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

Coincidence loss in a well counter occurs when the count rate from incoming radiation exceeds the detector's processing capability, leading to the inability to fully register all emitted events. This phenomena is particularly relevant when the activity of the radioactive source is high.

When the source activity surpasses a certain threshold, typically around 2 mCi for many well counters, the rate at which events are detected grows to a point where the system cannot process them all simultaneously. This is primarily due to the finite time required for the counter to register each event, leading to the loss of those events that occur in rapid succession. As the activity increases, the likelihood of simultaneous events also increases, amplifying the potential for coincidence loss.

Given this understanding, the correct choice indicates that coincidence loss is likely when the activity exceeds 2 mCi, reflecting the operational limits of the well counter in processing high count rates effectively.