A freshly prepared radioactive source of half life `2` hr 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. 6 hours
B. 12 hours
C. 24 hours
D. 128 hours

Answer

B

Explanation

(b) `(A)/(A_0) = (N)/(N_0)`
Let safe-level activity be `A`. Initial activity `= 64A`
Hence `(N)/(N_0) = (A)/(A_0) = (A)/(64 A) = (1)/(64)`
or `((1)/(2))^n = (1)/(64)`or `n = 6`
Hence `(t)/(T) = n = 6`
But `T = 2 hours`,
`:. t = 12 hours`.