Answer to Question #8032 Submitted to "Ask the Experts"

I am interested in using lithium glass to perform autoradiography of industrial gamma sources in water. There is no neutron emission. On the market there are enriched ⁶Li for neutron and gamma detection and depleted ⁶Li and enriched ⁷Li for gamma detection, both of which use photomultiplier tube and preamplification. Considering that the gamma sources have activities in the order of kilocuries (37 TBq), will the glass material produce enough fluorescence to be seen by the naked eye or by the use of an underwater visual camera without any photomultiplier tube or amplifications when the source is brought near this material?

There are a number of considerations that affect whether source-induced scintillations from the lithium glass could be seen by the naked eye or with the aid of a camera. Among other things, these include the size of the scintillator, the intensity of the radiation field, the wavelength of emitted visible radiation, the number of visible photons emitted per unit time, the darkness of the volume being viewed and of the general area, the viewing distance and use of viewing aids, and the influence of any other light produced by the radiation source.

The common cerium-activated glass scintillators are not extremely light efficient, typically yielding about 10 times fewer photons per unit energy absorbed than do some of the other common crystalline inorganic scintillators. However, in an intense field, possibly in excess of 10⁴ Gy h^-1, as might be present close to these kilocurie sources, I would expect that a reasonably sized detector would produce enough emission to be visible either to the naked dark-adapted eye, possibly requiring a viewing aid, or to a camera if the volume and general area are dark and no other interference is present.

The depth of water between the viewer and the detector may also be a limiting factor simply because of the fewer number of photons reaching the eye when the water depth is great. One potential light interference is the blue Cerenkov radiation that might be generated in the water by electrons set free by gamma radiation interactions in the source cladding and in the water. The light background from the Cerenkov might wash out the fluorescent visible light from the glass scintillator, which tends to have predominant wavelengths toward the blue-violet end of the visible-light spectrum.

Because the Cerenkov process is an energy threshold event, the Cerenkov radiation will tend to be noticeably more intense from a higher-energy photon source, such as ⁶⁰Co, as compared to a lower-energy source such as ¹³⁷Cs. Water is a polar material with a relatively high index of refraction, and this favors increased Cerenkov yields, so the water viewing may be a problem. Good luck.

George Chabot, PhD, CHP