The First

International Proficiency Testing Conference

Sinaia, România 11th − 13th October, 2007

393

PRINCIPLES AND METHODS OF ELECTRON PARAMAGNETIC

RESONANCE - DATING AND DOSIMETRY

Emilia Dana Seleţchi Faculty of Physics, University of Bucharest Măgurele, CP MG-11

RO - 077125, Bucharest, Romania seletchi@gmail.com

Abstract Electron Paramagnetic Resonance (EPR) is a non-interfering method of measuring the concentration of free radicals into a wide range of materials such as: carbonates, sulfates, phosphates, silica, silicates and organics. This study outlines the technical procedures for EPR dating regarding sample preparation, measurements and the additive dose method. The EPR retrospective dosimetry has proven to be a very useful technique for dose assessment in accident and also for detection of food treated with ionizing radiation. Key words EPR, free radicals, retrospective dosimetry, γ-irradiation 1 INTRODUCTION In the last years of XIX century a range of new dating techniques have been developed. These methodologies based on radiation exposure effects in host materials include Electron Paramagnetic Resonance (EPR) and various forms of luminescence dating: Thermoluminiscence (TL), Optically Stimulated Luminiscence dating (OSL) and Infrared Stimulated Luminescence (IRSL). These techniques require careful sampling strategies that are best accomplished by geochronologists and archaeologists working together. Electron Paramagnetic Resonance or Electron Spin Resonance (ESR) is based on the effect of ionizing radiation on the fossil material. The radiation creates paramagnetic centers with long lifetimes, whose concentration is proportional to the total radiation dose absorbed by the sample. Natural crystals contain 1016 to 1017 crystals defects per cm3 which are produced during growth or by diagenetic displacement of atoms. Ionizing radiation lifts

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electrons from the valence band to the conduction band. Most of them recombine immediately and a very small number fall into quasi-stable traps in forbidden energy levels. The traps occupied by a single electron act as paramagnetic centers, whose density can be determined by EPR [1]. In the absence of external magnetic field, electron magnetic moments are oriented casually and their energies do not differ from each other. When an external magnetic field is applied, electron magnetic moments will be oriented towards the field, depending on the value of spin magnetic moment and their energy level splits into two discrete energy levels (fig.1).

Figure 1 - Zeeman splitting of the energy levels of a free electron in an external magnetic field

In thermal equilibrium the population ratio of these two states is give by the Boltzmann distribution [3]:

TkEΔ

eNN −

↑

↓ = (1)

where and are the number of electrons on a higher or lower energetic level corresponding to the magnetic moment of electron with +1/2 and -1/2 spin, T is the temperature and k is the Boltzmann constant.

↓N ↑N

The absorption of the electromagnetic wave (microwave) by the unpaired electrons is called ’’Electron Paramagnetic Resonance’’. The Zeeman energy is written:

HμMHβgE ez −== (2)

where = spectroscopic splitting factor, β = Bohr magneton, H = the external magnetic field, M = magnetic quantum number, = magnetic moment.

g

eμThe direction of the spin is changed by the absorption of microwaves when the difference between two energy level lines HβgEΔ = is equal to the quantum energy of an electromagnetic wave . The resonance condition is: νh

νhHβg 0 = (3)

where H0 = the resonance magnetic field, ν = the frequency of the electromagnetic wave. An important parameter of an EPR signal is the spectroscopic splitting factor , since unpaired electrons in different environments have slightly different factors, resulting

gg

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in the appearance of signals for different centers at different magnetic field strengths. The factor of unknown signal is determined using a standard signal with a known g factor. The resonance of a standard signal with g

g1 and an unknown signal with g2

occurs at magnetic fields H01 and H02. By using the resonance condition we can write:

⎟⎟⎠

⎞⎜⎜⎝

⎛−=⇒==

0112022011 H

HΔ1ggβνhHgHg (4)

In EPR dating and dosimetry we can measure the intensity of an EPR signal and its enhancement by artificial irradiation or by the passage of time. The dose equivalent ED to that of α, β or γ rays used for artificial irradiation of an archaeological or geological material. ESR age is calculated as:

•=D

EDt EPR (5)

•D = annual dose rate of natural radiation.

2 ADDITIVE DOSE METHOD If we wish to know the defect concentration at a certain future time we must estimate the past passage of time from the present concentration. We use the artificial irradiation by γ rays from or source to let the defect concentration proceed to a future state [2]. The signal intensity enhance with the absorbed dose of artificial irradiation D (Fig. 2.a,b.).

Co60 Cs137

( ) ⎟⎠⎞

⎜⎝⎛ +=

EDD1II 0D (6)

where =( )DI signal intensities after irradiation and 0I = signal intensities before irradiation. On the other hand it has been demonstrated that a non-linear fit is a better choice:

( ) ⎥⎥

⎦

⎤

⎢⎢

⎣

⎡−=

⎟⎟⎠

⎞⎜⎜⎝

⎛ +−

0DEDD

SD e1II (7)

where = the saturation intensity, SI D0 is the dose corresponding to the IS(1-e-1) value (0.63 of the maximum intensity). ED is obtained by the extrapolation back to the zero ordinate.

a.

