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In case of radioisotopes the activity reduces to ........................ of initial value after 2 half lives.

  • A.\(\frac{1}{3}\)rd of the initial activity
  • B.\(\frac{1}{2}\) of the initial activity
  • C.\(\frac{1}{4}\) of the initial activity
  • D.\(\frac{1}{6}\) of the initial activity

Answer: C

After two half-lives the activity falls to 1/4 of the initial activity.

Each half-life halves the activity:

\[ \begin{aligned} A &= A_{0}\left(\tfrac{1}{2}\right)^{n} \\ &= A_{0}\left(\tfrac{1}{2}\right)^{2} = \tfrac{1}{4}A_{0} \end{aligned} \]

One half is the value after a single half-life. One third and one sixth come from treating the decay as linear or dividing by the number of half-lives, which is not how exponential decay works.

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