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Re: Pls Explain ESR
JamShreeve (jamshreeve@aol.com)
13 Nov 1995 03:44:05 -0500
You are not a plebe. I do not understand ESR really either, and I've had
several practioners of the technique try to explain it to me. Or perhaps
we're both plebes...
Maybe this helps; it's from my book (The Neandertal Enigma), which has
more info and discussion on the new dating techniques if you're
interested:
"All electrons 'spin' in one of two opposite directions -- let's call
them up and down. Metaphors are a must, because the nature of this
electron 'spinning' is quantum mechanical, and 'words cannot
describe it, only huge mathematical equations can,' according to one
expert I talked to. The spin of each electron creates a tiny magnetic
force pointing in one direction, something like a compass needle.
Under normal circumstances, the electrons are paired so that their
opposing spins and magnetic forces cancel each other out. But
trapped electrons are unpaired. By manipulating an external
magnetic field placed around a sample to be dated, the captive
electrons can be made to flip around and spin in the other direction.
When they flip, each electron absorbs a finite amount from a
microwave field also applied to the sample. This energy loss, called
resonance, can be measured with a detector. It is a direct count of
the number of electrons caught in the traps, and thus a measure of
time. ESR has been used to date everything from stalagmites to
stale potato chips, but it works particularly well on tooth enamel,
with an effective range of from a few hundred to two million years."
[Note that "trapped electrons" are electrons that are captured out of
their orbits by crystal impurities or electronic aberrations in the sample
material; these are known to accumulate at a steady rate through time,
which makes their measurement a geochronological "clock" revealed by both
ESR and thermoluminescent dating.]
James Shreeve
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