On Tuesday, March 09, 2004,
Jeff Grossman kindly wrote :
> The most reliable method, and the one you should have the most confidence
> in, is the thermoluminescence (TL) sensitivity measurement. Only a handful of
> labs around the world do this, notably Derek Sears' lab at the Univ. of Arkansas
> where the method was pioneered. Basically, the more metamorphic feldspar in
> a chondrite, the higher the induced TL. This method is good to +/- 0.1, although
> severe weathering or shock can cause problems.
Now, what is thermoluminescence? There was a very informative article for the
interested layperson by ..., well, of course, Derek Sears, in Sky and Telescope,
July, 1980, pp. 14-16:
Meteorites have a special fascination for students of planetary science. They formed
when the solar system began, 4.6 billion years ago, and have been influenced both by
the conditions then and by more recent events. No other source can tell us about those
events the way meteorites can, if only we can extract the information effectively.
Experimental techniques of meteorite study are many and various, and some have taken
a very long time to mature. A prime example is luminescence - the way some meteoritic
minerals emit light under certain laboratory conditions.
The first published report of meteorite luminescence appeared in 1802, when Edward
Charles
Howard observed that a sample of the Benares meteorite glowed in the dark when exposed
to
an electrical discharge. Some 87 years later, Alexander Herschel (grandson of Sir
William
Herschel) discovered that some grains from the Middlesbrough meteorite glowed
distinctly
when sprinkled onto a hot plate in the dark.
Although caused by different processes, the phenomena observed by Howard and Herschel
are
both evidence of the remarkable ability of meteorite minerals to be luminescent. But
only
within the last decade have scientists attempted to capitalize on this property.
Modern Techniques
Herschel observed thermoluminescence (TL) - a result of heat stimulation. The diagram
on
the facing page shows a typical layout for the equipment used to study it. A specimen
is
heated at a controlled rate in a nitrogen atmosphere. It emits light which is picked up
by a photomultiplier tube, whose electrical signal is amplified and displayed on a
chart
recorder. Usually, brightness is automatically plotted as a function of temperature.
Alternatively, the wavelengths of the light emitted may be analyzed with a
spectrometer.
Samples irradiated by an electron beam can glow with what is called cathodoluminescence
(CL). The electron gun and an optical microscope each point to a spot on the specimen's
surface, and the result is observed directly.
.. to be continued
Reference:
S.W.S. McKEEVER and D.W. SEARS (1980) Meteorites
That Glow (Sky and Telescope, July 1980, pp. 14-16).
Best wishes, Good night
(it's late here: 23:25 hrs)
Bernd
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