Minimum detectable activity

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Minimum detectable activity (MDA) is the smallest amount of radioactivity a detector can reliably detect at a chosen confidence level. It helps decide if a radioactive substance is present and allows comparison of detector performance, in tasks like whole-body counting and radiation monitoring.

Two common ways to estimate MDA exist. The Currie method gives:
MDA = L_D / (epsilon × P × t)

- L_D is the detection limit in counts
- epsilon is the detector efficiency (how well the detector catches the radiation)
- P is the emission probability of the radiation
- t is the live measurement time

L_D depends on the desired confidence level. For a typical choice of 5% false positives and 5% false negatives, L_D = 2.71 + 4.65 × sqrt(B), where B is the number of background counts.

In short, to lower MDA you want low background, high detector efficiency, a higher emission probability for the radiation you’re measuring, and a longer measurement time. Ways to reduce MDA include using coincidence measurements (detecting two related events together), using larger detectors, adding shielding, or measuring in underground facilities to cut background.