Problem 5.221

A narrow beam of X-ray radiation of wavelength 62 pm penetrates an aluminium screen $2.6 \mathrm{~cm}$ thick. How thick must a lead screen be to attenuate the beam just as much? The mass absorption coefficients of aluminium and lead for this radiation are equal to 3.48 and $72.0 \mathrm{~cm}^{2} / \mathrm{g}$ respectively.