Pim>Vernon mentioned, without proof might I say, that decay rates of
>radioactive substances might be variable.
>....
>Pressure, chemical state and temperature can change the decay rates but
>under these very extreme conditions, the maximum variation is a few percent.
> ....
Glenn>I want to note that the increase in decay rates which come from pressure
>apply only to one type of process--electron capture. This is where an
>electron is captured by a proton and turned into a neutron. Such processes
>are not used in dating, and the pressures of rocks is not enough to alter
>the basic decay rate of such things.
Glenn & Pim are correct in the thrust of their comments if not in all the
details. Nuclear radioactive decay rates are *not* adjustable by
environmental conditions (even by a few percent) short of placing the
sample in an environment such as the middle of an environment far more
extreme than can be found on earth, such as in the middle of a supernova
explosion. If a sample of radioactive nuclei *was* placed in the middle
of a supernova explosion any change in the rate transmutation to the final
nuclide would not be due changes in the original radioactive decay process
as much as due to stimulated conversion from other interactions of the
nucleus with the flux of other environmental particles that may be present
under such extreme conditions which then opens up many other new reaction
possibilities.
Electron capture is not strictly a form of a *nuclear* decay. It is a
reaction between a low-lying orbital electron and an otherwise (in the
absense of such orbital electrons) radio-stable nucleus. It is true that
an envirnmental condition of extreme pressure may slightly affect electron
capture rates in nuclei that are prone to such captures, but as Glenn
says, (and I recall) the pressure conditions found in crustal rocks is
too low to have a significant affect on them. (I have no reference for
this last statement other than my fading memory however.)
AFAIK the reaction: 40K --> 40Ar proceeds *via* electron capture, and this
reaction *is* relevant to the K/Ar dating method. I do not know of any
other radiometric dating methods where electron capture plays a role
though.
David Bowman
dbowman@georgetowncollege.edu