Pomelo detector pair configured in coincidence mode. Bottom one also houses the Am-241 source.

Two Pomelo detectors are configured in coincidence, in a fixed configuration with the Am-241 source. One of the detectors is sensitive just to the gamma rays that easily pass through the enclosures, whereas the other will see the alphas too.

Schematic representation of the two detectors and the physics process. A coincidence pulse is generated only when one gamma hits the top detector and one alpha hits the bottom one.

The bottom detector is made insensitive to gamma rays by setting its energy threshold above the gamma ray energy. Even slowed down by some air in between the source and detector, alphas still deposit more energy than the 59.5 keV gammas.

Changing the distance between the two detectors adjusts the coincidence rate by lowering the gamma count rate in the top detector. This distance has been adjusted to produce around one coincidence pulse per second.