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Author Topic:   Dependency of decay rates on chemical environment?
Member (Idle past 4198 days)
Posts: 766
From: Newcastle, Australia
Joined: 11-17-2002

Message 1 of 2 (34551)
03-17-2003 4:30 AM

One of the fundamental assumptions of radiometric dating is that decay rates have remained constant over time. There are various creationist writings which address this matter.

A widely cited article is by Tas Walker, available from Answers in Genesis, entitled Radioactive decay rate depends on chemical environment. This article cites recent measurements of variations in the decay rate of Beryllium-7, as reported in Dependence of the decay rate of 7Be on chemical forms by Dr Chih-An Huh.

Walker suggests that the research by Huh into variations of the Be-7 decay rate is an upset for existing assumptions of constant decay used in radiometric dating. He says: "contrary to previous thinking, the chemical environment noticeably affects the half-life of radioactive decay by electron capture".

The truth, however, is that the dependency of Be-7 decay rates on chemical environment had been both predicted and measured over 50 years earlier.

Huh's paper cites the original prediction of the effect, and the original experimental measurements, and a flurry of associated further work from around fifty years ago. His contribution is a new measurement of a well known phenomenon.

Walker also cites "Principles of Isotope Geology" by Gunter Faure (1986). This is a standard reference, and I looked it up in the University library. (Any member of the public is welcome to come along and do the same thing.) Walker quotes Faure as follows:

... there is no reason to doubt that the decay constants of the naturally occurring long-lived radioactive isotopes used for dating are invariant and independent of the physical and chemical conditions to which they have been subjected ...

In the context of his article, Walker is using Faure as an example of the assumption of constant decay rates supposedly disproved by Huh.

But guess what?

In the exact same paragraph from which Walker lifted that sentence fragment, Faure includes a discussion of varying decay rates of Beryllium-7, and also of several other isotopes. That is, Faure knew of this effect also. You can find it on page 41.

In fact, Faure's extract as given by Walker is correct, because Faure is explicitly speaking of decay constants of long lived isotopes used in dating. Berillium-7 is a short-lived isotope, with a half life of only about 53 days. It has been used for study of sedimentation rates in the present, but it cannot be used for dating.

There is only one long lived isotope used in dating which has a decay by the same electron capture mode: Potassium-40. Both Huh and Faure consider it specifically, and point out that standard nuclear physics, which predicted the Beryllium-7 dependency way back in 1947, also predicts that the same effect on Potassium-40 will be negligible due to the many additional electron shells around the nucleus. This is confirmed by experiment.

The interesting aspect of Huh's measurement is that he detects a change in halflife of about 1.5%, which is larger than previously observed. The earliest measurements were around 0.1%, and another more modern measurement found 0.8%. These are not necessarily contradictions; they are a range of measurements made under different conditions and different chemical environments. Huh's measurements are new and have excited some interest with specialists because they are larger than previous results; but they are not any radically new phenomenon. Such experiments have prompted new tests for variation in Potassium-40 decay rates, which (as expected) failed to detect any change whatsoever.

Huh's measurements are the latest in a long series of studies of decay rate variation, extending over fifty years, all of which is good agreement with existing scientific models of nuclear physics, and all of which helps confirm the processes and models of nuclear decay, which in turn predict the simple fact -- also confirmed by experiment -- that radioisotopes used in dating have no detectable variation in decay rate under any of the conditions of chemistry, pressure or temperature to which they might be subjected on Earth.

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Message 2 of 2 (35279)
03-26-2003 1:18 AM

Some theoretical background:

Be7 beta decay happens by way of electron capture; one of the atom's electrons gets turned into a neutrino.

However, the charged weak interaction has a range of 10^(-17) m [Compton wavelength of a W particle], meaning that it is effectively a contact interaction. Meaning that an electron has to be at the nucleus itself in order to be captured.

This becomes more likely if an atom gets squeezed, which is what accounts for the observed Be7 effect. However, the electrons most affected are those in the highest-most orbit shells, those with the smallest orbital energy and largest extent. This is why the effect is much more difficult to observe with potassium-40; it has more inner electrons than beryllium.

Beryllium has shell structure 2-2
Potassium has shell structure 2-8-8-1

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