Calibration Tests: Olduvai GorgeMuch has been done in the quest to discover more about the earliest humans, when they lived, and how they lived. To this end, many techniques, such as carbon dating, have been employed to obtain some idea about the relative ages of tissue remains found in archaeological sites. One such technique is called electron spin resonance (ESR) dating.
The principle of ESR dating is that radiation damage occurs in minerals as a result of uranium uptake, and external effects. This damage is usually repaired in living tissue, but in dead tissue it accumulates. If the method of uptake can be judged, then the approximate age of the tissue can be deduced from the extent of the radiation damage.
ESR dating is done primarily on teeth and since human teeth are currently considered too small, in particular the teeth of large animals, with the expectation that information about these animals would indirectly give information about the humans who lived at the same time.
In the Williams College laboratory run by Dr. Anne Skinner, experiments have been done to check the accuracy of ESR dating in tissue from older sites, sites that are approximately one million years old, and older. In order for this to be done, the teeth used were from a site that had already been dated using other methods: Olduvai Gorge in east Africa, the best-dated early hominid site. The calibration is needed because ESR dating is also currently being done on teeth from other African sites: Makapansgat, Sterkfontein, and Swartkrans, for which there is no comparison absolute dating method available.
Other projects include:
Calibration Tests: Olduvai Gorge
Central Europe Hominid and Mammal Sites
Technology
of Early Peoples: ESR examination of heated flint artifacts
The Robert F. Kennedy High School Summer Science Research Institute
ESR Spectroscopy
Due to its spin, each electron in a crystal lattice generates a small current, and hence, a small magnetic field associated with it. Normally, each magnetic moment for a given electron is cancelled out by a reversely spinning electron, but paramagnetic (EPR and ESR) centers have permanent magnetic moments due to an unpaired electron.
If a free electron encounters an external magnetic field, H, the external field splits the initial energy level for the electron's magnetic moment into two discrete energy levels. Transferring an electron from the lower to the higher energy level can be induced by adding energy. The difference in energy between the states is given by:
ΔE = g µB H = h ν (1)
where:
ΔE = the energy difference between the two levels,
g = the LandŽ factor, the g value,
µB = the Bohr magneton = 9.27314 x 10-21 erg/Gauss
H = the external field strength,
h = Planck's constant, 6.62554 x 10-27 erg/sec,
ν = the frequency of the energy.
To transfer electrons from the lower to the higher energy requires energy absorption at the frequency, ?.
Radiation Induced Centers
Radiation induced centers are created during sample irradiation with high-energy particles (e.g. α, β, or γ particles, and cosmic rays). Irradiation, natural or artificial, excites electrons from the valence band of a mineral to metastable sites in semi-conductance or conductance bands. Electron holes are positively charged deficient sites left by electron excitation. Both the excited electron and the hole can be paramagnetic; however, they will have different values of g, the LandŽ factor.
Electron Spin Resonance as a Dating Technique
Electron spin resonance (ESR) was proposed as a dating method by Zeller as early as 1967, but its practical application began with the work of Ikeya in 1975. Since then, there have been substantial contributions based on carbonate materials, bones, and quartz. GrŸn and Blackwell have written good general reviews of the subject. The LandŽ factor, g, is extremely sensitive to the electronic environment, and therefore, ESR has been used for archaeological provenance studies as well as for dating. For dating purposes, the unpaired electrons of interest are those created as a result of radiation damage, which implies that ESR has many similarities as a dating technique to thermoluminescence (TL). It has some advantages over TL, primarily in the fact that the signal is not destroyed during measurement, and so it is easier to study a given sample under a variety of experimental conditions. However ESR is at least in some cases less sensitive that TL; so far, for instance, it is not a practical method for dating pottery.
Any object is subject to radiation, but not all forms of radiation damage are sufficiently stable to serve as chronological markers. The general approach to establishing ESR as a dating tool has been to examine materials of interest to archaeologists and geologists to detect ESR signal related to radiation damage. If such a signal exists, it must meet the following criteria in order to be usable for dating studies:
Assuming that the material has been judged suitable for ESR dating, aliquots of the material are irradiated artificially and a growth curve of the ESR intensity is extrapolated to zero. This yields De ("equivalent dose" in Gy), or the amount of radiation (usually in gamma-equivalent form) needed to produce the observed natural signal. From De the age of the material in years can be found by dividing by the sum of the known radiation doses both external (Dext in Gy/yr) and internal (Dint in Gy/yr) to the material, as shown in the relationship:
Age = De / (Dext + Dint) (2)
This of course requires that samples of the matrix in which an artifact was found be available for isotopic analysis. It also implies that the dose rate has been constant with time, which is not generally true. For most applications, Dext and Dint are replaced with integrals of a time-dependent dose rate over time.
References
South African Hominid Sites
Non-destructive Techniques