Radiometric and Trapped-Charge Dating

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Radiometric and Trapped-Charge Dating R.A.
Varney Paleoresearch Institute
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• Presentation Format
• Basic Chemistry for Radiometric Dating
• Types of Radiometric Dating for Archaeology
• Primer on Radiocarbon and Radiocarbon Dating
• Potassium-Argon Dating
• Argon-Argon Dating
• Uranium Disequilibrium Dating
• Lead 210 Dating
• Intro to Trapped-Charge Dating
• Thermo-Luminescence and Optically-Stimulated
  Luminescence Dating.
• Question/Discussion Session.

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Relative dating is great for determining the
sequence of events within a culture, but to
compare cultures to other cultures or to place
events within calendar time you need to have a
method of absolutely dating an archaeological
context. This calls for absolute dating.
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• Basic Chemistry for Radiometric Dating
• Chemistry refresher Elements, Isotopes,
  Radioactive Decay, and Half-life
• Carbon Isotopes 14C formation and decay, Brief
  history of Radiocarbon Dating, Radiometric
  Dating, Accelerator Mass Spectrometry Dating
• Daughter products Uranium series and Potassium/
  Argon, Argon/Argon Dating

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• Basic Terms
• Atom a group of particles composed of two main
  parts electrons and the nucleus
• Electrons negatively charged particles which
  form the outer, mostly empty, portion of an atom.
  Different numbers of electrons change the charge
  of the atom and are responsible for most chemical
  reactions.
• Nucleus relatively densely packed inner portion
  of the atom. Composed of positively charged
  protons and generally uncharged neutrons.
• Protons Positively charged particles, the
  number of which, in an atom, defines that
  element.
• Neutrons Generally uncharged particles in the
  nucleus, different numbers of neutrons in the
  nucleus define the different isotopes of an
  element.
• Element A single type of matter, gold is an
  element, as is lead, copper, carbon, argon and
  hydrogen for example.

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• An element is defined by the number of protons in
  the nucleus.
• Hydrogen ALWAYS has one proton and Carbon ALWAYS
  has 6 protons. If there is a change in the number
  of protons is a different element.
• The number of electrons and neutrons can and does
  change.
• A change in the number of electrons changes the
  electrical charge of the atom.
• A change in the number of neutrons makes a
  different isotope of the same element.

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All of the nuclei pictured here are the element
Hydrogen, because they all contain a single
proton, but each is a different isotope of
Hydrogen, because they contain different numbers
of neutrons.
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Element and Isotope Nomenclature
Number of Neutrons and Protons This is how the
isotope if identified
Number of Protons This is how the element if
defined
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• Different isotopes of an element have different
  properties.
• For purposes of this discussion, different
  isotopes of an element have different levels of
  stability.
• A stable isotope will not change into another
  isotope or element under normal circumstances.
• An unstable isotope will change into another
  isotope of the same element or into another
  element within a statistically determined time.
• There are various levels of stability found in
  isotopes, resulting in different periods of time
  that the unstable isotope will exist.
• Isotopes that change into other isotopes or
  different elements by emitting radiation are
  called radioisotopes.

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• Radioisotopes give off radiation as they change
  in a process called decay.
• There are three main types of radioactive decay
• Alpha radiation is two protons and two neutrons.
  When an radioisotope decays by Alpha radiation a
  different element is created. This is the
  fallout that many of us grew up worrying about.
  Alpha radiation can be shielded by a piece of
  paper.
• Gamma radiation is an electromagnetic wave just
  as FM radio or light is a wave, but at a higher
  frequency. It is very difficult to block gamma
  radiation. This is the most dangerous, but
  shortest-lived component of a nuclear explosion.
  Gamma radiation causes changes to DNA.
• Beta radiation emits beta particles which can be
  either either high speed electrons or positrons.
  Beta radiation can be stopped by a sheet of
  aluminum.
• Carbon 14 decays by beta emission.

