The fluence of high-LET particles in space radiation was measured with plastic nuclear track detectors (PNTDs) in the brain of a life-size human phantom in the 9th Shuttle-Mir Mission (STS-91). The relationship between PNTD track-formation sensitivity (S) and LET∞·H2O was examined using heavy-ion beams at the NIRS-HIMAC and the incident-angle dependence of S was corrected using an empirical function. The particle fluence of space radiation with LET∞·H2O greater than 10keVμm-1 in the brain was evaluated as 1.3×104ncm-2 in a 9.8-day low-Earth-orbit mission at 400km×51.6°. Based on these data, the ratio (the number) of hits to hippocampus cells and cell-nuclei were calculated for long missions. As a result, it was estimated that 45% (2.2×107) cells and 7.0% (3.4×106) cell-nuclei will be hit by high-LET particles (LET∞·H2O>10keVμm-1) in a 90-day mission, and 91% (4.4×107) and 25% (1.2×107) of cell-nuclei will be hit in a 1-year mission.
We analyzed data on gamma-ray dose rates measured with an NaI(Tl) scintillator, and made 3 groups: dry days, rainy days, and rain. The hourly gamma-ray dose rate from April 1991 to December 1995 in Dazaifu City was 14.4cps on average. The average on dry days was 14.2cps. The daily variation of dose rates was inversely correlated to daily variation of temperature, showing cyclical variations with high values in the morning and low values in the afternoon. The difference between days was 0.8cps on average, which was relatively small. Monthly variations on dry days were generally high during the period from October to January, and low during the period from June to September. The difference between months was 1.9cps on average. Annual variations did not appear to be significant. The average on rainy days was 14.6cps and the range of change was high, 11.1cps on average. The average in the rain was 15.9cps and the highest was 24.1cps. Monthly variations in the rain were high in January and low in August. The gamma-ray dose rate increased rapidly with rain and was changed by the intensity, time and interval of the rainfall. After it stopped raining, it decreased gradually. Measurements that were higher than the value obtained by adding three times the standard deviation of the monthly gamma-ray dose rate to the monthly average gamma-ray dose rate were 2.2% a year on average. Most of them were observed when it rained. They were also observed when it rained after yellow sand was blown in from China.
An integrating radon progeny monitor developed by the JNC Ningyo-toge collects radon progeny on a filter using a pump with a lead storage battery and detects alpha particles by a CR-39 detector. However, it differs from others in that it uses interval operation for sampling and that it has the function of estimating the effect of components attached by diffusion during the interval. The monitor makes possible the long-term measurements of radon progeny concentration over a month without an AC power source. The monitor is suitable for environmental monitoring. Several experimental measurements were carried out at laboratories and at outdoor fields. The measurement results by the integrating monitors have agreed well with our previous investigations.
It is important to understand 222Rn concentrations in dwellings precisely for dose assessment. 222Rn concentrations were continuously measured in a reinforced-concrete house in Tokyo with three stories above ground and one basement for seven years, from October 1988 to September 1995. In the basement, temperature and humidity were also measured, which were used for analyzing the seasonal variation of the 222Rn concentration and its relationship with environmental factors. 222Rn concentrations on the 2nd and 3rd floors showed a statistically significant seasonal variation, i. e., higher in winter and lower in summer, but those on the 1st floor did not show any significant seasonal variation. The 222Rn concentration in the basement showed a reverse seasonal variation, i. e., higher in summer and lower in winter. The 222Rn concentrations on each floor showed a drastic decrease after the renewal of the dehumidifier in the basement, which suggests that the 222Rn concentration in the basement has an influence on that in the rooms above ground. A multiple regression analysis suggested that the 222Rn concentration in the basement and its seasonal variation can be expressed with statistical significance by the linear combination of temperature, humidity and atmospheric pressure. It was also revealed that the 222Rn concentration on the 1st floor can be expressed by the linear combination of the concentrations in the basement, 2nd and 3rd floors.