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Mechanism of thermoluminescence in natural barites
1. Phys. Chem. solids Vol. 40, pp. 405-412 Pergamon Press Ltl.. 1979. Printed in Great Britain
MECHANISM OF THERMOLUMINESCENCE NATURAL BARITES
IN
M. PROKIC Boris Kidrif Institute of Nuclear Sciences, VinPa, Belgrade, Yugoslavia (Received
14 he
1978: accepted
in revised form
8 November
1978)
Abstract-The mechanism of thermoluminescence in natural barites has been investigated by comparing the TL glow curve shapes, TL emission spectra and ESR spectra of natural barites and impurity doped synthetic barium sulphate phosphors. The results of TL-ESR correlation studies indicate that the anion radicals formed on irradiation act as hole traps and are responsible for the bar&es TL glow curve peaks, i.e. the temperatures at which anion radicals are anihilated correspond to the temperatures of the glow peaks of natural barites. The TL mechanism involves the release of holes during the thermal activation which recombine with electrons trapped at the host cation or at lattice defect sites. Energy thus released is non-radiatively transferred to the Pb” impurity ion which is identified to be responsible for TL emission of natural barites. On return to the normal state, the Pb*’ ion emits its characteristic emission.
I. INTRODUCTION 9
During the last few years thermoluminescent phosphors having high effective atomic numbers have become the subject of intensive studies since their dosimetric properties offer an.exceptionai potential application in the specific fields of radiation dosimetry, e.g. as an X-ray quality indicator for determination of the “effective” radiation energy of X-rays (Dixon[l] and Yamashita[2]). In view of the possibility of employing natural barite as a thermoluminescent radiation dosimeter with high effective atomic number (ProkiE [3,4]) it is necessary to have detailtid knowledge about thermoluminescence processes in this material. In the previous paper[51 the kinetics of the thermoluminescence process in natural barites have been examined. Since the mechanism responsible for the phenomenon of thermoluminescence emission of natural barites is insufficiently understood, the purpose of these investigations was the study of the basic thermoluminescent process in natural barites with the intention to propose a physical model to explain the thermoluminescent mechanism.
6
7
6
2. E!U’ERWEWl’ALMETHODANDRESULTS Measurements were performed
of
the
emitted
with natural
barite
thermoluminescence samples
annealed
to
in natural barites by surrounding soil radioactivity. Thermally treated barite samples, from the site Velika KladuSa, (recommended for use in radiation dosimeters, ProkiC[3,4]) irradiated to doses less than about 2000 rad, emitted strong thermoluminescence with two glow peaks, a main peak at about 205°C and a smaller peak around 115°C (Fig. 1). It was observed that after exposure of the barite samples to doses above some threshold dose (different for the barite samples of the different origin) a new peak appeared at about SO-170°C (Fig. 2). Some barite samples from different localities in Yugoslavia, for eliminate
thermoluminescence
accumulated
0
60
loo
1st
200
Temperature,
250
300
I
0
‘C
Fig. 1. Glow curve of annealed, gamma irradiated natural barite (site V. Kladu&a) after irradiation to 10 rads. 405
M. PROI&
406
20
Ii0
230
290
Temperature, *C
Fig. 2. Glow curve of annealed naturalbarite (site V. KladuSa) after irradiationto doses above 2OOOrads.
0
Temperature, *C
example from KreSevo, Gracac, Podkovac, Zletovo, etc. (except for samples from V. Kladuga and F&, the latter showing a single glow peak at about 185°C with a small low temperature peak) at all levels of irradiation show three glow peaks at approximately the same temperature as the glow peaks of V. KladuSa barite samples when the latter is irradiated to doses above 200rad (Fig. 3). The threshold dose for the appearance of the new peak at about 1%160°C for the barite samples near F0L!a is 50krad. For the barite samples examined by Gupta[6], the three glow peaks appeared for doses over 2Okrad. For doses above lvrad, the glow peak at about 1% 170°C becomes dominant for all barite samples regardless of the deposit locality, although there is a definite peak position shift toward lower temperatures (MO15O’C) for the high dose values. Depending on the deposit locality as well as on the crystalline form, the barite samples show different TL sensitivities-from a few mrad to a few tens of mrads; the upper detection limit amounts to about IO’ rads. All the glow curves f0r barite samples were recorded on a Harshaw Model 2000 TL Analyser. 2.1 Emission centres In order to explain the mechanism of the thermoluminescent process in barite it is necessary to know the nature of the charge and emission centres. An important and useful technique in studying emission centres involves incorporatiOn of the known impurities into pure synthetic samples. By comparing glow curves of the natural bar&e samples with those of synthetic thermoluminescent materials doped with the known activator
Fii. 3. Glow curve of annealed naturalbark of diierent origin (KAevo, Graeac,PodkovaE)after irradiationto 10rads. impurities, it is possible to identify the impurity responsible for the TL emission of the natural mineral barite. Studies of the TL emission spectra have provided additional data about the nature of the emission centres. To get the synthetic barium sulphate samples with incorporated activator impurities we have precipitated BaSOJ from a solution of BaC&-2Hz0 with dilute sulphuric acid. Undoped synthetic barium sulphate samples precipitated from dilute HISOJ acid irradiited to doses of 2Okrads give stronger ZZ peaks than barhim sulphate samples precipitated from a solution of BaCh-2Hz0 with dilute &SO,. However, those precipitated from dilute &SO. induced the strongest TL peak at about 115°C because potassium in some sulphate phosphors induce strong low temperature peaks. The studies were extended to the effect of the time of ageing the sediment on the peak intensities: in the case of the shortest ageing time (a few minutes) the samples obtained were inst&ciently pure. Following irradiation to doses of about one krad, the most intense peak was the peak at about 2OOY. It was also observed that solutions with low umcentrati0ns of the constituents in the basic solution and with low activator concentrations show a dominant peak at about 2OtPC.Higher umcentrati0ns of the basic solution show the appearance of three ZZ peaks (for all dose levels), and with samples precipitated from c0ncentrated stllphuric acid, the dominant peak was the one at about 15&17o”c. Impurity c0ncentrati0ns in samples obtained by chemical and spectr0graphical analysis are presented in Table 1.
