106 RADIOACTIVITY; BIOLOGICAL TRACERS 



atom, excitation can occur, accompanied by loss of speed, and hence loss of 

 energy ( = 1/2 mv 2 ). The definite changes in direction result from collisions, 

 and the energy and momentum transferred can cause ejection of the electron 

 hit; i.e., ionization. When collisions are ''favorable," the trail of ionization, 

 although sparse, may penetrate quite deeply into a tissue; but when unfavor- 

 able, it will be very intense but very short. (See Figure 5-1.) 



Very fast electrons may penetrate the atom as far as the nucleus, and by 

 interaction with the field of force about the nucleus lose energy, with the 

 production of secondary X rays. These X rays are called bremstrahlung. 

 Hence a hard beta source may produce a secondary radiation which is much 

 more penetrating than the impinging betas. 



The initial velocities of betas from a source vary widely because the 

 small neutral particle, the neutrino, is ejected from the nucleus along with 

 the beta, and the energy of the disintegration is split between the two. It is 

 the maximum energy of the betas which is usually given in tables of data. As 

 a result of the energy distribution and the deflection of betas as they enter 

 and lose energy in a target, the betas follow a nearly exponential law of 

 penetration. 



Gamma Rays 



The gamma ray is electromagnetic radiation, like light, but of very short 

 wavelength. Since it carries no charge, it is captured only by direct collision 

 or wave-like interaction with a target: with the nucleus or the electrons of an 

 atom. Some energy is transferred to the target electron and the gamma con- 

 tinues on, usually in a modified direction, at reduced energy (e, = hv t ) 

 where v, is frequency and h is Planck's constant. The recoil electrons are 

 relatively slow, and are therefore good ionizers (see Figure 5-1). Just as in 

 the case of X rays (see Figure 4-10 (a)), then, absorption of gammas arises 

 from essentially two processes: (1) "pair production": strong interaction 

 with the nucleus and production of a pair of electrons (e + and e~) — impor- 

 tant in water only if energy of the y is above 3 Mev; and (2) "Compton ab- 

 sorption": ejection of an electron at an angle, some of the energy of the 

 gamma being lost, and the remainder ("Compton scattering") proceeding, 

 usually in a changed direction, and always at lower frequency. The process 

 (2) is repeated until, finally, the energy left from a succession of collisions is 

 absorbed by the electron clouds of atoms (photoelectric absorption) and is 

 ultimately dissipated as heat. At energies below about 0.2 Mev, elastic 

 (Rayleigh) scattering reduces the absorption and increases the range of the 

 gamma in water and soft tissues. 



Neutrons 



The neutron is as heavy as the proton, but carries no charge. Energy is 

 lost only by collision with light nuclei, and hence it can penetrate as deeply 



