A freshly prepared radioactive source of half-life `2 h` emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safely with this source is

Options

A. `6h`
B. `12 h`
C. `24 h`
D. `128 h`

Answer

b

Explanation

`(A)/(A_(0)) =(N)/(N_(0))`
Let safe level avtivity be `A`. Initial activity =`64 A`.
Hence, `(N)/(N_(0)) =(A)/(A_(0))=(A)/(64A)=(1)/(64)`
or `((1)/(2))^(n)=(1)/(64)` or `n=6`
Hence, `(t)/(T) =n=6`
`becauseT=2h`
`:. t= 12 h`.