Because 7 ray absorption is by the discrete events described, rather than a slowing down process as with charged particles, 7 rays do not possess a definite range in matter. In all of the events, however, the probability of reaction between the radiation and mat­ter is constant. The constant of proportionality is called the linear absorption coefficient fi. Thus the decrease in intensity of radiation in a beam of intensity I in passing through a thickness of material 6x is given by:

51 = — pi 5x

or 1 = I0 exp(-fix) where I0 is the intensity of the radiation at x = o, generally the material surface. The form of the above equation applies to /3 radiation also because the j3 particles are emitted with a range of energy up to a maximum value for the radioactive source — unlike a particles which are mono-energetic.

In a similar analysis to the expression for radio­active decay in Section 1.4.4 of this chapter, t± = 0.693/(U, where u is the thickness of the material required to halve the intensity of the 0/y radiation. For comparison, 40 mm of aluminium would be required to halve the intensity of 1 MeV 7 ray and 0,2 mm for 1 MeV /3 radiation. An aluminium thickness of 0.002 mm would be more than sufficient to stop 1 MeV « radiation completely. The inverse of д is sometimes referred to as the mean distance travelled by the radiation,