407
Mechanism of thermoluminescence in natural barites Table 1. Impurity concentrations in synthetic BaSO, samples
Impurity
Cl
Si
As
Pb
Fe
Ca
Mn
Al
Concentration
0.5
-
0.1
5
1
50
-
-
N
2
20
ff
Na
Mg
-
-
6m) Specific impurities in barium sulphate samples were incorporated by coprecipitation with fixed amounts of pure synthetical BaSO, samples precipitated from a solution of BaC12.2Ht0 with dilute sulphuric acid. Since within all natural barite samples we identified the following impurities: Mn, Ag, Pb, Al, etc. (Table 2) which could act as good activators, the synthetic barium sulphate was doped with each of these impurity ions. The BaSO,:Mn phosphor samples show the most intense TL peaks in the low temperature region (about lloOC), and although Mn ions acted as a good activator, the characteristic Mn2+ luminescence is in the green-orange range of the optical spectrum, while all natural barite samples examined emitted an intense deep-blue light. BaSO.,:Ag and BaSO.:Ag, Pb phosphors emitted intense thermoluminescence, however, both of the TL materials showed an intense fluorescence, which is a property of Ag, since barite as well as BaS04:Pb did not show this effect. Irradiated with 6oCo y-rays (doses about 106rads) and heated to a temperature of about MO-15O”C,barite samples emitted a strong deep blue light. The emission spectra of all three glow peaks of barite samples are the same, so the same emission centre is responsible for all the TL peaks and it is likely that these centres are randomly distributed with respect to the large majority of the trapping centres. Within most barite samples could be seen several grains which emitted bright yellowishwhite light. TL emission spectra of the barite samples as well as doped synthetic barium sulphate samples show wide spectral bands with a maximum at a wavelength of about 370nm for natural barite samples, and at the corresponding wavelengths for BaS04:Ag, BaS04:Mn samples, BaSO,:Pb and BaSO&l doped barium sulphate samples. Emission spectra were observed using a SPM-2 Zeiss grating monochromator. Figure 4 also shows theTL emission spectrum of a BaSa:Pb phosphor, the shape of which corresponds to that of barite. It is thus evident that the mineral barite emission is most probably caused by Pb2+ ions. It is known that lead can act as an activator, as a sensitizer of emission of the host crystal[q, and can behave as a sensitizer of the activator emission[S]. Pb ions act as an activator at very low concentrations of about 1ppm. Depending on the host crystal the optimum concentration of Pb ion activators amounted to between 10 and 1000ppm. The luminescence emission of Pb centres is related to the atomic levels of the Pb2+ ions (the so called Hg-like emission) such as TI’, Pb2+ and Sb3’. Spectroscopic levels of the expected states of Hg-like cations are, in order of in-
creasing energy 3P0 c
3PI < 3P2 < ‘PI. There is a problem as to which of the allowed Pb2+ transitions can the observed emission maxima be related in barite and BaSO$b samples. This type of emission has been found (weakly) in KCkPb and, more intensely, in KClTl (Pb” ions are isoelectronic with Tl’). The emission in the two former compounds has been related to the ‘PI + ‘So transition. According to BettinaIli[9] and Medlin[lO], respectively the emission from Pb2’ ions of BaC12:Pb and lead-activated synthetic CaCOs:Pb sai.tples can be related to the ‘P, + ‘So transition. If, as is very probable, the TL emission of natural barites originates predominantly from Pb2’ ions, then the emission can be related to the ‘PI + “Sotransition. The spectral character of the thermoluminescence is found to change with the amount of radiation dose. After irradiation with extremely high doses of approximately 6 x lO’rad, there occurs a decrease of the deep blue emission in favour of a pale bluish-green emission which becomes predominant. Simultaneously at these values of the radiation dose the intensity of the emitted thermoluminescence drops rapidly, indicating that it is possibly due to radiation damage in the barite lattice. Moreover, the undoped synthetic barium sulphate samples and recrystallized natural barite samples which have first been totally dissolved in concentrated sulphuric acid both emit a pale bluish-green emission as the most intense one after irradiation with any amount of radiation dose. This emission could not be attributed to any particular impurity, so it is quite likely that it is associated with the host lattice. Smith[ll] has reported that natural barite samples from Palos Verdes (Cahiomia) have a yellowish-white emission (“afterglow”) while specimens from England have only a pale bluish-green emission. All other natural barites according to Przibram[12] emit a deep blue light.
