Article 47641 of talk.origins: Newsgroups: talk.origins Path: klaava!news.funet.fi!sunic!pipex!howland.reston.ans.net!math.ohio-state.edu!news.cyberstore.ca!nntp.cs.ubc.ca!unixg.ubc.ca!acs.ucalgary.ca!tuba!macrae From: macrae@tuba.geoph.ucalgary.ca (Andrew MacRae) Subject: varying radiometric decay rates Message-ID: Sender: macrae@pandora.geo.ucalgary.ca Date: Tue, 30 Nov 1993 07:39:01 GMT Reply-To: macrae@pandora.geo.ucalgary.ca Nntp-Posting-Host: tuba.geo.ucalgary.ca Organization: Dept. of Geology and Geophysics, University of Calgary Followup-To: talk.origins Lines: 82 For those just joining this thread, there really are certain conditions when radiometric decay rates can vary. It occurs in isotopic reactions that include decay by "electron capture". Read on for details. BTW, I goofed on the "electron capture" reference in a previous post. It is Gunter Faure's text, but it is "Principles of Isotope Geology", not "Isotope Geochemistry". Sigh. Anyway, here is a summary of what Faure has to say about electron capture (paraphrased): Electron capture occurs when an electron is "captured" by the nucleus of an atom. The net result is the reduction of the number of protons by one, an increase in the number of neutrons by one, plus the release of some energy (e.g., electromagnetic - gamma rays or x-rays), and a neutrino. In an equation form: proton + electron -> neutron + neutrino + energy According to Faure, electrons in the K shell are more prone to capture because they are closer to the nucleus (makes sense). Because electron capture is the only decay mechanism that involves particles from outside the nucleus, it is the _only_ mechanism that can be affected by processes "external" to the nucleus. In this case, pressure. Pressure has an effect (apparently) because the electrons are forced closer to the nucleus. As anyone with a basic understanding of atomic structure will know, we are talking about _high_ pressures in order to do this. Here are the effects of pressure on electron capture, and therefore decay rates, as explained by Faure (paraphrased from p.41): 1.) The decay rates of 7Be, 99mTc, and 131Ba increase when subjected to very high pressures, on the order of 100 kilobars or more (!). [This is outside any geologic processes in the Earth's crust, with the exception of meteorite impacts, which are too brief to have a significant effect.] 2.) As an example, 7Be decays by electron capture to 7Li, and Hensley et al. (1973) observed an increase in the decay rate of 0.59 percent (a little over 1 half of a percent), when a sample was subjected to a pressure of 270 kilobars in a diamond anvil press. For anyone wondering, 270 kilobars is an awful lot of pressure. 3.) 40K, whose decay is used for the K/Ar method of radiometric dating, can decay by electron capture. However, Faure points out that "There is no evidence that potassium now residing in the crust of the Earth has been subjected to pressures on the order of several hundred kilobars for a sufficient length of time [the _length_ is very important, because the decay goes back to normal when the pressure is released] to affect the amount of radiogenic 40Ar produced". 4.) K/Ar is the _only_ isotopic decay commonly used for radiometric dating that can be affected by electron capture - pressure processes. For example, the U/Pb isotopic decay _cannot_ involve electron capture, so its decay constants do not vary. It is important to note that K/Ar, U/Pb, and Rb/Sr all yield concordant ages in the appropriate conditions (undisturbed samples), confirming that K/Ar methods have not been significantly distorted by electron capture - pressure effects. 5.) For the 40K -> 40Ar reaction, electron capture is responsible for only 11% of 40K->40Ar decays, so the potential effect on radiometric dates due to pressure and electron capture is correspondingly reduced to an even smaller amount. 