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Radioactive decay does not happen all at once. A
certain percentage of the quantity of the
radioisotope will decay over a period of time.
The time that it takes for half of the original
mass of the radioisotope to decay is called the
half-life of the isotope. If the original mass
is 1000 grams, at one half-life in time there
will be 500 grams of the radioactive material
remaining. At two half-lives, there will be 250
grams remaining.
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• Selected Types of Radiometric dating.
• Radiocarbon Used for things that were once
  alive, including organic decay products like
  Humates and carbonates. Age range
  pre-aboveground atomic bomb test (1952) to 45000
  year ago and post-atomic bomb (1952) to modern.
• Argon dating (both Potassium/Argon and
  Argon/Argon) Used to date igneous rocks (rocks
  that formed directly from the molten state) as
  well as some metamorphic rocks (rocks that have
  been changed by heat and/or pressure) Used to
  great effectiveness in bracket dating pre-humans
  in volcanic regions. Olduvai for example . Age
  range 100,000 to 4 billion years
• Uranium lead dating Generally used on the
  element Zircon though other elements can be used.
  Age range 10 million years ago to 4.5 billion
  years. Not used for archaeology. Commonly used
  in dating geologic events.
• Uranium disequilibrium dating Measures the
  amount of uranium absorbed by buried porous
  materials such as teeth, eggshell, coral, etc. 1
  to 400,000 or 500,000 years. Not technically a
  radiometric method, but uses radioisotopes.
• Lead 210 dating Measures the decay of Radon xx
  into lead 210. generally used for ecological
  studies in lakes and marine sediments, but can be
  used to date sediments less than 200 years old

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Radiocarbon

• There are 15 known isotopes of Carbon, from 8C
  with only 2 neutrons, to 22C which contains 16
  neutrons in the nucleus.
• Only three of the Carbon isotopes occur in the
  natural world 12C, 13C, and 14C
• Carbon 12 And Carbon 13 are stable, with nearly
  99 percent of the carbon in the world being the
  Carbon 12 isotope and nearly 1 percent being
  Carbon 13.
• There is about one Carbon 14 atom for every 850
  billion Carbon 12 atoms in the atmosphere.

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• Carbon 14 is formed when cosmic radiation in the
  upper atmosphere excites a neutron, causing the
  neutron to impact a Nitrogen 14 atom and
  dislodges a proton forming carbon 14.
• This is an ongoing process, generating a
  relatively stable percentage of Carbon 14 atoms
  in the atmosphere.
• All living things are composed of this same
  fraction of the isotopes of carbon.
• When an organism dies, it is no longer taking in
  the carbon and the decay clock on the radiocarbon
  begins.
• The longer the time that has past since an
  organism has died, the smaller the percentage of
  radioactive carbon will remain in whatever is
  left of the organism.
• The half-life of Carbon 14 is 5730 years,
  therefore when there is half of the atmospheric
  percentage of carbon 14 remaining, the organism
  died 5730 years ago.
• We are able to measure the relative quantity of
  Carbon 14 very precisely.