2.2 Charge trapping centres Trapping centres in natural barite samples have been examined by two diRerent methods: (1) by measurement of the residual thermoluminescence after thermal treatment of irradiated barite samples which gives data about the energy levels associated with traps (presented in the previous paperIS]), and (2) by ESR (Electron Spin Resonance) studies of the irradiated and thermally treated samples, giving detailed information about the nature and identity of the defect sites which have a role in the charge traps. The investigations were performed for high irradiation doses, i.e. with all charge trapping
Table 2. Impurity concentrations in experimental natural bade specimens Impurity Concentration @pm)
Si 80
Cr 10
Mg 20
Mn 10
Al 5
Pb 15
Ca 15
Cu 100
Fe 200
Sr 500
M. PROKIC
(Mn)
300
4X)
400
300
Wavrisngth,X,
sso
nm
Fig. 4. Thermoluminescence emission spectra of natural barite and different impurity-doped barium sulphate samples.
1
r
--Earite
,Bo%:Mn BaSQ:Ag
Barite loso4: -_n An
BaSQ4:Pb !‘I .I
Temperature, ‘C
(a)
(b)
Fig. 5. (a) Glow curves of doped and undoped synthetic barium sulphate samplesirradiated to lo6 rad, (b) Glow curves of undoped synthetic barium sulphate samples irradiated to 20 rads.
Mechanismof thermoluminescencein natural barites
centres lilled, a condition necessary for correct interpretation. It is evident that all barite samples as well as doped and undoped synthetic barium sulphate exhibit TL peaks at similar temperatures (Fig. 5). It follows that the TL glow curve shape does not depend on activatorimpurity ions, and that these do not act as charge trapping centres, but instead that during the thermoluminescent emission the thermally released charge originates from BaS04 crystal lattice centres, while the activator ions play only a part in recombination (luminescent) centres. Namely, the various activators produce various emission TL. spectra but the glow peak temperatures are seen to be independent of dopant impurity, showing that the released charge carriers asso-
110
200
Temperature, ‘C Fig. 6(a).
409
ciated with different glow peaks are from the lattice defect centres of the host medium. The TL peak temperature may vary by as much as +2OT. The nonactivator impurities present, as well as different incorporated activator concentrations, may be responsible for shifting of the TL peak position. In order to analyse the observation that the natural barite samples show two glow 7Z peaks for doses below some “threshold” dose, the barite samples (V. KladuSa site) were subjected to the following treatments: (a) various thermal treatments of the barite samples prior to irradiation, (b) treatment by sulphuric acid, dilute and concentrated, (c) recrystallization of a natural barite sample, one sample being totally dissolved in sulphuric acid and crystallized. All the experiments performed showed that the first glow peak of the barite samples was greatly increased in all three cases, exactly as in almost all impurity-activated synthetic BaSO, samples (Figs. 6, a, b, c). It has also been observed that in several samples of recrystallized barite samples irradiated with doses far below 200rad a new peak appeared at about 1%170°C (shown in Fig. 6d), exactly like that in the synthetic sample and in some natural barite samples as shown in Fig. 3. It is evident that the initial density of the traps for capture of the released charge carriers for the peak at about 15fL17O”Cin certain samples of natural barites is negligible, and that during the thermal treatment there occurs a redistribution of the total number of traps in favour of the trap involved with that peak. Both effects can be ascribed to one of the following causes: (a) the influence of thermal treatment of the samples prior to irradiation, which is supposed to cause a change in the number of traps corresponding to one peak, or traps
110
ioo
Temperature, ‘C Fig. 6(b).
.
290
Fig. 6. tntens~tion of ihe lowtern~ratureglowpeakfor: (a) thermaltreated barite sampiesfv. KladuSa)effectof pre-irradiation thermal tnatment on the glow-curveof natural barite for diierent precept time (at 1WC) from I to 18hr, (b) naWraI bark (V. KIaduSasite) samplestreated in the sulphuricacid for differenttimes prior to irradiation(from 1to 24hr), (c) recrystallized natural barite samples, fd) recrystallized natural barite samples.showingthe appearanceof the glowpeak at about 160°C after irradiationto 60rads.
Temperature,
‘C
Fig. 6(c).
