Effect of mechanically induced background signal on EPR dosimetry of tooth enamel

Radiation Meawremenrs, Vol. 24, No. 3, pp. 249-254. 1995 Copyright 0 1995 Elsevier Science Ltd

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EFFECT OF MECHANICALLY INDUCED BACKGROUND SIGNAL ON EPR DOSIMETRY OF TOOTH ENAMEL V. POLYAKOV,*

E. HASKELL,?

G. KENNER,t

G. HUETT*

*Institute of Geology, EE-100 Tallinn, Estonia; and tuniversity

and R. HAYES?

of Utah, Salt Lake City,

UT 84112, U.S.A. (Received 1 August 1994; revised 8 December 1994; in final form

6 February 1995)

Abstract-The effect of the mechanically induced background ESR signal whose Lande factor is g = 2.0038, width = 0.791 mT, on absorbed dose estimation using the additive method was studied. The intensity and width of this signal increases with decreasing grain size. It was found to be thermally stable and sensitive to 90Sr radiation. The latter phenomenon should lead to its increasing contribution to the radiation-induced hydroxyapatite signal at g, = 2.0018 at irradiation with higher doses. However, it was found that the interference between mechanically induced and the hydroxyapatite signals may be interpreted as either ‘negative’ for larger grain size or ‘positive’ for finer grain size. This feature in turn leads to under and overestimation of the hydroxyapatite signal, respectively, and is apparently caused by the inverse relationship between the signal width and grain size. Enamel samples were irradiated with 44, 88, 220,440, 660 and 880 mGy from a “‘Cs gamma ray source. It was determined that 220 mGy was the lowest absorbed dose that could be reliably detected, while doses as low as 44 mGy could tentatively be= identified.

1. INTRODUCTION

Health Plan) Hospital. Teeth which showed signs of disease or discoloration were discarded. Fillings were removed by drilling. Approximately the bottom three-fourths of the teeth (roots) were removed with water irrigation using a 4” diamond saw blade mounted on a Buehler Isomet saw. A dental drill was used to remove the remaining dentine from the crowns of the teeth. The crowns were then crushed using an agate mortar and pestle. Different grain size fractions were separated by sieving. These fractions were designated as follows:

At present, tooth enamel of various origins (human, mammoth, etc.) is extensively used as a paleodosimeter for dating geological events by the ESR technique (Ikeya, 1993; Rhodes and Griin, 1991). This natural material is also being investigated for retrospective accident dosimetry purposes because of its high sensitivity to radiation exposure and its extensive dose response linearity (Aldrich and Pass, 1986; Ikeya et al., 1986; Nishiwaki and Shimano, 1990; Pass and Aldrich, 1985; Rink and Schwartz, 1994). Tooth enamel is a good natural dosimeter because of the relative simplicity of its ESR spectrum (Desroisiers and Skinner, 1993; Aldrich et al., 1992; Aoba et al., 1982; Bacquet et al., 1981; Callens et al., 1987). The main radiation-induced signal with principal values of g, = 2.0018 and g,, = 1.9980 is easily recognized. A major disadvantage is that sample preparation results in definite complications for interpretation of results (Desroisiers et al., 1989). In this paper, we report on the effect of sample preparation (removal of dentine from the tooth by drilling, subsequent crushing and grinding of enamel) and grain size on the results of absorbed dose reconstruction by the additive dose method.

2. MATERIALS

AND

dl = 0.25m.850 mm; d2 = [email protected] mm; d3 =
METHODS

The teeth used in this study were obtained from the dental clinic of the Salt Lake City FHP (Family 249

V. POLYAKOV

et al.

,

10 Gy irradiated

dl = 0.250-0.850

346.5

347

347.5 MqnetlcFbld

348

340.5

(mT)

Fig. 1. ESR spectra of background samples. Each spectrum is overlayed with a matching curve derived from a calculation of the derivative of the corresponding Gaussian curve. The equation used was y = m3*(m 1 - MO)+exp(-(MO ml)*(MO-ml)/(m2*m2));ml=347;m2=1;m3=3OO0. The resulting statistics are in Table 1.

