What is the primary characteristic of a paralyzable counting system?

In a paralyzable counting system, the primary characteristic is that it decreases in count rate after reaching a peak count. This occurs because, in a paralyzable system, the detector becomes overwhelmed by high activity, leading to the loss of counts as additional events cannot be processed immediately. As the count rate increases, there comes a point where the system cannot register every event, leading to a drop in the effective count rate. This behavior results in a non-linear response at high activity levels, as the detector cannot continuously record each incoming event once a certain threshold is exceeded.

In contrast to what might be suggested by the other options, a paralyzable counting system does not maintain a consistent plateau with increasing activity; instead, it experiences a decline in count efficiency. This characteristic differentiates it from other counting systems that may exhibit more stable counting rates even as activity levels rise significantly. Understanding this functional aspect is crucial for interpreting data from nuclear medicine instruments accurately.