Question

A 2.0 mm thick sheet of shielding allows 80% of incident gamma radiation to pass through it.
How many layers of this shielding are required such that less than 30% of incident gamma rays to pass through all layers? (to 2 s.f)

Answer 1

For $\gamma$ rays

$I=I_0e^{-\mu x}$

In our case

$0.8I_0=I_0e^{-\mu\cdot 0.002} \to \mu=111.57 m^{-1}$

So,

$0.3I_0=I_0e^{-111.57\cdot x} \to x=111.57 m^{-1}\approx0.011m$

$n=\frac{0.011}{0.002}=5.5\approx6$

6 plates are required.