Question

A radioactive sample had an initial activity of 56 DPM ( disintegration per min ). After 69.3 min, it was found to have an activity of 28 DPM. Find the number of atoms in a sample having an activity of 10 DPM.

• A. 693
• B. 1000
• C. 100
• D. 10,000

Answer 1

Answer: (B)

The expression for the activity is $A=\lambda N=\frac{0.693\times N}{t_{1/2}}$

$t_{\frac{1}{2}}=69.3min$

Substitute values in the above expression:
$10=\frac{0.693\times N}{69.3}$

$N=1000$