Two further experiments using the 0.150-0.250 mm grain size fractions were done to determine the dose response curve for enamel. In the low dose experiment, the sample was exposed to 0, 44, 88 and 220 mGy of 13’Cs with an exposure rate of 0.667 mGy/s (JL Shepherd, San Fernando, CA 91340-l 822). In the moderate dose experiment, doses of 0, 44, 220, 440, 660 and 880 mGy were used. The intensity of the main radiation-induced signal was measured as peak-to-peak (PTP) height of the signals. For cases where the signal was too weak to measure PTP directly, the center point of the signal was calculated and the extrema of the signal were located using the value of the spectral width. Since we do not have a magnetometer, the commonly accepted value of g, = 2.0018 was used as a standard for determining the g-values for the mechanically-induced spectra and g,, of the hydroxyapatite signal. The Gaussian curve derivatives were calculated using the Gaussian curve fit option of KaleidaGrapho (Synergy Software, Reading, PA 19606). Least squares analysis was used on the dose response curves. When not stated otherwise, plus/minus values are standard deviations. 3. RESULTS

AND DISCUSSION

The samples were not irradiated prior to sample preparation. Therefore, all absorbed doses, recon-

346

346.5

347

347.5

340

Magnetic

340.5

Intensitvt ml1

m2 m3 Chisq R

_’

349

349.5

350

field (mT)

Fig. 2. ESR spectra of sample with dl =0.2X1-0.850 mm grain size. Note that over the range of approximately 346.8 to 347.3, the spectrum for the difference lies above zero on the y-axis. This is a dose effect on the mechanically-induced signal caused by irradiation.

strutted by the additive dose method were expected to be zero although there may be an unknown contribution from diagnostic dental X-rays taken prior to extraction. A mechanically induced signal with a Lande factor of g = 2.0038 f 0.0003, width = 0.791 + 0.088 mT was identified. It was found to be thermally stable and did not saturate with microwave power up to 50mW and did not diminish with time. It is assumed that the finer grains have undergone greater mechanical stress than the larger grains. Figure 1 shows the background spectra of different grain size aliquots of the same tooth enamel sample. Overlying them are best fit plots of differentiated Gaussian curves corresponding to each spectra. As may be seen from these spectra and their associated Gaussian curve fits, greater mechanical stress causes the appearance of a more intensive mechanically induced signal at g = 2.0038. Table 1 shows the data for the Gaussian fit. The ml parameters which correspond to the center points of the mechanical spectra, shift to the right with decreasing grain size. Similarly, m2 increases in magnitude reflecting an increase in the width of spectra, while m3 whose

Table 1. Statistics for ESR background spectra of different grain sizes Grain sizes (mm) . ,

mm

0.2WO.850

0.150-0.250

<0.150

2625 347.43 + 0.007 0.625 k 0.0099 4482 + 140.9 5.80e + 07 0.855

3829 347.49 * 0.005 0.643 + 0.0074 6146 f 141.1 6.35e + 07 0.914

5787 347.55 f 0.004 0.666 k 0.0060 9729 f 174.6 10.8e + 07 0.944

tRelative ESR signal intensity $y = m3*(m 1 - MO)*exp( - (MO - m l)*(MO -m l)/(m2*m2)); m2 = 1; m3 = 3000. See Fig. 2 and text for explanation

ml = 347;

EPR DOSIMETRY

OF TOOTH

values are related to the PTP value of the spectra also increases with decreasing grain size. Previous studies of enamel have shown similar results (Desroisiers et al., 1989; Shimano et al., 1989; Tatsumi and Okajima, 1985). One possible explanation for the origin of the signal is that it is caused by free radicals generated during the grinding of the tooth enamel. The free radicals would be mostly distributed within a thin subsurface layer of the tooth enamel. The association of these paramagnetic centers with impurity defects of the tooth enamel seems unlikely due to the high (up to 95-97%) content of hydroxyapatite Ca,,(PO,),(OH), in the tooth enamel. The high level dose response study showed that the mechanically-induced signal is sensitive to radiation. This conclusion was derived from comparison of the lineshapes of radiation signals of the difference spectra of tooth enamel aliquots with grain size dl, d2 and d3. In the case of non-sensitivity of the mechanically-induced signal to irradiation, the value of the difference between the irradiated and background spectra would have been zero for a distance of width = 0.791 mT centered at g = 2.0038. This was not the case as is shown in Fig. 2 for grain size dl. This conclusion is also derived from comparison of the lineshapes of the difference spectra of tooth enamel aliquots with grain size d 1, d2 and d3, shown in Figs 2, 3 and 4, respectively. In the case of non-sensitivity of the mechanically-induced signal to irradiation, the main radiation signals in the difference spectra of all aliquots should have congruent lineshapes. However, the non-congruence of the lineshapes of the radiation signals for different grain size aliquots is unambiguously indicated. The contribution of the mechanically-induced signal of the sample with d 1 grain size to the main radiation signal results in a decrease of its intensity, and may be d2 = 0.150-0.250

