Steven,
You wrote:
>Richard,
> Another sharp question. For many of these radioactive isotopes, we are
>talking about minuscule amounts relative to the abundance of stable
isotopes
>of the same element. For example, my ancient CRC Handbook of Chemistry and
>Physics lists the natural abundances of potassium (K) isotopes as:
>K-39 -- 93.1%
>K-40 --- 0.00118%
>K-41 --- 6.88%
>This suggests that for a K-bearing mineral solidifying today, an average of
>12 K atoms out of every million K atoms will be K-40. Of course, the ratio
>would have been higher in the past. Yet, potassium (all isotopes combined)
>is also the 8th most abundant element in the Earth's crust.
>
>Therefore, your cogent mathematical observation caused me review my
sources.
>First, I could not find my source for that 10 half-life rule of thumb. I
>believe it was in some notes relating to a classroom laboratory experiment
>demonstrating the nature of radioactive decay. Thus it is very possible
>that the 10 half-life rule was based on detection limits rather than the
>"decay of the last few atoms" and that I mistakenly expanded that
>observation to a more general rule. I also found some exceptions to the "10
>half-life" rule. While scanning Dalrymple's "Radiometric Dating, Geologic
>Time, and the Age of the Earth: A Reply to 'Scientific' Creationists" (USGS
>Open-File Report 86-110), I see where he was optimistic that new analytical
>techniques may extend the limits of C-14 dating back to 100,000 years.
That
>is almost 20 half-lifes or 1 in 10^6 (2^20) original atoms.
I think you should drop the rule of thumb altogether, unless you can justify
it. And I fail to see why the same rule of thumb should be applicable to all
the isotopes, unless it's because all the isotopes originally occurred in
roughly similar concentrations (at least to within an order of magnitude or
two).
>But, how do these revelations affect my original point? Actually, if
>anything, they improve it. The question still stands. With the exceptions
>noted in my first post (and David Bowman's correction of my sunlight/cosmic
>ray blunder), why are there no naturally-occurring radioactive isotopes
>having half-lifes of 70 million or less? This question is valid whether
all
>Sm-146 (70 M.Y. half life) has decayed or whether it has simply decayed to
>the point that we can no longer detect it.
To justify that point, you need to show that the original concentrations
were well within today's detection limits. That may be obvious to you, but
not to a layman like me!
By the way, how do we know the half-lives of isotopes which are not found in
nature? Have they been created artificially?
>P.S. BTW, although humbling, I consider it an honor to be able to correct
>my errors. Almost everyone who has written peer-reviewed papers
understands
>and ultimately appreciates correction -- especially before publication.
>Perhaps someday, in another post, I will relate the story of person whom
>I've worked with here -- one of my heros of scientific integrity who
>personally and very publicly refuted his own published paper after redoing
>his experiment.
Please do. I look forward to reading it. :-)
Richard Wein (Tich)
See my web pages for various games at http://homepages.primex.co.uk/~tich/
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