The trap parameters of electrons in intermediate energy levels in quartz

Radiation Measurements 38 (2004) 743 – 746 www.elsevier.com/locate/radmeas

The trap parameters of electrons in intermediate energy levels in quartz I. Veronesea;∗ , A. Giussania , H.Y. G-oksub , M. Martinic a Universit a

degli Studi di Milano, Dipartimento di Fisica, Sezione di Fisica Medica, Via Celoria 16, Milano I-20133, Italy Forschungszentrum f&ur Umwelt und Gesundheit, Ingolst&adter Landstr. 1, Neuherberg D-85764, Germany c INFM and Universit a degli Studi di Milano-Bicocca, Dipartimento di Scienza dei Materiali, Via Cozzi 53, Milano I-20125, Italy b GSF,

Received 10 November 2003; received in revised form 10 November 2003; accepted 5 January 2004

Abstract The trap parameters (thermal activation energy Et and frequency factor s) of the glow peaks of quartz, occurring in the temperature range 420 –520 K and corresponding to intermediate energy levels, were evaluated using di6erent and complementary methods of analysis: peak shift, isothermal decay and fractional glow curve. The values of Et and of s derived with the isothermal decay method and with the peak shift technique agree quite well, within the error limits. The corresponding values obtained using the fractional glow curve analysis proved to be underestimated, probably as a consequence of the thermal quenching. Proper corrections were therefore applied to take into account this phenomenon. c 2004 Elsevier Ltd. All rights reserved.  Keywords: Quartz; Thermoluminescence; Trap parameters

1. Introduction A new challenge in retrospective dosimetry is the development of a reliable methodology for dose reconstruction applications using quartz extracted from un>red building materials, such as mortar and concrete. Various studies have been recently published regarding this new approach to retrospective dosimetry (Jain et al., 2002; G-oksu et al., 2003) and two main techniques have been employed. The >rst technique consisted in OSL single grain measurements, coupled with a probabilistic dose distribution analysis. The second approach consisted in thermoluminescence (TL) measurements and in the analysis of the peaks occurring in the intermediate temperature range of the quartz glow curve. These peaks can be used for dose determination since, due to their relatively short lifetimes, the geological TL signal is expected to be weak compared with that arising from arti>cial radiation. At present there is a little knowledge of the trap parameters of these peaks and a lack of agreement ∗ Corresponding author. Tel.: +39-02-5031-7398; fax: +39-025031-7630. E-mail address: [email protected] (I. Veronese).

c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter
doi:10.1016/j.radmeas.2004.01.012

concerning the published values. Di6erent methods of analysis of the glow peaks of interest were applied in this study in order to derive reliable values of the corresponding trap parameters.

2. Materials and methods 2.1. Sample preparation and TL measurements Analytical grade quartz purchased from MERCK Company was used in this study. Each sample, consisting of about 9 mg of powder (grain size 140 –200
m) was uniformly placed onto stainless-steel planchettes during the glow curve collection. The TL measurements were performed using a commercial automated TL reader (RisH-TLDA-12) at a 2 K s−1 heating rate, with a bialkali photomultiplier tube (Thorn-EMI 9235 QB) mounting a Schott BG-38 >lter. A proper procedure was followed to increase the sensitivity of the glow peaks of interest, consisting in several cycles of irradiation and heating treatments. The typical glow curve of sensitised quartz, irradiated with a gamma source to 0:5 Gy is shown in Fig. 1.

744

I. Veronese et al. / Radiation Measurements 38 (2004) 743 – 746

1000 Peak I

900 800 TL intensity

700 600 500 Peak II

400 300 200 100 0 325

350

375

400 425 450 475 Temperature (K)

500

525

550

Fig. 1. Glow curve of quartz sample below 550 K. Heating rate 2 K s−1 .

The peak at lower temperature (“110◦ C peak”) is not present in the glow curve because of its short lifetime at room conditions. Two main peaks can be observed: the >rst at a temperature of about 423 K (peak I) and the second just below the 473 K (peak II).

from which the values of Et and s can be derived recording several glow curves of a same sample at di6erent heating rates. Quartz samples irradiated with a 60 Co source to about 15 Gy were used for the analysis. Measurements were performed after the decay of the so-called “110◦ C peak” in order to avoid the presence of a strong luminescence signal which could a6ect the position and the shape of the two peaks of interest at higher temperature (peak I and II). The various heating rates used for the glow curves collections were setted to: 0.3– 0.5 –1–2–3–5 –8 and 10 K=s. • Isothermal decay (ID): if during the recombination processes involving the trapped electrons and the luminescent centres no re-trapping occurs, the TL signal ITL at a constant temperature will decay exponentially with time, according to the expression ITL (t) = I0 exp(−t=);

(3)

where I0 is the initial intensity at the time t0 = 0 and  is the lifetime. An estimation of the trap parameters (Et and s) can be obtained measuring the lifetime of a glow peak at di6erent temperatures of storage, being  dependent on Et and s according to the expression  = s−1 exp(Et =kT ):

