Radiodating using NaI(Tl)

[Steven Sesselmann]

Hi All,

I had a query this week from a Phd candidate asking me if it is possible to do radio dating using a NaI(Tl) setup, and frankly I haven't seen anyone attempt this here. I assume the method would be similar to that described on Wiki here: https://en.wikipedia.org/wiki/Uranium–lead_dating

Please help if anyone has experience in this area.

Steven

[isoenzyme]

Steven:

I hope you'll accept a best guess from someone with only experience in SIMS, but I don't know if anyone has experience doing radiodating of minerals using a sodium iodide scintillation detector. Gamma spectroscopy isn't usually the method used to do this kind of work but it's an interesting problem. The answer is (as it is always in science), "It depends." There are several challenges to consider:
(1) How much sample would you need? Zircon (the mineral that has to be isolated from the rock being dated) is difficult to separate from pyrite using Stokes Law though there are methods to do so if one has no other choice. Would a few grams be enough, or would one have to sift through a ton of finely ground rock to get enough (and at that point would self-shielding absorb too much signal). It literally depends on how much Uranium or Thorium were originally present in the mineral and if the source is low in pyrite.
(2) There is an IT gamma from meta-stable Pb-207 of around 1,630 keV that would be well suited for a NaI(Tl) setup, but as far as I can tell Pb-206 is a "no show" - the spectra for U-235 and U-238 should work OK with your NaI(Tl) setup but it would also depend what else happens to be in the sample. Could one use X-rays from the Pb in the sample to quantitate lead? Detection limit would be the driving issue for the Uranium and Lead isotopes. (If using X-rays one would have to have excellent shielding but the long analysis times might reveal any shielding deficiencies.)
(3) The traditional way of measuring isotopes in minerals is a SIMS (Secondary Ion Mass Spectrometer). If anyone tells you this is an excellent choice for quantitative work then you know they've never actually used one! Calibration methods are possible but one usually has to invent them for each sample and be good at doing it "on the fly." Certain ICP-MS units can be configured to give isotopic data but I don't have enough personal experience with them to know how well it works. Many years ago I was teaching an analytical chemistry class at the local university and had a student run acid-dissolved yellowcake Uranium samples through the ICP and was told by the operator that if I ever sent Uranium samples through his ICP again that he would punch me in the face. (Now I wished I had asked him why...)

All the usual negativity aside it is a fascinating question. It has peaked my interest enough to respond to your query and to bid on some natural (unpolished and unrefined) zircon crystals on eBay to stick into my setup to see if it might be possible. (However, if someone outbids me by this Sunday it probably won't happen.) I'm working on determining the lowest detection limit for Uranium in my recently-improved detector/shielding setup so looking at minerals for Uranium won't be too much of a detour from what I'm currently working on. I'll post again if anything comes of the experiment that might prove interesting!

[ealang25]

Hi All,

I'm the PhD mentioned above. The type of radiodating I am interested in is Cesium-137 in combination with lead-210. My research is from sediment cores in marshes along the tidal portion of the Potomac River in Viriginia. The cores are approximately 1 meter deep and have fairly low (less than 1 cm) accretion rates (best guest because need radiodating to calculate). The manner in which these cores are usually dated since they are relatively young geologically speaking is to use lead-210, provided that your sample is deep enough to reach the time period when lead-210 was stable in the atmosphere from radon degrading. That is pre-industrial revolution. I am fairly certain that none of my cores are that old, so I need a manner to calibrate the lead data to known decay rates. Thus cesium-137.

My location should yield one large peak that would correlate to 1963 when nuclear testing was banned and then the depth at which cesium was first detected would be approximately 1952, which correlates to the first testing. That gives me two dates related to depth that I can apply to the legacy pesticide concentration vs. depth profiles I have obtained. DDT first was introduced in the early 1940s and was widely used until its ban in 1972. There is a concentration maxima associated with the bans of legacy pesticides and I have tentatively assigned dates based on the concentration profile data for ~70 pesticides and their regulation histories. One of my goals is to determine how well the DDT (and its metabolites) 1972 peak aligns with the cesium-137 1963 maxima and to then determine if the pesticide profile can be used as a rough dating mechanism for the tidal Potomac.

I haven't seen in the forum anyone measuring lead-210. Have any of you tried to measure environmental levels of lead-210? It's gamma photo-peak is at 46.5 keV.

Thanks for any thoughts!

Liz

[GEOelectronics]

First question: Geologic radiometric dating or organic radiometric dating?

Organic can be definitely be ruled out (C-14) because sodium iodide (Tl) can't measure stable carbon C-12 to radioactive C-14 ratio.


George Dowell

Hi All,

I had a query this week from a Phd candidate asking me if it is possible to do radio dating using a NaI(Tl) setup, and frankly I haven't seen anyone attempt this here. I assume the method would be similar to that described on Wiki here: https://en.wikipedia.org/wiki/Uranium–lead_dating

Please help if anyone has experience in this area.

Steven

[Steven Sesselmann]

George,

If you read the post above you will see that this attempt to date sediment is based on trace amounts of Cs137 fallout from nuclear bomb testing in the 50's, so detecting this with a sensitive setup is definitely plausible.

Steven

[isoenzyme]

As for using gamma spectroscopy to date rocks, I did win the natural zircon crystals on eBay and ran them for a moderate time using a CsI(Tl) scintillating detector in a shielded GS-STANDUP to determine whether it was worth trying. Photographs of the huge zircon (ZnSiO4) crystals (> 6 carats) and the few I used to collect the spectrum are shown below:

Natural zircon crystals used to collect the spectrum.

The collected spectra are shown below:

Gamma spectrum of zircon crystals (with and without background subtracted).

ThereminoMCA_2019_05_12_22_30_28 Zircon.png (74.03 KiB) Viewed 77 times

Though the spectrum was collected using a relatively large mass of crystals there seems to be enough interesting peaks that are detectable. If I had my low-level Uranium calibration standards ready I might be able to pick out the relative peaks but there is definitely some uranium in that sample. Perhaps optimization of the experimental conditions would lead to lead isotope detection as well.

Curiosity sated - does anybody want to buy some natural zircon crystals to try before I put these back on eBay? ;)

[elemental]

Nice zircon specimens.

You don’t mention anything about dating. Did you try?
If you can send information about voltage volume etc and any other setup parameters, I’ll try with my zircons from Norway

Gary
Willunga south

[Peter-1]

hello,
if I understand correctly, zircon contains the elements uranium and thorium with their decay products. So you should see these lines. Or are there other lines? That has to be measured well.
Peter

[Addison.G]

Liz,
That research sounds very interesting, where along the river are the cores taken from? I recall seeing though that NaI(Tl) looses linearity below 100kev which may be an issue. That could be specifically calibrated for, but you might need two separate calibrations for 137Cs and 210Pb.

Addison

[Steven Sesselmann]

Hi all,

I found this web page explaining the Pb210 method, thought it might be of interest to this thread.

http://www.flettresearch.ca/UnderstandingPb210.html

Pb210 emits a single gamma which makes it easy: http://nucleardata.nuclear.lu.se/toi/nu ... iZA=820210

It should be possible to detect this in sediment if you have good shielding.

http://www.radiochemistry.org/periodict ... /pb210.pdf

Steven