It is an internationally accepted assumption that genetic or tumor-causing risk of exposure to radiation can never be zero at any low dose. Essential data supporting this assumption are reviewed and used for discussion on no increase in genetic diseases in F1 offspring of A-bomb survivors and on tumor incidence data of A-bomb survivors. A working hypothesis is proposed that radiation initiates carcinogenesis followed by “spontaneous” tumor promotion or that spontaneous tumor-initiation is followed by radiation-induced promotion, which occurs only after exposure with doses above threshold values. Supporting evidence for this assumption is given by reviewing experimental data of mice. The data clearly show that initiation is caused by somatic mutation, an indication that there should be no threshold effect in radiation-induced tumor-initiation, i. e., in some types of radiation-caused tumors. However, in other types of radiation-caused tumors that occur only when radiation works as tumorpromoter, there could be threshold effects.
The acceptable level of radiation risk for public members, that is 10-5/y, was proposed by ICRP and other international organizations. We studied to survey basic procedures of deriving this value and to derive an acceptable risk value in Japan by using similar procedures. The basic procedures to derive 10-5/y were found as follows; (1) 0.1 percent of annual mortality from all diseases, (2) 0.1 percent of life time risk, (3) one percent of mortality from all causes in each age cohort and (4) corresponding value to 1 mSv annual radiation exposure. From these bases we derived the value of 10-5/y as acceptable risk level in Japan. The perception to risk level of 10-5/y in conventional life was investigated by means of questionnaires for 1, 095 college students living in Tokyo. The risks considered in this study were natural background radiation, coffee, skiing, X-ray diagnosis, spontaneous cancer, passive smoking and air pollution. The most acceptable risk was the risk related with natural background radiation. And the risk of natural background radiation was more easily accepted by the students who had knowledges on natural background radiation. On the other hand, the risk from air pollution or passive smoking was the most adverse one.
Adsorption of radioactive elemental iodine gas (131I2) on incense stick aerosol particles has been studied to provide basic data for a realistic and precise assessment of dose to the public due to radio-iodine released from nuclear facilities. A mixture of iodine gas and aerosol was passed once through a glass vessel to cause the adsorption reaction. The adsorption was studied at different reaction times, initial I2 gas concentrations and particle number concentrations. The results showed that the adsorption reached an equilibrium in about 2min at the initial I2 gas concentration of 10-10g/cm3. The proportion of iodine adsorbed by the aerosol particles was almost constant, about 25% at the initial I2 gas concentrations below 10-11g/cm3 and at the reaction time of 1min, and decreased with increase of the initial concentration over 10-11g/cm3. The adsorption isotherm of the aerosol for the gas was obtained from the experimental data. The semi-empirical equations to explain the adsorption reaction were also obtained based on the experimental data and the FUCHS's equation relating to vaporization of a droplet. The sticking probability in the equations was estimated to be 7.6×10-3. The calculated results using the equations were in good agreement with the experimental ones.
β-ray irradiation fields for calibrating radiation protection instruments, such as survey meters and personnel dosimeters, have been established. The β-ray irradiation fields are formed with two types of plane sources; one is a thin ion exchange membrane source 100mm in diameter, and the other is an extended area source which has 42mm active diameter and is covered with silver foil. The radionuclides are 147Pm, 204Tl, 90Sr-90Y and 106Ru-106Rh. The β-ray irradiation fields are characterized by their β-ray absorbed dose rates and residual maximum energies. Absorbed dose rates were measured with an extrapolation chamber (the entrance window thickness is 6.37mg/cm2) for every source and obtained as a function of irradiation distance. Residual maximum energies were determined by the absorption method. The residual maximum energy which is larger than the standard minimum value recommended by ISO (International Organization for Standardization) was obtained for every source. The uniformity of the β-ray irradiation field was examined using large area industrial X-ray film. The examination indicated that the β-ray irradiation field was quite uniform and suitable for calibrating radiation protection instruments.
The analytical method of 127I and 129I in environmental samples has been studied and the background levels of these nuclides in soils, seaweeds and milk were measured. The analytical method consists of the combustion technique and neutron activation analysis (N. A. A.). The iodine was separated from samples by ignition at about 1, 000°C in a quartz combustion apparatus with an oxygen and a nitrogen stream, and was absorbed by small amounts of active charcoal. The iodine was then purified through the carbontetrachloride extraction method and the PdI2 decomposition method. Irradiation was conducted by JRR-4 (nth: 8×1013n/cm2·sec) for 40min. After irradiation, the iodine was purified by the solvent extraction method same as pre-irradiation extraction. Each activity of 126I and 130I, which was produced by 127I (n, 2n) 126I and 129I (n, γ) 130I reaction respectively, was measured by γ-spectrometry using a Ge-detector. The lower detection limits of 129I by this method were 4×10-7Bq/g for dry soil, 7×10-8Bq/g for fresh seaweeds and 7×10-6Bq/l for fresh milk, respectively. The relative standard deviation of 129I measurements in soil and milk were less than 10%.
This paper describes a convenient and easy method of evaluating the short-lived radon daughters, 214Pb(RaB) and 214Bi(RaC), deposition activity on ground surface carried by rainfall in situ. Data of variations in the deposition activity on ground surface calculated from the continuous gamma radiation made at several sites for a period of nine years are given together with data of precipitation. It is concluded from the observations that the annual Ra(B+C) deposition activity lies within the range 4, 600-12, 000pCicm-2y-1, which depends on the annual precipitation. From the annual mean radon (222Rn) exhalation rate (UNSCEAR 1982), the annual precipitation on the terrestrial parts of the earth (reference data), and the annual mean specific activity of Ra(B+C) in rainwater (observation), we calculate the ratio of Ra(B+C) atom deposited on the ground to 222Rn released from the ground. This ratio is less than 1%. This means that almost all of the short-lived radon daughters in the atmosphere are not caught by rain and decayed to the long-lived radon daughters in the atmosphere.
Iodine in the urban air was determined and it was found that the iodine was contained in the suspended particles. In addition, lead and copper were also determined in the same samples. From the correlation between the iodine and lead and copper concentrations and the fact that the lead in air is originated from the combustion of the fossil materials, the iodine was also appeared to be generated from the combustion of those materials. By the results of our experiments, the average value of iodine concentration in air at ground level in Tokyo was 7.8μg/1, 000m3, and the average values of lead and copper were 60μg/1, 000m3 and 59μg/1, 000m3, respectively.