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_Luminescence & Radiometric Dating_
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Luminescence is a phenomenon exhibited by many crystals, such as
diamond, quartz, feldspars and calcite. Energy absorbed from ionizing
radiation (alpha, beta, gamma, cosmic rays) frees electrons to move
through the crystal lattice and some are trapped at imperfections in
the lattice. Subsequent heating of the crystal, or stimulation by
absorption of light can release some of these trapped electrons with
an associated emission of light - thermoluminescence (TL) or optically
stimulated luminescence (OSL) respectively. This is the technology
used for dosimetry badges in areas where radiation safety is a
concern. The time over which the badge has been exposed is well known,
and the total radiation does controls the final luminescence. The
badges are heated (TL), luminescence recorded, and total dose derived.
Since we know the time period of exposure and total does, we know the
average dose per unit time. Now turn the process around; if you know
the average dose per unit time, and the total dose from the
luminescence, then you know the time period of exposure. This is the
fundamental process behind luminescence dating (TL and OSL), as well
as electron spin resonance (ESR) dating, which uses a different
technique to achieve the same result.

OSL and TL dating techniques don't get as much press in the
creation/evoluton game, because they cannot be used to measure the
billions of years time period characterized by the age of the Earth.
But they can be used to measure significant periods nonetheless,
especially in the range between about 40,000 to 50,000 years where
radiocarbon dating cuts off, and the 1,000,000 years or so required
for most radiometric techniques to become reliable. The idea here is
that all materials carry extremely low concentrations of radiogenic
isotopes, line Uranium, which in turn expose the material to extremely
low doses of radiation over a long time. That radiation frees
electrons that get trapped in crystal defects, just like dosimetry
badges. The total population of trapped electrons in turn determines
the total dose. If you know the average dose per unit time, by
studying the geology of the site, you can then use the ratio of total
dose over average dose, and get the time period.

Sunlight on a crystal will evict the trapped electrons much faster
than background radiation puts them in. So once the crystal is buried,
the "clock" starts. Dig up the crystal, measure its luminescence
(either optically or thermally stimulated), and you know the total
dose. Compare with the average dose per unit time, and you know how
long the crystal has been buried. This is a favorite means for dating
buried sediments that are often rich in quartz and feldspar. For other
materials, notably non translucent material, electrons become trapped
in defects where the lattice potential is too deep and the electrons
cannot be stimulated to come out. In those cases, electron spin
resonance (ESR), which is much more complicated that luminescence
techniques, can be used to count the number of trapped electrons by
using a combination of microwaves and a variable magnetic field. The
disadvantage of ESR is that it is much more complicated, and has
larger uncertainties than luminescence techniques. The advantage of
ESR is that, unlike luminescence, the electrons are not evicted from
their traps, so the measurement can be repeated as desired on the same
sample.

One of the key tests of reliability for any dating technique is the
ability to intercompare with other techniques; they should all give
the same age for the same sample, within the bounds of the usual
experimental uncertainties. There is a lot of literature available
that demonstrates intercomparison between these luminescence
techniques and radiometric dating. But here is one recent, and very
good example.

_Australia's oldest human remains: age of the Lake Mungo 3 skeleton_

Alan Thorne, Rainer Grün, Graham Mortimer, Nigel A. Spooner, John
J. Simpson, Malcolm McCulloch, Lois Taylor, Darren Curnoe

Journal of Human Evolution, v 36, n 6, June, 1999, p591-612
(ID jhev.1999.0305)

_Abstract:_
_We have carried out a comprehensive ESR and U-series dating study
on the Lake Mungo 3 (LM3) human skeleton. The isotopic Th/U and
Pa/U ratios indicate that some minor uranium mobilization may have
occurred in the past. Taking such effects into account, the best
age estimate for the human skeleton is obtained through the
combination of U-series and ESR analyses yielding 62,000±6000
years. This age is in close agreement with OSL age estimates on the
sediment into which the skeleton was buried of 61,000±2000 years.
Furthermore, we obtained a U-series age of 81,000±21,000 years for
the calcitic matrix that was precipitated on the bones after
burial. All age results are considerably older than the previously
assumed age of LM3 and demonstrate the necessity for directly
dating hominid remains. We conclude that the Lake Mungo 3 burial
documents the earliest known human presence on the Australian
continent. The age implies that people who were skeletally within
the range of the present Australian indigenous population colonized
the continent during or before oxygen isotope stage 4
(57,000-71,000 years)_

The authors performed the following dating measurements, with the
indicated resulting ages expressed in thousands of years.

