Minimum detectable activity

Minimum detectable activity (MDA) is the lowest activity of a radioactive nuclide that can be detected with a detector, to some confidence level.¹ It is a concept that is used in several circumstances, such as in whole-body counting or radiation monitoring, to aid in determining the presence or absence of a radioactive substance or comparing the performance of different detector systems. There are several ways to calculate MDA that are commonly used, including the ISO11929 standard and the Currie method.² The Currie method is given by³

${\displaystyle MDA={\frac {L_{D}}{\epsilon \cdot P\cdot t}}}$,

where ${\displaystyle L_{D}}$ is the detection limit in units of counts, ${\displaystyle \epsilon }$ is the detection efficiency of the detector, ${\displaystyle P}$ is the emission probability of the radiation, and ${\displaystyle t}$ is the live time of the measurement. The formula for ${\displaystyle L_{D}}$ will vary depending on the specified confidence level. For false-negative and false-positive rates of 5%,¹

${\displaystyle L_{D}=2.71+4.65{\sqrt {B}}}$,

where ${\displaystyle B}$ is the background counts. In other words, to achieve a low MDA, low background counts and a high efficiency are desired. Methods to decrease MDAs include utilizing coincidence measurements,⁴ large detection volumes,⁵ or using large shielding and underground measurement facilities.⁶