mm

346.5

347

347.5

346

346.5

349

349.5

350

Magneticfield (mT) Fig. 3. ESR spectra of sample with d2 = 0.1504250 mm grain size. The mechanically-induced signal at g = 2.0038 (ca 347.5 mT) is slightly larger than that seen in Fig. 2. Other comments are the same as for Fig. 2.

251

d3=<0.150mm

0

346

346.5

347

347.5

346

340.5

349

349.5

350

Magnetic field (mT)

Fig. 4. ESR spectra of sample with d3 = < 0. I50 mm grain size. The mechanically-induced signal at g = 2.0038 (ca 347.5 mT) is much larger than that seen in Figs 2 or 3. Other comments are the same as for Fig. 2.

characterized as ‘negative’. The PTP measurement for the 1OGy irradiation is 5957 while the value corrected for background subtraction obtained from the difference spectra is 6341. This is an error of 6%. This conclusion is further supported by the dose response curve for grain size dl, presented in Fig. 5 and Table 2. This negative interference leads to the systematic underestimation of absorbed dose for d 1 grain sizes. By contrast, Fig. 5 and Table 2 show that for finer d2 and d3 grain sizes, this contribution results in

0 346

ENAMEL

50

loo h

150

200

250

WY)

Fig. 5. Dose.-response curves and linear fits for different grain size ahquots of the same tooth enamel sample. The statistics for this figure are given in Table 2. Note that the d2 and d3 fractions intercept the y-axis at slightly above zero while the dl fraction intercepts at a value of less than zero. Also note that the greater the grain size the steeper the slope. Circle, d I; rectangle, d2; diamond, d3.

252

V. POLYAKOV

et al.

Table 2. Results of linear curve fitting on several samples of tooth enamel with different grain sizes. The systematic uncertainty AD and the slope coefficient of the linear fitting function are presented (negative and positive values of AD mean overestimation and underestimation of the absorbed dose, respectively) Sample # 1 Grain size (mm)

&I

Sample #2

Slope coeff.

r% +3.67

dl [email protected]

+2.25

1.00

d2

-0.56

0.95

Sample #3

Slope COeff.

Slopecoeff.

Kl

1.00 +2.0

1.00

- I .62

0.93

-7.46

0.79

0.150-0.250 d4

0.0754). 150 d5

+2.16

1.00

0.075-0.0106 d3

-0.38

0.82


decreasing PTP values for the hydroxyapatite signal. Thus, the systematic uncertainty of absorbed dose, originated from the presence of the mechanicallyinduced signal, may be reduced by the appropriate

choice of grain size. It was also found that different grain size samples have different regression coefficients (Table 2). The decreasing regression coefficient may be explained in terms of trap disintegration during crushing of the tooth enamel. This could result in significant error when assessing absorbed dose. This is illustrated by the sketch shown in Fig. 6, where the actual doseresponse curve is represented by the solid line (2). The extrapolation of line (2) towards the dose axis gives the overestimated dose D,,,,, if compared to the true dose D,,. The hypothetical dose-response curve (1) correctly describes the actual dose-response curve only within the dose range, before the mechanical grinding of the tooth enamel. The relative error of absorbed dose is easily evaluated as:

I

0

Absorbed dose

50

Fig. 6. Hypothetical dose-response dependencies of tooth enamel sample: (1) non-ground sample with larger trap concentrations; and (2) true dose-response curve, which coincides with curve (I) within the laboratory dose range prior to laboratory dosage and has a lower slope than curve (1) due to the trap disintegration during the tooth enamel grinding. D,, and D,, are the true and overestimated absorbed doses, respectively.

