Q 12-13-095JEE MainJEE Main 2022 (28 Jul, Shift 1)Easy
A freshly prepared radioactive source of half life $2$ hours $30$ minutes emits radiation which is $64$ times the permissible safe level. The minimum time, after which it would be possible to work safely with source, will be ______ hours.
Numerical value type. Enter your answer.
Answer: 15
$64 = 2^6$, so the activity must fall through $6$ half-lives:
$$t = 6\times2.5 = 15\ \text{hours}$$
Solution by Sreeraj P, M.Sc Physics