A freshly prepared radioactive source of half life period of 2 hour emits radiations of intensity which is 64 times the permissible safe level. Minimum time after which it would be possible to work with the source is

Options

A. 6 hr
B. 12 hr
C. 24 hr
D. 48 hr

Answer

B

Explanation

Intensity of radiation depends upon the amount of the substance. Thus , for permissible safe level, the amount left must be 1/64 of its initial amount , say a.
In n half periods, amount left `=((1)/(2))^(n) a`
`therefore ((1)/(2))^(n) a=(1)/(64)a`
`therefore n=6`
Total time `=6xx2 =12` hrs.