having equal or closely spaced activation energy values, (b) a change in efficiencyof the trappingcentres, or (c) a change in the efficiency of conversion of the trapped charge carriers into photons. For the same reasons, the cooling rate effect (the absolute and relative heights of the glow peaks depend upon the rate of coolingfrom the annealingtem~rat~) has an importantinfluenceon the g!ow curve shape. Charge mobility, concentration of competing defects and capture cross sections are all temperature dependent. In order to analyse the observed effect of recrystallization on synthetic barium sulphate samples, the undoped synthetic barium sulphate and Mn, Pb, Al or Ag activators were dissolved in concentrated sulphuricacid, boiled and evaporated to dryness. After the same annealii treatment and irradiation, it is observed that the recision method for synthetic barium sulphate sample preparation, as well as coprecipitation of barium stiphate with various impurities from concentrated acids, induced three glow peaks for every dose Ievel, with the exception of BaSOAI samples. These showed only two g&w peaks for doses under a few tens
of krads, at about 110°Cand 215°C(for coprecipitated BaSOA samples). Irradiation to doses of around 101’radalso induced in this phosphor a third peak at about 17OT,identicalto all other doped bariumsulphate and natural barite samples.Tbe observed effects indicate the complexity of the ‘IZ process, and particularly the need for caution in interpreting,analyziogand comparing the ~e~o~~s~n~ of natural and synthetic samples. 2.3 ESR A~lysfs
Defects may be observed by ESR (Electron Spin Resonance) analysis, or optical absorption techniques. However, the ESR method is employed as an especially sensitive detector of effects caused by radiation in crystals. The barite ESR spectra were recorded at room temperature, after having been irradiated with a dose of 2 x Wrad, using a Varian X band spectrometer with field modulation of lOOKC!s-‘.‘l%enatural barite ESR spectra were #mp~ed with the synthetic barium sulphate ESR spectra obtained by Spitsyn[l3], Gupta[l4] and Luthra[ls]. Four lines with the g-values 2.0333, 2.W, 2.oc#n and 2.0033were observed in samples of natural barite, pure barium sulphateand in doped barium sulphatesamples(Fig. 7). The signalshave been ascribed to anion radicals SO;, SO;, SO;, SO; and 0; vaulting from gamma~mtion which act as hole opt centres. Natural barite, barium sulphate and Pb** doped bariumstiphate sampleshave a similardecay of the ESR signalsat the titrates co~es~nd~ to the peaks of TL glow curves for the samples anatysed. Thermal arm&@ behaviour of ESR signalsand glowpeaks show
411
Mechanismof thermoluminescencein naturalbarites Natuml Barlm
undopod
Boso4
r
-“hr .
Baso4:
Pb
x 2.5
160% X5
+I/
Fig. 7. ESR spectraof naturalundopedbarites,and Pb-dopedbariumsulphatesamplesafter gammairradiationat room temperature;typical thermalanneali characteristicsof ESR signalsof irradiatednaturalbaritesamples.
that the temperatures at which these anion radicals are annihilatedcorrespond to the temperatures of the prominent glow peaks of the samples examined, indicating that the holes from these centres are responsiblefor the peaks of the barite glow curve. 3. DISCUSSION
A model for the 7” process in barite has been proposed on the assumptionthat the luminescentcenters do not act as charge trapping centres, taking into. account the fact that the emission center varies with the change of activator, while the TL glow curve peak positions are not inthrenced by the type of activator, indicating that the released charge carriers for all glow curve peaks originatefrom crystal lattice centres of the host material. It follows that the impuritiesin some way participate in the TL emission process, although they do not act as charge trapping centres. A similar type of emission process was proposed by Medlin[lOlfor natural calcite samples. Since the 77_,glow curve peaks are not iniluencedby activator impurity ions, the TL mechanism is believed to be a process involvingthe release of holes from different anion radicals followed by their recombination with electrons. The Z’LESR correlation studies have shown that the hole traps are responsible for the ZZ glow curve peaks. Since it has been shownthat at the annihilationtemperatures of the anion radicals produced by gamma-rays,which act as hole trapping centres, the TL glow peaks appear, it is suggestedthat the thermally released holes from each of these centres are mobile,and hence recombine with electrons from some of the electron trapping centers. Most probably, the electrons are trapped in centres formed by cations of the host crystal lattice or at the sites of lattice defects. Duringthe heating of the crystal, the released holes recombine with electrons from the trapping centres and the recombination energy thus released is non-radiativelytransferred to the nearest Pb” ion, raising it to an excited level. By
c. 8. =I
w
P
W”
Emirrlon
Lk
Fig. 8. Model for the thermoluminescentprocess in natural barites. returning to its normal state, the excited Pb*’ impurity ion acts as a thermoluminescent centre, giving its characteristicemission(Fig. 8). ttJmxmJcE9
1. DixonR. L. and EkstrandK. E., Phys. Med. Biol. 19. 1% (1974). 2. Yamashita T.. Ftvc. 4th Conf. Luminescent lbsimetry (Editedby T. Niewiadomski).p. 467. Krakow(1974). 3. Prokic hf.. bat. 3. Appl. Rod. Isot. 25,545 (1974). 4. ProLiE M., Ph.D Thesis, Belgrade University, Yugoslavia (1976). 5. Prokic hf., J. Phys. Chem. Solids 38,717 (1977). 6. GuptaN. hf., LuthraJ. hf. and ShankarJ., Indian J. Pure and Appl. Phys. 11,684 (1973).
412
M. PROKI~
7. Vtam C., Physica 15,609 (1949). 8. Schuiman J. H.. Br. L A&. Phys. 6, S64 (195.5). 9. Bettinafli C. and Ferraresso G.. Rend. Classe Scien. Fis. Mat. Nat, VIII, 569 (1%8). 10. Medlin W. L.. Thermoluminescenceof Geological Materials, Vol. 15. Academic Press, London (1968). 11.Smith 0. C., Ident~cation and ~uulitativ~ Chemical Analysis of Mine~ols. Van Nostrand, Toronto ~1946~.
12. Przibram K., ~~ad~attaa&lows and Lumtaescence. Pergamon Press, New York (f956).
13. Spitsyn V. I., Gromov V. V. and Karaseva L. G., Lbkl. Akad. Nauka SSSR, 159, 179(1964). 14. Gupta N. M., Luthra J. M. and Shankar J., Rad. Effects 21, 151 (1974). 15. Luthra J. M., GuptaN. M., .f. Lamin. 9,94 (1974).
