Radiation Measurements 42 (2007) 1181–1184www.elsevier.com/locate/radmeas

�- and UV-induced CO−2 radicals in tooth enamel

V.V. Rudko∗, I.P. Vorona, N.P. Baran, S.S. IshchenkoInstitute of Semiconductor Physics of National Academy of Sciences of Ukraine, 45, pr. Nauky, Kiev 03028, Ukraine

Abstract

Results of comparative EPR study of dental enamel powders and bulk samples irradiated by �-rays and by UV-light are reported. It is shownthat the EPR spectra of UV- and �-irradiated powders have a similar shape, but different saturation behavior, which implies that these signalsare different. Angular anisotropy of the EPR signals of enamel plates (bulk samples) irradiated by UV-light is considerably higher than for the�-irradiated ones. Annealing behavior of defects in irradiated enamel showed that both UV- and �-irradiation produce EPR signals primarilycaused by the same paramagnetic species, namely oriented and disoriented CO−

2 radicals. It can be shown that for each type of irradiation theratio between the amounts of these radicals is a fixed value. These values do not depend on the type of the enamel, nor on the irradiation dosein the range of 0.5–10 kGy. Differences in saturation curves for UV- and �-irradiated powdered samples as well as differences in anisotropyobserved in bulk ones may be explained by different ratios of the two CO−

2 species. Thus, it is possible to define the type of the radiation towhich the samples were exposed by measuring the anisotropy of the EPR spectra of dental enamel plates.© 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Dental enamel is a typical biological hydroxyapatites. Itsmineral component, 95–98% of the enamel weight, con-tains, mainly, hydroxyapatite and carbonate-hydroxyapatite(Borovsky and Leont’ev, 1991). Application of �-radiationcreates an intense EPR signal near g = 2 that depends linearlyon irradiation dose over a wide range of doses found in thera-peutic and accident applications. UV light is also able to forma similar spectrum in dental enamel (Liidja et al., 1996). Thusthe problem arises how to separate the contributions of thesetwo types of radiation to the EPR spectrum when both of themaffected the sample. Therefore the detailed knowledge of thestructure of the radiation-induced EPR spectrum and propertiesof the corresponding paramagnetic defects is necessary.

The radiation-induced EPR spectrum was assumed to be thesuperposition of the signals that originate from different param-agnetic defects. The main contributions are caused by so-called

∗ Corresponding author. Fax: +380 44 265 8342.E-mail address: vv_rudko@yahoo.com (V.V. Rudko).

1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2007.05.017

“background” signal (its nature is under the debate), and rad-icals such as CO3−

3 , CO−, CO−2 and some others (Callens,

1997). Progress in understanding of structure of this dosime-try EPR signal was achieved due to study of enamel plates(Callens et al., 1995). Lately it was shown that the �-inducedEPR spectrum of the enamel plates irradiated with large doses,can be fairly well described by the contributions of only twotypes of CO−

2 radicals: axial and orthorhombic (Vorona et al.,2006). When the width of EPR lines is 0.2–0.4 mT (typical val-ues for the irradiated enamel (Callens, 1997) the axial and theorthorhombic radicals reveal a very similar EPR lineshape inpowdered samples, but the spectra of enamel plates are sub-stantially different. Axial radicals are oriented in the enamelplates, and as a result, the spectral position of EPR signal ofaxial centers depends on the orientation of the enamel plate inthe external magnetic field. On the other hand, the orthorhom-bic radicals are oriented chaotically. Therefore they produce the“powder-like” EPR signal which does not depend on the plateorientation.

Here we report the results of the comparative study of �- andUV-irradiated enamel samples. Main attention is paid to thestudy of enamel plates in order to find out the differences inthe structure of the radiation EPR spectrum that is caused bydifferent types of radiation.

1182 V.V. Rudko et al. / Radiation Measurements 42 (2007) 1181–1184

2. Materials and methods

We studied the enamel of sound teeth of man and pig. Pow-der samples were prepared in accordance with the procedureaccepted in retrospective EPR dosimetry (Tatsumi-Miyajima,1987). Cleaned enamel was crushed by hand grinding with anagate mortar to a powder with a grain size between 100 and300 �m. Powdered enamel (approximately 80 mg) was placedin quartz tube of 4 mm inner diameter. Enamel plates approxi-mately 1×2×3 mm3 were cut out from enamel that was clearedof dentine by stomatological instruments. For the comparativestudy of influence of �- and UV-irradiation on EPR spectra ofenamel the plates cut from the same tooth were used in orderto avoid possible variation of angular anisotropy caused by dif-ferent crystallites ordering in different teeth. The samples wereirradiated at room temperature in air by 60Co �-rays and/or byUV-light from the mercury lamp of middle pressure (type PRK-2). The absorbed dose of �-irradiation was within the range0.5–10 kGy for different samples. Time of UV-irradiation var-ied in the range of 60–200 h in order to receive EPR spectra ofintensities comparable to the �-irradiated samples.

