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A quantity decaying with \( dN/dt = -\lambda N \) falls to half its initial value after a time

  • A.\( 1/\lambda \)
  • B.\( 2/\lambda \)
  • C.\( \ln 2/\lambda \)
  • D.\( \lambda \ln 2 \)

Answer: C

The solution is \( N = N_{0}e^{-\lambda t} \). Setting \( N = N_{0}/2 \),

\[ e^{-\lambda t} \;=\; \tfrac{1}{2} \quad \Longrightarrow \quad \lambda t \;=\; \ln 2 \quad \Longrightarrow \quad t \;=\; \frac{\ln 2}{\lambda} \;\approx\; \frac{0.693}{\lambda}. \]

The time \( 1/\lambda \) is the mean lifetime, at which the quantity has fallen to \( 1/e \) rather than a half

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