MECHANISM OF THERMOLUMINESCENCE NATURAL BARITES
IN
M. PROKIC Boris Kidrif Institute of Nuclear Sciences, VinPa, Belgrade, Yugoslavia (Received
14 he
1978: accepted
in revised form
8 November
1978)
Abstract-The mechanism of thermoluminescence in natural barites has been investigated by comparing the TL glow curve shapes, TL emission spectra and ESR spectra of natural barites and impurity doped synthetic barium sulphate phosphors. The results of TL-ESR correlation studies indicate that the anion radicals formed on irradiation act as hole traps and are responsible for the bar&es TL glow curve peaks, i.e. the temperatures at which anion radicals are anihilated correspond to the temperatures of the glow peaks of natural barites. The TL mechanism involves the release of holes during the thermal activation which recombine with electrons trapped at the host cation or at lattice defect sites. Energy thus released is non-radiatively transferred to the Pb” impurity ion which is identified to be responsible for TL emission of natural barites. On return to the normal state, the Pb*’ ion emits its characteristic emission.
I. INTRODUCTION 9
During the last few years thermoluminescent phosphors having high effective atomic numbers have become the subject of intensive studies since their dosimetric properties offer an.exceptionai potential application in the specific fields of radiation dosimetry, e.g. as an X-ray quality indicator for determination of the “effective” radiation energy of X-rays (Dixon[l] and Yamashita[2]). In view of the possibility of employing natural barite as a thermoluminescent radiation dosimeter with high effective atomic number (ProkiE [3,4]) it is necessary to have detailtid knowledge about thermoluminescence processes in this material. In the previous paper[51 the kinetics of the thermoluminescence process in natural barites have been examined. Since the mechanism responsible for the phenomenon of thermoluminescence emission of natural barites is insufficiently understood, the purpose of these investigations was the study of the basic thermoluminescent process in natural barites with the intention to propose a physical model to explain the thermoluminescent mechanism.
6
7
6
2. E!U’ERWEWl’ALMETHODANDRESULTS Measurements were performed
of
the
emitted
with natural
barite
thermoluminescence samples
annealed
to
in natural barites by surrounding soil radioactivity. Thermally treated barite samples, from the site Velika KladuSa, (recommended for use in radiation dosimeters, ProkiC[3,4]) irradiated to doses less than about 2000 rad, emitted strong thermoluminescence with two glow peaks, a main peak at about 205°C and a smaller peak around 115°C (Fig. 1). It was observed that after exposure of the barite samples to doses above some threshold dose (different for the barite samples of the different origin) a new peak appeared at about SO-170°C (Fig. 2). Some barite samples from different localities in Yugoslavia, for eliminate
thermoluminescence
accumulated
0
60
loo
1st
200
Temperature,
250
300
I
0
‘C
Fig. 1. Glow curve of annealed, gamma irradiated natural barite (site V. Kladu&a) after irradiation to 10 rads. 405
M. PROI&
406
20
Ii0
230
290
Temperature, *C
Fig. 2. Glow curve of annealed naturalbarite (site V. KladuSa) after irradiationto doses above 2OOOrads.
0
Temperature, *C
example from KreSevo, Gracac, Podkovac, Zletovo, etc. (except for samples from V. Kladuga and F&, the latter showing a single glow peak at about 185°C with a small low temperature peak) at all levels of irradiation show three glow peaks at approximately the same temperature as the glow peaks of V. KladuSa barite samples when the latter is irradiated to doses above 200rad (Fig. 3). The threshold dose for the appearance of the new peak at about 1%160°C for the barite samples near F0L!a is 50krad. For the barite samples examined by Gupta[6], the three glow peaks appeared for doses over 2Okrad. For doses above lvrad, the glow peak at about 1% 170°C becomes dominant for all barite samples regardless of the deposit locality, although there is a definite peak position shift toward lower temperatures (MO15O’C) for the high dose values. Depending on the deposit locality as well as on the crystalline form, the barite samples show different TL sensitivities-from a few mrad to a few tens of mrads; the upper detection limit amounts to about IO’ rads. All the glow curves f0r barite samples were recorded on a Harshaw Model 2000 TL Analyser. 2.1 Emission centres In order to explain the mechanism of the thermoluminescent process in barite it is necessary to know the nature of the charge and emission centres. An important and useful technique in studying emission centres involves incorporatiOn of the known impurities into pure synthetic samples. By comparing glow curves of the natural bar&e samples with those of synthetic thermoluminescent materials doped with the known activator
Fii. 3. Glow curve of annealed naturalbark of diierent origin (KAevo, Graeac,PodkovaE)after irradiationto 10rads. impurities, it is possible to identify the impurity responsible for the TL emission of the natural mineral barite. Studies of the TL emission spectra have provided additional data about the nature of the emission centres. To get the synthetic barium sulphate samples with incorporated activator impurities we have precipitated BaSOJ from a solution of BaC&-2Hz0 with dilute sulphuric acid. Undoped synthetic barium sulphate samples precipitated from dilute HISOJ acid irradiited to doses of 2Okrads give stronger ZZ peaks than barhim sulphate samples precipitated from a solution of BaCh-2Hz0 with dilute &SO,. However, those precipitated from dilute &SO. induced the strongest TL peak at about 115°C because potassium in some sulphate phosphors induce strong low temperature peaks. The studies were extended to the effect of the time of ageing the sediment on the peak intensities: in the case of the shortest ageing time (a few minutes) the samples obtained were inst&ciently pure. Following irradiation to doses of about one krad, the most intense peak was the peak at about 2OOY. It was also observed that solutions with low umcentrati0ns of the constituents in the basic solution and with low activator concentrations show a dominant peak at about 2OtPC.Higher umcentrati0ns of the basic solution show the appearance of three ZZ peaks (for all dose levels), and with samples precipitated from c0ncentrated stllphuric acid, the dominant peak was the one at about 15&17o”c. Impurity c0ncentrati0ns in samples obtained by chemical and spectr0graphical analysis are presented in Table 1.
