Electron Spin Resonance Dating

The Electron Spin Resonance (ESR) laboratory, one of the four geochronology laboratories at CENIEH, is mainly devoted to dating geological and/or archaeological materials such as tooth enamel or optically bleached quartz grains extracted from sediment. Occasionally, it also carries out work in dosimetry and characterization of modern or old geo-materials.

Although ESR spectroscopy gives the name to the dating method, it is only used to calculate the equivalent dose (DE), one of the two parameters necessary to calculate the final date. The ESR-based dosimetric reconstruction process is key to obtaining the second parameter: the dose rate (D). The dating protocol necessary to determine these parameters (DE and D) is divided into several stages:

  • Field sample collection
  • Physical and chemical preparation of the sample
  • Irradiation
  • ESR data acquisition
  • ESR data analysis to determine the ED.

The ESR laboratory at CENIEH has fully equipped independent areas in order to implement these stages and provide a final report on the results.

Methods

ESR dating is classified within the group of paleodosimetric methods, which are based on the detection and quantification of the defects generated and accumulated as a result of natural radioactivity. The sample is considered as a dosimeter which can register and, subsequently, restore the dose absorbed during its exposure to natural radioactivity. This ionizing radiation (gamma and cosmic rays, alpha and beta particles) leads to movements within the electronic structure of minerals, and some electrical charges become trapped in the crystal defects of the mineral, forming an entity known as paramagnetic center. These centers produce a signal that can be detected through ESR spectrometry.

The amount of trapped charges (equivalent dose; DE) is directly related to the dose of radiation absorbed by the sample which, in turn, depends on the intensity of the radiation (dose rate; D) and on the duration (time; T) of the exposure to radioactivity. In the case of a dose rate that is constant over time, the ESR age (T) is calculated by the following equation:

Fórmula

DE expressed in Gray (Gy) and D expressed in µGy/a or Gy/ka

Facilities and equipment

The ESR laboratory at CENIEH is formed by 6 independent working areas:

Field material area Equipo 1

Area designed for the storage of all field material necessary for sampling: PVC tubes, Minigrip® Anti UV opaque bags, hammer, GPS, tape measures and two Canberra portable gamma spectrometers: an Inspector1000 multichannel analyzer connected to a NaI(Tl) probe and an Inspector1000 multichannel analyzer connected to a LaBr3(Ce) probe

Sample storage area

Area designed for the storage of the samples collected on the field and of finalized samples for possible future projects.

Sample preparation area

Area dedicated to the preparation of samples where work is carried out under controlled lighting conditions.

The physical preparation area has all the necessary material (saws, dentist's drills, agate mortars, sieves, mills, hot plates, scales, magnetic separator, etc.) for the optimal treatment of the samples prior to chemical preparation.

The chemical preparation area has fume hoods along with all other laboratory material necessary for acid attacks or the separation of minerals through dense liquids.

Gamma irradiation area

Area dedicated to gamma irradiation through a Gammacell-1000 irradiator with a 137Cs source (dose rate ~ 8 Gy/min) (Best Theratronics). This area is part of the IRA 3015 Radioactive Facility.

Equipo 2

Environmental radioactivity evaluation area

Area dedicated to the evaluation and quantification of the natural radioactivity (gamma and cosmic rays, alpha and beta particles) present in the samples using two coaxial (well) and XtRa Germanio HpGe Canberra detectors.

ESR spectrometry area

Area dedicated to Electron Spin Resonance measurements, based on which it is possible to study the evolution of the ESR signal and build a growth curve. The room's temperature is controlled to ensure the most stable experimental conditions as possible, a fundamental requirement for any work involving ESR spectrometry. For that purpose, we have two Electron Spin Resonance Spectrometers:

  • Bruker EMXmicro-6/1 (X-band), which is equipped to work either at room temperature (300 K) or at liquid nitrogen temperature (77 K)
Equipo 6
  • Bruker Elexsys E500 (Q-band and X-band), which is equipped to work either at room temperature (300 K) or at liquid helium temperature (4K)

Equipo 5

We also have a Uvacube400 insolation unit (Dr HÖNLE) in order to bleach the samples.

Equipo 3

 

Applications and Services

Applications

  • Dating of geological/archaeological materials, encompassing the entire Quaternary
  • Dosimetric reconstructions of modern and old materials
  • Study of the damage and effect of radiations
  • Reaction kinetics, polymerization, spin trapping, etc

Services

  • Field sample collection
  • Dating of teeth
  • Dating of quartz
  • Gamma irradiation of materials
  • Evaluation and quantification of the natural radioactivity in sediments, dental tissue, waters, etc.