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Thermoluminescence studies on Sr2+ doped cesium halides and their application to dosimetry
__ RB
Nuclear instruments and Methods in Physics Research B 111 (1996) 290-296
NIOMI B
Beam Interactions wlth Materials&Atoms
ELSEVIER
Thermoluminescence studies on Sr2+ doped cesium halides and their application to dosimetry S. Selvasekarapandian
*,P. Christober
a’
Selvan a, P. Neelamegam
b
a Department ofPhysics, Bhuruthiur University. Coimbatore-641 046, India b AWM
Sri Pushpum College, Poondi. Thanjavur-603
103, India
Received 26 July 1995; revised form received 6 November 1995 Abstract Thermoluminescence (‘IL) and optical absorption studies on Sr ‘+ doped cesium halides have been carried out. The various glow peaks due to F, D,D, and Z, centers are identified by means of TJ_ and optical absorption studies. Z, centers are found to be formed on irradiation itself in both Sr*+ doped CsBr and CsCl crystals. The study of the dosimetric properties of the ‘IL glow peak at 403 K for irradiation indicated that the CsBr:Sr*+ phosphor satisfies the most basic need of an efficient TLD material. It is proposed that the CsBr:Sr’+ phosphor material can be used in dosimetry for the range IO’-lo3 R.
1. Introduction
2. Experimental
A coloured alkali halide crystal containing any of the alkaline earth impurities, if F bleached at room temperature, gives rise to a new series of electron centers called Z centers [I]. In addition to this, these impurities also introduce some new optical absorption bands due to the formation of D,, D, centers in alkali halides [2]. Acharya [3] and Berg and Frolich [4] observed Z centers in irradiated crystals even without F bleaching. These Z type centers are known to be affecting the TL properties of the host crystal also, indicating their involvement in the TL processes 15-71. Alkaline earth impurities are reported to increase the initial stage coloration of alkali halides. As a result of this enhanced sensitivity, the alkaline earth doped alkali halide phosphors have received the attention of a number of workers. The chief interest in studying these materials arose from their possible use in dosimetry. With this viewpoint, we have recently studied and reported [8,9] certain alkaline earth doped cesium halides. In this series, the current paper is dealing with the thermoluminescence and optical studies on strontium doped cesium halides. An attempt has been made in the present study to examine the TL behaviour of CsBr:Sr *+ from the dosimetric point of view and to explore its use in gamma dosimetry.
Single crystals of undoped and Sr*+ doped CsBr and CsCl crystals were grown in vacuum using the Bridgemann technique. A 0.808 X 10m4 mole fraction of SrBr, and 1.264 X 10e4 mole fraction of SrCl, have been added to the melt of CsBr and CsCl respectively, to get the crystals. Samples of approxiCsBr:Sr’+ and CsCl:S?+ mately 3.5 X 3.5 X 1 mm3 in size were used. All studies were made after quenching the crystal from 400°C to room temperature to have a homogeneous distribution of impurity ions. Samples were irradiated at room temperature using a 6oCo -y-source of 0.33 mR/h. TL glow curves were recorded at a heating rate of 30”C/min using a set up described elsewhere [lo]. Optical absorption measurements were done on a Hitachi V-3400 spectrophotometer. F bleaching studies were carried out with Ilford filters (603 and 650 nm) and a tungsten filament lamp (60 W). TL emissions were recorded using a Jarrel-Ash monochromator with omnidrive in conjunction with a R955 photomultiplier tube.
* Corresponding author. Tel. t91 + 9 1 422 422387.
422 422222 ext. 242, fax
3. Results 3.1. CsBr:Sr2 + crystals 3.1.1. TL glow Fig. 1 shows the TL glow curves of CsBr:Sr*+ crystals. For travel dose (time taken for the sample to reach and just to come out of the bottom of the gamma irradia-
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01339-3
291
S. Sefuasekarapandian et al. /Nucl. Instr. and Meth. in Phys. Res. 3 I I I (1996) 290-296
8
2
313
353
433
393 TEt4PERAlwRB
473
(IO
313
Fig. I. TL glow curves of gamma irradiated CsBr:Sr2+ crystals. Curve (a) travel dose, (b) 5 min irradiation.