407
Mechanism of thermoluminescence in natural barites Table 1. Impurity concentrations in synthetic BaSO, samples
Impurity
Cl
Si
As
Pb
Fe
Ca
Mn
Al
Concentration
0.5
-
0.1
5
1
50
-
-
N
2
20
ff
Na
Mg
-
-
6m) Specific impurities in barium sulphate samples were incorporated by coprecipitation with fixed amounts of pure synthetical BaSO, samples precipitated from a solution of BaC12.2Ht0 with dilute sulphuric acid. Since within all natural barite samples we identified the following impurities: Mn, Ag, Pb, Al, etc. (Table 2) which could act as good activators, the synthetic barium sulphate was doped with each of these impurity ions. The BaSO,:Mn phosphor samples show the most intense TL peaks in the low temperature region (about lloOC), and although Mn ions acted as a good activator, the characteristic Mn2+ luminescence is in the green-orange range of the optical spectrum, while all natural barite samples examined emitted an intense deep-blue light. BaSO.,:Ag and BaSO.:Ag, Pb phosphors emitted intense thermoluminescence, however, both of the TL materials showed an intense fluorescence, which is a property of Ag, since barite as well as BaS04:Pb did not show this effect. Irradiated with 6oCo y-rays (doses about 106rads) and heated to a temperature of about MO-15O”C,barite samples emitted a strong deep blue light. The emission spectra of all three glow peaks of barite samples are the same, so the same emission centre is responsible for all the TL peaks and it is likely that these centres are randomly distributed with respect to the large majority of the trapping centres. Within most barite samples could be seen several grains which emitted bright yellowishwhite light. TL emission spectra of the barite samples as well as doped synthetic barium sulphate samples show wide spectral bands with a maximum at a wavelength of about 370nm for natural barite samples, and at the corresponding wavelengths for BaS04:Ag, BaS04:Mn samples, BaSO,:Pb and BaSO&l doped barium sulphate samples. Emission spectra were observed using a SPM-2 Zeiss grating monochromator. Figure 4 also shows theTL emission spectrum of a BaSa:Pb phosphor, the shape of which corresponds to that of barite. It is thus evident that the mineral barite emission is most probably caused by Pb2+ ions. It is known that lead can act as an activator, as a sensitizer of emission of the host crystal[q, and can behave as a sensitizer of the activator emission[S]. Pb ions act as an activator at very low concentrations of about 1ppm. Depending on the host crystal the optimum concentration of Pb ion activators amounted to between 10 and 1000ppm. The luminescence emission of Pb centres is related to the atomic levels of the Pb2+ ions (the so called Hg-like emission) such as TI’, Pb2+ and Sb3’. Spectroscopic levels of the expected states of Hg-like cations are, in order of in-
creasing energy 3P0 c
3PI < 3P2 < ‘PI. There is a problem as to which of the allowed Pb2+ transitions can the observed emission maxima be related in barite and BaSO$b samples. This type of emission has been found (weakly) in KCkPb and, more intensely, in KClTl (Pb” ions are isoelectronic with Tl’). The emission in the two former compounds has been related to the ‘PI + ‘So transition. According to BettinaIli[9] and Medlin[lO], respectively the emission from Pb2’ ions of BaC12:Pb and lead-activated synthetic CaCOs:Pb sai.tples can be related to the ‘P, + ‘So transition. If, as is very probable, the TL emission of natural barites originates predominantly from Pb2’ ions, then the emission can be related to the ‘PI + “Sotransition. The spectral character of the thermoluminescence is found to change with the amount of radiation dose. After irradiation with extremely high doses of approximately 6 x lO’rad, there occurs a decrease of the deep blue emission in favour of a pale bluish-green emission which becomes predominant. Simultaneously at these values of the radiation dose the intensity of the emitted thermoluminescence drops rapidly, indicating that it is possibly due to radiation damage in the barite lattice. Moreover, the undoped synthetic barium sulphate samples and recrystallized natural barite samples which have first been totally dissolved in concentrated sulphuric acid both emit a pale bluish-green emission as the most intense one after irradiation with any amount of radiation dose. This emission could not be attributed to any particular impurity, so it is quite likely that it is associated with the host lattice. Smith[ll] has reported that natural barite samples from Palos Verdes (Cahiomia) have a yellowish-white emission (“afterglow”) while specimens from England have only a pale bluish-green emission. All other natural barites according to Przibram[12] emit a deep blue light.
