The Intensity of Gamma Rays Passing Through a Material as a Function

The intensity of gamma rays passing through a material as a function of thickness,x is given by where n is the number of atoms per cm³ and $\sigma$ is the absorption cross section.If the intensity of gamma rays of a particular energy is reduced by half when they passed through 1 cm of lead,what is the absorption cross section for the gamma rays? The density of lead is 11.3 g/cm³ and its molar mass is 207 g.

A) 3.5 $\times$ 10^-22 cm²
B) 6.8 $\times$ 10 ^-22 cm²
C) 4.6 $\times$ 10 ^-23 cm²
D) 2.1 $\times$ 10 ^-23 cm²
E) 8.4 $\times$ 10 ^-24 cm²