Abstract
Variations dose rates of X-rays and electron beams of the medical 15 MeV betatron with forcus-ionization chaber distance are studied. As for X-rays, the low energy component is found to be fairly large, especially near the donut. Therefore the dose rate is not inversely proportional to the square of the distance as is simply expected. But using 2.5cm thick acrylite or 3mm lead cap on the ionization chamber, the inversely proportional relation can be obtained. The effect of the scattering rays from the filter is also examined by this method. In the case of the electron beams, the dose rate is not inversely proportional to the square of distance at all, but to the m-th power of the distance (2<m<3). The value of m differs for various energies or methods of measurement. The location of the electron beam source, so called the equivalent focus, is estimated from the variation of the dose rate with distance. The equivalent focus almost coincides with the center of the donut. But the ones decided by photographic method are not same point on the orbital plane and on the vartical plane. And moreover they vary with energy.
