Potassium-argon radiometric dating

[Stefan Udrea]

Hello.
I've heard that electron capture rate, which is used in the dating with potassium-argon, can vary at ultra-high pressures.How is it possible that this nuclear phenomenon is influenced by pressure?

 

[Vanadium 50]

Where have you heard this?

 

[Stefan Udrea]

Where have you heard this?

I don't remember where I've first read it.
I've found the statement here, and it seems to be referenced by a serious book :

http://www.answers.com/topic/potassium-argon-dating#cite_note-McD-H_10-0

EDIT:
I also found it more exactly phrased here :

http://www.talkorigins.org/indexcc/CF/CF210.html

"Extreme pressure can cause electron-capture decay rates to increase slightly (less than 0.2 percent), but the change is small enough that it has no detectable effect on dates. "

I'm relieved that it doesn't have a detectable effect on dates, but I'm still curious about how it's possible at all.

 

[phyzguy]

Electron capture is when the nucleus absorbs an inner shell electron, decreasing the atomic number of the nucleus by 1. If you put the substance under pressure, the radius of the atomic orbitals will be decreased slightly, and this will slightly increase the magnitude of the electron wave function at the origin, and hence the probability of the electron to be 'inside' the nucleus where it can be captured. So it makes sense to me that the electron capture rate should increase slightly. Why doesn't this make sense to you?

 

[Bill_K]

It doesn't make sense to me because pressure should affect only the outer orbitals, not the inner ones. Also the use of the vague terms "extreme pressure" and "less than 0.2 percent".

 

[Stefan Udrea]

phyzguy, I didn't know that pressure can decrease the radius of a an atomic orbital.I thought pressure can only make atoms come closer to each others, not shrink them.

 

[phyzguy]

As the nuclei in the lattice come closer together, the distribution of positive charge changes, so the solution to the Schrödinger equation changes from what it was for an isolated atom. The outer orbitals change drastically, so that instead of the discrete energy levels we get for an isolated atom, in a solid we get energy bands. Look up any reference on solid state physics. While it is true that the impact on the inner orbitals is less, they are still affected. As the pressure goes up, the volume available to each atom is reduced, so the probability of finding an electron in the nucleus must increase. As stated, it is a small effect, but it is there.

 

[Bill_K]

As stated, it is a small effect, but it is there.

I'd like to believe you but from what I can find, the effect has been conceptualized only. Never verified, and not even estimated. I find several references saying it does not have a measurable effect.

 

[phyzguy]

There are several measurements showing this effect. Attached is one.

 

[Vanadium 50]

Thanks for the links. A few comments:

It is true that the environment can affect electron-capture rates. In the most extreme case, totally stripping a nucleus of its electrons, will (obviously) make that rate zero.

It is also true that by far the effect is largest in Be-7, because it has so few electrons. I am not sure if it's even been observed elsewhere. The 19 electrons of potassium mean that external factors have a much smaller influence on the K-shell electrons and thus the capture rate. So a 0.2% for beryllium would be much smaller for potassium.

We only know the half-life of K-40 to a few parts per mil anyway. Even if the change were that large, it would make little difference to radiometric dating using this technique. And techniques using isotopes that do not decay by K-capture (like uranium-lead) are of course immune.

 

[phyzguy]

Vanadium50 - I agree with everything you said. I never meant to imply that the effect is significant - it is probably far too small to have an impact on decay rates and dating. I was just saying that the effect is there and that it is not difficult to understand why.

On the topic of variations in nuclear decay rates, there is a lot of interesting data showing that nuclear decay rates are not constant, and show annual variations at the 0.1% level, presumably due to some unknown solar influence. Solar neutrino fluxes have been suggested as a cause, but this is just hypothesis. Here are some links to this interesting phenomenon. Since it has been confirmed in more than one isotope, in labs in both the US and Germany, it looks like a real effect.

arxiv:0808.3283
arxiv:0808.3156
arxiv:1007.3318
arxiv:1010.2225
arxiv:1105.1335

 

[Vanadium 50]

Those results are, not to put too fine a point on it, hogwash.

They are not from the experimenters - they are a re-analysis of data by an external group, one of whom has a long history of discovering spurious effects in other people's data. None of this has been replicated (despite searches with better sensitivity). As far as I know, nobody who has actually done the experiment themselves claims to see anything whatsoever.

As David Wark said, "If a student brought this to me I would advise them to understand their experiment before they start coming up with wild correlations of detector noise with natural phenomena picked at random. You have to prove that something needs to be explained before you come up with wild explanations.” Sound advice.

But in any event, the OP is talking about K-Ar dating. I think we can all agree that there is no reason to think that changes in nuclear decay rates causes any problems whatsoever.