Mass spectrometer U-series (Th/U) on 4 bone shavings
69.8 ± 2.1
58.3 ± 1.2
50.7 ± 0.9
54.5 ± 0.7

Gamma spectrometer U-series (Th/U and Pa/U) on the skull cap
69.5 ± 2.9 (Th/U) 74 ± 7 (Pa/U)
64.1 ± 3.7 (Th/U) 60 ± 5 (Pa/U)

Electron Spin Resonance (ESR) on tooth enamel
63 ± 6 (EU) [closed system, early U uptake]
78 ± 7 (LU) [open system, late U uptake]

Mass spectrometer U-series (Th/U) on calcitic matrix recovered from
the original skeletal material
81 ± 21 (isochron)

Optically Stimulated Luminescence (OSL) on sediment recovered
directly under the soil horizon
59 ± 3
63 ± 4
61 ± 2 [weighted mean of previous 2 ages]

Some discussion of the results is in order. The authors have a long
discussion of the 4 Th/U dates on bone shavings, which cover a fairly
wide spread. Bones, especially teeth, do not normally lose uranium,
but take it in ("uranium uptake"), which would make the radiometric
ages appear younger than the true age. However, the geological setting
of the skeleton, in a rapidly eroding area, can indicate that uranium
in this case was lost due to oxidation, which would make the ages
appear too old. The authors conclude that uranium mobility has
affected these ages, but they cannot be sure in which direction
(whether or not net uranium was lost or gained). The OSL age should
represent the maximum age of the specimen, since sediment deposit
occurred before burial. Since uranium mobility is more of an issue for
the bones (which are permeable) that for the crystals (which are not),
it is not surprising that the OSL ages agree with each other fairly
well, and lie in the range of bone Th/U ages, which indicates that
different bone samples experienced different uranium mobility. Using
the mass spectrometer to study other isotope ratios, and model uranium
mobility, the authors decided that the "late uptake" model was not
applicable here, an idea bolstered by the discordant ESR date if one
assume late uptake. They conclude a weighted mean age 62 ± 6 as
indicated in the abstract.

The paper includes lengthy and detailed discussions of techniques,
assumptions, and tests of assumptions that lead to their final
conclusion. You really need to read the paper to get a good feel for
how carefully the work was done. My point here is that the OSL and ESR
dates are consistent across the board with the radiometric dates.
Radiometric dating is an obvious target for young-Earth creationists,
but criticisms are more effective when oriented towards geologic
assumptions (like isotope abundances or mobility, or "excess argon),
than towards fundamental concepts (like variable decay rates). But
this kind of concordance between extremely different techniques is
common, and illustrated by far more papers than this one. My own
Radiometric Dating Resource List includes a link to an EOS paper,
"Breakthrough Made in Dating of the Geological Record", which shows
concordance between radiometric dates and astronomical dates via the
Milankovitch cycles.

This is a real fundamental problem for the continued assertion that
there is scientific validity to the argument that the Earth is only
about 10,000 years old . This is not just concordance between
different radiometric techniques, this is a wide concordance between
techniques that share no common fundamental conception beyond physics
itself. If they are so wrong, why are they so consistent? This
question has to be addressed if there is any hope for the scientific
revival of a "young Earth".
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[1]Tim Thompson's Home Page
[2]Tim Thompson's Collected Writings
[3]Radiometric Dating Resource List

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References

1. file://localhost/www/sat/files/tim_thompson/index.html
2. file://localhost/www/sat/files/tim_thompson/faqs.html
3. file://localhost/www/sat/files/tim_thompson/radiometric.html