To: All Nov2993 08:50AM Subject: Re: Science and Evolution (really decay rates) kgs@maths.

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From: Andrew MacRae To: All Nov-29-93 08:50AM Subject: Re: Science and Evolution (really decay rates) Organization: Dept. of Geology and Geophysics, University of Calgary From: (Andrew MacRae) Message-ID: Reply-To: Newsgroups: (Ken Smith) [Decay rates can vary in high pressure conditions, due to effects on electrons] Surprise! Yup, decay rates can, indeed, vary. I was rather surprised to learn this just a couple of weeks ago, when I was reading up on radiometric dating methods for my candidacy. Now, as Ken mentions, the conditions under which this occurs are very specific, and the effect is always small. The process is explained in Gunter Faure's "Isotope Geochemistry" text. One of the forms of decay is by "electron capture" - i.e. an electron in "orbit" around the nucleus of an atom is captured by the nucleus. I can't remember exactly what happens at that point, but I think it results in some sort of Beta decay (the physicists can correct me on this :-). The basic idea is to "squish" the electrons closer to the nucleus, so capture is more likely. There are a couple of important points to realize: 1.) Electron capture is the _only_ decay mechanism known to be effected by non-nuclear processes (Pressure, in this case). 2.) Electron capture is a possible decay mechanism for only some isotopes. In other words, the alteration of the decay rate is _only_ possible for isotopes that include electron capture as a possible decay mechanism. This is determined by isotopic properties. [I am going to check which of the isotopic reactions commonly used for dating can use this mechanism] 3.) You need _extremely_ high pressures for any measurable effect at all. We are talking about squishing electrons so they are closer to the nucleus than normal. As anyone with basic physics knows, you need _incredibly_ high pressures to have any apprecible effect. We are talking about pressures like those found in diamond anvil presses here - well beyond the pressures found in near-surface (<10km deep) geologic processes. More like the core of the Earth, or the Sun. 4.) Electron capture is usually responsible for a very small percentage of individual decays in a radioactive isotope. I.e. other decay mechanisms predominate - by _far_. It is a fairly rare decay mechanism. 5.) As Ken mentions, the potential effect for this process is less than 1%. So, as you can see, there _is_ a genuine effect, but as Ken points out, it does _nothing_ to transform 4.5 billion years into 6000 years. Not withstanding the extra heat that would be produced if the decay rates were that much faster (the Earth would probably still be molten :-)). Despite these qualifications, I'm sure you will see creation "scientists" telling people - "radiometric dating is bogus because decay rates are not constant". Sigh. -Andrew or: I thought the same thing when I composed it, but was too lazy to look up the conversions. Since you have done that, I have no excuse now :-) So, I looked up a reference on diamond-anvil presses from Scientific American, which has a great "order of magnitude" plot of pressures. As a bonus, diamond-anvil presses are the tools used to study the high-pressure effects on electron capture decay, so you can learn about the techniques used in the decay rate experiments at the same time. Here is a summary of common pressures from Jayaraman (1984). Unfortunately they are on a graphic scale, so the values are estimated (guaranteed to be within 1 order of magnitude :-), + means a little over, - means a little less): 1 atmosphere = .987 bars = .000987 kilobars - pressure at sea level 10+ atm. = .00987+ kbars - high pressure bicylce tire 100+ atm. = .0987+ kbars - steam pressure in boiler of a power plant 1000+ atm. = .987+ kbars - pressure at greatest depth in oceans 100 000- atm = 98.7- kbars - graphite->diamond 100 000+ atm = 98.7+ kbars - spinel->perovskite, 670 km deep in Earth 1 500 000+ atm = 1481+ kbars - approximate maximum for diamond anvil (in 1984) 4 000 000 atm = 3948 kbars - pressure at centre of the Earth So, for example, the 270 kilobars that produced the 0.59 percent increase in the 7Be -> 7Li decay is about 270 000 atmospheres, and would be found deep in the mantle of the Earth, hundreds of kilometres below the crust. It is 270 times the pressure at the greatest depth in the ocean. Clearly, these pressure effects are insignificant for any radiometric dating that has occurred, because _all_ terrestrial samples come from the Earth's crust, and virtually all dated samples are also from within 1km of the surface of the Earth. Since K/Ar is the only common radiometric dating method with potential for even these effects, they would be easy to detect by comparison with other methods anyway. -Andrew or: References: Jayaraman, A., 1984 (April). The diamond-anvil high-pressure cell. Scientific American, v.250, no.4, p.54-62.


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