6.) Electron capture is not not likely in areas where atoms have been ionized, for example, the interior of the Sun. -Andrew macrae@pandora.geo.ucalgary.ca or: macrae@geo.ucalgary.ca References: Faure, Gunter, 1986. Principles of Isotope Geology, 2nd Edition. John Wiley & Sons, Inc.: New York, p.1-589. Hensley, W.K.; Bassett, W.A.; and Huizenga, J.R., 1973. Pressure dependence of the radioactive decay constant of beryllium-7. Science, v.181, p.1164-1165. Article 47750 of talk.origins: Path: klaava!news.funet.fi!sunic!EU.net!howland.reston.ans.net!vixen.cso.uiuc.edu!moe.ksu.ksu.edu!phys.ksu.edu!jsanders From: jsanders@phys.ksu.edu (Justin M. Sanders) Newsgroups: talk.origins Subject: Re: varying radiometric decay rates Date: 30 Nov 1993 20:53:35 GMT Organization: Department of Physics, Kansas State University, Manhattan KS, USA Lines: 46 Message-ID: <2dgbsf$f9m@newserv.ksu.ksu.edu> References: NNTP-Posting-Host: compton.phys.ksu.edu In article macrae@pandora.geo.ucalgary.ca writes: > >5.) For the 40K -> 40Ar reaction, electron capture is responsible for only >11% of 40K->40Ar decays, so the potential effect on radiometric dates due >to pressure and electron capture is correspondingly reduced to an >even smaller amount. This is incorrect. Of 40K decays, 89% are via beta-minus decay resulting in 40Ca, essentially all of the remaining 11% of 40K decays are via electron capture resulting in 40Ar. The other 40K -> 40Ar decay mechanism, beta-plus (positron decay) accounts for only 0.001% of the decays. In other words, all of the decays of interest for dating occur via electron capture. [ref. P.M. Endt, Nuclear Physics, vol 521A, p1 (1990), and in particular p604]. Andrew's point that variations in the Electron Capture rate for 40K are negligible still holds, however. >References: > >Faure, Gunter, 1986. Principles of Isotope Geology, 2nd Edition. >John Wiley & Sons, Inc.: New York, p.1-589. > >Hensley, W.K.; Bassett, W.A.; and Huizenga, J.R., 1973. Pressure >dependence of the radioactive decay constant of beryllium-7. >Science, v.181, p.1164-1165. Very extensive reviews of measurements of decay constant perturbations can be found in: G.T. Emery, "Perturbation of Nuclear Decay Rates", _Annual Review of Nuclear Physics_, v22, pp165-202 (1972). H.P. Hahn, H.J. Born, & J.I. Kim, "Survey on the Rate Perturbation of Nuclear Decay," _Radiochimica Acta_, v23, pp23-37 (1976). H.Daniel, "Influence of Chemical Environment on Lifetimes in Nuclear Physics," _Atomic Energy Review_, v17, pp287-343 (1979). Thus far, 85Sr with a half-life of 64 days is the longest-lived isotope in which any half-life variation has been observed. 7Be (which Andrew discussed) has been extensively investigated and has a 53 day half-life. No variation has been reported in half-lives of isotopes used for radioactive dating. -- Justin M. Sanders "[The theory] seems a very dramatically Research Associate appealing and simple explanation to me, Physics Division, ORNL which therefore should be regarded with jsanders@orph14.phy.ornl.gov deep suspicion."-- Roger M. Squires Article 47781 of talk.origins: Path: klaava!news.funet.fi!sunic!pipex!howland.reston.ans.net!agate!ames!koriel!lll-winken.llnl.gov!imager!dk From: dk@imager (Dave Knapp) Newsgroups: talk.origins Subject: Re: varying radiometric decay rates Date: 30 Nov 1993 23:04:16 GMT Organization: Laboratory for Experimental Astrophysics Lines: 50 Message-ID: <2dgjhh$bpf@lll-winken.llnl.gov> References: <2dgbsf$f9m@newserv.ksu.ksu.edu> NNTP-Posting-Host: imager.llnl.gov In article <2dgbsf$f9m@newserv.ksu.ksu.edu> jsanders@phys.ksu.edu (Justin M. Sanders) writes: > Thus far, 85Sr with a half-life of 64 days is the longest-lived > isotope in which any half-life variation has been observed. Oooh, goody! Now I get a chance to make pedant points! A few comments: 1.) The sources are out of date. It might still be true that 85Sr is the longest-lived electron-capture isotope to have demonstrated chemical-environment-related half-life decreases, but, as I will show later, it isn't the longest-lived such isotope to show any such changes. 