Question

A freshly prepared radioactive source (half life = 4 hrs) emits radiations of intensity which is 32 times the permissible level. The minimum time (in hr) after which it would be safe to work with this source is:

• A. 4
• B. 8
• C. 10
• D. 20

Answer 1

The correct option is D 20

Let permissible level $=R$

And it current intensity $32R=2^{5}R$

Half life$=4hr$

$2^{5}R$ It will take $5$ halflife to be safe.

$5$ half life $=5\times 4=20hrs$