INTERACTIONS OF RADIATION WITH MATTER 



atomic electrons will be spaced much closer together. If, however, the range 

 of penetration is calculated in units of weight per unit area (i.e. distance X 

 density) it is found to be nearly independent of the nature of the absorbing 

 material, provided that the absorber is not a very heavy element. Some 

 idea of the penetrating power of homogeneous /5-particles of different 

 energies may be obtained from Table 2. The density of aluminium is 2,600 



TABLE 2 

 Extrapolated Ranges for ^-particles of Various Energies (from Kamen^) 



{By courtesy of Academic Press) 



mg/cm^, so that a 1 MeV /9-particle would be brought to rest by about 1-6 

 mm of aluminium or 4-2 mm of water; in air its range would be about 

 350 cm. 



a-Particles, by virtue of their high charge and low velocity, cause a rela- 

 tively intense ionization of the matter through which they pass, and their 

 total range is correspondingly small. Thus the range of a 1 MeV a-particle 

 in air is only about 5 mm. This results in some differences between methods 

 for detecting a- and /5-particles, but these need not concern us here since 

 those isotopes which emit a-particles are not of major importance in 

 biological research. 



When y-rays pass through matter they do not cause ionizations directly 

 as the charged particles do, but instead they interact in one of three main ways 

 with the electrons in the absorbing material. For y-rays of relatively low 

 energy the principal mechanism of interaction is photoelectric absorption. 

 This consists in an encounter between the y-ray photon and an extranuclear 

 (orbital) electron, as a result of which all the y-ray energy is transferred to 

 the electron, ejecting it from its parent atom with a kinetic energy equal 

 to the difference between the y-ray energy and the energy with which it was 

 bound to the atom. The electron subsequently loses the energy by causing 

 ionizations in the manner just described for /9-particles. The probability 

 that photoelectric absorption will occur increases sharply with the atomic 

 number of the absorber, being proportional to Z^. The second mechanism of 

 energy transfer is termed Compton scattering, this being the dominant process 

 in y-ray absorption for energies between about 0-5 and 5 MeV. It consists of 

 the transference of some of the y-ray energy to an electron which may be con- 

 sidered as free (the quantity of energy transferred is generally large compared 

 with the atomic binding energy, so that whether the electron is initially free 

 or extranuclear makes little difference). After collision, the y-ray photon 



421 



