48 



BIOPHYSICALLY ACTIVE X-RAYS 



Narrow beam 

 / X , Surface of water 



human tissue to water is about 800/830, which is sufficiently near to 

 unity so that, for all purposes, water may be substituted as an absorbing 

 medium for tissue in measurements of intensity at varying situations 

 and distances from an x-ray tube. In order to avoid errors arising from 

 the effects contributed by the walls and bottom of the water container, 

 the phantom is preferably constructed of a material having a scattering 



power approximating that of water 

 (Quimby [1939]), such as sheets 

 of wax or " presswood." 



The procedure in determining 

 depth dosage is then as follows. 

 Ionization measurements are made 

 at various depths below the sur- 

 face of the water to determine the 

 variation of intensity with depth. 

 The intensity at a point beneath 

 the surface will also depend upon 

 the horizontal and vertical coordi- 

 nates of the point, since we are 

 dealing with a volume effect*. This 

 intensity will depend on the hard- 

 ness of the radiation, the distance 

 of the x-ray tube above the surface 

 of the water, and the size of the 

 portal of entry. The size of the 

 portal is determined by a lead dia- 

 phragm placed at the surface of 

 the water. The distribution of the 

 intensity with depth for a narrow 

 cone of x-rays is shown in Fig. 

 1-20. 



The radiation observed at the points outside the geometric beam, 

 indicated by the dotted lines, is due to scattered radiation produced 

 by the material lying in the path of the primary beam, while the radia- 

 tion intensity inside the geometrical beam is due to unabsorbed radiation 

 reaching that depth plus that due to scattering. Weatherwax [1934] 

 has shown with the aid of water phantoms that when large ports of entry 

 are used 30 to 35 per cent of the intensity just below the surface is due 

 to scattered radiation, coming from the deeper layers of water, while 

 50 to 60 per cent of the radiation reaching 10-cm depth is made up of 

 scattered radiation. These results show the impossibility of predicting 

 predetermined intensities in irradiated tissue on the basis of absorption 

 alone. 



2 4 

 Distance across beam in cm 



Fig. 1-20. A narrow beam of x-rays 

 is directed perpendicularly to the water 

 surface. These so-called "Isodose" 

 curves were obtained from a water phan- 

 tom using a thimble chamber ionization 

 meter. (By courtesy of J. L. Weather- 

 wax [1934]). 



