Answer to Question #1215 Submitted to "Ask the Experts"

Category: Historical Issues/Applications

The following question was answered by an expert in the appropriate field:

Does anyone know anything about the radioactive constituents of old camera lenses (rare earths containing radium perhaps) and/or possible neutron activation of nonradioactive lens materials? This relates to an interesting relic (supposedly of the Manhattan District or one of the later programs) that was used on a high-speed camera in one of the early tests. The optics are discolored in the manner of glass exposed to high gamma doses, and the assembly emits significant levels of gamma radiation which is not the result of any luminescing agent. It's a neat item for radiation demonstrations and I would like to provide a sensible account of the origin of the radiation. Any ideas?

The most likely explanation is that your device incorporates a thorium-containing (thoriated) lens. Many of the lenses used by the military, some of which date from WWII, were quite radioactive due to their thorium content (²³²Th)—up to 30 percent by weight. Although ²³²Th does not emit gamma rays, its short-lived decay products, which are also present, do.

The thorium was primarily used in higher-quality lenses. Because thorium increased the index of refraction, the lenses could be built with less curvature—this made them thinner and lighter. Production of thorium-containing lenses pretty much ceased by the late 1980s.

It is not uncommon for old thoriated lenses to be colored (reddish-brown?) in the manner you describe due to self-irradiation. For what it's worth, this color can be removed by heating.

A second, more remote, possibility is that a radium-containing radioluminescent marker is present. Although you ruled this possibility out, these markers can sometimes be very difficult to identify.

Neutron activation is not a viable explanation.

References

Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources. National Council on Radiation Protection and Measurements Report 95
Frame P, Kolb W. Living with Radiation: The First Hundred Years
Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials. NUREG-1717.

Paul Frame, CHP, PhD

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