Effects of specific counting parameters on minimal detectable activity of instruments

Objectives Regulatory agencies require that counting efficiencies be performed on all counting instruments used to evaluate removable contamination levels. In addition, minimal detectable activity (MDA), which is defined as the minimal amount of activity that can be measured by a counting instrument, must be calculated when determining counting efficiencies. The objective of the research project was to evaluate the effect of specific counting parameters, including instrument counting efficiency, background count rate (cpm), background count time (min), wipe count time (min), and area wiped (cm2) on MDA.

Methods The effect of low (15%) and high (80%) counting efficiency, increasing background count rates ranging from 20 cpm to 100 cpm and increasing background/sample count times from 1 to 5 minutes on MDA was calculated. For each MDA evaluation, specific parameters were altered while other parameters remained fixed. In addition, the effect of background counts and background/wipe count times on MDA of instruments with low counting (15%) and high counting efficiency (80%) was evaluated.

Results The counting efficiency of the instrument was the most important factor influencing MDA, demonstrating an inverse relationship between instrument efficiency and MDA. With increasing counting efficiency, MDA was significantly reduced. When background count rate was increased, MDA also increased. A decrease in MDA was observed when background count time and wipe count time was increased. The effect of background count rate, background/wipe count time was more significant with low counting efficiency instruments as opposed to high counting efficiency instruments.

Conclusions Specific parameters associated with MDA of counting instruments were investigated. The study showed that counting efficiencies of instruments dramatically affected MDA. For low counting efficiency instruments, minimizing background counts and maximizing background count time and wipe count time greatly reduced MDA