I have no idea why Po210 is used, but I think detecting Ra226 would
certainly indicate that it must be Po210 in the spark plugs. I am
not a radiochemist; my comment about Po208 was made after a casual
glance at the Rubber Bible. Many years ago I used a Po210-Be neutron
source. It seems, in retrospect, that Po208 would have been a better
choice for that application, too.
I used my neutron source in a field application measuring the
moisture content of soil *in situ*. I also used a gamma source (for
measuring soil density) which I kept in a lead shield. The neutron
source was kept in a paraffin-filled aluminum container that looked
sort of like a roundish 25-liter liquid nitrogen dewar, if my
memory serves me. Gammas were not a problem with the neutron source
so you may have spotted the advantage of Po210 at that. The
paraffin was used as a moderator and calibrator. The relatively
short half life of Po210 was an inconvenience as I recall.
Still, there is a nuclear chemist reading this list (Truman Kohman)
who should be able to cast some light on the questions. Perhaps
tradition played an important role in choosing Po210 over Po208, or
maybe the recipe that Mme. Curie left for cooking the stuff out of
pitchblende was the only one that could be found?