353
423
393
TFJU'ERATLJRE
(K)
Fig. 2. TL glow curves of CsBr:Sr2+ crystals before and after F bleaching. Curve (a) gamma irradiated for 5 min. Curve (b) gamma irradiated for 5 min and F bleaching for 20 min tion chamber = 18 s> (a), three glow peaks at 363, 403 and 418 K were observed. As the time of irradiation is increased to 5 minutes (b) an additional glow peak at 388 K has been observed in addition to the former three. ln order to study the nature of the trapping centers responsible for the observed peaks in these doped samples Fbleaching studies were carried out (Fig. 2). From these curves it is observed that on F bleaching subsequent to irradiation (b), the TL intensity of the 363 and 418 K glow peaks is completely suppressed whereas the intensity of the glow peak at 403 K found to be enhanced and the 388 K peak appears as a weak shoulder. The intensities of the TL peaks are plotted against the time of F bleaching to study the behaviour of these centers (Fig. 3). The result shows that the TL intensity of the 403 K peak (b) increases up to 3 minutes of F bleaching and then starts decreasing whereas that of 378 K continuously decreases.
8.
2
3.1.2.
Optical absorption
Fig. 4 shows the optical absorption spectra of CsBr:Sr2+ crystals before (a) and after (b) gamma irradiation, measured at room temperature. After irradiation of the crystal the spectrum exhibits two bands, one in the visible region at 650 nm and another in the UV region at 280 run, the former one being very broad. On heating the crystal up to 388 K, the 280 nm band is found to disappear.
0
10
5 TIM3
15
(usin)
Fig. 3. The effect of F bleaching on the intensities of TL peaks for the glow peaks at (a) 363 K and (b) 403 K.
S. Seluasekurapandian
292
et ul./Nucl.
Instr. md Meth. in Phys. Res. 3 I1 I (1996) 290-296
0.8
0.6
0.4
0.2
0 100
500
300
WAvEL.ERGTB
2
700
(nml
Fig. 4. Optical absorption spectra of CsBr:Sr” crystals. (a) Before irradiation, (b) after 15 min gamma irradiation.
273
323
373 TmPERATmE
423 [lc)
Fig. 6. TL glow curves of gamma irradiated Curve (a) travel dose, (b) 5 min irradiation. 3.1.3.
,J
CsCl:Sr*+
crystals.
TL emission
From the TL emission spectral data displayed in Fig. 5, it can be observed that the 363 K glow peak (a) gives emission with a peak maximum at 425 nm accompanied by a shoulder at 450 nm. An emission band at 450 nm with a shoulder at 475 nm was observed for the glow peak at 403 K (b). For the 418 K glow peak (c), a single band at 425 nm has been noticed.
3.2. CsCl:Sr2 + crystals 3.2.1.
TL glow
Fig. 6 shows the TL. glow curves of CsCl:Sr*+
crystals. For travel dose (a), two glow peaks at 373 and 404 K with a shoulder at 353 K were observed. As the time of
9
3 ..I
(a)
I
E
26
F t:
& H3
0 300
AhA 420
500
300
420
WAVE
Fig. 5. TL emission spectra of gamma irradiated
CsBr:Sr2+
crystals.
LEmGTB
500
300
420
500
(nm)
Emission spectra under glow peaks at (a) 363 K, (b) 403 K, (c) 418 K.
S. Selvasekarapandian et ai./Nucl. Instr. and Meth. in Phys. Res. 3 1II (19961290-296
293
ure). On irradiation, a broad F band develops around 600 nm and the broadness is removed on heating the crystals up to 353 K. On F bleaching the crystals subsequent to irradiation, the F band is found to diminish and the decrease is also accompanied by an increase in absorption on the longer wavelength side. 3.2.3. TL emission From the TL emission spectral data displayed in Fig. 8, it can be observed that the 353 K glow peak (a) gives emission with a peak maximum at 445 nm accompanied by a shoulder at 425 nrn. A single emission band peaking at 425 nm has been observed for the two glow peaks at 373 and 404 K (b).
4. Discussion
. 4.1. Undoped CsBr and CsCl crystals
J 273
323
373 TmlPEnAlmlE
423
IK)
The glow peaks observed at 378 and 428 K in the undoped CsBr and the glow peaks at 373 and 404 K in undoped CsCl crystals have been attributed to thermal decay of F centers. An emission band at 425 nm (2.91 eV> has been observed for all the glow peaks in both the cesium halides and attributed to the recombination of F electrons with V type centers [8,9].
