Nondestructive ESR Dating

Non-destructive Electron Spin Resonance Dating

The aim of this branch of ESR research in the Williams College lab is to find answers to two related questions, namely, can teeth be tested without being destroyed, and can human teeth be used in ESR testing. Currently, teeth being dated using ESR are crushed to form a powder before being irradiated. This destructive method means that precious hominid teeth found at archeological sites are unable to be dated. Added to this is the fact that human teeth are currently considered too small, because the yield of enamel is not very large. If it is proven that both that teeth no longer need to be destroyed, and that human teeth can be successfully dated, then the scope of ESR dating will be greatly improved. It will no longer depend solely on the indirect method of dating human hominid sites by dating the animal remains found at those sites, but rather on the more immediate process of dating the hominid remains themselves.

Alan Velander, a Williams College student, began the study in June 1999.

ABSTRACT

To determine the age of an entire fossilized tooth nondestructively using electron spin resonance (ESR) spectrometry, we designed three tooth-holding apparatuses. Each apparatus aimed to position a tooth within the cavity of a JEOL JES-REIX ESR spectrometer, allowing signal reproduction at every angle at an appropriate height. Relatively large diameters disrupted the microwave signal, allowing only the tooth's centered tip to enter the cavity when fitted below on a narrow mount held by the apparatus. Ultimately the apparatus needed a micrometer and a goniometer attached directly to the cavity to reposition the tooth. The tooth signal then displayed reproducibility and growth with subsequent irradiation. Growth curves at many angles of a tooth, coupled with neutron activation analysis (NAA) of dentine and measurement of enamel thickness with a computerized tomography (CT) scan microscope should allow nondestructive ESR dating of teeth.

Figure 1: Created by ionising radiation, point defects accumulate predictably in enamel. Imperfections in the crystal lattice exist between the valence and conduction bands, trapping free radicals.

Figure 2: A free radical occupies one magnetic spin number, +1/2 or -1/2. The static magnetic field, H0 increases the energetic difference between those dipoles. At some frequency of microwave energy, the electron resonates and a magnetic dipole transition occurs.

INTRODUCTION

The use of ESR spectrometry to date fossilized teeth is effective and codified but severely destructive. For each sample, the enamel and dentine must be separated entirely, measured and powdered. Understandably, archaeologists refuse to expose rare fossils to such treatment. However, ESR could date many significant samples, i.e. early hominid teeth, which other techniques (¹⁴C, U-series, ⁴⁰Ar/³⁹Ar, TL) cannot. Clearly, the challenge is to develop a nondestructive method of ESR dating, that is, to date an entire tooth-an obvious and promising possibility with manifold difficulties:

The enamel and dentine are inseparable. Research by Min showed that the dentine signal merely dilutes the enamel signal. We will use a CT scan microscope to calculate the enamel/dentine ratio and compensate.
A tooth is irregular and asymmetrical. We found that tooth volume and orientation within the cavity affect the signal. We made each apparatus to reposition the tooth, recording peak heights at numerous heights and orientations and expanding the research of Grün.
The cavity is not designed to hold a tooth. We designed three apparatuses to raise the tooth into the cavity exclusively while measuring and locking its radial and vertical positions reproducibly.

Figure 3: The cavity of an ESR spectrometer contains uniform magnetic and microwave fields that initiate magnetic dipole transitions. The resulting signal is unique to enamel and gauged by its first-derivative peak height.

PROCEDURE AND RESULTS

After irradiation, a powdered sample requires three days of annealing to dispel short-lived ESR signals. We suspected that the strength of these signals was a function of surface area to volume, comparatively minuscule for an entire tooth. The unannealed and annealed peak heights showed no trends and their errors feel within our necessarily low standards. We concluded the ephemeral signals as negligible, saving precious time.

TABLE 1.1 ANNEAL NO MORE

99006-8 krads run

Orientation	Unannealed	Annealed
315°	3670	3976
315°	3954	3922
0°	4342	3288
0°	4574	3282

Figure 5: We constructed growth curves for both 0° and 315°. The polynomial regression for each orientation agreed nicely with the actual data, as evidenced by high R-values. The extrapolated accumulated doses (5.6516 and 5.6929 krads) also concurred closely. The actual dose, admittedly, was 8.702 krads, but that figure is within one standard error of both predicted doses. Considering the plentiful sources of instability and irreproducibility, coupled with only four data (most curves use a dozen), the findings were promising.

