Sec. 2.2] GAMMA RAYS 35 



These factors, together with the fact that scattering causes degradation of 

 energy as well as directional divergence, make it impracticable to attempt to 

 compute the reduction in energy flux or intensity of beams traversing absorb- 

 ing media, and experimental measurements of the attenuation must be made. 

 However, for the detection and measurement of gamma rays emitted by 

 radioisotopes, the requirements of homogeneity and parallel beams can be 

 met in the experimental arrangement and the simple exponential absorption 

 described above is more nearly valid. Only scattered radiation remains a 

 possible source of error, but this is largely eliminated by adequate collima- 

 tion. Detailed calculations of gamma-ray absorption in thick slabs when 

 single and multiple scattering are included have been given by Hirschfelder 

 and Adams [13]. 



2.2. Photoelectric Effect. Gamma rays of low energy are absorbed mainly 

 by photoelectric ejection of orbital electrons from atoms of the absorbing 

 medium. This is a resonance phenomenon in which the energy of a photon 

 is transferred to a single electron, ejecting it from the atom with a kinetic 

 energy E e equal to the difference between the gamma energy hv and the 

 electron's ionization potential /, or E e — hv — I. The mechanism of the 

 interaction is most easily explained in terms of the influence of the electric and 

 magnetic components of the gamma ray on an electron. At very low energies 

 only the electric component is important, and since, as in other forms of 

 radiation, it is oriented normally to the direction of the ray, the most probable 

 direction taken by the ejected electron is also normal to the incident gamma 

 ray and along the electric vector. As the energy is increased, the influence of 

 the magnetic component becomes appreciable. The electron is accelerated 

 by the electric field as before, but it is also deflected more in the forward 

 direction of the incident photon by the magnetic-field component. From 

 the direction taken by the electron it is evident that the total momentum 

 involved in the process can be preserved only if it is shared in part with the 

 atom from which the electron is ejected. 



The orbital electrons most likely to participate in the photoelectric effect 

 are those with binding energies nearest in magnitude to that of the incident 

 photon, provided that the binding energy is less than hv. When the gamma- 

 ray energy is considerably greater than the K x-ray absorption limit, the 

 probability for interaction with electrons of the various atomic shells, K, L, 

 M, . . . , decreases with increasing principal quantum number. Con- 

 sequently the K electrons are the most likely to participate in the photo- 

 electric effect when the gamma-ray energy is greater than the K absorption 

 limit. 



When the gamma-ray energy is comparable to the K, L, . . . electron 

 ionization potentials, the photoelectric cross section is complicated by the 

 characteristic x-ray absorption limits. However, for energies greater than 



