A new radioilluminant

Interest Groups Atomic Energy
#1

I happened to find out, the other day, that the new “glow-in-the-dark” material of choice is strontium aluminate, replacing the old favourite zinc sulphide. Now, ZnS is also the material used as a phosphor for radioilluminants such as radium-paint & tritium-plastic. It occurs to me that a small proportion of the fission product, radioactive strontium-90, could be used in the strontium aluminate. By the very nature of isotopes, this would be effectively inseparable from the much greater mass of radioinert strontium.

To evaluate this idea, we really need four pieces of information. The first is the energy release of radiostrontium, in watts per gram. This can easily be found from the specific activity & the decay energy. The second is the luminous efficiency, in lumens per watt, of SrAl2O4 when activated by the beta rays of 90-Sr, which will have to be determined by experiment. The third is the desired luminous intensity, in lumens per square centimeter, of the illuminant. And the fourth is the desired application density of the illuminant, in grams per square centimeter. These last two points can perhaps be determined by comparison with existing products. Taken together, these figures will give us the proportion of 90-Sr, in grams per gram, required for the radioilluminant.

90-Sr, as a fission product, is plentiful enough, & this type of application would provide a new use for it, thus improving the overall economics of the nuclear fuel cycle. It also has a half-life almost 2.5 times that of tritium, currently the most common radioilluminant, & unlike tritium, it doesn’t escape through the walls of containers. Some consideration must be given to the question of biological hazard, but this hinges on three points : first, the bioavailability of strontium from the aluminate ; second, the proportion of radiostrontium used ; and third, the physical form of the material. Radiostrontium-laced “glow powder” might not be sold to the general public, but if incorporated in a solid form not readily inhalable or digestible, it might well be completely acceptable.

#2

Sr-90 was used in radiolumenescent paint by the US military in the '50s and '60s, but it apparently is no longer used.

http://orau.org/ptp/collection/radioluminescent/radioluminescentinfo.htm

http://health.phys.iit.edu/extended_archive/0003/msg00336.html

A concern one would have is meeting OSHA (or any safety) compliance in the workplace, where much larger amounts of Sr-90 would be warehoused than in the consumer marketplace.

#3

As far as I can tell, that was made with 90-Sr & zinc sulphide. Diluting the radiostrontium by a factor of likely a million or more with inert strontium should go a long way toward making it safe by any reasonable standard ; in any case, all facilities which handle radioactive material have special problems, but the mere fact that it’s easier to handle than tritium ought to make it less of a health & safety issue. What that translates to in terms of regulation, of course, is not always easy to say, but regulatory reform is another question!

#4

I think you addressed in class the issue of Sr-90 absorption in the body, but I don’t remember how that applies here. I know that in any nuclear plant leak, Sr-90 absorption is one of those things that is of most concern. Yes, it would be diluted in radioinert Sr, but what would be a reasonable threshhold of ingestion one would not want to cross?

#5

The old “permissible quarterly intake for occupational exposure” list gives : for ingestion, 0.7 microcuries for soluble forms, 70 microcuries for insoluble ; for inhalation, 0.73 (soluble) / 3.5 (insoluble). It might take a specialist to provide a meaningful opinion on the effects of dilution, although one would expect them to be less important for insoluble compounds. And my CRC handbook doesn’t give a solubility figure for SrAl2O4.