Yup, the 3 microcurie source makes sense. With a pair of diodes we could make the detector 4 pi, but no big deal. 3 microcuries is not a problem health-wise as long as you don't eat it. As it's an alpha source, any shiledingor even a few centimeters of air will stop the rays. Howerver, I just encountered a new angle on it. According to my father, that's on the order of $100 worth of radium :(. If we're willing to go with a much slower source, we can use thorium which is only $2 per gram or so. Alternativly, anyone have an old clock with a radium dial? <grin> BTW: Cosmic ray background is only 1 event per square foot per minute. Plus, they occur in showers. So that isn't going to work. -- Vercotti: I was terrified of him. Everyone was terrified of Doug. I've seen grown men pull their own heads off rather than see Doug. Even Dinsdale was frightened of Doug. Interviewer: What did he do? Vercotti: He used sarcasm. He knew all the tricks, dramatic irony, metaphor, bathos, puns, parody, litotes and satire. -- Monty Python, Episode 14 PGP 2.1 Key by finger