Tadahiro Kurosawa

Relationship between Individual External Doses, Ambient Dose Rates and Individuals' Activity-Patterns in Affected Areas in Fukushima following the Fukushima Daiichi Nuclear Power Plant Accident.

The accident at Fukushima Daiichi Nuclear Power Plant on March 11, 2011, released radioactive material into the atmosphere and contaminated the land in Fukushima and several neighboring prefectures. Five years after the nuclear disaster, the radiation levels have greatly decreased due to physical decay, weathering, and decontamination operations in Fukushima. The populations of 12 communities were forced to evacuate after the accident; as of March 2016, the evacuation order has been lifted in only a limited area, and permanent habitation is still prohibited in most of the areas. In order for the government to lift the evacuation order and for individuals to return to their original residential areas, it is important to assess current and future realistic individual external doses. Here, we used personal dosimeters along with the Global Positioning System and Geographic Information System to understand realistic individual external doses and to relate individual external doses, ambient doses, and activity-patterns of individuals in the affected areas in Fukushima. The results showed that the additional individual external doses were well correlated to the additional ambient doses based on the airborne monitoring survey. The results of linear regression analysis suggested that the additional individual external doses were on average about one-fifth that of the additional ambient doses. The reduction factors, which are defined as the ratios of the additional individual external doses to the additional ambient doses, were calculated to be on average 0.14 and 0.32 for time spent at home and outdoors, respectively. Analysis of the contribution of various activity patterns to the total individual external dose demonstrated good agreement with the average fraction of time spent daily in each activity, but the contribution due to being outdoors varied widely. These results are a valuable contribution to understanding realistic individual external doses and the corresponding airborne monitoring-based ambient doses and time-activity patterns of individuals. Moreover, the results provide important information for predicting future cumulative doses after the return of residents to evacuation order areas in Fukushima.

Estimation of Electron-loss and Photon-scattering Corrections for Parallel-plate Free-air Chambers

Parallel-plate free-air ionization chambers are used for X-ray air-kerma rate standards at the National Metrology Institute of Japan (NMIJ), AIST. The electron-loss and photon-scattering correction factors are needed for the evaluation of air-kerma rate from measured current. The electron-loss correction factor (Ke) is a correction of the charge loss by giving energy to the electrode part without the high-speed electron stopping in the air area where the charges are collected. The scattering correction factor (Ksc) is for a correction of extra charges produced by scattered photons generated after an incidence photon is interactive. The electron-loss and photon-scattering correction factors for 3 different size parallel-plate free-air chambers are estimated by the EGS4 code. One chamber is used to get primary standards for absolute measurements of air kerma in beams of medium-energy X-rays and the other two are used for the same purpose in beams of low-energy X-rays. These correction factors are calculated for mono-energetic photons. It is found that electron-loss and photon-scattering correction factors depend on the chamber size, and the latter especially changes greatly depending on the size. The Ke and Ksc value for medium-and low-energy X-ray fields at AIST are estimated by averaging the energy deposition contributions over the X-ray spectrum.

Field photon energy spectra in Fukushima after the nuclear accident

After the accident of the Fukushima Daiichi Nuclear Power Plant, radionuclide were spread out over large area. It has been past almost two years since the nuclear accident. Therefore, Cs-137 and Cs-134 nuclei are the main sources of gamma rays. The field gamma rays, however, are not mono-energy due to photons scattered from the ground, the air, etc. The effective dose for external exposure depends on the energy of radiations, thus photon energy spectra are important for the evaluation of effective dose for the people who live in Fukushima. In the present study, the photon energy spectra have been measured at several points in Fukushima to evaluate reference energy spectra after the nuclear accident. The energy spectra in Fukushima area were measured using a cadmium zinc telluride (CdZnTe) detector, and an unfolding method was applied to evaluate photon energy spectra. The fraction of low-energy photon is increased by decontamination around the measurement point. This can be used to estimate the efficiency of decontamination effect by removing the radionuclides on the target surface. The photon energy spectra measured by the present study would be useful for the evaluation of the effective dose for the people who live in Fukushima.

Measuring and assessing individual external doses during the rehabilitation phase in Iitate village after the Fukushima Daiichi nuclear power plant accident

After the Fukushima Daiichi nuclear power plant accident in 2011, Iitate village was placed under an evacuation order because the level of radioactive materials drifting from the nuclear plant to the village was above a government-set level for allowing residents to live in the area. The evacuation advisory for most of the village was lifted on 31 March 2017. For displaced residents deciding whether or not to return to their homes, it is important to correctly understand and estimate the realistic individual external doses they will receive after returning to the village. In this study, with the support of residents of the village, we used a personal dosimeter (D-Shuttle) coupled with a global positioning system device to measure and thus understand realistic individual external doses while the residents were in Iitate village and to project the individual external doses for different administrative districts as of 1 April 2017. The measured individual external doses measured by D-Shuttle for 38 study participants showed that the doses measured during time spent inside the village were higher and more widely distributed than the doses measured during time spent outside the village. The exposure ratio (ER) was defined as the ratio of additional individual external dose measured by D-Shuttle to the additional ambient dose based on an airborne monitoring survey. The medians of the average ERs were 0.13 (min-max 0.06-0.27) for time spent at home and 0.18 (min-max 0.08-0.36) for time spent outdoors. Projected additional annual individual external doses as of 1 April 2017 for different administrative districts in the village were calculated using ERs obtained in this study. Assuming that individuals spent 8 h per day on outdoor activities and 16 h on indoor activities, additional annual individual external doses were estimated to be below 3 mSv using the mean of the average ERs for most districts in the village, and these values were well below the individual external doses estimated using the approach taken by the central government. The results of this study provide valuable information both for understanding realistic radiological situations in the village and for those who want to know their future individual external dose in order to make a decision on whether or not to live in the village.

