Answer to Question #498 Submitted to "Ask the Experts"

Is there any unwanted x-ray radiation from radio tubes used in old (1930s ->) electronic equipment, such as power supplies (kvolt), high-power radio transmitters that use cathodic radio tubes, etc.?

X rays can be produced in vacuum tubes when the electrons are accelerated and strike the anode. In the vast majority of situations, the intensity and energy of such x rays are so low that any radiation is effectively nonexistent. However, when very high voltages and currents are employed, the x-ray production can become significant. The prime example of such a situation was the generation of x rays in early color television sets (ca. 1965-1970). The three major sources of x rays from these sets were the picture tube, the vacuum tube rectifier, and the shunt regulator tube. The latter (designations 6EF4 and 6LC6) were a particular problem. Over a third of the 6EF4 tubes tested produced exposure rates above 50 mR/hr at a distance of nine inches, and exposure rates up to 8 R/hr were observed at seven inches with one defective tube! Modern television sets have essentially eliminated the problem of x-ray production however.

As a matter of interest, an article appeared in the July 1956 issue of Scientific American that described how an ordinary vacuum tube could be turned into a source of "Homemade X Rays."

Over the years, beginning around 1940, a large number of vacuum tubes have incorporated radioactive sources, often up to a microcurie, as a means to ionize the gas within the tube. The most commonly employed radionuclides were ²²⁶Ra, ¹³⁷Cs, ⁶⁰Co, ³H, and ⁸⁵Kr. Many of these tubes were designed for military use, e.g., with radar equipment, for voltage regulation, for surge protection, as trigger tubes, etc. Most were coated with an opaque lacquer that gave them a black color and they usually had a label identifying the radioactive component inside. Such tubes emitted low levels of radiation even when not in use. In fact, they would be more of a concern when broken because this would increase the opportunity for the accidental ingestion of the radioactive material. Tubes containing ⁶³Ni and ³H are still being manufactured.