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Carbon 14 is created most intensely at altitudes
from 30,000 to 50, 000 feet and at high
latitudes. This is a plasma model of the magnet
fields of the earth showing the relatively less
protected polar areas where gamma radiation is
able to most easily interact with the Nitrogen in
the atmosphere and change the Nitrogen 14 to
Carbon 14.
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• Radiocarbon dating was developed in 1949 by
  Willard Libby, one of the developers of the
  atomic bomb who wanted to explore peaceful uses
  of radiation after his involvement in the bomb.
  In 1960, Dr. Libby received the Nobel prize for
  his radiocarbon research.
• Originally, radiocarbon dating was conducted by
  counting the radioactive emissions over a period
  of time and calculating the quantity of Carbon 14
  remaining in the sample, thus determining the age
  of the sample. This method requires a large
  sample and returns fairly broad time periods for
  an age of the sample (plus or minus around 90 to
  150 years). This method is called proportional
  counting and works on the same principal as the
  Geiger counter.
• Liquid scintillation counting is conducted by
  dissolving the sample in either benzene or
  toluene with fluors ( material which emits light
  when excited by the beta emissions) the light
  emitted is then multiplied and provides the data
  for the age of the sample. More light
  corresponds to more beta emissions and a younger
  age.
• Proportional and liquid scintillation counting
  are relatively insensitive, and yield larger
  uncertainties than AMS dating. These methods
  generally require over a gram of carbon.
• Accelerator Mass Spectrometry (AMS) counts the
  actual numbers of atoms of each isotope,
  requires only very small samples (gt0.0002 grams
  of processed carbon, or about 0.001 grams of
  field sample), and returns very precise ages for
  the sample (plus or minus 15 to 30 years).
• Paleoresearch Institute only utilizes the AMS
  method of Radiocarbon dating.

Carbon 14
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• The Graphite sample is placed on a sample wheel,
  where a Cesium ion beam turns the sample into
  atoms.
• Powerful magnets pull the atoms into the
  accelerator, then strip all of the non-carbon
  atoms from the plasma, and send a beam of carbon
  atoms through a series of bends.
• Due to the different atomic weights of the
  isotopes, they will deflect at different angles
  and be received at different detectors.
• The detectors count the impact of the different
  isotopes and establish the ratio of each. The age
  of the sample is then straightforward math.

Carbon 14
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• Radiocarbon Sample Pretreatment
• Radiocarbon samples collected in the field
  contain various contaminants that must be removed
  in order to get an accurate date.
• Contaminations sources include old carbon from
  percolation of water through sediments containing
  coal, limestone or other sources of carbon 14
  depleted sources. These have the risk of making
  the age that you get too old. The sample can
  also be contaminated with young carbon that is
  richer in carbon 14 from sources such as decaying
  organic matter.
• Organic matter decays into a rich soup of carbon
  compounds including humins, humates and foelvic
  acids.
• If no particulate carbon is found, humate
  dates can usually be processed to get a rough
  age for the sample. These humate or soil organic
  matter dates are problematic because they are
  high affected by water soluable carbon
  percolating through the sediments.
• Pretreatment of charcoal samples require the use
  of an acid/base/acid process to remove
  contaminates.
• Pretreatment of bone samples requires removal of
  the mineral component of the bone to enrich the
  carbon content of the sample.
• There are many methods of pre-treating samples to
  ensure an accurate date, all of them need to be
  conducted in a specialized lab.

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Radiocarbon date calibration Radiocarbon dates
returned from the lab need to be calibrated to
calendar years. Radiocarbon creation in the
atmosphere has varied a little through time, due
to changes in the levels of gamma radiation based
variations in the earths magnetic field and the
frequency and intensity of solar storms. These
variations in the quantity of radiocarbon in the
atmosphere are called Seuss Squiggles. Radiocarb
on mixing in the atmosphere and old carbon influx
into the atmosphere has varied a little through
time primarily due to changes in climate ie. Rock
weathering and exposure after glacial periods
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Carbon 14
Statistical Introduction This is a normal curve,
the curve is the same shape on either side of the
mean or center point of the distribution.
Mean
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Radiocarbon dates from the lab are usually
returned in one standard deviation.
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Carbon 14
This is read as zero plus or minus one half (0
/- 1/2) to two standard deviations
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Carbon 14

• The result from the counter or accelerator will
  be reported as a number( the mean7630) and a
  range (Standard Deviation or the sigma 15).
• The first part, the mean, tells you the middle of
  the distribution of the probability of the
  number of radiocarbon years ago that the organism
  died.
• Present is agreed to be AD 1950 to prevent
  confusion as time goes on.

