Luminescence Dating Laboratory

What is Luminescence Dating?

Luminescence dating is a form of geochronology that measures the energy of photons being released. In natural settings, ionizing radiation (U, Th, Rb, & K ) is absorbed and stored by sediments in the crystal lattice. This stored radiation dose can be evicted with stimulation and released as luminescence. The calculated age is the time since the last exposure to sunlight or intense heat. The sunlight bleaches away the luminescence signal and resets the time 'clock'. As time passes, the luminescence signal increases through exposure to the ionizing radiation and cosmic rays. Luminescence dating is based on quantifying both the radiation dose received by a sample since its zeroing event, and the dose rate which it has experienced during the accumulation period (See the Luminescence Age Equation). The principal minerals used in luminescence dating are quartz and potassium feldspar.

Types of Luminescence Dating Techniques

• Thermal (TL)
• Optically Stimulated (OSL)
  • Green Light (GSL) - Feldspar & Quartz
  • Blue Light (BSL) - Quartz
  • Red Light (RSL) - Volcanic Feldspar & Quartz
• Infrared Stimulated (IRSL) (IRSL)
  • post-infrared infrared Stimulated (pIRIR) – Feldspar

Luminescence Age Equation, Equivalent Dose, and Dose Rate

Controlling Assumptions - TL and OSL Method

• Complete Zeroing - Exposure to sunlight or intense heat (>150°C) for an extended period of time
• Stable Luminescence Present - Plateau Test (TL only)
• Accurate Determination of the Dose Rate (D_R) and Past Moisture Content

The Luminescence Age Equation:  Age = D_E / D_R

D_E is measured in grays (absorbed dose) and commonly known as the Equivalent Dose or paleodose

D_R is measured in grays/ka and commonly known as the Dose Rate. Comprised of K , U, Th, Rb, and cosmic ray components

Determination of Equivalent Dose (D_E)

• Additive Dose - usually multiple aliquots
• Partial Bleach - assumes a mix of grains with varying partial bleaches
• Total Bleach - assumes complete zero
• Regeneration - usually single aliquot or more rarely single grain

Determination of Dose Rate (D_R)

• Neutron Activitation - U, Th, K , Rb
• γ Spectrometry - U, Th, K (Lab & Field)
• Atomic Absorption - K , Rb
• Alpha Counting - ZnS scintillation user for α
• Inductively Coupled Plasma - Mass Spectrometry (ICP-MS)
• Flame Photometry - K

Contributions to Sediments

The application of luminescence to dating archaeological or geological materials relies on determining two quantities. The first is the amount of radiation absorbed by the sample during the period since the event being dated, measured as D_e. To determine the age of the sample in years, De has to be divided by the radiation dose received by the sample each year – the dose rate.

There are four types of environmental radiation: alpha particles (α), beta particles (β), gamma rays (γ) and cosmic rays. The first three originate from naturally occurring elements in the sample itself and its surroundings. The most important of these sources are radioactive isotopes of uranium (U), thorium (Th) and potassium (K).

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Once the concentrations of these three elements are known, conversion factors enable the calculation of the radiation dose rate (Adamiec and Aitken 1998). For example, 1% potassium in sediment will produce a gamma radiation dose rate of 0.243Gy per thousand years (Gy/ka), a beta dose rate of 0.782Gy/ka, but no alpha dose rate, as the decay of 40K does not result in the emission of alpha particles. Adding together the alpha, beta and gamma dose rates gives the total radiation dose rate.

Sampling

Excavation Process

• Excavate Back ∼20 cm
• Expose a fresh face
• Soft Sediment - auger/push in PVC or aluminum tubes
• Hard Sediment - carve out block
• Moisture and light tight, black cloth cover during collection

Sample Amount Requirements

• Need 100 mg silt (4-11 microns) and 1 gram of fine sand size (90-125 microns or dominant grain size) for OSL (corresponds to 10-100 grams initial sample)
• Usually an extra bulk sample is needed (about 400-600 grams) for moisture