2.2. Order of kinetics

(4) 60

No shift in the position of the peaks was observed with the increase of the dose (from approximately 1–120 Gy). A >rst-order kinetics was therefore assumed to describe the recombination processes of the trapped electrons. 2.3. Trap parameters analysis The following methods of analysis were employed to derive the trap parameters of the glow peaks of interest: • Peak shift (PS): the change of the heating rate at which the glow curve is measured causes a shift in the position of the peaks. The Randall–Wilkins analytical expression of a glow curve ITL (T ), under the assumption of >rst-order kinetics is (McKeever, 1985) ITL (T ) = n0 s exp(−Et =kT ) 
 s T exp(−Et =k#) d# ; ×exp −

T0

(1)

where n0 is the initial concentration of trapped electrons, s (s−1 ) is the frequency factor, Et (eV) the activation energy of the trap, k the Boltzman constant (eV K −1 ), T the temperature (K), the heating rate (K s−1 ) and is a dummy variable representing the temperature. Setting the derivate of (1) with respect to temperature equal to zero at T = Tm (where Tm is the temperature at the maximum of the peak), the following expression is obtained:

Et = s exp(−Et =kTm ) (2) kTm2

Quartz samples were irradiated with a Co source to 0.6 –1.2– 6.2–12.4 –24.8 and 124:2 Gy and stored to the controlled temperature of 317 K in an oven. Glow curves were measured at >xed times post irradiation, over a period of about 1 week. Similar measurements were performed on samples irradiated to about 2 Gy and stored in an oven to temperatures of 343 and 373 K, respectively. The decay of the TL signals of the peaks I and II was monitored also at higher temperatures keeping the irradiated samples at 393 and 413 K using the heater of the TL reader. • Fractional glow curve (FGC): this method is an improvement of the initial rise technique, which consists in considering the initial rising part of a TL peak dependent on temperature according to the expression ITL (T ) = constant exp(−Et =kT ):

(5)

The simple Eq. (5) is valid till the trapped electrons concentration can be considered constant. Therefore, this approximation can no longer be used when the signal intensity reaches the 15 –20% of the maximum intensity. An enlargement of the application of this method to the analysis of not well resolved TL peaks can be obtained performing many thermal cleaning cycles, each time using a slightly higher temperature (TSTOP ), before the collection of the remaining glow curves. A value of Et can be therefore calculated for each cycle (i.e. for each value of TSTOP ) using Eq. (5). Quartz samples were irradiated to 5 Gy using a calibrated 90 Sr beta source mounted on the TL reader. The samples were then heated to a >xed temperature, cooled and heated again to a temperature of 553 K

I. Veronese et al. / Radiation Measurements 38 (2004) 743 – 746

3.1. Peak shift Fig. 2 shows as an example the behaviour of ln(Tm2 = ) versus Tm−1 for peak II. The trap parameters, derived >tting a linear function to the experimental data, are given in Table 1. The various heating rates method is quite simple and does not require great experimental e6orts. Nevertheless important precautions are needed in order to obtain reliable results. The samples to use for this type of analysis have to be thin, thus to avoid signi>cant thermal gradient inside the samples. The use of few mg of powder placed as a thin layer on the planchettes proved to assure a homogeneity in the temperature distribution inside the sample. Moreover, the lag between the temperature of the heater and the temperature of the sample cannot be neglected for high values of heating rates. As a consequence of this lag, the Tm reported in the glow curve will be systematically higher than its actual value and the plot of ln(Tm2 = ) versus Tm−1 will deviate from a straight line in the region of high heating rates. This deviation was observed for heating rates greater than 10 K s−1 , in the experimental conditions used for the measurements. 3.2. Isothermal decay Fig. 3 shows as an example the behaviour of ln(−1 ) versus T −1 for the peak I. In the inset the typical time course of the TL signal of this peak at a constant temperature is

Peak temperature (K)

Activation energy (eV)

Frequency factor (s−1 )

Method of analysis

423 ± 2 423 ± 2 423 ± 2 473 ± 2 473 ± 2 473 ± 2

1:13 ± 0:03 1:22 ± 0:02 1:05 ± 0:02 1:46 ± 0:06 1:36 ± 0:02 1:19 ± 0:02

(3:35 ± 2:99) × 1012 (3:39 ± 1:46) × 1013 — (7:79 ± 10:91) × 1014 (2:23 ± 0:75) × 1013 —

PS ID FGC PS ID FGC

1 120

Peak I Linear fit

0

TL remaining (%)

3. Results and discussion

Table 1 Trap parameters of peaks I and II derived with the various techniques

-1 -2 -3 ln (τ-1)

recording the glow curve. A heating rate of 2 K s−1 was used both for the thermal cleaning and for the TL measurements. The same procedure was repeated with other samples of the same aliquot, using slightly higher cleaning temperatures. The range of temperature 403–487 K was investigated, each time increasing the value of TSTOP of 4 K.