Figure 4: A graph of peak heights for sub-samples of increasing irradiation make a growth curve to extrapolate the accumulated dose (the number of point defects). NAA and other geological considerations determine the dose rate. We may then calculate the tooth's age by using: AD (t) = ò(dose rate) dt

We also found that any large object near the 1-cm diameter resonant cavity disrupts the electromagnetic fields. This condition has no exception-with or without an ESR signal, solid or hollow, large objects interfere destructively with the fields, making it impossible to tune and lock a signal. The restriction also applies to a tooth, allowing only its very tip to enter the cavity.

We had hoped that as a substitute for NAA we could assume a 5% ratio of enamel to dentine uranium. Unfortunately, a statistical survey of this assumption disproved it handily and showed little predictability to the ratio. We propose a refined NAA procedure, demanding only a small dentine splinter obtained by laser oblation of the tooth.

To our dismay, we cannot locate a CT scan microscope. Should anyone have a lead, please contact us.

APPARATUS ONE

Materials: Nonmagnetic-Teflon, aluminum, wood

Method:

Insert the mount, turn to first orientation.
Raise tooth slowly, turning continuously to keep signal, lock height.
Tune, run, record peak height.
Lower tooth, turn to second orientation.
Raise tooth slowly, tuning continuously, to same height.
Tune, run, record peak height.
Repeat method until two peak heights for each orientation agree with 10% error.

Flaws:

Apparatus not parallel with cavity, tooth not centered.
Rushing water hoses made signal oscillate.
Three stabilizing feet sunk into spongy floor.
Base difficult to reposition.

Thin, long, cylindrical, nonmagnetic-a toothpick did not disturb the resonant cavity and proved an expedient mount. We drilled a sized hole in each tooth and fit it on such a skewer, marking its position. Ultimately we cannot use a toothpick, but here it performed admirably.

APPARATUS TWO

Materials: nonmagnetic-aluminum

Method:

Place tooth on mount, tape securely at bottom, insert mount.
Attach to cavity, turn to first orientation, raise tooth slowly, tuning continuously to keep signal, fix height.
Tune, run, record peak height-repeat three or four times.
Do not lower tooth, remove apparatus carefully, measure height with caliper exactly.
Turn to second orientation, reproduce height exactly using caliper.
Attach to cavity, tune, run, record peak height-repeat three or four times.

Flaws:

Caliper use onerous.
Measurement still insufficiently precise.

Using a caliper to fix the height, we reproduced signals exactly. The slightest difference (>0.1mm) in height and orientation, however, changed the signal noticeably. We need a yet more precise and adjustable apparatus.

PLAN OF THIRD APPARATUS

Figure 6: We will make out third apparatus to provide absolute stability and precision. It will resemble the second apparatus except for an integrated micrometer. The outer cylinder will possess the same docking mechanism. The inner cylinder will consist of three parts. The lowest component will be the micrometer stalk, its end threaded to screw into the next part. This second part will attach to the outer cylinder by two small screws. The third weighted part will have a centred hole on top to accommodate an improved mount and a larger hole beneath where the end of the micrometer stalk will sit freely, adjusting the sample height while still allowing free radical movement. When built it will allow reproducible changes of 0.01mm in height and one degree in orientation.

Figure 7: We have design a truly non-destructive mount. Six tiny aluminum screws will hold the tooth securely. Two thin aluminum rings will hold these screws, connected by plastic netting to another ring and an aluminum stalk below. The tooth will not leave the mount until it is dated completely. Hopefully its fine construction (diameter < 1cm) will not disrupt the resonant cavity.

CONCLUSIONS

Any relatively large diameter disrupted the electromagnetic fields of the resonant cavity.
We isolated enamel ESR signals from the tips of entire teeth at mild parameters.
Annealing the teeth after irradiation was unnecessary because the short-lived ESR signal was surface area dependent and thus negligible in whole teeth.
The stability and precision of each apparatus was paramount to achieving accurate, reproducible results. Vertical changes of 0.1mm and radial changes of a few degrees from the original yielded significantly different intensities.
Growth curves of peak heights at different orientations showed consistency, regular growth, and accuracy within one standard error, indicating the viability of nondestructive ESR dating.

ACTUAL THIRD APPARATUS (July 2000)

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