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Author: Subject: Detectable radioactivity from rubidium?
Heptylene
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[*] posted on 3-1-2019 at 10:04
Detectable radioactivity from rubidium?


Naturally occuring rubidium exists as about 72 % of Rb-85 which is stable, and 28 % of Rb-87 which has a half-life of about 50 billion years.

I was wondering if rubidium salts are radioactive enough to detect. So I did some research:

According to "Half-Life and Beta Spectrum of Rb-87", K. F. Flynn; L. E. Glendenin, Phys. Rev. Vol 116(3) 1959 pp. 744-748, the maximum beta decay energy is about 272 keV.

Half life is 50 billion years. That's about 5x1019 seconds.
1 mole of rubidium chloride (121 g) contains about 5x1023 atoms of rubidium.

So there should be on the order of 103 beta decay events per second in a mole of RbCl.

Now, I expect these beta particles to generate X-rays by Bremsstrahlung given that rubidium is a somewhat heavy atom. Copper (molar mass 63.5 Da), molybdenum (96 Da), rhodium (103 Da) are used as targets in X-ray tubes, so I think Rubidium would behave in the same way.

Does anyone have a geiger counter and rubidium-containing substances on hand to test whether the radioactivity can be detected? I have a Geiger counter (SBM-20 tube, sensitive to beta and gamma radiation) but no rubidium, as its quite expensive.

As a side-note I have successfully detected radiation from 500 g of potassium chloride (beta decay at around 1.3 MeV and gamma at 1.5 MeV) using the SBM-20 tube.

Feel free to move the thread to radiochemistry if appropriate, although there is no real chemistry here.
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[*] posted on 3-1-2019 at 12:52


I've tried with an ampoule of rubidium metal and rubidium iodide, but did not observe a clear increase in counts.
In the case of the ampoule of metal, the glass was not very thick, but it would probably still block most of the beta.
The rubidium iodide was in air with the GM tube almost touching it. The iodide ions probably block some of the beta and x-rays.

I have not tried counting for extended periods of time (my counter is analog). I don't know the model of my GM tube, but it is easily able to pick up the activity of potassium salts. When I hold the tube above any potassium salt, the increase in count rate is immediately obvious.




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[*] posted on 3-1-2019 at 14:26


mhhh i have a digital geiger counter with an SBM-20 tube, and i couldn't detect any counts above background near my 5kg of potassium nitrate, anyway i'll try again tomorrow as i come back home from the holidays




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[*] posted on 3-1-2019 at 19:49


It looks like it's a fairly weak beta particle with an average energy of 81.67keV. I'd recommend an end window tube or organic scintillator probe.

I'll have access to quite a few decent detectors including a liquid scintillation counter in a week and a half, all I need to do is find some rubidium.
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Texium (zts16)
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4-1-2019 at 07:57
Heptylene
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[*] posted on 5-1-2019 at 08:52


Interesting, so the beta is too easily stopped. If I get around to buying a rubidium salt I'll try to pick something light, not as likely to shield the Bremsstrahlung (nitrate or chloride).

Ubya I should state that in my case the potassium salt was in a thin plastic bag. The probe was placed in a fold of the bag to surround it with potassium. The max rate I got that way was about 300 CPM (about 10 times background). The dose rate drops to normal at a few tens of centimeters

Spock be sure to post your results if you can detect anything!
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[*] posted on 5-1-2019 at 11:16


Quote: Originally posted by Heptylene  


Ubya I should state that in my case the potassium salt was in a thin plastic bag. The probe was placed in a fold of the bag to surround it with potassium. The max rate I got that way was about 300 CPM (about 10 times background). The dose rate drops to normal at a few tens of centimeters


i'm home now, and i tested again, maybe when i checked a few years ago i used a smaller quantity of potassium nitrate or maybe i just put the probe too distant. i tried again putting the GM tube between 2 plastic bags each full with 1kg of KNO3, and i got 240CPM when my background is around 60CPM so yea potassium 40 hello





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