In this report are discussed the characteristics of a radiation field generated by a small gamma-ray source and the depth-dose curve in a material placed there.The experiments with TLDs and a 137Cs source resulted in a depth-dependence of about 30% decrease by aluminum wall of 100mg/cm2, to the contrary of a well-known pattern, i.e.saturation after a rapid build-up for parallel photon beam.In order to investigate the discrepancy, a Monte Carlo simulation was employed to calculate the energy distribution of the secondary electrons in an approximate geometry of a spherical source of glass sealed in a stainless-steel capsule.It was confirmed that the surface dose may be affected considerably by secondary electrons generated in the capsule and/or the air between the source and the detector. The angular distribution of secondary electrons and scattered photons have also been calculated, with which the function of a collimator was discussed.

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In order to measure the spatial distribution of neutrons in an accelerator room, the combined use of activation detectors and an imaging plate was tried.Activation detectors were set on monitoring points in the accelerator room. After the end of operation, the detectors were collected and pasted on the corresponding monitoring points on a layout plan of the accelerator room. The plan was overlaid with an imaging plate to develop the activity of activation detectors. This method is suitable for the simultaneous measurement of the activities of many detectors without any decay correction. The obtained image data are very useful for grasping the propagation of neutrons from the beamloss points and to make an effective design for reducing the activation of the accelerator components and accelerator buildings.

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The budding yeast Saccharomyces cerevisiae was used as a model organism to study the effects of 60 Hz magnetic fields on growth and mutagenicity.We designed an extremely low-frequency magnetic field (ELF-MF) exposure system for biological experiments.Using this system, we studied the effect of the ELF-MF on various yeast strains including radiation-sensitive mutants, rad and rev.The exposure to 60Hz magnetic fields of up to 0.5T had no effect on yeast cell growth.Neither a mutagenic potential of 60Hz nor static magnetic fields of up to 0.5 T could be detected by ade2→ADE2⁺reversion assay.

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A method for determining an individual subject's counting efficiency of a thyroid ¹²⁵I monitoring system was developed on condition that health physicists can obtain the subject's parameters, thyroid shape, thyroid weight, and prethyroid tissue thickness, by applying high resolution ultrasonography.The features of counting efficiency and detection limit for anthropomorphic thyroid-neck phantoms were experimentally investigated for various combinations of seven factors; thyroid volume, thyroid shape, prethyroid tissue thickness, neck diameter, rotation angle of the thyroid in the neck, angle between the thyroid and the detector, and the distance between the detector and neck surface.A procedure to determine a unique subject's counting efficiency and detection limit for any detector arrangement was elucidated for a reference man by using a data base on counting efficiency and detection limit.

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