area at a distance x from the surface is given by
The total energy W1 per unit area radiated to the surface per sec. by a thickness d is given by
if λ1d is large.
In a similar way it may be shown that the energy W2 of the β rays reaching the surface is given by
where E2 and λ2 are the values for the β rays corresponding to E1 and λ1 for the α rays. Thus it follows that
λ1 and λ2 are difficult to determine directly for the radio-active substance itself, but it is probable that the ratio λ1/λ2 is not very different from the ratio for the absorption coefficients for another substance like aluminium. This follows from the general result that the absorption of both α and β rays is proportional to the density of the substance; for it has already been shown in the case of the β rays from uranium that the absorption of the rays in the radio-active material is about the same as for non-radio-active matter of the same density.
With a thick layer of uranium oxide spread over an area of