CHAPTER 2 

 GAMMA RAYS 



2.1. Properties. Gamma rays are an electromagnetic radiation produced 

 only in nuclear processes, either in processes such as neutron capture or from 

 the decay of excited nuclei by isomeric transitions. This distinguishes 

 gamma radiation from x-rays only in the sense of its origin. 



The energy of a gamma photon is directly proportional to the frequency: 

 E = hv = hc/\, where the factor of the proportionality, h, is Planck's con- 

 stant, c is the velocity of light, and X is the wavelength. In units of mev, 

 E = 0.012354/A where X is in angstroms. Also associated with the photon 

 is a momentum of magnitude p = hv/c = h/\. In any process involving 

 the interaction of a gamma photon with an atom or any elementary particle, 

 both the energy and momentum must be distributed according to the laws of 

 conservation among the various particles and radiations participating. In 

 the collision of a photon with an atom, a large proportion of the momentum 

 is transferred to the atom and the greater part of energy is transferred to an 

 orbital electron which is ejected from the atom. 



Gamma-ray absorption is due almost entirely to interaction of photons 

 with free and bound electrons in an absorbing medium. Each absorption 

 event, involving a single photon and an electron, takes place by one of three 

 distinct interaction processes: (1) photoelectric effect, cross section designated 

 by t; (2) scattering (Compton effect), cross section designated by a; (3) pair 

 formation, cross section designated by k. The relative importance of each 

 process as well as the absolute probability for its occurrence bears a strong 

 dependence on gamma-ray energy and the atomic number of the absorber. 

 In principle, the total effective electronic cross section over the entire energy 

 range is represented by the sum of t, a, and k, but in limited ranges contribu- 

 tions from more than one process may be negligible. At very low energies 

 only the photoelectric effect is important, particularly in heavy elements. 

 Scattering becomes the dominant process at medium energies, and for high 

 energies pair formation is mainly responsible for gamma-ray absorption. 

 Again, in certain portions of the energy range, pairs of processes, either t and 

 a or a and k, must be considered in gamma-ray absorption because the cross 

 sections for the processes are then comparable in magnitude. 



The simplest problem in gamma-ray absorption and the one most fre- 

 quently encountered in experimental arrangements is that of a well-colli- 

 mated beam of radiation. Geometrical reduction in intensity does not enter, 



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