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b.

Figure 2 - a,b - The enhancement of EPR signal intensities by an additive artificial irradiation with the dose

3 ISOTHERMAL ANNEALING STUDIES The decrease of the EPR intensities at high temperatures is appreciated as a regress in the population of radiation-induced radicals. Mathematical expression which describes the time dependency of EPR signal intensity during the isothermal annealing can also be written:

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−−=

STD T

texp1III(t) (8)

where ID is the EPR signal intensity corresponding to the absorbed dose D before the annealing treatment, IT is the variation of the EPR signal intensity during the thermal treatment, t is the annealing time and TS is the time corresponding to the IS(1-e-1) value (0.63 of the maximum intensity). 4 EXPERIMENTAL The technical procedures for dose reconstruction by EPR method are illustrated in figure 3.

Figure 3 – Protocol steps for retrospective dose assessment

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4.1 Sample preparation

Geological samples must be subjected to the following standard treatment: 10 min etching in hydrochloric acid (0.5 N), washing with deionized water, drying and then gentle grinding in an agate mortar (grain size 63 – 180 μm). The weigh dry samples are in the range of 100-200 mg for carbonates. Severe mechanical treatment must be avoided, since it has proved to induce essential alterations in the marble spectrum [4]. 4.2 EPR measurements

The EPR spectra of carbonate samples were performed at ambient conditions by using an X-band JEOL JES MX 3X spectrometer provided with a TE011 cylindrical resonant cavity and 100 kHz modulation frequency. The samples in powder form were placed separately into quartz tubes with a 3 mm inner tube diameter. The position of tube in the cavity was not changed during the whole experiment in order to ensure the uniform and reproducible measurements of the sample. The optimal conditions of EPR measurement were: microwave frequency 9.50 GHz, modulation amplitude 1-2 G, microwave power 4 mW, sweep time 5 min and response time 0.3 s. The g- factors of the observed signals have been determined by using a 2.2-diphenyl-1-picryl-hydrazil (DPPH) as well as CaO:Mn2+. The block diagram for a typical EPR spectrometer is illustrated in figure 4.

Figure 4 – Scheme of an EPR spectrometer 1. Microwave source (Klystron), 2. & 5. Isolator, 3. & 4. Variable attenuator, 6. Electromagnet, 7. Field sensor, 8. Zeeman modulation coils, 9. Cavity with sample, 10. Magnetic field control & Power supply, 11. Zeeman modulation generator, 12. Microwave phase sensitive detector, 13. Zeeman modulation phase sensitive detector, 14. Amplifier, 15. Recorder or computer.

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5 APPLICATIONS OF ELECTRON PARAMAGNETIC RESONANCE METHOD EPR technique is an absolute dating method suitable for the Quaternary which can be applied to a wide range of materials (Table 1).

Table 1 - Materials used for EPR dating and dosimetry

Materials Applications Carbonates stalactite, travertine, coral, shell, foraminifera, egg shell Sulphates anhydride, gypsum (desert deposit, cave deposit)

Phosphates hydroxyapatite, tooth, bone, phosphate nodule Silica geological fault, volcanic rock, altered rock

Silicates zircon, feldspar, clay minerals Ice & Dry Ice comet, solid and OH2 2CO

Organics food, crop, leather, paper, alanine, sugar, mummy, blood Chemicals, pharmaceuticals, dyes, pigments

* EPR signals produced by chemical reactions rather than natural radiation.

The EPR spectra of carbonates consist mainly of a group of six lines of Mn2+ (Fig. 5.). An additional free radical peak identified as the radiation-induced defects ( , ,

, and ) stabilized by impurities in the range 1.996 - 2.0063 was observed at the center of this group for some un-irradiated carbonate samples.

−2CO −3

3CO−3CO −

2SO

Figure 5 - A typical EPR spectrum of a carbonate sample showing Mn2+ peaks It consists of six double lines plus 10 single ones between the doublets

Electron Paramagnetic Resonance dosimetry is a very useful technique for dose estimation in accidents. The retrospective dosimetry of bones and teeth is based on the measurement of radiation-induced radicals in hydroxyapatite . Tooth enamel retrospective dosimetry has been first used for assessment of the A-bomb radiation dose to survivors at Hiroshima and Nagasaki and later it has been

( ) ( )[ ]26410 OHPOCa

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also used for the victims of Chernobyl accident. The majority of radiation–induced radicals in tooth enamel are carbonate derived, but also radicals derived from phosphate and oxygen had been identified. The studies showed that not all radiation-induced radicals are thermally stable. In addition, several types of ionizing radiation (gamma, beta, alpha and X rays) and ultraviolet light produce essentially the same EPR signal, most likely the same types of radiation-induced radicals in hydroxyapatite and it is not possible to distinguish the radiation type by the EPR spectrum [5]. White granulated sugar is also a cumulative dosimeter in the case of radiation accident [9]. The fast neutrons from had a lower EPR sensitivity for a sugar dosimeter than gamma rays from source [8].

