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The half-life periods of two radioactive materials A and B are 1500 years and 1200 years respectively. If their mean life periods are \(\tau_{A}\) and \(\tau_{B}\) respectively, then the value of the ratio \(\frac{\tau_{A}}{\tau_{B}}\)

  • A.\(\frac{5}{4}\)
  • B.\(\frac{2}{3}\)
  • C.\(\frac{3}{5}\)
  • D.\(\frac{5}{7}\)
  • E.\(\frac{2}{5}\)

Answer: A

The ratio is \(\frac{5}{4}\).

Mean life and half-life are proportional: \(\tau = \frac{1}{\lambda}\) and \(T_{1/2} = \frac{\ln 2}{\lambda}\), so \(\tau = \frac{T_{1/2}}{\ln 2}\).

\[ \frac{\tau_{A}}{\tau_{B}} = \frac{1500}{1200} = \frac{5}{4} \]

The factor \(\ln 2\) cancels in the ratio. The other fractions, all less than 1, would make A shorter-lived even though its half-life is longer.

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