Fig. 7. TL glow curves of C&I: Sr2+ crystals before and after F bleaching. Curve (a) gamma irradiated for 5 min. Curve (b) gamma irradiated for 5 min and F bleaching for 20 min. irradiation is increased to 5 minutes (b), the intensity of all the glow peaks was found to be increased. From the F bleaching studies (Fig. 7) it could be observed that on F bleaching subsequent to irradiation, the intensity of the 373 and 404 K glow peaks is suppressed whereas that of the 353 K glow peak was found to be enhanced. The glow peak at 353 K which appeared as a shoulder before F bleaching, is now seen to be a prominent glow peak.
4.2. CsBr:Sr’ ’ crystals The TL pattern of the CsBr:Sr’+ crystals irradiated with gamma rays exhibit four glow peaks at 363, 388, 403 and 418 K for different doses of irradiation. F bleaching after irradiation results in the increase of the intensity of the glow peak at 403 K whereas all the others were found to decrease. Similar results were obtained in the case of Ba2+ and Ca 2f doped CsBr crystals (8,9]. Sastry et al. [ 11,121 and Gartia and Acharya [3] also observed this type
3.2.2. Optical absorption Optical absorption spectra of CsCl:Sr*+ crystals were measured at room temperature (results not shown in fig-
0
1 300
540
420
300 WAVE
Fig. 8. TL emission
spectra of gamma irradiated
CsCl:Sr*+
crystals.
420
LENGTH
Emission
540
(IId
spectra under glow peaks at (a) 353 K, (b) 373 and 404 K.
S. Seluusekurapandian
294
et cd./ Nucl. Insir. and Mesh. in Phys. Rex B I I I 119961290-296
of behaviour in alkaline earth doped rubidium halide crystals. They attributed this type of behaviour of TL glow to the formation of Z, centers during irradiation and its annihilation on TL heating at this particular temperature. Based on the above argument, it may be noted that in the present CsBr:Sr*’ system also a small number of Z, centers are formed on irradiation and their concentration increases considerably on F bleaching at room temperature. The 403 K glow peak may be attributed to the thermal annealing of Z, centers. The other glow peaks found to be suppressed much on F bleaching could be ascribed to F centers. On studying the bleaching kinetics of the glow peaks (Fig. 3), it is obvious that the number of Z, centers increases on F bleaching (subsequent to irradiation) up to 3 minutes, which is the optimum dose for getting the maximum number of Z, centers and then decreases. In general it is inferred that the alkaline earth cations in crystal lattices enhance the rate of coloration. In Sr2+ doped cesium halides, the F center concentration and the emission intensity were found to be higher than the pure samples. The emission peak positions are only slightly modified probably due to the presence of impurities. Similar effects have been reported in alkaline earth doped KC1 [ 131 and RbCl [ 1 I] systems. In divalent impurity doped crystals, one or more of the cations of the host lattice surrounding the F center could be replaced by an isolated impurity ion or by an impurity-vacancy (I-V) dipole. The impurity present in different states of dispersion surrounding the F centers will perturb them to different extents. This perturbation perhaps causes a change in the configurational coordinate of the F center resulting in the shift of the position of the glow peak. Lakshmipathy Rao and Haribabu [6] and Selvasekarapandian et al. [8] observed such a shift in glow peak temperatures in the case of alkaline earth doped alkali
halides. Hence the shift of the TL glow peaks at 363 and 403 K towards lower temperature in doped samples compared to that of a pure sample may be due to the distortion of the configurational coordinate curve of the F center by the unassociated divalent strontium impurity surrounding the F center. Optical absorption measurements reveal the presence of a broad F band with an asymmetry in the longer wavelength side. F bleaching results in a diminishing of the band accompanied by an increase in the intensity on the higher wavelength side. By heating the crystal up to the peak temperature of 403 K, the asymmetry is found to disappear. On further F-bleaching, the Z, centers are regenerated. These results confirm the formation of Z, centers during irradiation itself and its increase on F bleaching. Hence the glow peak observed at 403 K could be safely ascribed to Z, centers. The optical absorption band appearing at 280 nm is the one observed in the earlier cases of Cd, Ba and Ca doped CsBr crystals [8,9] which has been attributed to the formation of D,D, centers on irradiation. The glow peak observed at 388 K in the above alkaline earth doped CsBr systems has also been ascribed to these centers 181. Hence in the present case also the optical absorption band at 280 nm and the glow peak at 388 K could be attributed to D,D, centers. The TL emission can be explained on the basis of the model proposed by Hagseth [ 141. In the TL emission spectra of CsBr:Sr*+, the observed 425 nm emission band is the one observed in pure CsBr itself [8]. Hence it could be assigned to the recombination of thermally detrapped electrons with V type centers and the two emissions at 450 and 475 nm to the V centers perturbed by the presence of Sr*+ impurities. A tentative energy level diagram is proposed for the various observed emissions (Fig. 9).
