IONIZING RADIATIONS 7 



pairs, it will form a secondary track, far from straight, tortuously 

 branching off from the main primary track. Such recognizable secondary 

 electron tracks are called delta rays, distinguished from the secondary 

 ion pairs made close to the primary path by low-energy secondaries only 

 by their length. Delta rays are responsible for about half of the energy 

 transfer for all ionizing particles, except the slowest electrons, and 

 extend considerably the volume affected by the concentrated ionizing 

 forces of a heavy alpha particle. Lea [(6), esp. pp. 26 ff.] has estimated 

 that delta rays extend the length of the alpha-ionized column by a factor 

 of 2 or 3, that of recoil protons by 20 per cent or so, that of fast electrons 

 very little. This means that the ions and excited atoms lie in a narrow 

 column a few atom-diameters wide along the actual path of an ionizing 

 particle, but out of this main more or less linear spine there come 

 numerous short branches, producing a feathery structure, especially for 

 the heavier particles. (See Fig. 4.) 



Energy-Transfer Density, kev/ju tissue 



Minimum ionization particles 0.22 kev/n 



20-mev betatron gamma rays . 28 



Cobalt gamma rays . 42 



1-mev supervoltage x-rays 0.5 



200-kev deep-therapy rays 2 . 8 



X-ray region 3 . 5 



12-mev protons 10.0 



Cyclotron neutrons: Be + D 23.0 

 Thermal-neutron capture recoil ions -^100.0 



Polonium alphas 150.0 



U fission fragments '^4.0 mev 



Fig. 4. Space rate of energy transfer for a variety of types of beam. Note the 



wide variation possible. 



In the time that any such projectile crosses the cell, some 10"^"* sec, 

 the main energy transfer takes place. Out of the atoms in the wake of 

 the particle there then come secondaries, while molecular transitions 

 and "free-radical" formation take place in the excited atoms. This 

 stage takes perhaps even less time, say about the time of a few electron 

 orbit passages. Some molecular disassociation, now already sure to 

 occur, will not be complete for a considerably longer period, since the 

 nuclei must move. Meanwhile the secondaries are moving out, most of 

 them slowly, and ionizing and exciting as they go, again for such a 

 period of time. After not more than 10~^^ sec, then, there is a feathery 

 arrangement of excited molecules and atoms with free electrons and 

 positive ions, none of them possessing enough energy to ionize further, 



