A Spectrum to Ponder

[Steve Dubyk]

I have a real interest in the primordial radionuclides, and have attached a spectrum from one of them. This radionuclide is in very low abundance, although the element is not rare. It has a very long half life. I used a 100g sample to obtain this spectrum, and run times were very long. Peaks below 500 keV may be associated with a contaminant, even thought the sample is supposedly very pure. Has anyone seen this spectrum?

Steve Dubyk

[iRad]

Not that I have seen it before personally, but I think you might have Lanthanum (La138).

[Steve Dubyk]

Tom, you are correct, and that was fast! To obtain this spectrum you need a good size detector and sample, and a long run time, definitely the most challenging one I've done yet. I should mention that the peaks toward the lower end of the spectrum, below about 400 keV, are likely to be from Ac-227. The lanthanum was derived from a thorium bearing mineral, and although the thorium may have been refined out, Ac-227 remains, at least in small amounts. Lanthanum used in scintillation crystals such as the bromide have to be highly refined to remove it.

Steve Dubyk

[iRad]

You can get about 20 to 30 pounds of lanthanum out of the batteries from a Toyota Prius...

[Sesselmann]

Nice one, naturally occurring too.

http://nucleardata.nuclear.lu.se/toi/nu ... iZA=570138

[brehwens]

Nice spectrum!

I recently bought some lutetium and lantanum metal from Ebay, both 1 g samples. I got a very nice spectrum from the lutetium sample using my 1" well probe, but I could see nothing from the lanthanum sample using my 3x3" probe. I guess sample amount and detector geometry is to blame here...

[Nephrondoctor]

I had the same experience. I also have 1 g of 175/176 Lu and 1 g of La. I calculated the activity of the 176 Lu: it is 51 Bq in 1g, for the Lanthanum it appeared to be 0.82 Bq for 1 g. So I think you need at least 30 g of Lanthanum to get a spectrum within reasonable time and without 6 cm of lead.

[Steve Dubyk]

The sample amount is critical. I initially used just a few grams, then went to about 30 grams, at which point I was able to collect a spectrum after about an hour, but it was weak. Keep in mind my largest detector is a 40x40 mm NaI. I then went to a 100 gram sample, since it was inexpensive. Supposedly this sample is 99.99% pure. Still, even with this sample size data collection time was over 5 hours. The gamma spectrum of La-138 is typically collected using a HPGe, so it is pretty neat that we can get it with our equipment! I'm sure with your larger detectors, you should be able to get a nicer spectrum. I posted a good paper on the lanthanum spectrum in the library.

Still looking into what other spectra we can obtain from the primordial elements, but most are beta and alpha emitters. Indium 115 looks out for sure with our equipment, I don't think we will be seeing that 497 keV peak any time soon.

Steve Dubyk

[brehwens]

I tried getting a spectrum using my 3x3" NaI detector, but with greater detector volume comes more background, and the 3 cm lead I can provide for this detector wasn't enough. And my 1" well, while surrounded by 5 cm lead and 2mm copper, is not seeing these higher energies very well, so that was also not successful.

More sample, and better sample geometry would be nice to have, but I am not sure it is worth for me to go to any extremes to see these two peaks. Maybe in the near future when I am upgrading to a bigger well detector, will it become feasible.

[Nephrondoctor]

@Steve, with respect to primordial elements, I also have 9 or 10 g of Samarium, which is an alpha emitter. However, I can't measure any alpha's neither gamma. Does anyone has an explanation? (When I use my 241 Am source I get thousands cps alpha's )