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Archaeology > Unit 5: Unpaired Electrons Dating Methods > Flashcards

Unit 5: Unpaired Electrons Dating Methods Flashcards

1
Q

What is ESR?

Ionising

A
  • Electron spin resonance (ESR)
  • A repeatable dating method that measures the amount of unpaired electrons in the artefact that have been created over time by ionising radiation.
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2
Q

What is TL?

Ionising

A
  • Thermoluminescence (TL)
  • A non-repeatable dating method that depends on the presence of unpaired electrons in the artefact that have been created over time by ionising radiation.
  • Artefacts must be electrical insulators so it can’t date metals.
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3
Q

The concentration of unpaired…

A

The concentration of unpaired electrons increases with time

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4
Q

Potassium-40

A
  • Potassium-40 is a radioactive isotope of potassium which has a very long half-life.
  • It is a beta emitter.
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5
Q

Thorium-232

A

The isotope decays by alpha decay

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6
Q

Uranium-238

A
  • Decays by alpha decay to daughter isotopes
  • Extensively used in radiometric dating, particularly for material older than ~ 1 million years.
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7
Q

ESR and Tl dating range

A

Age range 40,000 - 1,000,000 years is often quoted

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8
Q

After about 1 million years…

trapping

A
  • After about 1 million years, the population of unpaired electrons is maximum.
  • All the available electron trapping sites are occupied.
  • This is called saturation.
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9
Q

ESR and Tl minimum age

Detection & Interfering

A

The minimum age depends on spectrometer detection limit, and absence of interfering signals.

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10
Q

What can TL and ESR be used to study?

A

Can study “inorganic” samples, lacking carbon.

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11
Q

ESR and Tl dating applications

ate

A
  • Bones
  • Teeths
  • Shells
  • Carbonate deposits
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12
Q

The intensity of the ESR spectrum measures…

A

The intensity of the ESR spectrum measures the concentration of unpaired electrons

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13
Q

Thermoluminescence dating

Heating, trapped

A
  • Heating sample discharges the trapped unpaired electrons
  • Heating gives electrons the activation energy to escape their trap sites. They become mobile and recombine with ions
  • Energy stored in the solid is released as light
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14
Q

The intensity of light emitted measures

In thermoluminescence

A

The intensity of light emitted measures the concentration of unpaired electrons

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15
Q

Benefits of ESR and TL?

A
  • Study solid samples as it is
  • Non-destructive
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16
Q

TL measurement

Discharges

A

TL measurement discharges the unpaired electrons so the dating measurement can be done only once

17
Q

ESR measurements

Irradiation

A
  • ESR does not discharge the unpaired electrons
  • The dating measurement can be repeated
  • Calibration may involve extra irradiation, changing the number of unpaired electrons
18
Q

ESR methodology

Magnetic field, MR

A
  • Samples are placed in a magnetic field. The unpaired electrons now have different energies/absorption frequencies, depending on whether their spins are “up” or “down”
  • Sample absorbs microwave radiation that flip spins from low energy to high energy state
19
Q

An ESR spectrum is recorded by…

Magnetic, absorption

A
  • An ESR spectrum is recorded by scanning the magnetic field and measuring the absorption of microwave radiation at a fixed frequency.
  • The ESR spectrum provides an absorption peak and its total area provides the quantitative intensity that can be used for dating.
20
Q

Rad

A
  • Rad is the Radiation Absorbed Dose
21
Q

1 RAD =

A

1 Rad = 0.01 Joules absorbed in 1 kg of material

22
Q

TL dating method

Heating, activation energy

A
  • Heating sample discharges trapped electrons by providing the activation energy to escape their metastable trap sites by recombination with ions.
  • TL discharges electrons so can only be done once
  • Energy stored in the solid is released as light.
23
Q

TL detectors measures…

A

TL detectors measures intensity giving electron concentration

24
Q

Benefits of ESR

A
  • The ESR measurements do not discharge unpaired electron so can be remeasured
  • No need to chemically characterise samples for dating purposes

Archaeology (15 decks)

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