Annealing of the samples was carried out in a muffle fur-nace in air. The temperatures range was 100–300 ◦C. Theannealing duration was 40 min at each selected temperature.The temperature was controlled with the accuracy of ±1 ◦C bya thermocouple.

EPR measurements were carried out using an X-band EPRspectrometer at room temperature. Except for the experimentson continuous saturation, the typical experimental conditions ofEPR dosimetry were used: 100 kHz modulation of the magneticfield with 0.2 mT amplitude modulation and microwave powerof 1–3 mW. The EPR spectra of irradiated enamel were regis-tered together with the spectrum of reference sample, MgO :Cr3+, that allowed direct comparison of experimental spectralintensity.

3. Results and discussion

Fig. 1 shows the typical EPR spectra of the �- and UV-irradiated enamel powders. It is clear that their lineshapes aresimilar. It agrees with published data (see, for example, Liidjaet al., 1996). However, on increasing microwave power theEPR signal from �-irradiated enamel samples saturated con-siderably quickly, than signal from UV-irradiated samples (seeFig. 1b). This suggests that relaxation characteristics of the sig-nals induced in enamel by �- and UV-irradiation are different.Considering that the EPR signal in the �-irradiated enamel isformed, mainly, by contributions of two types of CO−

2 radicals(Vorona et al., 2006), it is possible to assume that UV-lightproduces the same radicals. Therefore, most probably explana-tion of difference in the saturation curves of investigated EPRspectra, in our opinion, is different contributions of axial andorthorhombic radicals under different types of irradiation. Thisis supported by previous research into EPR spectra saturationof irradiated enamel powders before and after annealing (Briket al., 1997). In these experiments, the EPR spectrum ofannealed enamel was characterized by smaller relaxation times

as compared to the spectrum of unannealed enamel. On theother hand, it was recently shown that there is the increase ofrelative contribution of axial radicals in the total EPR spectrumas a result of annealing (Ishchenko et al., 2002; Vorona et al.,2006). This implies axial CO−

2 is characterized by smaller timesof relaxation, than orthorhombic once. If under different typesof irradiation the ratio between axial and orthorhombic radicalsin enamel is different, it will cause differences in saturationcurves of total EPR signals. Other differences can be attributedto the presence of additional components with smaller relax-ation time in the case of UV-irradiation. More detailed infor-mation about this issue may be obtained from the study of bulkmaterials.

As previously noted, studying samples cut from enamel intoplates gave the possibility of defining the structure of radiation-stimulated EPR spectrum in �-irradiated samples (Callenset al., 1995; Vorona et al., 2006). And so in order to compareEPR spectra caused by �-rays with those caused by UV light,we studied enamel plates irradiated by different types of irradi-ation. To describe these results we used a Cartesian system ofcoordinates hpv, suggested in work (Brik et al., 2000), whereaxis v is the axis of the tooth growth and axis p is perpendic-ular to the enamel surface. The sizes of the studied plate alongh, p and v axes were 2, 1 and 3 mm, respectively. EPR spectraof �- and UV-irradiated enamel plates for different sample ori-entations within the magnetic field, in a plane perpendicular tothe v-axis, are shown in Fig. 2. It is clear, that for both samplesthe dependence of spectral shape, i.e. anisotropy, is similar.One must note that anisotropy of EPR spectra of irradiatedenamel depends on the plane rotation of the magnetic field andis maximal in the vh plane (Brik et al., 2000).

For a quantitative estimation of anisotropy we calculated thevalue I⊥/(I⊥)max, where I⊥ is amplitude of the EPR spectralmaximum near g = g⊥ at arbitrary orientation of samples, and(I⊥)max is the maximal value of I⊥ (see Fig. 2). At the relativelylarge doses of irradiation which were used in this work, itis possible to consider that the radiation-induced spectrum ofenamel is determined only by contribution from the two types ofCO−

