Modern Physics Ques 110
- The half-life of ${ }^{215} At$ is $100 \mu$ s. The time taken for the activity of a sample of ${ }^{215}$ At to decay to $\frac{1}{16}$ th of its initial value is
(2002,2 M)
(a) $400 $ $\mu s$
(b) $63 $ $\mu s$
(c) $40 $ $\mu s$
(d) $300 $ $\mu s$
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Answer:
Correct Answer: 110.(a)
Solution:
Formula:
$ R=R _0 \quad (\frac{1}{2}){ }^{n} $ $\quad$ …….(i)
Here $R$ = activity of radioactive substance after $n$ half-lives
$ =\frac{R _0}{16} $
Substituting in Eq. (i), we get $n=4$
$ \therefore \quad t=(n) t _{1 / 2}=(4)(100$ $ \mu s)=400 $ $\mu s $
BETA
