Radioluminescent dosimetry of α-quartz

Radiation Measurements, Vol. 24, No. 4, pp. 565-569, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1350-4487/95 $9.50 + .00

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RADIOLUMINESCENT DOSIMETRY OF 0~-QUARTZ YU. A. MARAZUEV, A, B. BRIK and V. YA. DEGODA Kiev University, Department of Physics, Prosp. Akad. Glushkova, 6, 252127 Kiev, Ukraine (Received 27 September 1994; in final form 3 May 1995) Abstract--The variation in the ultraviolet component of the radioluminescence intensity during X-ray excitation of a-quartz has been found to be dependent on the dose of previous ionizing irradiation. A new method of radiation dosimetry using UV radioluminescence is described. Radioluminescent dosimetry measurements of 7-rays produced by Chernobyl accident and background irradiation were made using natural and extracted crystalline quartz.

1. INTRODUCTION After the Chernobyl accident it was necessary to apply a complex of instrumental methods, working out new experimental approaches to retrospective dosimetry. It is known that the radioluminescent (RL) spectrum and intensity are sensitive to previous ionizing irradiation (Lietz and Matheja, 1964; Zimmerman, 1971). In the present paper we report a new method of radiation dosimetry using the UV radioluminescence emission of or-quartz. It was shown (Brik et al., 1994), that the variation in the component AJ (RL response) of the UV emission intensity during the X-ray excitation at room temperature was proportional to the dose D Oof previous irradiation AJ = k D 0,

(1)

where k is the sensitivity of radioluminescent dosimetry (RLD), This dependence is linear in the dose range 0.2-10 Gy. The RLD sensitivity and the equivalent dose for each sample may be determined by a comparative method, e.g. by preparing several aliquots of each or-quartz sample and administering a variety of ?,-Co6° laboratory doses.

2. EXPERIMENTAL

result of quartz grains extracted from the tile samples, the RLD sensitivity was measured for natural quartz from west Ukraine. Quartz grain sizes of 100-160/~m were tested. The results are listed in Table 1. 2.2. Experimental technique All luminescence measurements described here were performed at room temperature. An X-ray Cu-set operated at 35 kV was used. The dose rate Px in the material was variable from 0.6 to 60 Gy/min. The RL signal produced during X-ray excitation was recorded via a monochromator MDR-2 (A2 = +_4nm). All spectra and peak intensities J(2max) were corrected for instrument response. The dose rate P~. for v-Co6° additional irradiation was equal to 1 Gy/min. According to the TL pre-dose methodology (McKeever et al., 1985), all samples have been rapidly heated up to ~ 500°C before RL measurement. The manifestation of this effect is a strong enhancement of the s-quartz sensitivity to X-ray excitation (thermal activation). 3. RESULTS 3.1. Luminescence spectra and its variation Radioluminescence spectra of investigated samples

2.1. Specimens

Nos 1-4 in the visible are shown in Fig. 1. The

Radioluminescent dosimetry measurements of gamma rays produced by the Chernobyl accident and natural (background) irradiation were made using samples of crystalline or-quartz. The doses obtained by different methods for Chernobyl samples had values 0.2-40 Gy (Brik et al., 1995). In this paper the specimens with 'accidental' dose D 0 < 0 . 5 G y are considered to illustrate the sensitivity of the RL method. To prove that the RLD effect is attributable to crystalline quartz in general and is not the unique

spectral emission indicates that there is the superposition of two groups of emission bands: ultraviolet bands with ,k~,x= 350-400nm (so-called 380nm emission) and a blue band ()'m,x~ 480 rim). The spectra and Table I represent the complicated structure of the UV band and its different halfwidths in different crystals. For all investigated samples the peak intensity J(2m~) for UV emission decreased during X-ray excitation and the intensity of the blue emission increased for the time interval 0-60 min. UV luminescence intensity variation depends on the dose rate Px 565

566

YU. A. M A R A Z U E V et al. Table 1. Luminescent and dosimetric characteristics of the samples used for the measurements

No. sample

Origin

DO(Gy) reconst.

RL peak 2m~x(rim)

~,-sensitivity AJRL/D (a.u.)

