0.20 



>- 



-. 0.15 - 



98 RADIATION BIOLOGY 



electrons and photons travel on deeper into the material. The further 

 penetration of the electrons and positrons proceeds as described in the 

 preceding section. The further penetration of photons proceeds as will 

 be described in Sect. 4-3b. The lower energy photons, having little 

 chance of forming pairs, turn out to constitute the most penetrating 

 secondary component of a shower. 



4-2d. Positive Electrons. The penetration and the distribution of 

 energy by positive electrons take place in the same manner as for ordinary 

 negative electrons, except for a few rather minor differences. 



A positron is constantly exposed to annihilation as a result of collision 

 with an atomic electron anywhere along its track. However, the chance 

 of annihilation remains small until the positron has spent nearly all its 



kinetic energy. Figure 1-59 gives 



data on the chance of annihilation 



along the track. 

 < 0.10 - / As indicated in Sect. 2-2b, positron 



2^^^[ / annihilation may yield one or two 



X-ray photons. Emission of a single 



Q5 5 5Q photon is never very likely. Its 



ENERGY, Mev maximum probability is estimated to 



Fig. 1-59. Probability of annihilation be at most 20 per cent of the prob- 



before stopping for positrons of differ- ability of two-photon annihilation for 



ent energies. {Heitler, 1944.) 5-Mev positrons in lead. 



The elementary processes of collision of charged particles, as described 

 in earlier sections, have equal probability irrespective of whether the 

 charge of the incident particle is positive or negative [see Eq. (13), Sect. 

 2-2c]. Accordingly, only the strength of the electric forces between the 

 incident particle and the atomic particles matters, irrespective of their 

 sign. This accounts for the basic similarity of the behavior of electrons 

 and positrons traversing matter. 



However, attractive and repulsive forces do produce different effects 

 when the forces are very strong and act on particles whose speed 

 approaches the speed of Ught. The formulas of Sec. 2-2c do not indicate 

 this difference of action because they are derived under the assumption 

 that the velocity of the incident particle remains much smaller than the 

 speed of light. An improved analysis must take into account the special 

 effects of inertia which arise when the speed of light is approached. These 

 effects turn out to increase the chance of sharp deflections of electrons and 

 to reduce the corresponding chance for positrons when the energy of the 

 incident particles amounts to hundreds of thousands of electron volts or 

 more. 



Therefore scattering effects are less important, as compared to the 

 energy dissipation, for high-energy positrons than for electrons. The 

 probal)ility of backscattering of positrons has been observed to Ue about 