(a)
(b)
CONDUC!l!ION BAND
I-
I-
CONDUCTION
I- -Terl
1.91 lv
BAND
-
w 2.91
2.18cv
Cl
F
v
39 l" l
VALENCE
BAND
5.09 ev
I
5.22 ll
I VALENCE
BAWD
Fig. 9. Energy level diagrams for TL process of calcium doped (a) CsBr and (b) CsCl crystals.
S. Selvasekarapandian et al./Nucl.
Instr.
4.3. CsCl:Sr2 + crystals
and
Meth. in Phys. Res. B II I (1996) 290-296
295
type centers and the 445 nm emission to the V centers perturbed by the presence of impurities.
The TL pattern of the CsCl:Sr’+ crystals irradiated with gamma rays exhibit three glow peaks at 353,373 and 404 K for different doses of irradiation. F bleaching after irradiation results in the increase of the intensity of the glow peak at 353 K whereas the other two were found to decrease. Similar results have been obtained in the case of Ba*+ and Ca*+ doped CsCl crystals. As discussed in the case of Sr *+ doped CsBr cry stals, it may be noted that the glow peak at 353 K in the present CsCl:Sr*+ system may also be attributed to the formation of a small number of 2, centers formed on irradiation and their concentration increases considerably on F bleaching at room temperature. The other two glow peaks could be ascribed to F centers. As observed in the previous case, optical absorption measurements reveal the presence of a broad F band peaking at 600 nm and F bleaching results in diminishing of the band accompanied by an increase in the intensity on the higher wavelength side. On studying the bleaching kinetics, it is observed that the number of Z, centers increases on F bleaching in the initial stages and then decreases. In the TL emission spectra of CsCl:Sr*‘, the observed 425 nm emission band is the one observed in pure CsCl itself. Hence it could be assigned to the recombination of thermally detrapped electrons from different traps with V
5. Experimentation a TLD phosphor
on the use of CsBr:Sr2+
Among the four glow peaks observed in the CsBr:Sr*+ system, the behaviour of the 403 K glow peak with size of irradiation dose is particularly striking from the dosimetric viewpoint (Fig. 10). The phosphor fully satisfied all the basic requirements of a good TLD material described elsewhere [15] like other alkaline earth doped cesium bromide systems reported recently [8,9]. The various features of the phosphor can be listed as follows. The phosphor has substantial TL output at 403 K. The shape of the glow curve clearly indicates that the trap is single valued, which is a basic requirement of a good TLD material. There is no change in the TL peak position with respect to size of irradiation dose. It is, therefore, inferred that the material under investigation is resistive to radiation damage. The shape of the glow curve remains the same on increasing the concentration of the strontium impurity.
10-3.
lo-'. g
6
k lo-5_ I2
10
102
crystals as
103 WUQ4R EXPOSURE
10'
105
(R)
Fig. 10. Linear dependence of TL intensity of the 403 K glow peak in CsBr:Sr*+ crystals on gamma dose.
296
S. Selvasekarapandiun et al./Nucl.
Instr. and Meth. in Phys. Res. B III (1996) 290-296
The TL emission exhibited by the material around the glow at 403 K occurs around 450 nm. This matches with the spectral sensitivity of the photomultiplier with the spectral distribution of the luminescence emission. Storage of the coloured samples at room temperature up to 20 days does not alter the TL response of the phosphor by more than 30%. It is obvious from the growth studies of the 403 K glow peak with dose that the TL intensity is having a linear dose response up to 1000 R and thereafter supralinear. The other requirements like reproducibility of the glow curves, desirable shape and size of the specimen and very low cost of the material are additional factors which strengthen the claim of CsBr:Sr’+ phosphor as a y-dosimetry TLD material. Since most of the desirable characteristics of a TLD material are satisfied by the phosphor it is felt that the CsBr:Sr’+ material can be of use in y-dosimetry in the range loo-lo3 R.