Measurement of Neutron Production Cross Sections by High-Energy Heavy Ions

The double-differential cross sections (DDX) of neutron production by high energy heavy ions were measured using the RIKEN Ring Cyclotron of the Institute of Physical and Chemical Research, Japan, and the Heavy Ion Medical Accelerator in Chiba (HIMAC), the National Institute of Radiological Sciences. The neutron energy spectra from 0 to 110 degrees were obtained by using the time-of-flight method. The neutron spectra in the forward direction have a sharp peak which is located at the incident particle energy per nucleon due to the knock-on process, and spreads out to about 3 times the incident particle energy per nucleon. The total neutron production cross sections were obtained by integrating the energy spectra above 20 MeV over a hemisphere from 0 to 90 degrees. The experimental results are compared with calculations using the intranuclear cascade-evaporation Monte Carlo code, HIC, and the quantum molecular dynamics code, QMD. The QMD codes give better agreement with the experimental results than the HIC code.

Absolute radiant power measurement of the X-ray free-electron laser at SACLA

The Japanese hard X-ray free-electron laser (XFEL), SACLA (SPring-8 Angstrom Compact free-electron LAser), reached laser amplification at 10 keV photon energy in June 2011. SACLA can provide XFELs with its wavelength of shorter than 0.1 nm. Since the radiant power is a fundamental parameter of the XFEL beam which strongly influences nonlinear effects, its measurement in absolute terms is of significant importance. In the present study, the absolute radiant power of the XFEL was measured using a cryogenic radiometer at the BL3 in SACLA. The radiant power as a function of a silicon attenuator thickness was also measured to estimate the contribution of the higher harmonics. The radiant power in the range between 8 μW and 1005 μW was measured in the photon energies of 4.4 keV, 5.8 keV, 9.6 keV, 13.6 keV and 16.8 keV. The contribution of the higher harmonics is negligible except for the photon energy of 4.4 keV. The third harmonics component in the photon energy of 4.4 keV is about 1 %.

Measurement of anisotropic angular distributions of photon energy spectra for I-125 brachytherapy sources

The angular distribution of photon energy spectra emitted from an I-125 brachytherapy source was measured using a specially designed jig in the range of ±70° in the plane of the long axis of the source. It is important to investigate the angular dependence of photon emissions from these sources for the calibration of the air kerma rate. The results show that the influence of the distributions between 0° and ±8° is small enough to allow a calibration using current primary instruments which have a large entrance window.

High sensitive standard measurement to determine strength of an I-125 brachytherapy source

We are encouraged to contribute to strengthen quality assurance which keeps brachytherapy safe and available. For the purpose, we aspire to determine the source strength and establish national metrological standard in Japan. It is necessary to assemble a high sensitive free air chamber for measurement of ionization current derived from the source which is very weak especially in Japan. Total uncertainty for the source strength is promising to meet required value from users of measurement instruments.

Development of improved free-air ionisation chamber as absolute dosimetry standard for low-energy X rays in INER

The National Radiation Standard Laboratory of the Institute of Nuclear Energy Research (INER) designed and constructed an improved Attix style free-air ionisation chamber (FAC) for low-energy X-ray measurements. Clinically, X rays in this energy range are used in mammography radiology. This chamber is also used to perform air-kerma measurements. The original Attix two-sectional design was redesigned by INER using the piston design. The correction factors were determined experimentally for volume estimation, ion recombination and air attenuation. The aperture transmission, wall transmission, electron loss and photon scatter correction factors were determined using Monte Carlo calculations. INER established the Bureau International des Poids et Mesures (BIPM) X-ray beam code and performed a comparison of secondary standard air-kerma calibration factors for 10-50 kV low- energy X rays to verify the experimental accuracy and measurement consistency of the improved chamber. The INER-NMIJ/National Institute of Advanced Industrial Science and Technology (AIST) experimental results comparison using a transfer chamber yielded a difference <1.0% at the 95% confidence level in calibration factors. The overall uncertainty for the X-ray measurement in terms of air kerma was <0.6% at the 95% confidence level. These results indicated that the improved FAC is capable of serving as a primary standard as well as a trace standard in low-energy X-ray calibration services in Taiwan and even forming a basis for the future mammography X-ray air-kerma primary standard.

Current situations and discussions in Japan in relation to the new occupational equivalent dose limit for the lens of the eye

Since the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens of the eye in 2011, there have been extensive discussions in various countries. This paper reviews the current situation in radiation protection of the ocular lens and the discussions on the potential impact of the new lens dose limit in Japan. Topics include historical changes to the lens dose limit, the current situation with occupational lens exposures (e.g., in medical workers, nuclear workers, and Fukushima nuclear power plant workers) and measurements, and the current status of biological studies and epidemiological studies on radiation cataracts. Our focus is on the situation in Japan, but we believe such information sharing will be useful in many other countries.