the Hiroshima/Nagasaki atomic bombs, because the half-life of 63 Ni is 100 years and 70% of the 63 Ni produced in a copper piece presently exists after 50 years. Using the neutron spectrum given in DS86 and the estimated cross section, we found that a piece of copper of about 10 g which was exposed at a point around 100 m from the hypocenter gives a measurable amount of 63 Ni using a low-background liquid scintillation counter. For the measurement of 63 Ni, accelerator mass spectrometry also seems to be applicable.

137Cs concentration in soil samples from an early survey of Hiroshima atomic bomb and cumulative dose estimation from the fallout

Low background gamma-ray measurement has been performed to determine the 137Cs content in soil samples collected in a very early survey of the Hiroshima atomic bomb. These soil samples were collected just 3 d after the explosion within 5 km from the hypocenter and were not exposed to the global fallout from nuclear weapon tests. Out of 22 samples, 137Cs was detected for 11 samples, and their radioactivities ranged from 0.16-10.6 mBq g-1 at the time of the measurement. A comparison of the 137Cs deposition with the rainfall area within Hiroshima city indicates that the rainfall area was wider than the previously proposed one. Cumulative exposure by the fallout has been estimated to be 0.31 mC kg-1 (0.12 R) in Hiroshima city except for the heavy fallout area and at most 1.0 mC kg-1 (4 R) in the heavy fallout area.