Acknowledgements One of the authors (P.C.) gratefully acknowledges UGC, New Delhi for a fellowship through a scheme.
the
References
[I] S. Radhakrishna
and B.V.R. Chowdari, Phys. Status Solidi A 14(1972) 11. [2] S. Radhakrishna and R. Narayanan, Phys. Status Solidi A 26 (1974) 337. (31 R.K. Gartia and B.S. Acharya, Phys. Status Solidi 47 (1978) K165. [41 K.J. Berg and F. Frolich, Krish. Tech. 13 (1978) 481. I51 K. Inabe and N. Taekeuchi, Jpn. J. Appl. Phys. 17 (1978) 1549. [61M. Lakshmipathy Rao and V. Haribabu, Ind. J. Pure and Appl. Phys. 17 (1979) 765. [71 P. Veeresham, U.V. Subbarao and V. Haribabu, Phys. Status Solidi A 73 (1982) 565. P. Neelamegam and P. Christober [81S. Selvasekarapandian, Selvan, J. Lumin. 54 ( 1992) 12 1. P. Neelamegam and P. Christober ]91 S. Selvasekarapandian, Selvan, Phys. Status Solidi A 148 (1995) 311. K. Padmanabhan and S. Selvasekarapan[lOI P. Neelamegam, dian, Meas. Sci. Technol. 3 (19921 581. [I II S.B.S. Sastry and S. Sapru, Cryst. Latt. Def. 9 (1980) 39. S.B.S. Sastry, V. Viswanathan and C. Ramasastry, J. Phys. C [121 5 (1972) 3552. iI31 A. Rascon and J.L. Alvarez Rivas, J. Phys. C 11 (1978) 1239. [I41 G.T. Hagseth, Phys. Rev. B. 5 (1972) 4060. [151 R.V. Joshi, T.R. Joshi, K.P. Dhake and S.P. Kathuria, Health Phys. 44 (1983) 29.
Nuclear instruments and Methods in Physics Research B 111 (1996) 290-296
NIOMI B
Beam Interactions wlth Materials&Atoms
ELSEVIER
Thermoluminescence studies on Sr2+ doped cesium halides and their application to dosimetry S. Selvasekarapandian
*,P. Christober
a’
Selvan a, P. Neelamegam
b
a Department ofPhysics, Bhuruthiur University. Coimbatore-641 046, India b AWM
Sri Pushpum College, Poondi. Thanjavur-603
103, India
Received 26 July 1995; revised form received 6 November 1995 Abstract Thermoluminescence (‘IL) and optical absorption studies on Sr ‘+ doped cesium halides have been carried out. The various glow peaks due to F, D,D, and Z, centers are identified by means of TJ_ and optical absorption studies. Z, centers are found to be formed on irradiation itself in both Sr*+ doped CsBr and CsCl crystals. The study of the dosimetric properties of the ‘IL glow peak at 403 K for irradiation indicated that the CsBr:Sr*+ phosphor satisfies the most basic need of an efficient TLD material. It is proposed that the CsBr:Sr’+ phosphor material can be used in dosimetry for the range IO’-lo3 R.
1. Introduction
2. Experimental
A coloured alkali halide crystal containing any of the alkaline earth impurities, if F bleached at room temperature, gives rise to a new series of electron centers called Z centers [I]. In addition to this, these impurities also introduce some new optical absorption bands due to the formation of D,, D, centers in alkali halides [2]. Acharya [3] and Berg and Frolich [4] observed Z centers in irradiated crystals even without F bleaching. These Z type centers are known to be affecting the TL properties of the host crystal also, indicating their involvement in the TL processes 15-71. Alkaline earth impurities are reported to increase the initial stage coloration of alkali halides. As a result of this enhanced sensitivity, the alkaline earth doped alkali halide phosphors have received the attention of a number of workers. The chief interest in studying these materials arose from their possible use in dosimetry. With this viewpoint, we have recently studied and reported [8,9] certain alkaline earth doped cesium halides. In this series, the current paper is dealing with the thermoluminescence and optical studies on strontium doped cesium halides. An attempt has been made in the present study to examine the TL behaviour of CsBr:Sr *+ from the dosimetric point of view and to explore its use in gamma dosimetry.