X-sensitivity AJRL/D (a.u.)

I

Natural quartz from west Ukraine

14 + 3*

390

--

--

2

Natural quartz from west Ukraine

9 ___2

370

27.5

i>4.5

3

Extracted quartz from Chernobyl

0.4 + 0.1

350

26.0

6.1

4

Extracted quartz from Chernobyl

0.2 + 0.1

358

25.5

6.4

* The equivalent dose was reconstructed by the HT-TSL method. (intensity) of X-ray excitation (Fig. 2, sample No. 1). The initial parts of the curves 1-4 may be characterized by a decay time r. In the case Px = 100 Gy/min (Fig. 2, curve 1), r ~ = 3 . 0 + 0 . 3 s ; Z z = 8 - - - l s for Px = 40 Gy/min; z 3 = 28 _+ 3 s for Px = 10 Gy/min; ~4 : 3 0 0 -~- 5 0 S for Px = 1 Gy/min. At the low X-ray dose rate of 1 Gy/min the luminescence intensities decrease (for UV emission) and increase (for blue 4 8 0 n m emission) very slowly. Under this condition the initial luminescence spectra (not yet perturbed by X-ray excitation) were monitored in the time interval 0-3 min. The 350 nm luminescence signals observed from one of our investigated samples (No. 3) are shown in Fig. 3. After the X-ray excitation is switched on there is an intense signal (Jmax) which decays during excitation (J ---, Jmi,). After administering the laboratory calibrated y-doses, the initial intensity Jmax increases

100

2 1

(Fig. 3, curves 2-4). Larger previous irradiation doses give a proportionally larger initial intensity of UVR L signals. According to the experimental results, the variative R L component A J = Jmax - Jmin

(2)

A J = koD o + kND N.

(3)

is given by

By the assumption k 0 = k s = k we determine the so-called 'equivalent dose', which is the laboratory y-dose that produces the same signal as the 'natural' environmental dose. 3.2. Thermal activation characteristics X-ray induced U V luminescence response (AJ ) and the RLD-sensitivity (K = A J / D ) increase after rapid

4*

"-~ o

3 0.5

Z

0

300

I

I

400 500 Wavelength (nm)

f

600

0.0

I

10

I

20

I

30

I

40

I

50

I

60

Time (rain) Fig. 1. Emission spectra at the initial moments of X-ray excitation for investigated samples Nos 14, (curves 1~,), 4*-----emissionspectrum at t = 60 rain of X-ray excitation for the sample No. 4. The spectra have been corrected for instrument response. T = 295 K.

Fig. 2. UV peak intensity variation during X-ray excitation (sample No. 1 at T = 295 K) as the function of X-ray dose rate Px: (1) 100 Gy/min; (2) 40 Gy/min; (3) 10 Gy/min; (4) 1 Gy/min.

RADIOLUMINESCENT

DOSIMETRY OF ~-QUARTZ

200

567

G a m m a ray d o s e (Gy)

0

I

50

I00

I

I

i.0

e.

,~ I00

0,5

Jmin

I

I

I

I

I

I

I0

20

30

40

50

60

Dc

I

I

I

I

2

4

6

8

G a m m a ray dose (Gy)

T i m e (rain)

Fig. 3. UV radioluminescence peak (2m~. = 350 nm) intensity variation during X-ray excitation (Px = 100Gy/min) for aliquots 1-4 of the sample No. 3: (1) Chernobyl irradiated initial aliquot with unknown dose Do; (2) aliquot 1 was irradiated by y-Co 6° laboratory dose D n = I Gy; (3) D n = 4 Gy; (4) D N = 8 Gy. Thermal activation at 510°C was used before all luminescence measurements.

Fig. 5. RL response to gamma ray dose 0 100Gy (1) and its fragment 0-8 Gy (2) for equivalent dose reconstruction Sample No. 3, TA 510C.

