ESR Dating, Dosimetry and Microscopy for Terrestrial and Planetary Materials

BY M. IKEYA

1 Introduction

Historical developments of ESR dating, dosimetry and microscopy in the 20th century are described in reviews and the book updated in 2002. New works in early 21st century can be seen in recent and forthcoming proceedings. This report describes a brief introduction of the quantitative use of ESR in earth and planetary science, especially in dating, aimed in part at those who are not familiar with these applications and discuss new prospects and development of these interdisciplinary fields in the 21st century.

Radiation dose can be determined from the electron spin resonance (ESR) signal intensity of radiation-induced paramagnetic radicals or lattice defects. Similar paramagnetic species produced by natural radiation have accumulated in archaeological and geological materials. Radiation dosimetry of these objects has been used to determine ages in chronology science. ESR is a method of spectroscopy conducted by sweeping the magnetic field and dealing with the interaction of spins through the shift and splitting of the signal as a function of magnetic field. Dosimetry and dating with ESR are concerned with the quantitative measurement of the signal intensity, which was not reliable at an early stage of instrumentation, because of the poor S/N ratio due to the drift and instability of the microwave oscillator and vacuum-tube amplifier systems.

Dating with ESR by detecting radicals was proposed and attempted for coals and old geological materials through calibration of the signal intensity by additive artificial irradiation. However, no meaningful age was obtained since the age of samples were beyond the thermal lifetime of radicals in nature and too old to demonstrate the dating feasibility to geologists and archaeologists as potential users.

Crystallization ages of carbonate stalactites in the Akiyoshi limestone cave, Japan and Petralona cave, Greece have been successfully dated with ESR. The signal intensity of CO2- generated by natural radiation is essentially zero at the surface of the growing speleothems, while the older inner portion showed gradually more intense signals. The possibility of ESR dating has thus been proved by the 'stratigraphic' evidence and accepted by geologists involved in Quaternary era and by paleo-anthropologists excavating Petralona and Arago (Tautavel) caves inhabited by ancient homonids.

Biological materials of shells, corals, and microfossil foraminifela were dated in addition to fossil bones and teeth in paleo-anthropology. A field-swept electron spin echo (ESE) spectrum in pulse ESR was applied to remove the interfering signals of paramagnetic impurities such as Mn2+ and Fe3+ common in geological materials. Dating of some geological events was carried out for geothermal and volcanic materials and geological fault gouge using signals in SiO2. Evaporates (gypsum CaSO4. 2H2O and nahcolite NaHCO3) in deserts were also dated, suggesting the future use of ESR on a rover in Mars' surveys. Radiolysis of ice, solid CO2, SO2 and CH4 has been studied for future ESR dating of outer planets and their satellites, as well as for environmental studies of Antarctic ice cores.

Dosimetry of A-bomb radiation to survivors has been achieved using their tooth enamel and shell buttons and extended to accident dosimetry of residents near the Chernobyl reactor and of personnel in the former Soviet Union countries. Radiation dosimetry using the ESR signal of irradiated alanine amino acid and sugar has been developed and the former became a standard in IAEA transfer dosimeter. However, the sensitivity was not sufficient to replace conventional dosimeters. High-sensitive dosimeter materials with a tissue equivalent response to radiation energy are being investigated and a low-cost and light-weight reliable spectrometer has been fabricated using a permanent magnet of Nd-B-Fe alloys. Irradiated foodstuffs have also been monitored with ESR.

ESR specialists have developed L-band imaging systems largely for biomedical use. However, attempts to image the distribution of unpaired electrons in minerals and fossils were made at X-band frequency by ESR microscopy using small-field gradient coils or scanning the localized modulation or magnetic fields. Localized microwaves using a cavity with an aperture, which is a near-field ESR microscope, has been scanned by moving a fat sample over the aperture to get the image of spin concentration. Distribution of dangling bonds in CVD and diamond films and porous silicon have also been imaged for electronics engineers.

2 Principle and General Scope of ESR Dating and Dosimetry

2.1.1 Principle and Procedures of ESR Dating: Natural Radiation Effect.

2.1.2 Principle of ESR Dosimetry and Dating. Ionizing radiation generates electron and holes in the materials leaving radicals or defects. The trapped electron and hole centres, as well as Frenkel defect pair of a vacancy and an interstitial, are sometimes paramagnetic (as radicals) and give rise to ESR signals. The signal intensity of these radiation-induced species is proportional to the spin concentration and so related to the radiation dose, mostly linearly at the initial stage. Radiation dose of some chemicals is thus used as an element of an ESR radiation dosimeter, while tooth enamel and other samples are used for retrospective dose evaluation of A-bomb, reactor accident, and personnel exposure.

Natural radiation, α-, β- and γ-rays from radioactive elements in the environment or intrinsic to materials, ionizes the material and produces paramagnetic defects or radicals. They are often quite stable and accumulate with time. The ESR signal intensity is proportional to the total dose of natural radiation, i.e., to the product between the annual radiation dose rate and the time elapsed after their formation or an event which zeroed the spin concentration.

2.1.2 Calibration of the Intensity by 'Additive Dose'.In quantitative ESR analysis, the number of spins is measured either absolutely or relatively from the signal intensity of a standard sample. The intensity itself indicates neither the radiation dose in the unit of absorbed radiation energy, Gray [Gy: 1 Gy = 1 J/kg] nor the age in years in chronology science. The artificial irradiation is used in the 'additive dose method' to calibrate the signal intensity. It is a 'time machine' to lead from the spin concentration to a future state in ESR dating. Known additive doses, Q in Gy, usually by γ-rays from a 60Co or a 137Cs source at the dose rate D' (usually 1 kGy/h) for the irradiation time, t' (usually in hour), produce additional spins to calibrate the concentration and their production yield.

The signal intensity is enhanced linearly as a function of the absorbed dose of arti?cial irradiation, Q, (Q= D't')

I(Q) = Io(1 + Q/DE) (1)

where Io and I(Q) are the observed signal intensities before and after irradiation, respectively. The equivalent dose, DE is obtained by linear extrapolation of the data points to the zero ordinate using the least-square fitting method. The growth curve may be fitted to a simple saturation curve,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where Is is the saturation intensity and DS the saturation dose in a laboratory irradiation.

2.1.3 Radiation Assessment in ESR Dating. The absorbed radiation energy Q should be comparable to the paleo-dose, the product of the age, T [a] and the natural dose rate D (usually in [mGy/a]). If the natural dose rate D is obtained, the ESR age may be expressed in [ka] as TESR [ka] = DE [Gy] / D]mGy/a]. The annual dose rate for radioactive equilibrium of the uranium–238 (238U) and thorium-232 (232Th)-series disintegration are for the contents of 238U, 232Th and potassium-40 (40K). For known disequilibrium conditions, DE is given in an analytical form as a function of the age. The estimation of DE and D for geological and archaeological materials constitutes the main part of ESR dating after the identification of the appropriate signal for ESR dating.