Look for Co-60. I've seen this in Spark Gaps before. Radioactivity is used to make the discharge happen sooner, and allow it to do it's job better. The units I saw were of 15 yr or older vintage, so I don't know if that represents current [pun] practice. And, I don't know why Co-60 was used, instead of a different nuclide. frazier bronson canberra