テレコバルト-60の照射方向に垂直な平面の線量分布

Abstract

This paper deals with the γ ray intensity distributions on the plane normal to the central ray by the one type Telecobalt apparatus (Type RI 103,300C, 2cm^2φ, wafers type). Method : The γ ray dose was calculated by means of film method, corrected by Siemens-dosemeter. One of the 2 sheets of industrial film type RR was placed holizontally on the surface of the water phantom (20×30×20cm^3), and the other one in the 10cm depth. Results : 1) It seems the intensity distributions are nearly homogeneous within 5×5cm^2 diaphram width. It is observed that relative intensity of the peripheral area to the central area at the region of the umbra are more than 90%. In the penumbra area diminished the intensity to 80〜50%. But the shape of irradiation fields are seemed almost rectangular, similarly to th shape of diaphram. 2) If the diaphragm width are more than 6×6cm^2,the diminishments of intensity at the corners of the irradiation fields are observed (from 80% to 60% of center intensity). This matter is same in the umbral area as the penumbral area. According to this facts, the shape of irradiation fields are jnst like round, in spite of rectangular diaphragm. 3) It is caused by the shape and geometrical structure of the cylindrical hole of the rotating shutter that the isodose curves are not rectangular at large irradiation fields. 4) To perform the shape of irradiation fields rectangular similar to the diaphrgm pattern are there some improvements necessary. (1) To widen the diameter of cylindrical hole of the rotating shutter. (2) To make alternation of shape of the cylindlical hole to the rectangle. (3) To the shorten the length of the hole of the rotating shutter.