Another option I considererd before settling on the coincidence setup was to use a higher enerrgy gamma line where there is lower background, as this was the main problem identified in the previous log entry.

The natural background quiets down at higher energies. Above the 1.46 MeV peak from K-40 there are not enough counts in the detector to interfere with a one count per second source, if we can find it.

Background gamma spectrum taken with Pomelo. The K-40 is visible at 1.46 MeV

The only source I have that gives such high energies are thorium welding rods. Unfortunately they are way too weak, the highlighted peak in the Th-232 spectrum below, around 1.6 MeV, only has ~0.5 counts per second including its background.

Th-232 gamma spectrum taken with Pomelo from thorium welding rods. Bump around 1.6 MeV is a collection of closely spaced gamma lines from thorium decay

Unfortunately for my setup this means this method is unavailable for the clock timebase.