Part III.

Problem #$1.(15$ pts$)$ A beam with a specified intensity is incident on a slab of graphite such that the

total cross section of carbon at this energy is $5\mathrm{.}2$ barn. Assuming that the density of graphite is $20\frac{g}{c}{m}^3,$

$($a$)$ Calculate the macroscopic cross section of graphite.

$($b$)$ How thick must the slab be to reduce the intensity of uncollided beam by a factor of $4.$