W2 total number of ions due to β rays
12·7 × 6·1
= ------------- = 0·5 approximately.
154
Now the value of λ1 for aluminium is 2740 and of λ2 for the same metal 14, thus
E1 λ1W1
---- = ------- = 100 approximately
E2 λ2W2
This shows that the energy radiated from a thick layer of material by the β rays is only about 1 per cent. of the energy radiated in the form of α rays.
This estimate is confirmed by calculations based on independent data. Let m1, m2 be the masses of the α and β particles respectively and v1, v2 their velocities.