Radioactive Tubes!

Certain gas-filled electron tubes depend on radioactive ionization in order to function properly. Modern devices with this requirement usually contain Pm-147, H-3, or Ni-63, which are all low-energy beta emitters. However, older tubes may contain Cs-137, Co-60, or Ra-226. The gamma emissions from these isotopes are easily detected outside the envelopes of the tubes. What radioisotopes, if any, were used in the fabrication of a particular tube type depend on the date and the manufacturer. The best way to find radioactive tubes is to actually take a Geiger counter to a tube dealer and pick through his stock.

All tubes shown above contain Ra-226 in relatively large quantities. The hottest tube shown is the 1B22, containing 2.95 +/- 5% microcuries as measured on a calibrated HPGe counter. Certain 423-A tubes actually have an external foil source taped to the glass! The picture shows two 423-A tubes, the first of which has the foil source with an activity of 1.24 +/- 7% microcuries. The second tube has an internal radium source. The foil on the first tube produces enough gamma radiation to be visible on a ZnS:Ag screen! The KP-96 is a krytron that probably found use in defense applications. These contain approximately 0.7 microcuries.

Most tubes, especially high-power transmitting tubes like the magnetron in a microwave oven, contain a thoriated filament or cathode. This is very mildly radioactive. Also, many tubes employed uranium glass lead-in seals. Again, this is only very mildly radioactive.