Single crystals of undoped and Sr*+ doped CsBr and CsCl crystals were grown in vacuum using the Bridgemann technique. A 0.808 X 10m4 mole fraction of SrBr, and 1.264 X 10e4 mole fraction of SrCl, have been added to the melt of CsBr and CsCl respectively, to get the crystals. Samples of approxiCsBr:Sr’+ and CsCl:S?+ mately 3.5 X 3.5 X 1 mm3 in size were used. All studies were made after quenching the crystal from 400°C to room temperature to have a homogeneous distribution of impurity ions. Samples were irradiated at room temperature using a 6oCo -y-source of 0.33 mR/h. TL glow curves were recorded at a heating rate of 30”C/min using a set up described elsewhere [lo]. Optical absorption measurements were done on a Hitachi V-3400 spectrophotometer. F bleaching studies were carried out with Ilford filters (603 and 650 nm) and a tungsten filament lamp (60 W). TL emissions were recorded using a Jarrel-Ash monochromator with omnidrive in conjunction with a R955 photomultiplier tube.
* Corresponding author. Tel. t91 + 9 1 422 422387.
422 422222 ext. 242, fax
3. Results 3.1. CsBr:Sr2 + crystals 3.1.1. TL glow Fig. 1 shows the TL glow curves of CsBr:Sr*+ crystals. For travel dose (time taken for the sample to reach and just to come out of the bottom of the gamma irradia-
0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01339-3
291
S. Sefuasekarapandian et al. /Nucl. Instr. and Meth. in Phys. Res. 3 I I I (1996) 290-296
8
2
313
353
433
393 TEt4PERAlwRB
473
(IO
313
Fig. I. TL glow curves of gamma irradiated CsBr:Sr2+ crystals. Curve (a) travel dose, (b) 5 min irradiation.
353
423
393
TFJU'ERATLJRE
(K)
Fig. 2. TL glow curves of CsBr:Sr2+ crystals before and after F bleaching. Curve (a) gamma irradiated for 5 min. Curve (b) gamma irradiated for 5 min and F bleaching for 20 min tion chamber = 18 s> (a), three glow peaks at 363, 403 and 418 K were observed. As the time of irradiation is increased to 5 minutes (b) an additional glow peak at 388 K has been observed in addition to the former three. ln order to study the nature of the trapping centers responsible for the observed peaks in these doped samples Fbleaching studies were carried out (Fig. 2). From these curves it is observed that on F bleaching subsequent to irradiation (b), the TL intensity of the 363 and 418 K glow peaks is completely suppressed whereas the intensity of the glow peak at 403 K found to be enhanced and the 388 K peak appears as a weak shoulder. The intensities of the TL peaks are plotted against the time of F bleaching to study the behaviour of these centers (Fig. 3). The result shows that the TL intensity of the 403 K peak (b) increases up to 3 minutes of F bleaching and then starts decreasing whereas that of 378 K continuously decreases.
8.
2
3.1.2.
Optical absorption
Fig. 4 shows the optical absorption spectra of CsBr:Sr2+ crystals before (a) and after (b) gamma irradiation, measured at room temperature. After irradiation of the crystal the spectrum exhibits two bands, one in the visible region at 650 nm and another in the UV region at 280 run, the former one being very broad. On heating the crystal up to 388 K, the 280 nm band is found to disappear.
0
10
5 TIM3
15
(usin)
Fig. 3. The effect of F bleaching on the intensities of TL peaks for the glow peaks at (a) 363 K and (b) 403 K.
S. Seluasekurapandian
292
et ul./Nucl.
Instr. md Meth. in Phys. Res. 3 I1 I (1996) 290-296
0.8
0.6
0.4
0.2
0 100
500
300
WAvEL.ERGTB
2
700
(nml
Fig. 4. Optical absorption spectra of CsBr:Sr” crystals. (a) Before irradiation, (b) after 15 min gamma irradiation.
273
323
373 TmPERATmE
423 [lc)
Fig. 6. TL glow curves of gamma irradiated Curve (a) travel dose, (b) 5 min irradiation. 3.1.3.
,J
CsCl:Sr*+
crystals.
