The test points are the circular pad seen here:
Left is the analog, right is the digital. The red trace is the outoput of a long chain of ion pumps that produce about 25 vdc. The blue is the pulse signal from the photo diode.
Radon decays in the sensitive volume of the canister, the Po-218 is electrostatically deposited onto the diode. Since it - and its decay series - are in contact with the diode, this greatly reduces the loss of energy ( and broadening of the peaks ) due to alpha particles traveling in air. For "real" sources, like contamination swabs, rocks, or an Am-241 button... a collimation screen can be used ( I'm going to go that route ) or pulling a vacuum. Since "normal" levels of radon produce a pulse every 10 minutes, and I am the impatient sort, rigged an emanation jar to dial up the radon to 11. The shoved 2000 pCi of radon in there... Its a bit of autunite in a porous sintered metal filter ( ~5 micron ) with a press fit stainless cap.
Initially I used the logic level pulse, and oscilloscope sillyness to act as a MCA, this let me see if this was worth continuing with.
With that encouraging result, I worked out the right resistance to get the pulses into the MCA... The pulses are about 10ms wide - so just barely fit into PRAs window - and the audio devices I have don't let you do less than 44K. But it worked well enough.
The Po-218 peak cutoff is nice and sharp for these in-air sorts of measurements. The Po-214 is actually interesting, the right side slope reflects the very short half life beta decay ( 0.1 ms half life ) - and the coincidence summing of the alpha and beta stretch the peak. When you get higher rates you encounter pileup - this starts at about 20 - 30 cps. The spectrum is still nice. ( You can also see the buildup of radon in the emanation jar in the pulse vs time graph. )
