Unit 5: Unpaired Electrons Dating Methods Flashcards
What is ESR?
Ionising
- 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.
What is TL?
Ionising
- 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.
The concentration of unpaired…
The concentration of unpaired electrons increases with time
Potassium-40
- Potassium-40 is a radioactive isotope of potassium which has a very long half-life.
- It is a beta emitter.
Thorium-232
The isotope decays by alpha decay
Uranium-238
- Decays by alpha decay to daughter isotopes
- Extensively used in radiometric dating, particularly for material older than ~ 1 million years.
ESR and Tl dating range
Age range 40,000 - 1,000,000 years is often quoted
After about 1 million years…
trapping
- After about 1 million years, the population of unpaired electrons is maximum.
- All the available electron trapping sites are occupied.
- This is called saturation.
ESR and Tl minimum age
Detection & Interfering
The minimum age depends on spectrometer detection limit, and absence of interfering signals.
What can TL and ESR be used to study?
Can study “inorganic” samples, lacking carbon.
ESR and Tl dating applications
ate
- Bones
- Teeths
- Shells
- Carbonate deposits
The intensity of the ESR spectrum measures…
The intensity of the ESR spectrum measures the concentration of unpaired electrons
Thermoluminescence dating
Heating, trapped
- 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
The intensity of light emitted measures
In thermoluminescence
The intensity of light emitted measures the concentration of unpaired electrons
Benefits of ESR and TL?
- Study solid samples as it is
- Non-destructive
TL measurement
Discharges
TL measurement discharges the unpaired electrons so the dating measurement can be done only once
ESR measurements
Irradiation
- ESR does not discharge the unpaired electrons
- The dating measurement can be repeated
- Calibration may involve extra irradiation, changing the number of unpaired electrons
ESR methodology
Magnetic field, MR
- 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
An ESR spectrum is recorded by…
Magnetic, absorption
- 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.
Rad
- Rad is the Radiation Absorbed Dose
1 RAD =
1 Rad = 0.01 Joules absorbed in 1 kg of material
TL dating method
Heating, activation energy
- 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.
TL detectors measures…
TL detectors measures intensity giving electron concentration
Benefits of ESR
- The ESR measurements do not discharge unpaired electron so can be remeasured
- No need to chemically characterise samples for dating purposes