3.3. Measurement steps o f dose reconstruction

b) For the initial specimen (with unknown dose D 0 of previous irradiation) after it's rapid heating up to 510 _ 5°C (thermal activation) the variation of X-ray induced luminescence intensity during X-excitation is measured. The variative component AJ (i.e. RE response) is determined. c) F o r the next aliquots 1, 2 . . . . . N after administration of a laboratory calibrated 7-dose D N and thermal activation, the variative component is measured as a function of DN-value. d) The equivalent dose D o is determined by extrapolation of AJ-response versus laboratory calibrated dose to zero intensity as shown in Fig. 5(2) and Fig. 6(2).

a) Each sample was divided into N + 1 aliquots (index 0, 1, 2 . . . . . N).

3.4. Results of equh~alent doses reconstruction

heating of a particular sample to a selected temperature (thermal activation). The normalized sensitivity increases effectively in the range 300-500°C and a small plateau (500-550°C) is measured for investigated specimens Nos 1-4. A thermal activation duration (the time of thermal annealing) ttA = 10 min was used (see Fig. 4).

1.0

¢0 0.5 u

I 5

I 10 Time (mia)

I 15

I 20

Fig. 4. RLD sensitivity variation as a function of the time of thermal annealing for the sample No. 1 (curve 1) and the sample No. 3 (curve 2). The samples were placed in the warmed-up muffle furnace a n d w e r e kept at 510°C for different time tTA.

The R L response A J of Chernobyl quartz No. 3 exposed to an additional 7-ray dose is shown in Fig. 5(1). The dependence AJ(DN) is linear up to the dose DN ~ 8 Gy and its fragment for the dose reconstruction (Do evaluation) is represented by the curve 2. The analogous R L D response for sample No. 4 is shown in Fig. 6 and the Do values for sample Nos 2 4 are presented in Table 1. In the sample No. 4, the R L dose reconstruction (luminescence measurements) were made within 10 days of the ~,-ray calibrated irradiation, and in the sample No. 3 this time interval was greater than 7 months. The slope k;. = d(AJ)/d(D•) of the dose dependence (Fig. 5(2) and Fig. 6(2)) is a ";-sensitivity (RL response per unit 7-dose) and is listed in Table I. In sample N o . l (with Do= 14Gy) the R L D response was not linear in the dose Dn, and the D O value was reconstructed using the TSL method (using the TSL peak at Tm,x ~ 520 K).

568

YU. A. M A R A Z U E V et al. G a m m a ray dose (Gy) 3 6 9 12 t5

0

,-, 1

•

5

I

0

~

/ /

X-ray excitation (Fig. 7, curve 1). After previous exposure to an X-ray dose D x (instead of laboratory y-irradiation) before the luminescence measurements and after thermal activation, the initial intensity Jmax increases (Fig. 7, curve 3). As the dose D x increases, the AJ response increases (Fig. 6, curve 3), and X-ray sensitivity k x = d(AJ)/d(Dx) may be measured. Its values are shown in the last column in Table 1. It is evident that y-sensitivity is 4 times larger than X-sensitivity. 4. DISCUSSION

.g-<<"

3 .

G a m m a ray dose (Gy)

Fig. 6. RL response to gamma ray dose 0-15 Gy (1) and its fragment 0-4 Gy (2) for the sample No. 4 after TA--510°C. (3) RL response to X-ray dose 0~00 Gy as a dose of initial calibrated irradiation before RL measurements. The dose reconstructions (in the dose range 1-10Gy) in or-quartz samples (not discussed here) from the Chernobyl accident zone have been performed by different methods (the pre-dose method, HT TSL, EPR with external electric field) and the results obtained are satisfactorily comparable as a rule (Brik et al., 1995). In sample No. 2 the TSL method gives the value Do = 8 _ 2 Gy, but in sample Nos 3 and 4 the pre-dose effect was small for registration with our technique.