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This is the current radiocarbon calibration curve
going back to about 14800 radiocarbon years.
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This is INTCAL 2004 calibration curve for the
past 14000 Radiocarbon years Radiocarbon age
calibration is the process of drawing a line from
the 14C determination to the curve and then
dropping a line to the calendar year. Sounds
simple right? Example 14C determination of 12500
RCYBP becomes calendar year 15000 BP.
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CAL 86
INTCAL 98
CAL 93
INTCAL 04
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• Potential Complicating Factors for Radiocarbon
  Dating
• You must remember that you are dating the death
  of the organism. This can create an old wood
  problem where archaeological people collect wood
  or other organic materials that have been dead
  for hundreds or thousands of years.
• Water percolation either up or down through the
  sediment column can carry soluable carbons of an
  older or younger age than the item you are
  dating. These soluable carbons can contaminate
  your sample and skew the age.
• Coal contamination can skew the radiocarbon age
  to an older date. For an example of this problem
  read about Meadowcroft rockshelter in
  Pennsylvannia.

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• Radiocarbon Review
• Carbon 14 is produced in the atmosphere by gamma
  ray exciting a neutron which displaces a proton
  in the Nitrogen 14 atom creating carbon 14.
• Carbon 14 mixes in the atmosphere and becomes
  incorporated in organisms through respiration and
  eating.
• Carbon 14 is an unstable isotope and decays by
  beta radiation to form nitrogen 14.
• Radiocarbon dates can be returned for things that
  used to be alive.
• Radiocarbon can be used for objects as old as
  about 45000 years old.
• The amount of carbon 14 in the atmosphere has
  been nearly, but not completely, stable through
  time.
• Radiocarbon determinations returned from the lab
  need to be calibrated to provide calendar years
  before present.
• You must remember that you are dating the death
  of the organism.

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• Potassium/Argon dating
• Potassium (K) is the seventh most common element
  in the earths crust and makes up 1.5 of the
  weight of the crust.
• There are three naturally occurring isotopes of
  Potassium
• 39K the most common form occurs 93.3 of the
  time
• 40K the unstable form of potassium (0.0117)
• 41K Stable occurs 6.7 of the time
• 40K decays into stable Argon 40 and stable
  Calcium 40
• Calcium 40 is too common to be used for dating
• 40K has a half-life of 1.25 x 109 years and is
  useful for dating rocks older than 100,000 years.

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• What can be dated with Potassium/Argon dating?
• Volcanic rocks high in potassium, particularly
  the micas (muscovite and biotite), volcanic
  feldspar, and plutonic or highly metamorphic
  horneblend
• While the rock is liquid, the gas argon can
  escape, but when the rock solidifies, the argon
  is trapped and accumulates as the potassium 40
  decays into argon.
• Measuring the relative quantities of potassium
  and argon 40 gives the age of the rock. This
  makes the assumption that there was no argon in
  the rock when it was liquid.
• The sample is split into two aliquots The Argon
  aliquot is melted and measured on a mass
  spectrometer, and the potassium aliquot is
  measured using a an atomic absorption
  spectrometer or flame photometry .
• The ratio of the two returns the age of the rock.
• You are actually measuring the time when the rock
  solidified and argon was no longer able to
  escape.

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Difficulties with Potassium/argon dating When
the sample is prepared for dating the rock is
split in a potassium and an argon sub-sample. If
the rock is not completely homogeneous the ratios
of argon and potassium in the two sub-samples may
not be the same and the date will not be
accurate. Argon/ Argon dating prevents this
problem by measuring the same sample for both
isotopes.
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• Argon / Argon Dating
• Argon / Argon dating eliminates the potential for
  heterogeneous mixing of elements in rocks by
  sampling the same crystal.
• As in Potassium / Argon dating the argon 40 gas
  can escape when the rock is molten, so you are
  dating the time that the rock solidified.
• A single crystal of a potassium bearing mineral
  is selected and irradiated in a nuclear reactor
  to turn a known portion of the potassium 40 into
  Argon 39. The crystal is then degassed under
  heat and in high vacuum and the ratio of argon 40
  to Argon 39 is measured in a mass spectrometer.
• The conversion rate from Potassium 40 to Argon 39
  must be precisely controlled to obtain an
  accurate date.
• This method has been effectively used to date the
  Pompeii volcanic eruption that buried the Roman
  city of Herculaneum.
• Argon / Argon dating measures the last time that
  the rock solidified and may not record all of the
  events that the rock has undergone. Discrete
  sampling is needed to ensure that you are dating
  the event in which you are interested.

