Which action is NOT a method to lower the Minimum Detectable Activity (MDA)?

• A. Increase the count time
• B. Increase the sample volume
• C. Use a higher efficiency counter
• D. Increase the sample pH

Answer: (D)

Minimum Detectable Activity is driven by how many decays you actually count relative to the background and how efficiently your detector collects those counts. To lower the MDA, you want more true signal counts and less statistical uncertainty from background. Extending the counting time lets you accumulate more decays, so the signal stands out more clearly. Increasing the sample volume puts more radioactive atoms in the detector’s view, boosting expected counts. Using a detector with higher efficiency captures a larger fraction of emitted radiation, improving the signal-to-noise ratio. Changing the sample pH, however, does not directly improve counting statistics or detector performance; it’s a chemical parameter that may affect sample preparation but does not reduce the MDA.