Modern Physics 3 Question 22
22. 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
(1988, 2M)
(a) $6 h$
(b) $12 h$
(c) $24 h$
(d) $128 h$
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Solution:
- From $R=R _0 \frac{1}{2}^{n}$
we have, $1=64 \frac{1}{2}^{n}$
or $\quad n=6=$ number of half lives
$\therefore \quad t=n \times t _{t _{1 / 2}}=6 \times 2=12 h$
