$5.$A gamma radiation field impinges on a $2\mathrm{.}54$ cm thick absorber with an intensity of $2000$ Mev$/$s$/$cm$2.$ The HVL of the absorber is $5$ cm$.($a$)$ What is the attenuated intensity? $($b$)$ What thickness is needed to reduce the intensity to $2\%?$

$6)$ The mean$-$free$-$path of a copper absorber is $6\mathrm{.}45$ g$/$cm$2.($a$)$ What is the attenuation coefficient? $($b$)$ Convert the mean$-$free$-$path and attenuation coefficient to linear units. The density of copper is $8\mathrm{.}92$ g$/$cm$3.$