TL emission
From the TL emission spectral data displayed in Fig. 5, it can be observed that the 363 K glow peak (a) gives emission with a peak maximum at 425 nm accompanied by a shoulder at 450 nm. An emission band at 450 nm with a shoulder at 475 nm was observed for the glow peak at 403 K (b). For the 418 K glow peak (c), a single band at 425 nm has been noticed.
3.2. CsCl:Sr2 + crystals 3.2.1.
TL glow
Fig. 6 shows the TL. glow curves of CsCl:Sr*+
crystals. For travel dose (a), two glow peaks at 373 and 404 K with a shoulder at 353 K were observed. As the time of
9
3 ..I
(a)
I
E
26
F t:
& H3
0 300
AhA 420
500
300
420
WAVE
Fig. 5. TL emission spectra of gamma irradiated
CsBr:Sr2+
crystals.
LEmGTB
500
300
420
500
(nm)
Emission spectra under glow peaks at (a) 363 K, (b) 403 K, (c) 418 K.
S. Selvasekarapandian et ai./Nucl. Instr. and Meth. in Phys. Res. 3 1II (19961290-296
293
ure). On irradiation, a broad F band develops around 600 nm and the broadness is removed on heating the crystals up to 353 K. On F bleaching the crystals subsequent to irradiation, the F band is found to diminish and the decrease is also accompanied by an increase in absorption on the longer wavelength side. 3.2.3. TL emission From the TL emission spectral data displayed in Fig. 8, it can be observed that the 353 K glow peak (a) gives emission with a peak maximum at 445 nm accompanied by a shoulder at 425 nrn. A single emission band peaking at 425 nm has been observed for the two glow peaks at 373 and 404 K (b).
4. Discussion
. 4.1. Undoped CsBr and CsCl crystals
J 273
323
373 TmlPEnAlmlE
423
IK)
The glow peaks observed at 378 and 428 K in the undoped CsBr and the glow peaks at 373 and 404 K in undoped CsCl crystals have been attributed to thermal decay of F centers. An emission band at 425 nm (2.91 eV> has been observed for all the glow peaks in both the cesium halides and attributed to the recombination of F electrons with V type centers [8,9].
Fig. 7. TL glow curves of C&I: Sr2+ crystals before and after F bleaching. Curve (a) gamma irradiated for 5 min. Curve (b) gamma irradiated for 5 min and F bleaching for 20 min. irradiation is increased to 5 minutes (b), the intensity of all the glow peaks was found to be increased. From the F bleaching studies (Fig. 7) it could be observed that on F bleaching subsequent to irradiation, the intensity of the 373 and 404 K glow peaks is suppressed whereas that of the 353 K glow peak was found to be enhanced. The glow peak at 353 K which appeared as a shoulder before F bleaching, is now seen to be a prominent glow peak.
4.2. CsBr:Sr’ ’ crystals The TL pattern of the CsBr:Sr’+ crystals irradiated with gamma rays exhibit four glow peaks at 363, 388, 403 and 418 K for different doses of irradiation. F bleaching after irradiation results in the increase of the intensity of the glow peak at 403 K whereas all the others were found to decrease. Similar results were obtained in the case of Ba2+ and Ca 2f doped CsBr crystals (8,9]. Sastry et al. [ 11,121 and Gartia and Acharya [3] also observed this type
3.2.2. Optical absorption Optical absorption spectra of CsCl:Sr*+ crystals were measured at room temperature (results not shown in fig-
0
1 300
540
420
300 WAVE
Fig. 8. TL emission
spectra of gamma irradiated
CsCl:Sr*+
crystals.
420
LENGTH
Emission
540
(IId
spectra under glow peaks at (a) 353 K, (b) 373 and 404 K.