AD a,

l

_=--

4,

a2

’

where AD = D,,, - D,, and a,, a2 are the slope coefficients of the lines (I) and (2) respectively. For instance, the error for a, = 1.0 and a, = 0.82 (data from Table 2) exceeds 20%. These observations are further supported by the data given in Table 3 which shows that the center point of the spectra shifts to the right with decreasing grain size while the signal width increases. The resulting flattening of the mechanically-induced signal results in a decreased effect on the hydroxyapatite signal. An explanation for how this can occur is given in Fig. 7. These findings are in some disagreement with Tatsumi and Okajima (1985) who concluded that the mechanically induced signal has no influence on the radiation-induced signal. Unfortunately, these authors did not present the results of linear fitting on the unexposed samples. Two experiments were run to estimate the minimum detectable dose. Figures 8 and 9 show the spectra of unirradiated and irradiated tooth enamel samples from the low and medium dose experiments, respectively. As one can see from these figures, doses of 220 mGy and higher can be reliably detected. It is, however, extremely difficult to detect doses of 44 and 88 mGy using the spectra (Fig. 8); just a very slight change of lineshape of the spectra at the field position of the radiation signal at g, = 2.0018 may be indiTable 3. Effects of irradiation on the Lande factor (g-value) and width of the mechanically-induced signal and the PTP of the hydroxyapatite signal Grain fraction dl d2 d3

g-value

Width

PTP,,,

PTP,,,

% Error

2.0038 2.0035 2.0035

0.703 0.791 0.879

5957 5787 585 1

6341 5957 6063

6.06 2.85 3.50

PTP,,-Gbserved PTP,,--Corrccted

peak-to-peak value peak-to-peak value

EPR

DOSIMETRY

OF TOOTH

346

ENAMEL

346.5

253

347

347.5

348

346.5

349

Magnetk Field (mT)

Fig. 8. Results of low level dose-response study. ESR spectra of sample with grain size d2 = 0.150-0.2SO mm irradiated with 0, 44, 88 and 220mGy. The center points of g, = 2.0018 for hydroxyapatite and g = 2.0038 for the mechanically-induced signal are indicated.

4. SUMMARY

MagndcfiiM

Fig. 7. Effect of increasing signal width of the mechanical signal on the final shape of the apparent hydroxyapatite signal. The PTP of the mechanically-induced signals of the large grains (ml) and the small grains (ms) are assumed to be of the same magnitude as that of the hydroxyapatite (ha) signal. In these cases, the PTP is 2.0. (A) The thin line (ml) represents an idealized, true spectrum of the large grains due to mechanical trauma and irradiation. The thick line (ms) is the true spectrum for the small grains. The spectrum for the small grains is broader than the spectrum for the large grains. (B) Idealized true spectrum of hydroxyapatite of both the large and small grains which have been irradiated. The minimum for the mechanical spectrum of the large grains coincides with the midpoint of the hydroxyapatite spectrum. The minimum for the mechanical spectrum of the small grains approximates the minimum for the hydroxyapatite spectrum. (C) Summation of the idealized true spectra of the mechanical and hydroxyapatite signals. This results in an apparent PTP value of I.5 for the summation of the large grain signals (negative effect) and 3.0 for the small grains (positive effect). The values seen in Table 3 are not so extreme as in this idealized case because the effect of radiation on the mechanically-induced signal is only about one-tenth of that on the hydroxyapatite signal.

1. The mechanically-induced signal at g = 2.0038, which superimposes the hydroxyapatite signal at g, = 2.0018, was found to be thermally stable and sensitive to radiation exposure. 2. The intensity of the mechanically-induced signal increased with time duration of mechanical grinding. This results, finally, in its dependence on the grain size of the tooth enamel sample.

6di”

346

cated. Figure 10 shows the results of subtracting the background spectra and then calculating the PTP. In this instance, the 44mGy dose can be tentatively detected.

2.0016

346.5

346 347 347.5 M_ywl!s FWJ (mT)

346.5

349

Fig. 9. Results of the medium level dose-response study. ESR spectra of sample with grain size d2 = 0.15&0.250 mm irradiated at 0, 220, 440, 660 and 880 mGy. The center points of g, = 2.0018 for hydroxyapatite and g = 2.0038 for the mechanically-induced signal are indicated,

254

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et al.