2.2 Charge trapping centres Trapping centres in natural barite samples have been examined by two diRerent methods: (1) by measurement of the residual thermoluminescence after thermal treatment of irradiated barite samples which gives data about the energy levels associated with traps (presented in the previous paperIS]), and (2) by ESR (Electron Spin Resonance) studies of the irradiated and thermally treated samples, giving detailed information about the nature and identity of the defect sites which have a role in the charge traps. The investigations were performed for high irradiation doses, i.e. with all charge trapping
Table 2. Impurity concentrations in experimental natural bade specimens Impurity Concentration @pm)
Si 80
Cr 10
Mg 20
Mn 10
Al 5
Pb 15
Ca 15
Cu 100
Fe 200
Sr 500
M. PROKIC
(Mn)
300
4X)
400
300
Wavrisngth,X,
sso
nm
Fig. 4. Thermoluminescence emission spectra of natural barite and different impurity-doped barium sulphate samples.
1
r
--Earite
,Bo%:Mn BaSQ:Ag
Barite loso4: -_n An
BaSQ4:Pb !‘I .I
Temperature, ‘C
(a)
(b)
Fig. 5. (a) Glow curves of doped and undoped synthetic barium sulphate samplesirradiated to lo6 rad, (b) Glow curves of undoped synthetic barium sulphate samples irradiated to 20 rads.
Mechanismof thermoluminescencein natural barites
centres lilled, a condition necessary for correct interpretation. It is evident that all barite samples as well as doped and undoped synthetic barium sulphate exhibit TL peaks at similar temperatures (Fig. 5). It follows that the TL glow curve shape does not depend on activatorimpurity ions, and that these do not act as charge trapping centres, but instead that during the thermoluminescent emission the thermally released charge originates from BaS04 crystal lattice centres, while the activator ions play only a part in recombination (luminescent) centres. Namely, the various activators produce various emission TL. spectra but the glow peak temperatures are seen to be independent of dopant impurity, showing that the released charge carriers asso-
110
200
Temperature, ‘C Fig. 6(a).
409
ciated with different glow peaks are from the lattice defect centres of the host medium. The TL peak temperature may vary by as much as +2OT. The nonactivator impurities present, as well as different incorporated activator concentrations, may be responsible for shifting of the TL peak position. In order to analyse the observation that the natural barite samples show two glow 7Z peaks for doses below some “threshold” dose, the barite samples (V. KladuSa site) were subjected to the following treatments: (a) various thermal treatments of the barite samples prior to irradiation, (b) treatment by sulphuric acid, dilute and concentrated, (c) recrystallization of a natural barite sample, one sample being totally dissolved in sulphuric acid and crystallized. All the experiments performed showed that the first glow peak of the barite samples was greatly increased in all three cases, exactly as in almost all impurity-activated synthetic BaSO, samples (Figs. 6, a, b, c). It has also been observed that in several samples of recrystallized barite samples irradiated with doses far below 200rad a new peak appeared at about 1%170°C (shown in Fig. 6d), exactly like that in the synthetic sample and in some natural barite samples as shown in Fig. 3. It is evident that the initial density of the traps for capture of the released charge carriers for the peak at about 15fL17O”Cin certain samples of natural barites is negligible, and that during the thermal treatment there occurs a redistribution of the total number of traps in favour of the trap involved with that peak. Both effects can be ascribed to one of the following causes: (a) the influence of thermal treatment of the samples prior to irradiation, which is supposed to cause a change in the number of traps corresponding to one peak, or traps
110
ioo
Temperature, ‘C Fig. 6(b).
.
290
Fig. 6. tntens~tion of ihe lowtern~ratureglowpeakfor: (a) thermaltreated barite sampiesfv. KladuSa)effectof pre-irradiation thermal tnatment on the glow-curveof natural barite for diierent precept time (at 1WC) from I to 18hr, (b) naWraI bark (V. KIaduSasite) samplestreated in the sulphuricacid for differenttimes prior to irradiation(from 1to 24hr), (c) recrystallized natural barite samples, fd) recrystallized natural barite samples.showingthe appearanceof the glowpeak at about 160°C after irradiationto 60rads.
Temperature,
‘C
Fig. 6(c).
