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. 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. The Currie method is given by

$MDA=\frac{L_D}{ϵ\cdot P\cdot t}$,

where $L_D$ ins the detection limit in units of counts, $ϵ$ is the detection efficiency of the detector, $P$ is the emission probability of the radiation, and $t$ is the live time of the measurement. The formula for $L_D$ will vary depending on the specified confidence level. For false-negative and false-positive rates of 5%,

$L_D=2.71+4.65\sqrt{B}$,

where $B$ 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, large detection volumes, or using large shielding and underground measurement facilities.