An outline of the general-purpose electron-gamma shower Monte Carlo code, EGS4, is given together with the code's applications to various types of research related to health physics such as for radiation detectors, dosimeters, and dose evaluations, including effective dose calculations. The applications to medical physics and education concerning radiation are also introduced.

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We developed a clearance level inspection method using laser-based shape measurement and Monte Carlo calculations for quantifying very low radioactivity in metal wastes. The practical apparatus, the Clearance Automatic Laser Inspection System (CLALIS), is comprised of a shape measurement part and a radiation measurement part. The shape measurement part consists of four laser scanners and four controllers connected to a personal computer. The radiation measurement part consists of eight large plastic scintillation detectors at the upper and lower sides of the radiation measurement area, surrounded by a 5-cm-thick lead shield. Using the digital configuration data of wastes, the calibration factor is calculated by the Monte Carlo simulation code, assuming that the source type is either secondary contamination or activation. The correction factor for background (BG) reduction, due to the self-shielding effect of the waste during radiation measurement, is also calculated by the code. The uncertainties of calibration and BG count rate were experimentally evaluated using mock-metal wastes of various shapes, numbers and sizes with radioactive sources of ⁶⁰Co and ¹³⁷Cs. As a result, it was clarified that the detection limit of CLALIS, which is three times the measurement uncertainty, is approximately 100 Bq for ⁶⁰Co by considering the uncertainties of calibration and BG count rate, which makes it possible to achieve simultaneous inspections to confirm whether the radioactivity level is less than both the clearance level and the surface contamination level. In addition, the possibilities of overestimating and underestimating radioactivity due to the measurement method were evaluated. CLALIS can give objectivity to measured results through completely automatic operation without a manual survey, which would be helpful in the decommissioning of nuclear power plants.

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Some aircrews are certainly anxious about exposure to cosmic radiation. It is thus desired for experts of radiation protection to offer relevant correct information such as route doses and the health effects of low-dose radiation, so as not to make those concerned to suffer from excessive anxiety. In the present study, effective doses on round-trip flights from Tokyo's New International Airport (Narita) to major cities in the world are calculated under conservative conditions. Based on the calculation results, a method for screening the aircrews that need radiation exposure management is discussed.

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Field measurements of the cosmic-ray neutron dose rate on the ground using the neutron dose-equivalent counter developed by Nakamura were conducted in order to allow an understanding of neutron dose rate distribution in Japan. The effects of altitude, geomagnetic latitude, snowfall and rainfall on the neutron dose rate were confirmed from the results of the field measurements. Altitude variation can be fitted to an exponential function (e^αZ), where Z is altitude (km). The α value for the neutron dose rate increased with increasing geomagnetic latitude, 0.6-0.8(km^-1) at latitudes 20°-40°. Roughly, α=0.01×L+0.4 (for Z under 2km), where L is the geomagnetic latitude(°). The effect of geomagnetic latitude on the neutron dose rate was approximately 2% per degree(°) at latitudes 20°-40° Reductions in neutron dose rate due to snowfall and rainfall are caused by a decrease in component backscattering from wet ground surfaces. The neutron dose rate (H*(10)) at sea level at a geomagnetic latitude near 25° in 2002 was approximately 4nSv/h, and the neutron dose rate obtained from the neutron energy spectrum up to 400MeV with the Bonner sphere counter developed by Uwamino was 5.7nSv/h. This difference may by due to the low sensitivity of the dose-equivalent counter to neutrons with energies above 15MeV.

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The micronuclear test was used to study cytogenetic effects in Techa riverside residents exposed to accidental irradiation (the study group) and residents of uncontaminated regions in the Southern Urals characterized by similar socio-economic status and health care standards (the control group). In the course of the study it was possible to assess the spontaneous level of damage to blood lymphocytes, the role played by the radiation factor in the induction of damage to blood lymphocytes, sensitivity of blood lymphocytes to acute irradiation, and their capacity for adaptive response. It has been shown that the baseline (pre-test) level of damage to lymphocytes observed in exposed residents does not differ significantly from the spontaneous level observed in residents of uncontaminated villages. Lymphocytes from exposed subjects have been noted to manifest reduced radiosensitivity and capacity for adaptive response as compared to controls. A conclusion is drawn that the chronic radiation exposure factor makes a major contribution to the formation of radiosensitivity and adaptive response induction in exposed persons.

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A computer code system has been developed to evaluate the environmental radiological impact of accidental releases of radionuclides from a nuclear facility in real time. The system is based on a three-dimensional mass-conserving wind field model and a particle dispersion model with a nesting function in order to calculate the atmospheric dispersion of radionuclides with high spatial resolution in the vicinity of the facility. Input includes on-line local meteorological and effluent data at 10-min intervals, as observed and monitored at the site of the facility, and hourly local meteorological data as predicted by an atmospheric dynamic model. Contour maps and tables of internal and external doses are the output. The performance of the system was validated using environmental monitoring data for the air absorbed dose rate as observed at monitoring stations and monitoring posts around the Tokai Reprocessing Plant when ⁸⁵Kr was discharged during its operation, based on the safety regulations. The observed increase of the dose rate was simulated well. The agreement of calculated and observed dose rate was within a factor of 2 for 42% of observations, and within a factor of 5 for 74%.

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