Vertical distribution of alpha-ray intensity near ground surface

To investigate the alpha-ray flux intensity near the ground surface, integrating measurement of it has been made every month, using the CR-39 track detector at various heights. The track densities below the height of 5cm have sometimes shown more than five times of those at height of 200cm. The vertical distribution of the radon daughters near the ground surface measured at the same place was found to be almost uniform, and the content at the various heights showed the diurnal variation similar to each other. On the other hand, the thoron content measured with the two filter method showed a variation with the height; the values at the positions nearest to the ground surface were high and values at the upper positions were low. Therefore, it is confirmed that the difference of the vertical distribution of the alpha-ray intensity near the ground surface mainly depends upon the thoron distribution. The data obtained in this experiment are considered to be useful for the study not only of the vertical mixing of the atmosphere near the ground surface but also of the effect of the radioactivity to the living things near the ground surface. (author)