2 radicals. Therefore at any arbitrary orientation of samples,the amplitude I⊥ is determined by contributions from both typesof centers. It is well known (Ishchenko et al., 1992; Machoet al., 2003), that axial radicals in enamel are so oriented thattheir axes of symmetry formed a cone about axis p with anacute angle at vertex. Therefore, when the magnetic field B

is parallel to the p-axis, (B‖p) the contribution of orientedradicals is concentrated near g = g‖. At that the amplitudeI⊥ is minimum and is determined only by contribution of thedisordered radicals Idis. At an orientation of the magnetic fieldB‖h, the signal of the oriented radicals is concentrated nearg = g⊥ and the amplitude I⊥ will be maximal and equal toIdis + Ior, where Ior is contribution of the oriented radicals.Consequently, upon rotation of the magnetic field in a planeperpendicular to the axis v, the value I⊥/(I⊥)max will changefrom unity at B‖h to Ior/(Idis + Ior) at B‖p. Dependence ofthis value on angle between the magnetic field and axis h at therotation of samples around axis v for four samples is shown inFig. 3.

V.V. Rudko et al. / Radiation Measurements 42 (2007) 1181–1184 1183

Fig. 1. Typical EPR spectra of �- and UV- irradiated enamel powders (a). Saturation curves of EPR signal for �- and UV- irradiated enamel powders (b).

Fig. 2. Angular variation of EPR spectra lineshape of �- and UV- irradiated enamel plates at the rotation of the magnetic field in the plane perpendicular tothe tooth growth axis (axis v).

The difference between the maximal and minimum valuesI⊥/(I⊥)max, which in future we will designate by �, repre-sents relative contribution of oriented radicals in the amplitudeI⊥:

� = Ior/(Idis + Ior).

For a �- and UV-irradiated enamel it is ��=0.35, and �UV =0.50, respectively. Additional research into enamel plates cutout from different teeth of man and pig and irradiated by �-and UV-irradiation showed that the value � did not depend ontype of enamel or irradiation dose in this dose range. Irradiationof samples by �-rays and UV-light simultaneously results in avalue of � which depends on the ratio between the doses ofeach type of irradiation (see Fig. 3). Consequently this valuecan be characteristic of the irradiation type.

Taking into consideration that contributions of oriented anddisordered radicals in the amplitude I⊥ is proportional to theirconcentrations, that is Ior = kornor, Idis = kdisndis, then

� = 1

1 + �(ndis/nor),

where � = kdis/kor. The coefficients kdis and kor represent theproportionality between EPR signal intensity of these radicalsat point g =g⊥ and their concentration. Thus they depend onlyon the model which is used to simulate the different radicalEPR spectra. In other words, given a definite model, � = constand value � is proportional to the ratio of concentrations of thetwo radical types. The fact that �� < �UV demonstrates thatUV-light creates more axial centers than does �-rays.

Annealing studies provide additional confirmation that thesame radicals appear with different types of radiation. Fig. 4shows the changing contributions of orthorhombic (a) and

axial (b) CO−2 radicals to the total EPR spectrum as a result

of isochronal annealing. It is considered that � represents con-tribution of oriented CO−

2 , while amplitude at point g = g‖ atI⊥/(I⊥)max = 1 represents contribution of disordered radicals.The total contribution of CO−

2 in the spectrum of each samplebefore annealing is used to normalize the results. Note that sep-aration of the components of total EPR spectra of the enamelannealed at different temperatures is similar to previous work(Vorona et al., 2006), gives an extremely similar result. Thussimilar annealing characteristics of oriented and disordered

1184 V.V. Rudko et al. / Radiation Measurements 42 (2007) 1181–1184

Fig. 3. The dependence of � on the angle between the axis h and the magneticfield for �- and/or UV- irradiated samples.

Fig. 4. Changes of relative contributions from disordered (a) and oriented(b) CO−

2 under annealing.

radicals in the enamel samples irradiated by different types ofradiation, in our view, testifies to the identity of defects that

appear under influence of �-rays and UV-light. Some differencein annealing of orthorhombic CO−

2 is related to greater effec-tiveness in their transformation to axial for the �-irradiated sam-ples as a result of their greater amount in the original samples.

4. Conclusions

Thus, irradiation of enamel by �-rays and UV-light formsat least two types of the CO−

2 radicals in it: oriented (ax-ial) and disordered (orthorhombic). Their ratio depends on thetype of sample irradiation. For every type of irradiation in thedose range studied this ratio is a constant value that does notdepend on type of enamel, and can be descriptive of irradiationtype. Axial radicals have shorter relaxation times compared toorthorhombic, that can explain differences in the EPR spec-tral shapes that were recorded at different microwave power inpowdered samples. Therefore anisotropy can reveal the type ofradiation to which a tooth has been exposed.

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