When is coincidence loss most likely to occur when using a well counter?

Coincidence loss occurs when two gamma photons from the same radioactive decay event are detected by a well counter, but only one of the two photons is registered due to timing and spatial resolution limitations of the detection system. This is more common at high activity levels because with increased activity, the probability of multiple photons arriving at the detector simultaneously increases.

When the activity of the source is above a certain threshold, such as 2 mCi, the likelihood of these simultaneous events—or coincidences—occurring rises significantly. If the well counter is unable to process multiple photons closely in time, some events can be lost or undetected, leading to a misrepresentation of the actual activity in the sample.

In contrast, lower activities would produce fewer photon emissions, thereby reducing the chance of coincidence loss since the counting events can be more easily resolved. Hence, the activity of the source being above 2 mCi directly correlates to an increased chance of coincidence loss within the detection system of a well counter.