As some of you may recall, Andrew Back created a rather nice low-power radiation detector for OpenRelief last May. He refined the design around the turn of the year with the removal of the need for a large battery pack to provide a reference charge to the ionisation chamber.
The OpenRelief website now has an updated dedicated page discussing the detector, linking to the schematics and code, and providing plenty of pictures:
http://www.opendawn.com/openrelief/radiation-detector/