[thewanderer1983]

>What that commenter misses is that nearly all hobbyist grade detectors (Geiger tubes) are not sensitive to alpha

from the original post: "If your Geiger counter is actually detecting radiation, it's almost certainly the half-hour lead and bismuth. " If you look at the table he provided lead and bismuth are beta decay. It is likely he is specifying that on the fact that most home geiger counters only detect beta and gamma not alpha.

[Turskarama]

Could also be a happy accident though. Half an hour is short enough that the amount of radiation emitted is pretty high, but long enough that it hasn't pretty much all decayed by the time you try to measure it.

[BetterGeiger]

You are right, the commenter implied that.