having equal or closely spaced activation energy values, (b) a change in efficiencyof the trappingcentres, or (c) a change in the efficiency of conversion of the trapped charge carriers into photons. For the same reasons, the cooling rate effect (the absolute and relative heights of the glow peaks depend upon the rate of coolingfrom the annealingtem~rat~) has an importantinfluenceon the g!ow curve shape. Charge mobility, concentration of competing defects and capture cross sections are all temperature dependent. In order to analyse the observed effect of recrystallization on synthetic barium sulphate samples, the undoped synthetic barium sulphate and Mn, Pb, Al or Ag activators were dissolved in concentrated sulphuricacid, boiled and evaporated to dryness. After the same annealii treatment and irradiation, it is observed that the recision method for synthetic barium sulphate sample preparation, as well as coprecipitation of barium stiphate with various impurities from concentrated acids, induced three glow peaks for every dose Ievel, with the exception of BaSOAI samples. These showed only two g&w peaks for doses under a few tens
of krads, at about 110°Cand 215°C(for coprecipitated BaSOA samples). Irradiation to doses of around 101’radalso induced in this phosphor a third peak at about 17OT,identicalto all other doped bariumsulphate and natural barite samples.Tbe observed effects indicate the complexity of the ‘IZ process, and particularly the need for caution in interpreting,analyziogand comparing the ~e~o~~s~n~ of natural and synthetic samples. 2.3 ESR A~lysfs
Defects may be observed by ESR (Electron Spin Resonance) analysis, or optical absorption techniques. However, the ESR method is employed as an especially sensitive detector of effects caused by radiation in crystals. The barite ESR spectra were recorded at room temperature, after having been irradiated with a dose of 2 x Wrad, using a Varian X band spectrometer with field modulation of lOOKC!s-‘.‘l%enatural barite ESR spectra were #mp~ed with the synthetic barium sulphate ESR spectra obtained by Spitsyn[l3], Gupta[l4] and Luthra[ls]. Four lines with the g-values 2.0333, 2.W, 2.oc#n and 2.0033were observed in samples of natural barite, pure barium sulphateand in doped barium sulphatesamples(Fig. 7). The signalshave been ascribed to anion radicals SO;, SO;, SO;, SO; and 0; vaulting from gamma~mtion which act as hole opt centres. Natural barite, barium sulphate and Pb** doped bariumstiphate sampleshave a similardecay of the ESR signalsat the titrates co~es~nd~ to the peaks of TL glow curves for the samples anatysed. Thermal arm&@ behaviour of ESR signalsand glowpeaks show
411
Mechanismof thermoluminescencein naturalbarites Natuml Barlm
undopod
Boso4
r
-“hr .
Baso4:
Pb
x 2.5
160% X5
+I/
Fig. 7. ESR spectraof naturalundopedbarites,and Pb-dopedbariumsulphatesamplesafter gammairradiationat room temperature;typical thermalanneali characteristicsof ESR signalsof irradiatednaturalbaritesamples.
that the temperatures at which these anion radicals are annihilatedcorrespond to the temperatures of the prominent glow peaks of the samples examined, indicating that the holes from these centres are responsiblefor the peaks of the barite glow curve. 3. DISCUSSION
A model for the 7” process in barite has been proposed on the assumptionthat the luminescentcenters do not act as charge trapping centres, taking into. account the fact that the emission center varies with the change of activator, while the TL glow curve peak positions are not inthrenced by the type of activator, indicating that the released charge carriers for all glow curve peaks originatefrom crystal lattice centres of the host material. It follows that the impuritiesin some way participate in the TL emission process, although they do not act as charge trapping centres. A similar type of emission process was proposed by Medlin[lOlfor natural calcite samples. Since the 77_,glow curve peaks are not iniluencedby activator impurity ions, the TL mechanism is believed to be a process involvingthe release of holes from different anion radicals followed by their recombination with electrons. The Z’LESR correlation studies have shown that the hole traps are responsible for the ZZ glow curve peaks. Since it has been shownthat at the annihilationtemperatures of the anion radicals produced by gamma-rays,which act as hole trapping centres, the TL glow peaks appear, it is suggestedthat the thermally released holes from each of these centres are mobile,and hence recombine with electrons from some of the electron trapping centers. Most probably, the electrons are trapped in centres formed by cations of the host crystal lattice or at the sites of lattice defects. Duringthe heating of the crystal, the released holes recombine with electrons from the trapping centres and the recombination energy thus released is non-radiativelytransferred to the nearest Pb” ion, raising it to an excited level. By
c. 8. =I
w
P
W”
Emirrlon
Lk
Fig. 8. Model for the thermoluminescentprocess in natural barites. returning to its normal state, the excited Pb*’ impurity ion acts as a thermoluminescent centre, giving its characteristicemission(Fig. 8). ttJmxmJcE9
1. DixonR. L. and EkstrandK. E., Phys. Med. Biol. 19. 1% (1974). 2. Yamashita T.. Ftvc. 4th Conf. Luminescent lbsimetry (Editedby T. Niewiadomski).p. 467. Krakow(1974). 3. Prokic hf.. bat. 3. Appl. Rod. Isot. 25,545 (1974). 4. ProLiE M., Ph.D Thesis, Belgrade University, Yugoslavia (1976). 5. Prokic hf., J. Phys. Chem. Solids 38,717 (1977). 6. GuptaN. hf., LuthraJ. hf. and ShankarJ., Indian J. Pure and Appl. Phys. 11,684 (1973).
412
M. PROKI~
7. Vtam C., Physica 15,609 (1949). 8. Schuiman J. H.. Br. L A&. Phys. 6, S64 (195.5). 9. Bettinafli C. and Ferraresso G.. Rend. Classe Scien. Fis. Mat. Nat, VIII, 569 (1%8). 10. Medlin W. L.. Thermoluminescenceof Geological Materials, Vol. 15. Academic Press, London (1968). 11.Smith 0. C., Ident~cation and ~uulitativ~ Chemical Analysis of Mine~ols. Van Nostrand, Toronto ~1946~.
12. Przibram K., ~~ad~attaa&lows and Lumtaescence. Pergamon Press, New York (f956).
13. Spitsyn V. I., Gromov V. V. and Karaseva L. G., Lbkl. Akad. Nauka SSSR, 159, 179(1964). 14. Gupta N. M., Luthra J. M. and Shankar J., Rad. Effects 21, 151 (1974). 15. Luthra J. M., GuptaN. M., .f. Lamin. 9,94 (1974).