Answer to Question #8165 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

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

Q

What is the effect of bringing a commercially available 6Li and enriched 7Li cerium-activated lithium glass close, approximately (1 cm), to a gamma source that has activities in the order of kilocuries (37 TBq) in water? Besides the fluorescence activated by the high gamma radiation as well as the Cerenkov radiation that would appear, will the glass matrix of this lithium glass suffer from creation of defects such that the whole glass material will darken or the amount of defect created causes a permanently structure damage? Is there any evidence glass materials will run the risk of becoming gamma activated with accumulated and high gamma dosage?

A

The answer to your question depends on what the dose rate at the irradiation point is and for how long the glass detector is irradiated. The dose rate at a given distance from the source depends on the geometry of the source and the radionuclide composition of the source as well as the activity of the source. A source of the activity you describe (37 TBq) comprised of a commonly used gamma-emitting radionuclide, such as 60Co or 137Cs, may not be well represented by a point source geometry when the dose point is only 1 cm from the source. For simplicity and conservatism, however, we can assume a point source geometry.

For a 60Co source of this activity the unattenuated dose rate at 1 cm would be about 37 Gy s-1; for a comparable-activity 137Cs source the dose rate would be 8.9 Gy s-1. Attenuation in the source matrix and in the water would reduce these dose rates somewhat, but the dose rates are sufficiently high that for modest exposure times (possibly in excess of a few seconds) you could produce some effects; the most important effect you would likely experience would be some degradation in light transmission properties as a result of the induction of color centers in the glass material that present as a darkening of the glass.

Some studies that have been done on different glasses have tended to demonstrate that as the concentration of the cerium ion activator increases, the glass becomes more resistant to the effects of radiation. This effect is more notable for silicate-based glasses than for phosphate-based systems. To my knowledge, the lithium-doped glasses are generally silicate-based. There are practical limits to how much cerium can be added before other detrimental effects, such as possible shifts in the absorption wavelength interval, begin to occur. If you have access to Science Direct, here is a link to a paper by Bacarro et.al that discusses some experimental measurements involving silicate and phosphate-based glasses. Their results show that for a silicate-based glass the light absorption coefficient at 410 nm increases from about 0 to 3 m-1 after a dose of about 60 Gy; the coefficient increased with dose and reached a value of about 12.5 m-1 at a dose of 252 Gy. These changes will be more significant the larger the dimensions of the glass scintillator being used, and the effective path length of the emitted light through the glass increases.

Some of the darkening effects of elevated doses may often be relieved by putting the glasses through an annealing process that involves heating the glasses to an appropriate temperature for a specified period of time. In addition to changes in coloration, other effects may occur at high doses that can result in physical strain in the glass and changes in charges and charge distribution in the glass that might interfere with the fundamental scintillation process. Some of the damage of high doses may not be removable by any practical means.

Regarding your last question about gamma activation of the glass, if you mean is there a likelihood of production of radioactive species through activation, the general answer is “no.” A possible exception would be if you were irradiating with very high-energy photons (e.g., 8 MeV or greater) that could produce photoneutrons, which could then possibly induce nuclear activation reactions in some of the glass constituents. At the typical energies of common gamma sources, such as the 60Co or 137Cs we mentioned, activation would not be a concern.

Happy irradiating.

George Chabot, PhD, CHP 

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