Sparkfun.com sells a 712 end-window geiger tube from LND, Inc. It's
sensitive to alpha and beta particles as well as gamma radiation, and
it's something like $70. They have a complete geiger counter kit which
includes a DC-DC converter to get the 500V for $140.

They currently have a promotion going on which is causing their site to be
hammered, but it should be back to normal in a day or two.
--scott

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"C'est un Nagra. C'est suisse, et tres, tres precis."