Cf252

Co60

Radiation sterilization is applied in many countries to improve the hygienic quality of foodstuffs and to extend their shelf life [10]. Although it is considered to be an inexpensive and effective method for sterilization, the applicability of this technique is strongly limited by the life time of the radiation-induced free radicals. We know that the recombination processes of the free radicals yielded during irradiation could generate new, unidentified substances with unknown effects on human health. 6 CONCLUDING REMARKS A comparative study on EPR spectra of limestone, marble, stalactites, coral skeletons and mollusk shells with different geographical origins carried-out the typical spectrum of carbonates. The spectra of gamma-irradiated samples revealed an additional free radical peak between the third and fourth hf lines of Mn2+. No significant variation in linewidth of carbonates spectra was observed, but a remarkable variation in EPR amplitudes has been observed. The most important paramagnetic species existent in the EPR spectra of carbonates are: , , , and radicals in the range 2.0010 – 2.0062 [6].

−2CO −3

3CO −3CO −

2SO −3SO

Strong similarities in EPR spectra of limestone and marbles were observed, while the EPR spectra of mollusc shells and coral skeletons showed great differences due their complex structures. The EPR spectra of azo dye samples (control and irradiated with accelerated electron beams) show a single line in the central field at a calculated g = 2.0035 value due radical which increase with the irradiation dose. Superposed on this broad line we have noticed another two narrow satellite lines which of them characterized by gyromagnetic factors g

−3SO

1= 1.9932 and g2= 2.0218. The EPR spectra of sediment and soil samples are essentially the same as in the majority of carbonates, revealing an additional free radical peak in the center of Mn2+ sextet, at a g = 2.0038, attributed to species as well a very intense and broad signal due to Fe

−3SO

3+ ions [7]. The results concerning the shape of EPR spectrum are in good agreement with literature data.

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7 REFERENCES [1] Geyh, M. A.; Schleicher, H.: Absolute Age Determinations – Physical and

Chemical Dating Methods and Their Applications, Springer, Verlag, Berlin, Heidelberg, (1990)

[2] Ikeya, M.: New Applications of Electron Spin Resonance – Dating, Dosimetry and Microscopy, World Scientific, Singapore, New Jersey, London, Hong Kong, 2- 420, (1993)

[3] Jonas, M.: Concepts and Methods of ESR Dating, Radiation Measurements, Vol. 27 (No. 5-6), 943-973, Elsevier Science Ltd.. Great Britain, (1998)

[4] Maniatis, Y.; Mandi, V.: Electron Paramagneticc Resonance signals and effects in marble induced by working, J. Appl. Phys., Vol. 71 (No. 10), 4859-4867, (1992)

[5] Nilsson, J.; Lund, E.; Lund, A.: The effects of UV-irradiation on the ESR-dosimetry of tooth enamel, Applied Radiation and Isotopes, Vol. 54, 131-139, Elsevier Science Ltd. , (2001)

[6] Seleţchi, E.D.; Duliu, O.G.: Comparative Study of ESR Spectra of Carbonates, Romanian Journal of Physics, Vol. 52, Environment Physics, 611-621, (2007).

[7] Seleţchi, E.D.; Negrilă, C.; Duliu, O.G.; Ţurcaş, C.V.: XPS, AES and ESR studies on sediments in Herăstrău Lake, Bucharest, Romania, AIP Proceedings 899, Springer, Environmental Physics, 411-413, (2007)

[8] Shiraishi, K.; Iwasaki, M.; Miyazawa, C.; Yonehara, H.; Matsumoto, M.: ESR Dosimetry in the JCO Criticality Accident, Advances in ESR Applications, Vol. 18, 203-206, The Society of ESR Applied metrology, Osaka, Japan, (2002)

[9] Shiraishi, K.; Wanitsuksombut, W.; Chinudomsub, K.; Suzuki, G.; Nishizawa, K.: ESR Dose Estimation of the Radiological Accident in Samut Prakarn, Thailand Using Sugar Samples and an ESR Method, Advances in ESR Applications, Vol. 18, 207-209, The Society of ESR Applied metrology, Osaka, Japan, (2002)

[10] Yordanov, N.D.; Gancheva, V.: Some New Approaches in the Field of Solid State/ EPR Dosimetry, Advances in ESR Applications, Vol. 18, 227-231, The Society of ESR Applied metrology, Osaka, Japan, (2002)