The initial intensity Jmax of UV emission increases as the initial dose D o increases. After thermal activation UV intensity decreases during 0-60rain of X-ray excitation with Px = 100 Gy/min. The main question of the model of the RLD effect is the dynamic behavior of the luminescence: why does the UV luminescence decay with time of X-ray exposition? Combined with thermal activation, y-rays and X-rays may produce (activate) the centers of effective radiation recombination (UV emission). During irradiation there is evidently a parallel process of de-activation of the centers. After initial irradiation (dose Do) and thermal activation, an initial equilibrium is established between the 'activated' and 'de-activated' centers, and RL intensity has its starting value Jmax" In accordance with our results (Table 1) the RLD response of a-quartz exposed to 25 keV X-rays is 4

100

3.5. Lowest doses measurable (sensitivity limit) As the RLD response is measured as a variable component of UV-RL intensity AJ = Jmax--Jrain, it's lowest value measurable is determined by the ratio AJ/Jmi n. For sample No. 4 with additional calibrated y-dose DN = 0.5 Gy (Fig. 7, curve 2) the AJ value is easily detected but for the initial aliquot (with Do ~ 0.2 Gy) there is a variation of kinetics sign (Fig. 7, initial part of the curve 1) and the AJ value is not measurable. This is the main limiting factor to measuring even lower doses. The second disadvantage of the RLD technique is a superposition of the UV and blue emission bands, which is problematic for small dose evaluation (see Fig. 1, curve 4 and Fig. 7, curve 4). We consider the initial dose D o ~ 0.2 Gy as the lowest dose measurable in these crystals. 3.6. X-rays as a calibrated previous irradiation The initial sample No. 4 with Do ~ 0.2 Gy shows the slow decrease of the UV emission intensity during

i°

3

0

4

I

l0

I

20

I

30

I

40 T i m e (min)

I

50

r

60

Fig. 7. Luminescence intensity variation during X-ray excitation for the sample No.4 after thermal activation at 510°C. (1) UV emission peak (2ma~= 360 nm) intensity for initial aliquot; (2) aliquot 1 after ~,-irradiation Dr~ = 0.5 Gy; (3) aliquot 1 after X-ray calibrated initial irradiation and TA before luminescence measurements; (4) blue emission ( 2 ~ = 480 rim) peak intensity after TA for aliquot 1.

RADIOLUMINESCENT DOSIMETRY OF a-QUARTZ times smaller than that of the quartz exposed to Co 6° 7-rays for the same exposures. During X-ray excitation the initial equilibrium is shifted to the 'de-activation' side, and the intensity Jm~n is established, which corresponds to new kinetic equilibrium. The decay time r of UV emission intensity variation depends on X-ray dose rate Px (Fig. 2, curves 1-4), but P x z = constant. It means that to establish the final 'X-induced' kinetic equilibrium (J ~ Jmin), it is necessary to generate the same quantity of electron-hole pairs for de-activation of the same quantity of 'D0-induced' UV emission centers. Therefore, (Jmax--Jmin)= A J = k D o . If a procedure of R L D measurements is repeated (after the first X-ray exposition and luminescence measurements of Jm,~.~ the sample is submitted to a second thermal activation), the initial intensity increases (Jmax.~> Jmax,l). This indicates, that the R L D method is not a reproducible one. As a pre-dose method, the R L D method needs further study and evaluation. The questions of RL signal stability, radiation quenching and other problems must be considered.

569

We consider that the main advantage of the R L D technique is its high sensitivity. We suppose that the kinetic X-ray induced luminescence method described here may be suitable for use in the field of dosimetry and dating. Acknowledgement--The authors express their gratitude to

Prof. S. W. S. McKeever for fruitful comments on the original manuscript.

REFERENCES Brik A. B., Degoda V. Ya and Marazuev Yu. A. (1994) Radioluminescent dosimetry of irradiated quartz. J. Appl. Spectrosc. 60, 506-508. Brik A. B., Degoda V. Ya., Marazuev Yu. A. and Scherbina O. I. (1995) About dose loading of quartz from Chernobyl accident zone. Unpublished. Lietz J. and Matheja J. (1964) Uber Rontgenlumineszenz von Quarz. Naturwissenschaften 51, 503-504. McKeever S. W. S., Chen G. Y. and Haliburton L. E. (1985) Point defects and pre-dose effect in natural quartz. Nucl. Tracks 29, 489-495. Zimmerman J. (1971) The radiation-induced increase of the 100°C thermoluminescence sensitivity of fired quartz. J. Phys. C: Solid St. Phys. 4, 3265 3276.