ASSAY OF RADIOACTIVITY 



recoils with reduced energy in one direction, and the electron is ejected in 

 another; the interaction behaves as a classical two-body collision, the angle 

 through which the photon is scattered being that required for proper 

 conservation of energy and momentum. A given y-ray photon may take 

 part in several collisions of this sort, losing some energy on each occasion, 

 thus being degraded to a longer wavelength until in the end it is completely 

 absorbed by the photoelectric effect. As before, the electrons involved in 

 Compton scattering dissipate their energy in secondary ionizations. The 

 third mechanism of energy transfer, known as pair production, is most 

 important for very energetic y-rays. If the photon has an energy greater 

 than that equivalent to twice the rest mass of an electron (just over 1 MeV) 

 it is possible for it to disappear within the Coulomb field of an atomic nucleus, 

 and for an electron pair consisting of one positron and one electron to be 

 formed instead. These receive the excess energy as kinetic energy, not 

 necessarily in equal shares, the nucleus playing a part in conserving momen- 

 tum. The probabihty that pair production will occur increases with 7-ray 

 energy, and is proportional to the square of the atomic number of the 

 absorber. 



The range of penetration of y-rays into an absorbing material depends on 

 the sum of the probabilities of these three processes of energy transfer, and 

 the overall absorption coefficient varies in a complicated way with y-ray 

 energy. In general, )/-rays lose much less of their energy per unit path length 

 than do /5-particles, so that their penetrating power is much greater; also, 

 in contrast to /S-particles, they are absorbed more effectively in materials of 

 high atomic number, so that in air or water they can traverse great distances. 

 Since the absorption by any given material increases exponentially with 

 absorber thickness, y-rays cannot strictly be said to be completely absorbed, 

 but are attenuated in passing through matter. Their penetrating power is 

 best expressed in terms of half-thickness values, that is the thickness of 

 absorber required to reduce the energy of a beam of y-rays to half the inci- 

 dent value. For 1 MeV y-radiation the half-thickness of an aluminium 

 screen is about 12 g/cm^, or about 45 mm; for comparison, the half-thick- 

 ness for 1 MeV /9-particles is only 0-3 mm Al. 



DETECTION OF IONIZING RADIATION 



All methods for the quantitative determination of radioactivity depend on 

 detection of the ionizations produced when a-, j3- or y-rays pass through 

 solid, liquid or gaseous media. The techniques available may be divided 

 into three main classes as follows: 



Radioautography 



The path of ionizing radiation through a photographic film is marked by a 

 blackening of the grains in the emulsion similar to that produced by visible 

 light. This effect was first exploited over 60 years ago to demonstrate the 

 presence of radioactivity. It has not often been used for precise quantitative 

 measurements, but provides the most convenient basis for visualizing the 

 distribution of radioactivity in tissue sections or small whole organisms. 

 Developments in recent years have improved the spatial discrimination of 



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