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Candlestick II

Dear Reader,

I took my candlestick to work and I quickly found it was radioactive, it was emitting beta particles according to a quick check with a contamination meter. As it was emitting that nice yellow/green light when exposed to UV light and it was emitting beta particles I quickly decided it was genuine uranium glass.

The next step in the characterization of the candle stick was to use gamma spectroscopy on it, now before we get going I would like to point out that gamma spectroscopy is not a press the button and get the result type of machine. For those of you who are proper traditional chemists / scientists you will be aware that for a new type of sample it is very hard with most machines to create a method with a spectrometer where you just put in the sample and press go before getting the final answer.

One of the problems is the issue of self adsorption, for the lower energy gamma lines many of the photons will never escape from a large sample. The ideal sample for gamma spectroscopy would be a tiny spec (a point source) which would be at a well defined distance from the detector.

The candlestick is anything but well defined in distance from the detector and it is far from being a point source. I did not want to melt it down to make a lump with a more simple shape so I decided that we should measure it in its native form.

One of my questions about the candle stick was “is the uranium a depleted uranium, or is it a natural uranium which is likely to predate the nuclear age ?”

I reason that as DU is less valuable than natural uranium it would be the logical uranium to use if you were making a uranium glass candlestick in the 1950s or later. But if it was a more early candlestick then it would be more likely to have a natural isotope signature for its uranium.

We need to consider three uranium isotopes

238U which is the bulk of natural uranium, this does not have any useful gamma lines but its daughter (234Th) which emits gamma rays, as the half life of 234Th is short when compared with the age of the candle stick it can be treated as an extension of the radioactive decay of the parent 238U. 70% of the 234Th will decay to the meta stable state of 234Pa (234mPa). It is important to note that the 234Pa (both forms) give a forest of gamma lines (hedgehog spectrum).

Nuclide Half life Decay mode Main gamma lines
238U 4.468 x 109 years alpha No gamma
234Th 24.1 days beta 63.3 (4.8 %), 92.4 (2.8 %) and 92.8 (2.8 %)
234mPa 1.17 minutes beta 258.3 (0.73 %), Hedgehog spectrum
234Pa 6.7 hours beta Hedgehog spectrum

 

If the uranium had been a depleted uranium then I would expect that almost all the 234U and 235U would have been removed. As the 234U has a long half life it serves to block the decay chain of 238U if the sample is not old on a geological time scale.

I reasoned that by looking for the decay products of 234U that I could test the hypothesis that the uranium was a prenuclear age natural mixture of isotopes.

This uranium will decay to form a long lived radium (226Ra) which will then slowly on the timescale of the candlestick’s age decay further.

234U –> 230Th –> 226Ra –> 222Rn –> 218Po –> 214Pb

Nuclide Half life Decay mode Main gamma lines
234U 245500 years alpha No gamma
230Th 75380 years alpha 67.7 (37 %)
226Ra 1600 years alpha 186 (3.6 %)
222Rn 3.8 days alpha No gamma
218Po 3.1 minutes alpha No gamma
214Pb 26.8 minutes beta 242 (7.4 %), 295 (19.3 %), 352 (37.6 %),
214Bi 19.9 minutes beta Forest of lines
214Po 0.1643 ms alpha No gamma

 

The 214Pb will decay by beta emission to form 214Bi and then 214Po which then decays to form 210Pb. As after 226Ra no nuclide has a half life longer than a few days until you reach 210Pb we can treat these decays as extensions of the radium decay if we make a kinetic model of the candlestick.

The fissile 235U does have a useful gamma emission of its own, this can be used to confirm if the uranium was natural or depleted.

It will decay by alpha emission according to the following mechanism.

Nuclide Half life Decay mode Main gamma lines
235U 703800000 years alpha 109 (1.5 %), 144 (11 %), 163 (5.1 %), 186 (57 %), 205 (5%),
231Th 25.52 hours beta No gamma
231Pa 32760 years alpha Forest of lines
227Ac 21.773 years beta No gamma
227Th 18.72 days alpha Forest of lines

 

I hope to now be able to go through the spectrum and then hunt for lines, I recall that the 186 keV line for 235U was present. So far I think the uranium is from before the nuclear age.

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4 Responses

  1. Impressive~ 🙂

  2. Hi Dr. Foreman, I have a pair of those exact candlesticks! Would be very interested to know the results of your depleted vs natural U. tests. Kathy

    • Thank you for your comment, so far I have been able to tell from gamma spectrscopy that the candle sticks do contain a mixture of U-238 and U-235. I will publish some more on the subject of these candle sticks soon.

      • Thanks Dr. Foreman – I’ll look forward to seeing the results of your future investigations on the uranium composition of this glass!

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