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Electron Spin Resonance (ESR) dating

1. Introduction

The Electron spin resonance (ESR) or Electron paramagnetic resonance (EPR) dating method belongs to the group of radiation-induced dating methods (e.g. luminescence). Depending on the kind of mineral (predominately aragonite, carbonate and quartz) this method is used to date geological events such as the time of mineralisation and exposure to pressure (for example: volcanic events and fault gauge) or the burial time of sedimentation. 

2. Natural Radiation

As in luminescence, the mineral operates as a dosimeter by storing the rate of ubiquitous, natural radiation (a-, ß- and ?-rays from 238U, 232Th-series disintegration and 40K in soils and cosmic rays). The differences to TL/OSL are the kind of defects that can be detected, the detection method (microwave absorption spectroscopy) and the signal-intensity resulting from microwave absorption that is represented in a typical ESR-spectrum.

Source: after Wagner, G.A. (1995)

3. Electron Spins

Radioactive irradiation ionises paired electrons in the mineral and separates them from each other. The now unpaired and free electron can be trapped in a lattice defect (e.g. an impurity-defect such as Titan in natural alpha-quartz) and form a paramagnetic centre. In classical physics, an electron can be regarded as a negatively charged, rotating sphere. The circular current produces a magnetic moment in the vicinity of this electron. While two paired electrons neutralise their magnetic moments, the unpaired and trapped electron creates a paramagnetic moment. Paramagnetic since it tends to align in the direction of a magnetic field that can be externally installed.

vergrößern: Methodik_ESR_Beispielspektrum
Example of a typical ESR-Spectrum in quartz, representing the Ti-centre with its two denoted subcentres Ti-Li and Ti-H. The middle absorption line represents an overlap of the two subcentres. (X-axis: magnetic field in gauss; Y-axis: ESR-signal intensity). The g-value gives information on the electronic structure of the paramagnetic centre.

4. Electron Spin Resonance

The unpaired electrons in a mineral aligns either parallel or anti-parallel to the external static magnetic field. When a microwave at the same frequency as the circular current of the electrons is induced perpendicular to the sample, resonance, which causes the spins to flip, is generated. The energy that is spent during this process can be detected with the ESR-Spectrometer and converted in an ESR-spectrum.

5. Dosimetry

An ESR-age is a function of the radiation rate and the atomic lattice defects which have been produced by radiation over time and in which unpaired 'free' electrons are trapped. The ESR-signal intensity is proportional to the concentration of trapped electrons in the mineral. But though this defect concentration does not yet tell anything about the age unless it is put into relation to the radiation does. Thus, we need two parameters to calculate a ESR-age. The equivalent dose (derived from the ESR-signal) and the annual dose rate (derived from cosmic rays and radionuclides in the vicinity of the sample). The equivalent dose is calibrated by construction so-called growth curves. The sample is artificially irradiated and is thus made artificially older. From the ESR-signal intensities derived from these alquots, a relation into the past can be made by either extrapolation (additive method) or by intrapolation (regenerative method) of the growth curve on the x-axis. The dose rate, on the other hand, is determined by ICP-MS analysis and gamma-spectrometry.

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6. ESR-Spectrometer

The ESR-spectrometer quantifies the number of trapped electrons in a mineral and transfers the concentration into a ESR-signal. At the Geography Department of the University of Cologne, two ESR-Spectrometers are at use: the Bruker ELEXYS500-Spectrometer and the Bruker ESP300-E X-band-Spectrometer. Both allow low temperature measurements for the ESR-dating of sedimentary quartz. In addition, a high-sensitivity-cavity can be used next to the regular one for the new ELEXSYS500-Spectrometer. With Xepr, this spectrometer provides the prerequisites of innovative Spectrometer control, data acquisition and data processing.