L. H. GRAY 



air was exposed to the rays, I was delighted to see the cloud chamljer filled 

 with little wisps and threads of clouds — the tracks of electrons, ejected by the 

 action of the rays'. Photographs of these tracks, scaled down in the ratio of 

 the electron stopping powers of gas and tissue, still provide us with the best 

 available information as to the approximate distribution of ionizing events 

 in matter exposed to all types of ionizing radiation. We are still not too sure 

 of the precise value of the scaling factor because we do not know the exact 

 value of the average energy expended per ion-pair produced in condensed 

 systems. Moreover, these photographs tell us nothing about the location of the 

 excited molecules which occur along the tracks with greater frequency than 

 the ions. Nevertheless, photographs of the type reproduced in Figures 1 and 2 



10 micron 



Figure 1. A, proton projected by a neutron; B, slow electron (20 kV) 

 X-ray; C, fast electron (200 kV) y-ray 



suffice as a basis for the discussion of the physical factors to which differences 

 between biological response to equal doses delivered by fast and by slow 

 ionizing particles must be related. They provide us with a picture of the 

 dose distribution within the living cell at the level of resolution of the 

 electron-microscope. 



The most obvious features of such pictures are the large amounts of empty 

 space and the comparatively small number of particles which contribute 

 to the energy deposited within a cell exposed to moderate doses of radiation. 

 It is also important to note that the slower the particle, the higher the rate of 

 loss of energy along the track, and the smaller the total number of tracks which 

 contribute to a given dose. This has important biological consequences. 



Consider, for example, the nucleus of a cell about 10 microns in diameter 

 exposed to 25 rad of soft X radiation (AgK). The ionizing particles in this 

 case will be photo-electrons having an energy of 2 1 kV and range slightly 



3 



