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. 6 hr
B. 12 hr
C. 24 hr
D. 128 hr

Answer

B

Explanation

To work safely , intensity must reduce by 1/64 `therefore (N)/(N_(0)) = (1)/(64) ((1)/(2))^(t//T) i.e, ((1)/(2))^(6) = ((1)/(2))^(t//T)`
or `(t)/(T) = 6` or `t = 6T = 12 ` hrs