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• Uranium / Lead Dating in Brief
• The Uranium Lead decay series is the oldest and
  most reliable radiometric aging method.
• This series is useful for dating geological
  events, not archaeological events, as the entire
  series has a half-life of 4.6 billion years.
• Used for rock older than 10 million years
• Some of the daughter products have short enough
  half-lives to be used in archaeological contexts.
  These are called the uranium disequilibrium
  dating and will be discussed next.
• An assumption in Uranium / lead dating is that
  there was no lead in the rock when it was formed.
  For this reason, zircon crystal are the first
  choice for dating because they do not bond with
  lead, but preferentially bond with uranium.
• The ratio of Uranium 238 to lead 206 establishes
  the age of the rock.
• This method dates the formation of the rock.

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This is the complex Uranium Lead series. All of
these daughter products are radioactive except
Lead 206 (Pb 206) in the lower left corner. The
half-life of each isotope is given in the number
below the isotope. Several components of this
decay series are used for dating.
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• Uranium Disequilibrium Dating
• One of the properties of Uranium that helps in
  dating absorption-prone archaeological materials
  is its solubility in water.
• These materials are items such as bone, eggshell
  and other porous materials.
• These materials, when buried, absorb water
  containing uranium and the uranium is
  preferentially deposited in the porous material.
  This creates a disequilibrium in the quantity of
  uranium in the archaeological material relative
  to the sediment in which it is buried and by
  measuring the difference between the quantity of
  uranium in the buried object and the sediment the
  length of time that the item has been buried can
  be determined.
• This method measures the time that the porous
  object has been buried.
• This assumes that the object has only been buried
  once. If an abject has been buried more than once
  the date will reflect a combination of the
  uranium absorptions.
• This method is used for objects that have been
  buried from 1 to 4 or 500,000 years.

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• Lead 210 Dating
• Lead 210 is part of the uranium series.
• Radon 222 is a gas released from sediments and
  has a half-life of 3.8 days. It decays into
  Polonium 218 with a half- life of 3.05 minutes,
  which decays into lead 214 with a half-life of
  26.8 minutes, which decays into Bismuth 214 with
  a half-life of 19.8 minutes, which decays into
  Polonium 214 with a half-life of 0.16
  milliseconds, which decays into Lead 210 with a
  half-life of 22 years.
• Lead 210 forms in the atmosphere and is deposited
  along with other fine particles onto the land and
  surface of bodies of water where it is
  incorporated in sediments.
• Given the relatively short half-life of lead 210,
  it is used to date the age of sediments back to
  about 200 years ago.
• This method is primarily used in studies of
  recent ecological change and can be used for
  archaeology to date the sediments in which
  historical archaeological material has been
  buried.
• Lead 210 dates the time since the sediments were
  deposited by measuring the amount of lead 210
  remaining in the sediment.

