Question

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

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

Answer 1

Answer: (B)

12 h

Answer 2

From $R=R_0\bigg(\frac{1}{2}\bigg)^2$
we have, $1=64\bigg(\frac{1}{2}\bigg)$
or n = 6 = number of half lives
$\therefore t=n\times t_{t1/2}=6\times2=12h$