Aldrich J. E., Pass B. and Mailer C. (1992) Changes in the paramagnetic centers in irradiated and heated enamel studied using electron paramagnetic resonance. 1111. J. Radiat. Biol. 61, 433437.

Aoba T., Doi Y., Yagi T., Okazaki M., Takahashi J. and Moriwaki Y. (1982) Electron spin resonance study of sound and carious enamel. Calcif: Tbs. Ini. 34, S88-S92.

ot”““““““““” 0

200

400

600

Doss(mSv)

600

1000

Fig. 10. Dose response curve for medium level study. Values were obtained by subtracting the background spectrum from the irradiated spectrum. Note that the line intercepts the y-axis at a value greater than zero. This is due to the contribution to the hydroxyapatite signal from the overlapping mechanically-induced signal.

3. The contribution of the mechanically-induced signal to the radiation-induced signal at g, = 2.0018 ‘negative’ is considered for the grain size dl = 0.250-0.850mm, which leads to the underestimation of the absorbed dose. However, the value of that systematic uncertainty depends on grain size; lower for finer grain size despite the total enhancement of the intensity of the mechanically-induced signal. 4. Long-term mechanical stressing may result in a decrease of the initial trap concentration. This leads to a significant overestimation of absorbed dose. 5. The minimum detectable dose is estimated to be about 44mGy under the measurement conditions used in the present study.

Bacquet G., Truong V. Q., Vignoles M., Trombe J. C. and Bone1 G. (1981) ESR of CO*- in X-irradiated tooth enamel and A-type carbonated apatite. C&if. Tiss. Inr. 33, 105-109. Callens F. J., Verbeeck R. M. H., Matthys P. F. A., Martens L. C. and Boesman E. R. (1987) The contribution of CO:- and CO*- to the ESR spectrum near g = 2 of powdered human tooth enamel. Cu/c$ Tiss. Int. 41, 124129. Desroisiers M. F., Simic M. G., Eichmiller F. C., Johnston A. D. and Bowen R. L. (1989) Mechanically induced generation of radicals in tooth enamel. Inr. J. Rad. Appl. Instrum., Series A 40, 1195-I 197. Desroisiers M. F. and Skinner A. F. (Eds) (1993) ESR Dosimetry and Applications, Proceedings of the 3rd Int. Symp. 1418 October 1991, Gaithersburg, MD. Published in Appl. Radial. Isot. 44. Ikeya M. (1993) New Applications of Electron Spin Resonance: Dating, Dosimetry and Microscopy. World Scientific, New Jersey. Ikeya M., Miki T., Kai A. and Hoshi M. (1986) ESR dosimetry of A-bomb radiation using tooth enamel and granite rocks. Radial. Prot. Dosim. 17, 181-184. Nishiwaki Y. and Shimano T. (1990) Uncertainties in dose estimation under emergency conditions and ESR dosimetry with human teeth. Rudiar. Pro?. Dosim. 34, 295-297.

Pass B. and Aldrich J. E. (1985) Dental enamel as an in uiuo radiation dosimeter. Med. Phys. 12, 305-307. Rhodes E. J. and Grtin R. (1991) ESR behavior of the paramagnetic center at g = 2.0018 in tooth enamel. Ancient TL 9, 14-18.

Rink W. J. and Schwartz H. P. (1994) Dose response of ESR signals in tooth enamel. Radiut. Meas. 23, 481484.

Shimano T., Iwasaki M., Miyazawa C., Miki T., Kai A. and Ikeya M. (1989) Human tooth dosimetry for gamma rays and dental X-rays using ESR. Int. J. Radiat. Appl. Instrum. Part A 40, 1035-1038.

REFERENCES Aldrich J. E. and Pass B. (1986) Dental enamel as an in oiuo radiation dosimeter: separation of the diagnostic X-ray dose from the dose due to natural sources. Radial. Prot. Dosim. 17, 175-179.

Tatsumi M. J. and Okajima S. (1985) ESR dosimetry using human teeth. In ESR Dating and Dosimerry (Edited by Ikeya M. and Miki T.), pp. 397-405. IONICS, Tokyo. Tatsumi M. J. and Okajima S. (1991) Physical dosimetry at Nagasaki-Europium-152 of stone embankment and electron spin resonance of teeth from atomic bomb survivors. J. Radial. Res. Tokyo 32, 83-98.