Before settling on the coincidence setup I considered using just the gamma line from Am-241 decay. As Pomelo can perform gamma spectroscopy we can look for the 59.5 keV gamma line and use that for our timebase. In the example gamma spectrum below we see we would have to look for counts in the energy range ~40 keV to ~80 keV, because of limited detector energy resolution.

Gamma spectrum of Am-241 with source pretty close to the detector

While we can increase the source distance to the detector to get on average 1 count per second in that region, natural radioactive background also has a significant contribution there. The background spectrum below is accumulated over a very long time without the Am-241 source close by.

Background gamma spectrum with no Am-241 source. Position of the Am-241 gamma energy is still indicated for reference.

This spectrum has ~2.6 counts per second in the energy region of interest. This means background alone gives more signal than we need for the timebase. One fix for this would be to shield the detector and source in lead to remove the background contribution, but I have not explored this option.