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• Non-Carbon 14 Radiometric Dating Review
• Potassium /Argon dating Dates the time since the
  rock was molten, Useful for dating rock older
  than 100,000 years, used at Olduvai Gorge for
  dating human ancestors. The rock must be
  homogenous for Potassium and Argon for the date
  to be accurate. Method works on Potassium rich
  volcanic or high-grade metamorphic rocks.
• Argon / Argon dating Dates the time since the
  rock was molten, Useful for dating rocks older
  than 2000 years. Used on volcanic flows at
  Pompeii. Method works on same types of rocks as
  Potassium / Argon dating
• Uranium /Lead dating Not used for archaeology,
  Dates the formation of the rock. Useful for rocks
  older than 1 million year to 4.5 billion years.
  Parts of the series are useful to archaeologists.
• Uranium disequilibrium dating Dates the amount
  of soluble uranium the porous material has
  absorbed. Useful for material buried from 1 to
  400,000 or 500,000 years. Method works on porous
  material like bone or eggshell. This method is
  not technically a radiometric dating method, but
  it uses a radioisotope, so it is included here.
• Lead 210 dating Dates the amount of time since
  the sediment was deposited. Primarily used for
  ecology, but can be used to date sediments for
  archaeology.

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• Concepts for Trapped-Charge Dating
• All natural crystalline minerals contain
  Imperfections with the crystal lattice. For
  example, quartz crystals.
• Natural radiation exists in all sediments from
  the radioisotopes contained in the sediment.
• Radiation frees electron from atoms within the
  crystal lattice and these electron become trapped
  in the areas of imperfection.
• The longer that an crystal has been exposed to
  radiation, and not exposed to heat (over about
  350 degrees C) or light, the greater number of
  trapped electron will exist in the crystal.
• Exposure to light or heat mobilizes these
  electron and allows them to either leave the
  crystal entirely or become captured by another
  atom reducing to zero the trapped charges in the
  crystal.
• When the electron becomes free to move from its
  trapped position, it releases a photon, which,
  conceptually is a tiny amount of light.
• This tiny bit of light can be multiplied and
  quantified on very sensitive equipment.
• The quantity of radiation in the sediment around
  the crystal must be known for the radiation dose
  over time to be assessed.

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• Optically Stimulated and Thermo Luminescence (OSL
  and TL) Dating
• Optically Stimulated Luminescence can be
  conducted on quartz or feldspar crystals. Both
  are very common in sand and generally in the
  crust of the earth.
• On quartz, the crystal is exposed to blue or
  green light and the luminescence is measured in
  the near ultraviolet.
• On feldspar, the crystal is exposed to near
  infrared light and the luminescence is measured
  in violet light.
• Thermo luminescence can be conducted on heated
  flint or chert, or other quartz rich rocks,
  calcite from caves, quartz or feldspars in
  sediments that have been buried. But the sample
  is heated to release the photons.
• Both methods require that some event has set the
  electron clock to zero. This can be exposure to
  either heat or intense sunlight .
• For archaeological dating both methods can be
  applied to heated flint knapping stone, buried
  pottery with a sand matrix or can be applied to
  sediments in the archaeological context for a
  bracketing date.
• Samples for dating can not be exposed to heat or
  light before being submitted for dating.
• Thermo Luminescence on a single grain of crystal
  (selected from a large sample by the dating lab)
  is the most accurate of these methods.

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• Trapped charge dating review
• Optically stimulated and Thermo luminescence
  dating are very similar techniques. Optically
  stimulated luminescence uses light and Thermo
  luminescence uses heat to release the photons.
• Both methods require that the item dated has not
  been exposed to heat or sunlight since the object
  was used by people.
• Either method is used on quartz or feldspar
  crystal which are common in sand, flint knapping
  stone, and as matrix in pottery, as well as
  bracket dating sediments for archaeological
  contexts.
• Artifacts found in caves are ideal candidates for
  both methods.

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• The Big Concepts
• Absolute dating places the culture or cultural
  event in calendar time rather than in a floating
  chronology.
• There are many methods of placing archaeological
  materials in time, all of them date different
  events, and all of them have limitations and
  assumptions that must be taken into account when
  considering the results that you receive from the
  lab.
• All absolute dates must be evaluated for
  feasibility and appropriateness for your
  particular situation.
• There are many more methods of obtaining absolute
  dates than have been presented here, but these
  are the main methods that are commonly used in
  archaeology.

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Questions/ Discussion ?