Q. If one starts with one curie of radioactive substance $(T_{1/2} = 12hrs)$ the activity left after a period of 1 week will be about
A) 1 curie
B) 120 micro curie
C) 60 micro curie
D) 8 mili curie
Explanation
(c) 1 week = 7 days $= 7 \times 24hr \simeq 14$ half lives
Number of atoms left $= \frac{N_{0}}{(2)^{14}}$, Activity $= N\lambda$
$\therefore$ Activity left is $\frac{1}{(2)^{14}}$ times the initial
$\Rightarrow \frac{1}{(2)^{14}} \times 1curie = \frac{1}{16384} \times 1\ $curie $\cong 61 \times 10^{- 6}\ $curie
$\approx 60\mu\ curie$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.