The element lead, Mass = 207.2 g/mol , has a density of 11.35 g/cm3. Find the number of atoms per cubic centimeter.

The element lead, Mass = 207.2 g/mol , has a density of 11.35 g/cm3. Find the number of atoms per cubiccentimeter. If the total gamma ray cross section at 3 MeV is 14 barns,

what are the linearattenuation coefficient and the half-thickness?

I need help on finding the linear attentuation coefficient most of all! And then the half-thickness, I know there are closely related