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12 December 2013

Uranium Glass: Radioactivity and Fluorescence

With the availability of two Geiger counters purchased from GQ-electronics, I have been looking for radioactive sources to "play" with. Other than the Am-241 alpha source I've extracted from smoke detectors, I came to know some vintage household products are actually doped with radioactive materials for aesthetic or practical reasons. One particularly curious example is the uranium glass.

Uranium glass are essentially glass that contains small amount of uranium (typically within a few percent) in the form of oxide uranate, U(VI) state within its amorphous glassy matrix. Long before the discovery of radioactivity, it was found that the mineral contained within blackish yellow ore of uranium - pitchblende, when added to clear transparent glass melt will solidify to a yellowish green tint. These coloured glass were priced for their aesthetic appeal and up to a certain period in our history it was even used as tableware and household decorative exhibits until the realization of its potential health hazards or more likely political influence on the control of uranium related compounds that brings production to halt.

Today, uranium doped glass are still sold worldwide but in much smaller sizes such as beads or cubes for scientific novelties. Despite the possible health hazards (even though they are in minute concentration within the glass), I went to look for such material in Malacca city - a place famous for its cuisine, art and antiques.

 Vintage handmade uranium glass bowl fluoresce bright green under shortwave ultraviolet light

Scouring the old rustic stores, it took a tour of more than ten different antique shops for me to finally come by a sample (image above) that is not too physically large and of course affordable under my budget. It was that unmistakable yellow-green colour, nicely handcrafted and from its construction, must have been blown with old techniques because there are air bubbles still trapped within the glass. I recognized the glass through its bright green fluorescence induced by a cheap violet LED lamp. The shop keeper charged me MYR 80 after a casual bargain, little did he knew about the true value of these radioactive glassware. When I got home, I carefully washed it and prepared it for radiation detection. 

Fluorescence in uranium doped glass are excited by ultraviolet radiation. Because a sheet of glass blocks completely the shortwave UV, the side of the bowl facing the glass did not glow in green. Note that visible light from the mercury lamp still passes through the glass (cyan reflection on the uranium glass bowl).

As mentioned, I have two Geiger counters. The detecting elements are different for both tubes because one of it has a small active diameter of only 6.4 mm with alpha-sensitive mica window, the other tube has a longer length and slightly higher volume for cross section (I did not measure it) but the tube was encased entirely with glass, so it is only sensitive to beta and gamma radiation. I was thrilled when the glass GM tube detects some low level radiation but I was frustrated with the lack of sensitivity when I used the GM tube with relatively small alpha-sensitive mica window because I expected the radiation emitted from such sources primarily consist of alpha particles (just like the Am-241 source). 

The lack of radioactivity when detected using the small CBON 6107/BS-212 GM tube can be attributed to the relatively dilute amount of uranium added to the glass thus result in a very weak radioactive source. The glass GM tube picks up more counts than background for three reasons: 

1. It has higher volume compared to CBON 6107/BS-212 GM tube, thus higher probability for detecting any non-alpha ionizing particles zipping through the tube. This increases the sensitivity towards relatively weak sources like our uranium glass bowl. 

2. The first radioactive element from the decay series of Uranium-238 (most abundant isotope of uranium) is Thorium-234 and Protactinium-234. Subsequent decay mode in Thorium-234 emits mostly beta negative particles with energy 0.273 MeV; and Protactinium for the probability of 99.84 % emission of beta negative particles at 2.271 MeV coupled with 0.16 % gamma (0.074 MeV) emission due to isomeric transition. 

3. Further radioactive decay products of U-238 other than the fore-mentioned Th-234 and Pa-234 even though very very small, but there is a probability of its existence within the glass matrix (vintage product), emitting beta and gamma ray particles. 

In any case, someone worked out the decay equilibrium for one mole of U-238. Because the half-life of uranium is so unbelievably long, within our lifetimes only 3 of the first decay products were taken into account. Thus, for one mole of U-238 (either pure sample or embedded within a compound like our case) one alpha, two beta and one gamma ray particles are emitted 3,000,000 times per second. This explains why my beta + gamma detecting GM tube picks up higher counts than the CBON 6107/BS-212 GM tube. The weak activity also provide a hint on how much uranium atoms were present in the bowl itself. 

Ortec systems: GM tube assembly and the uranium glass sample

Next, being unsatisfied with the performance of my GM tubes, I brought the bowl down to our radiation lab where it has GM tube with substantially larger surface area of about 35.6 mm and 19.8 mm effective length and diameter respectively. It was a Saint-Gobain N204/BNC GM tube driven by Ortec systems with alpha enabled mica window. I literally stick the mica end of the tube to the surface of the bowl and did a 4 minute integration time exposure to measure counts (figure above). Later I repeated the measurement using calibrated attenuators of different densities.

Detecting radiation through a sheet of aluminium attenuator.

Figure above shows the actual set up with an aluminium sheet attenuator. As expected, when aluminium was introduced, the counts did not drop drastically as exhibited by Am-241 alpha sources but decreases in an exponential profile in increasing attenuation density indicating radiation interaction with matter but during the process loses significant amount of energy per particle.

For this case since primary radiation are beta particles, its interaction with attenuators are mostly contributed by ionization and especially bremsstrahlung. Such interactions dictates the maximum length where the particle will be completely stopped; and it is proportional to the attenuators' density. For beta particle with kinetic energy higher than 0.6 MeV, empirical evidence has found the range of the particles given by: 

Range [kg/m^2] = (5.42 * E) - 1.33 , where E is the kinetic energy in MeV

This gives the maximum range of our beta particles from the uranium glass stopped at 109,700 mg/cm^2. Which is pretty thick! Nevertheless, even though thick shielding are technically required to block those particles emanating from the glass bowl but because the number of particles originating from it is so small, such precautionary steps becomes unnecessary.

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