745

-4

100 80 60 40 20 0

0

-5

50 100 150 200 250 300 350 Storage time (min)

-6 -7 -8 -9 -10 0.0025

0.0026

0.0027

0.0028

0.0029

0.0030

0.0031

0.0032

1/T (K -1)

Fig. 3. Isothermal dacay plot of ln(−1 ) versus T −1 for peak I. In the inset the time course of the TL signal of this peak at the storage temperature of 343 K is given.

depicted. The various values of  were derived >tting monoexponential functions to the experimental data representing the decay of the TL signal at the >xed temperatures of storage. The estimated values of the thermal activation energy and frequency factor for this peak, as well as for peak II are given in Table 1. 3.3. Fractional glow curve

15.0 14.5

Experimental data Linear Fit

14.0 13.5 ln (T 2m /
)

13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 0.00205

0.00210

0.00215

0.00220

0.00225

1/Tm (K -1)

Fig. 2. Heating rate plot of ln(Tm2 = ) versus Tm−1 for peak II.

The behaviour of the activation energy as a function of TSTOP is depicted in Fig. 4. Two plateau regions appear around 423 and 473 K and correspond to peaks I and II of the quartz glow curve, respectively. The mean values of Et for peaks I and II in these stable regions were equal to 1:05 ± 0:02 and 1:19 ± 0:02, respectively. The comparison of these values with those obtained with the two previous methods of analysis suggests that the Fractional Glow Curve technique leads to a systematic underestimation of the thermal activation energy. This characteristic, observed also in previous studies by other authors (Petrov and Baili6, 1997), is probably due to the e6ect of thermal quenching (i.e. phenomenon in which the luminescence ePciency decreases as the temperature increases). According to the Mott and Seitz

746

I. Veronese et al. / Radiation Measurements 38 (2004) 743 – 746 1.4

Thermal activation energy (eV)

1.3

1.2

1.1

1.0

0.9

0.8 400

410

420

430

440

450

460

470

480

490

TSTOP (K)

Fig. 4. Thermal activation energy as a function of TSTOP .

4. Conclusions The trap parameters of glow peaks of quartz corresponding to intermediate energy levels were derived using three di6erent methods of analysis. The PS method and the ID analysis provided results in agreement within the error limits. The FGC technique led on the contrary to an underestimation of the activation energies. A correction was therefore applied in order to take into account the thermal quenching of the TL.

1.6e+5 Peak I Peak II Fit curve

1.4e+5

Peak area

1.2e+5

The quenching parameters (W and C) were determined analysing the glow curves measured at di6erent heating rates, since the quenching induce a decrease of the peak intensities with increasing heating rates (Fig. 5). The values of W and C, derived >tting Eq. (6) to the experimental data, were equal to 0:65 ± 0:04 eV and (1:79 ± 1:69) × 107 , respectively. The corresponding corrections to Et were evaluated using Eq. (7) as QEt (423 K) = 0:16 eV and QEt (473 K) = 0:44 eV. The corrected value for peak I was therefore 1:21 eV, in reasonable agreement with the values derived with the other techniques. The correction introduced by Eq. (7) for peak II seems on the contrary to be excessively high.

1.0e+5 8.0e+4 6.0e+4 4.0e+4 2.0e+4

Acknowledgements

0.0 400

420

440 460 Temperature (K)

480

500

Fig. 5. Scaled TL peaks area as a function of peaks maximum temperature.

This research has been partially supported by a Marie Curie Fellowship of the European Community under the contract number HPMT-CT-2001-00416. References

description of the mechanism of internal quenching (Chen and McKeever, 1997), the luminescence ePciency (T ) is given by: (T ) =

1 I (T ) = ; I0 1 + C exp(−W=kT )

(6)

where I0 and I (T ) are the intensities of luminescence in absence and in presence of quenching, respectively, C is a constant and W is the thermal activation energy of quenching. The value of Et , determined by the initial rise of a glow peak, results therefore underestimated by a factor QEt , which can be calculated (Petrov and Baili6, 1997) as W QEt = : (7) 1 + C −1 exp(W=kT )

Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scienti>c, Singapore. G-oksu, H.Y., Baili6, I.K., Mikhailik, V.B., 2003. New approaches to retrospective dosimetry using cementitious building materials. Radiat. Meas. 37, 323–327. Jain, M., BHtter-Jensen, L., Murray, A.S., Jungner, H., 2002. Retrospective dosimetry: dose evaluation using unheated and heated quartz from a radioactive waste storage building. Radiat. Prot. Dosim. 101, 525–530. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. Petrov, S.A., Baili6, I.K., 1997. Determination of trap depth associated with TL peaks in synthetic quartz (350 –550 K). Radiat. Meas. 27, 185–191.