2.) The chemical environment also has an effect on beta-decay rates. For tritium, the difference in total decay rate between the atomic and molecular forms differs by something like a part in 1e6 or so, mostly from differences in bound-state beta decay, which I will talk more about later. 3.) Electron capture can be turned off by ionization; in fact, in certain isotopes, the electron capture can be turned off and beta decay back the other way can be turned on via beta decay to a now-empty atomic orbital. These effects have relative sizes of 100%. The first direct observation of this process, known as "bound-state beta decay" came this last year at an ion storage ring at GSI in Germany. (It's in Phys. Rev. Lett. somewhere, but I don't have it right now.) 4.) Naturally, enough ionization to have a major effect on half-lives is not very common in the Earth's interior, so it is usually neglected; also, pressure variations tend to decrease the half-life, while ionization increases it. However, in the case of bound-state beta decay, the ionization can dramatically decrease the half-life; in the case of 163Dy, it goes from infinity to about 50 days. 5.) At the risk of losing all my pedant points, I'll say something germane to t.o. That is that bound-state beta decay does come in to dating. Not rocks, but galaxies. The 187Re/187Os ratio in meteorites is used as a cosmochronometer, because 187Re has a half-life of 40 billion years or so. But if the material ever spent any time in a supernova, where it would be highly ionized, its half-life is reduced to 120 days or so, which is a whole lot less. Of course, as we all know, nothing spends much time in a supernova :-) but even a few seconds can throw the clock off significantly. -- Dave -- *-------------------------------------------------------------* * David Knapp dk@imager.llnl.gov (510) 422-1023 * * 98.7% of all statistics are made up. * *-------------------------------------------------------------* Article 47875 of talk.origins: Path: klaava!news.funet.fi!sunic!pipex!howland.reston.ans.net!vixen.cso.uiuc.edu!moe.ksu.ksu.edu!phys.ksu.edu!jsanders From: jsanders@phys.ksu.edu (Justin M. Sanders) Newsgroups: talk.origins Subject: Re: varying radiometric decay rates Date: 1 Dec 1993 06:20:19 GMT Organization: Department of Physics, Kansas State University, Manhattan KS, USA Lines: 53 Message-ID: <2dhd33$gau@newserv.ksu.ksu.edu> References: <2dgbsf$f9m@newserv.ksu.ksu.edu> <2dgjhh$bpf@lll-winken.llnl.gov> NNTP-Posting-Host: oppy.phys.ksu.edu In article <2dgjhh$bpf@lll-winken.llnl.gov> dk@imager (Dave Knapp) writes: >In article <2dgbsf$f9m@newserv.ksu.ksu.edu> jsanders@phys.ksu.edu >(Justin M. Sanders) writes: > >> Thus far, 85Sr with a half-life of 64 days is the longest-lived >> isotope in which any half-life variation has been observed. > > Oooh, goody! Now I get a chance to make pedant points! I cheerfully award you 1 pedant point (don't spend it all in one place). Anyone who brings up bound-state beta (one of my favorite "neato" experiments) deserves at least that. >A few comments: > > 1.) The sources are out of date. It might still be true that 85Sr is >the longest-lived electron-capture isotope to have demonstrated >chemical-environment-related half-life decreases, but, as I will show >later, it isn't the longest-lived such isotope to show any such changes. Yes, the references I quoted were primarily concerned with electron-capture and internal conversion rates-- variation of beta decay was "speculative" at that time. > 4.) Naturally, enough ionization to have a major effect on half-lives is >not very common in the Earth's interior, so it is usually neglected; also, >pressure variations tend to decrease the half-life, while ionization >increases it. However, in the case of bound-state beta decay, the >ionization can dramatically decrease the half-life; in the case of 163Dy, >it goes from infinity to about 50 days. If I remember correctly, the bound-state beta decay experiment used helium-like or hydrogen-like ions. That degree of ionization (for high Z atoms) is definitely not what you would see outside a star; certainly ocurring nowhere naturally on earth. > 5.) At the risk of losing all my pedant points, I'll say something >germane to t.o. That is that bound-state beta decay does come in to >dating. Not rocks, but galaxies. The 187Re/187Os ratio in meteorites is >used as a cosmochronometer, because 187Re has a half-life of 40 billion >years or so. But if the material ever spent any time in a supernova, where >it would be highly ionized, its half-life is reduced to 120 days or so, >which is a whole lot less. Of course, as we all know, nothing spends much >time in a supernova :-) but even a few seconds can throw the clock off >significantly. Does this problem have a solution, or must one simply look with some reserve upon Re/Os ratios? -- Justin M. Sanders "[The theory] seems a very dramatically Research Associate appealing and simple explanation to me, Physics Division, ORNL which therefore should be regarded with jsanders@orph14.phy.ornl.gov deep suspicion."-- Roger M. Squires Article 47926 of talk.origins: Path: klaava!news.funet.fi!sunic!uunet!tadpole.com!news.dell.com!swrinde!pirates.cs.swt.edu!cs.utexas.edu!not-for-mail From: farrar@mistral.noo.navy.mil (Paul Farrar) Newsgroups: talk.origins Subject: Re: varying radiometric decay rates Date: 1 Dec 1993 09:00:22 -0600 Organization: UTexas Mail-to-News Gateway Lines: 41 Sender: daemon@cs.utexas.edu Message-ID: <9312011502.AA10494@mistral.noo.navy.mil> NNTP-Posting-Host: cs.utexas.edu In article <2dhd33$gau@newserv.ksu.ksu.edu>jsanders@phys.ksu.edu (Justin M. Sanders) writes: > In article <2dgjhh$bpf@lll-winken.llnl.gov> dk@imager (Dave Knapp) writes: [deletions] > > 5.) At the risk of losing all my pedant points, I'll say something > >germane to t.o. That is that bound-state beta decay does come in to > >dating. Not rocks, but galaxies. The 187Re/187Os ratio in meteorites is > >used as a cosmochronometer, because 187Re has a half-life of 40 billion > >years or so. But if the material ever spent any time in a supernova, where > >it would be highly ionized, its half-life is reduced to 120 days or so, > >which is a whole lot less. Of course, as we all know, nothing spends much > >time in a supernova :-) but even a few seconds can throw the clock off > >significantly. > Does this problem have a solution, or must one simply look with some reserve > upon Re/Os ratios? > -- > Justin M. Sanders "[The theory] seems a very dramatically > Research Associate appealing and simple explanation to me, > Physics Division, ORNL which therefore should be regarded with > jsanders@orph14.phy.ornl.gov deep suspicion."-- Roger M. Squires This would not pose a problem for dating meteorites. The Re/Os decay is usually used with isochron methods, so what is being measured is the time since the coalescence of the meteorite. Although the Re, and everything else above Fe, was probably formed in a supernova, the meteorite would not have been. Re/Os is used for the metallic phases of meteorites, and is one of the best ways to date an iron meteorite, because the chemical similarity of Re to Mo allows concentration in the Fe-rich phases. A Re/Os isochron formed from several meteorites gives one of the best guesses for the age of the solar system (Fig. 6.14, Dalrymple, The Age of the Earth), 4.57 +- 0.21 Ga, a value which is quite consistent with other radiometric dates. The event being dated by this isochron is the differentiation into core and mantle of a presumed parent body of the iron meteorites in question, which would be the era of planetary formation in the solar system, and thus an age for the earth. Paul Farrar not an official spokesman