Back in May, Explora!, the local science center for which I occasionally volunteer, referred me to the local public TV station to lead a weekend “Science Cafe”. The discussion subject was lightning and its connection with cosmic rays. Trying to augment my usual hands-on electrostatics program with something perhaps more topical, my mind wandered back to a fascinating radiation detector that I’d first encountered in an embodiment built by the consummate craftsman Tim Raney of Richmond, Virginia: an open-air spark counter for alpha particles.
In this type of detector, thin negatively-charged wires are strung through atmospheric air above a planar anode, and sparking occurs when strongly ionizing radiation like alpha particles passes through the high-field region near the wires. The concept was first described by Chang and Rosenblum in Phys. Rev. 67 (1945). Click to download the paper. My version is pictured above, the left hand photo showing its response to a radium source from a Walkie Record-All and the right hand photo the response to a Nuclespot 5-mCi Po-210 source. Note that this is not a traditional spark chamber; it’s much simpler than a spark chamber because it is self-triggering. It also only responds to alpha particles—no beta or gamma sensitivity at all. (I should also mention that it is not closely representative of the runaway relativistic breakdown mechanism postulated to trigger lightning, although it does obviously exploit the ionization effects of radiation to trigger avalanche breakdown.)
Construction and operation details are discussed in the video below:
Special Nuclear Material 