http://www.galacticelectronics.com/:.HTML
And using a ROBOTRON 70 013 Geiger tube.
TODO: serial interface for RNG based on the design at:
http://www.fourmilab.ch/hotbits/hardware.html
Further research:
http://www.cosmicrays.org/muon-detector.php
http://hometown.aol.com/redmo/geiger.html
http://www.kronjaeger.com/hv/rad/det/geiger/index.html
http://www.techlib.com/science/geiger.html
http://www.teralab.co.uk/Electronics/Geiger/Geiger_Page1.htm