In this paper, an outline of dose estimations for boron neutron capture therapy (BNCT) is presented from standpoint medical physics and biological field. BNCT is a sort of the radiation therapy for an obstinate cancer like malignant brain tumor. To perform clinical trials of the BNCT, characteristics of doses given to a patient by neutron irradiation have to be comprehended accurately. To investigate RBE for several organs with neutron, in-vitro biological experiments involving neutron irradiations were carried out. The dose estimation method for the BNCT and the dose characteristics by neutron irradiations are introduced. First doses were analyzed based on experimental measurement. Distributions of thermal neutron flux and gamma-ray dose rate that are fundamental values of the dosimetry were determined by experiments with a water phantom, then dose distributions and the characteristics in the phantom were analyzed based on the measurements and the RBE values. Based on the dosimetry techniques, a treatment planning system which can determine accurately the doses and its distributions in a patient using Monte-Carlo transport code was developed in JAEA. To verify the accuracy of the dose calculation, the phantom experiments was reprinted using the system, then the calculation values were compared with the measurement values obtained from the phantom experiments. This report describes the dosimetry method for BNCT based on experimental measurements and outline of the computational dosimetry system for BNCT.
The expert committee on development and utilization of phantoms had activities from April, 2004 to March, 2006 sponsored by the Japan Health Physics Society. The purpose of the committee was to accumulate and share the latest information concerning phantoms and related technical issues, and to discuss how and for what phantoms should be utilized in future. The committee had twelve meetings and one symposium. Many people took part in the activity including the committee members, observers and other participants; we had a certain number of lectures covering versatile topics and chances to discuss essential problems including sophistication and standardization of phantoms. Some parts of the information we obtained in this activity have been presented in the articles in series.
We compared the periodic characteristics of the atmospheric radon concentration on the land and ocean, using data simultaneously measured at Oki Island in the Sea of Japan and Matsue, which is located 70km south of the island and on the Shimane peninsula. The radon concentration was the highest in the fall at Matsue and in the winter at Oki Island and the lowest during the summer at both locations. The frequency distributions and the annual periodic variations of the arriving pathways of the air masses and these radon concentrations at Matsue and Oki Island showed the same tendency. At Matsue, the monthly average of diurnal minimum of the radon concentration was highest in the winter that was three month late from the maximum season of the monthly average of all measurements. The monthly average of radon concentration at Matsue showed the positive correlation between the monthly occurrence times of temperature inversion. The annual periodic variation of the radon concentration at Oki Island depends on the long-term variation of the distribution of arriving pathways of the air masses.
In order to contribute for the establishment of criteria whether the high level of gamma (γ) ray dose rate is an unusual value, statistical analysis for its frequency distribution was carried out. The γ ray dose rates monitored at six sites (Minamihoroni, Kariwa, Wajima, Asahikawa, Ohno, and Tokai-mura Oshinobe) in Japan during the periods from 2000 to 2004 were subjected to the analysis. The γ ray dose rates were not normally distributed but the distribution distorted to higher side. Our statistical analysis suggests that the γ ray dose rates above “average+3σ” or “100nGy/h”, which are the conventional definition, occurred frequently. Here we propose the use of more suitable method to detect the unusual value of the enhanced γ ray dose rates on the basis of the “return period”. It is the recurrence frequency of high γ ray dose rates. Estimate of the return periods, which is usually used for the extreme event of meteorology, is given for the first time in this field. In this study, we compared two methods to calculate the return periods: empirical return period (TH) based on Hazen plot by using the enhanced γ ray dose rates and theoretical estimated return period (TG) based on Gumbel distribution by using average and standard deviation of the enhanced γ ray dose rates. There were significant differences of TH as well as TG between monitoring sites. It turned out that the judgment of an unusual value must be considered based on TH and TG at each monitoring site.
Gamma ray spectra analyses using unfolding techniques and the dose evaluation are performed widely in the fields of environmental radiation monitoring and the relating environmental research. Various sizes and the shapes of NaI (T1) scintillation detectors can be used depending on the dose levels. However, available response functions are generally limited to 3″φ×3″ cylindrical and 3″φ spherical types and the energy range is roughly up to 3MeV. In this paper, the response functions of eight types of cylindrical and spherical NaI (T1) scintillation detectors (1″φ×1″, 2″φ×2″, 3″φ×3″, 4″φ×4″, 5″φ×4″ cylindrical, and 2″φ, 3″φ, 5″φ spherical) were determined. The energy ranges were extended to 10MeV in maximum. The some response functions reported in literatures were unfolded using our response matrices for comparison and the relative characteristics were discussed.
The concentrations of 137Cs and 239, 240Pu in shellfishes collected from the coast around the Tokai Reprocessing Plant (TRP) from 1975 to 2005 were analyzed for environmental radiological monitoring. The measured concentrations of 238Pu were under the detection limits and these concentrations of 137Cs and 239, 240Pu showed no significant short-term increase tendency. It was confirmed that 137Cs and 239, 240Pu in shellfishes around the TRP were derived from the past atmospheric nuclear weapons tests. Also the probability plot of 239, 240Pu concentration was dependent on the shellfish species (bivalve and abalone). It was presumed that the difference of feeding behavior of bivalve and abalone caused the difference of the probability plot of 239, 240Pu concentration.
The property of soil material was measured in order to clarify the relation between the radon and thoron exhalation rate and the physical and chemical characteristics of soil material. The examined factor is the physical property (particle size and porosity) of soil, the mineral component (quartz, microcline, biotite and kaolinite) of soil particle, and the chemical component of soil. The weathered granite soil was used in this experiment. The soil particle size was classified by using the standard mesh into six classes, i. e. >1000μm, 500-1000μm, 355-500μm, 250-355μm, 106-250μm and <106μm. The weight ratios of the classified soil were 48%, 12%, 10%, 9%, 13%, and 8% in each class, respectively. The exhalation rates showed a tendency to decrease with an increase in soil particle size. The measurement of the porosity suited the range of 0.46-0.62. A positive correlation between the exhalation rate and the porosity was found. Also, a positive correlation between the exhalation rate and kaolinite was obtained in each class. Moreover, a negative correlation between the exhalation rate and quartz and microcline was shown in each class. The relationship between the exhalation rate and the chemical components of SiO2 and Al2O3 that are the major elements in soil were negative and positive correlations, respectively.
The 222Rn concentrations in soil and 222Rn flux were measured in the campus of Nagoya University. The measurements were carried out discontinuously between December 1999 and March 2002 at depths of 15, 30, 50, 80, 120, and 200cm. The 222Rn concentrations in soil were analyzed for seasonal-, day-to-day-, and diurnal variations. The average values of 222Rn concentrations in soil were 1.6, 6.9, 13.6, 24.2, 27.7, and 34.3kBq·m-3 for soil depths of 15, 30, 50, 80, 120, and 200cm, respectively. The 222Rn flux ranged from 2.5 to 14mBq·m-2·s-1 with the average 7.2mBq·m-2·s-1. The variations of 222Rn concentrations in soil were larger in shallow depths. It seems that the 222Rn concentration in soil during night-time was slight lower than day-time especially at depth 15cm. The precipitation may have affected the 222Rn concentration in soil differently depending on its amounts. The large decrease of air pressure during one day increased the 222Rn concentration in soil at the depth 15, 30, and 50cm. The effect of soil water content on 222Rn concentration in soil was also observed.