S. Seluusekurapandian
294
et cd./ Nucl. Insir. and Mesh. in Phys. Rex B I I I 119961290-296
of behaviour in alkaline earth doped rubidium halide crystals. They attributed this type of behaviour of TL glow to the formation of Z, centers during irradiation and its annihilation on TL heating at this particular temperature. Based on the above argument, it may be noted that in the present CsBr:Sr*’ system also a small number of Z, centers are formed on irradiation and their concentration increases considerably on F bleaching at room temperature. The 403 K glow peak may be attributed to the thermal annealing of Z, centers. The other glow peaks found to be suppressed much on F bleaching could be ascribed to F centers. On studying the bleaching kinetics of the glow peaks (Fig. 3), it is obvious that the number of Z, centers increases on F bleaching (subsequent to irradiation) up to 3 minutes, which is the optimum dose for getting the maximum number of Z, centers and then decreases. In general it is inferred that the alkaline earth cations in crystal lattices enhance the rate of coloration. In Sr2+ doped cesium halides, the F center concentration and the emission intensity were found to be higher than the pure samples. The emission peak positions are only slightly modified probably due to the presence of impurities. Similar effects have been reported in alkaline earth doped KC1 [ 131 and RbCl [ 1 I] systems. In divalent impurity doped crystals, one or more of the cations of the host lattice surrounding the F center could be replaced by an isolated impurity ion or by an impurity-vacancy (I-V) dipole. The impurity present in different states of dispersion surrounding the F centers will perturb them to different extents. This perturbation perhaps causes a change in the configurational coordinate of the F center resulting in the shift of the position of the glow peak. Lakshmipathy Rao and Haribabu [6] and Selvasekarapandian et al. [8] observed such a shift in glow peak temperatures in the case of alkaline earth doped alkali
halides. Hence the shift of the TL glow peaks at 363 and 403 K towards lower temperature in doped samples compared to that of a pure sample may be due to the distortion of the configurational coordinate curve of the F center by the unassociated divalent strontium impurity surrounding the F center. Optical absorption measurements reveal the presence of a broad F band with an asymmetry in the longer wavelength side. F bleaching results in a diminishing of the band accompanied by an increase in the intensity on the higher wavelength side. By heating the crystal up to the peak temperature of 403 K, the asymmetry is found to disappear. On further F-bleaching, the Z, centers are regenerated. These results confirm the formation of Z, centers during irradiation itself and its increase on F bleaching. Hence the glow peak observed at 403 K could be safely ascribed to Z, centers. The optical absorption band appearing at 280 nm is the one observed in the earlier cases of Cd, Ba and Ca doped CsBr crystals [8,9] which has been attributed to the formation of D,D, centers on irradiation. The glow peak observed at 388 K in the above alkaline earth doped CsBr systems has also been ascribed to these centers 181. Hence in the present case also the optical absorption band at 280 nm and the glow peak at 388 K could be attributed to D,D, centers. The TL emission can be explained on the basis of the model proposed by Hagseth [ 141. In the TL emission spectra of CsBr:Sr*+, the observed 425 nm emission band is the one observed in pure CsBr itself [8]. Hence it could be assigned to the recombination of thermally detrapped electrons with V type centers and the two emissions at 450 and 475 nm to the V centers perturbed by the presence of Sr*+ impurities. A tentative energy level diagram is proposed for the various observed emissions (Fig. 9).
(a)
(b)
CONDUC!l!ION BAND
I-
I-
CONDUCTION
I- -Terl
1.91 lv
BAND
-
w 2.91
2.18cv
Cl
F
v
39 l" l
VALENCE
BAND
5.09 ev
I
5.22 ll
I VALENCE
BAWD
Fig. 9. Energy level diagrams for TL process of calcium doped (a) CsBr and (b) CsCl crystals.
S. Selvasekarapandian et al./Nucl.
Instr.
4.3. CsCl:Sr2 + crystals
and
Meth. in Phys. Res. B II I (1996) 290-296
295
type centers and the 445 nm emission to the V centers perturbed by the presence of impurities.
The TL pattern of the CsCl:Sr’+ crystals irradiated with gamma rays exhibit three glow peaks at 353,373 and 404 K for different doses of irradiation. F bleaching after irradiation results in the increase of the intensity of the glow peak at 353 K whereas the other two were found to decrease. Similar results have been obtained in the case of Ba*+ and Ca*+ doped CsCl crystals. As discussed in the case of Sr *+ doped CsBr cry stals, it may be noted that the glow peak at 353 K in the present CsCl:Sr*+ system may also be attributed to the formation of a small number of 2, centers formed on irradiation and their concentration increases considerably on F bleaching at room temperature. The other two glow peaks could be ascribed to F centers. As observed in the previous case, optical absorption measurements reveal the presence of a broad F band peaking at 600 nm and F bleaching results in diminishing of the band accompanied by an increase in the intensity on the higher wavelength side. On studying the bleaching kinetics, it is observed that the number of Z, centers increases on F bleaching in the initial stages and then decreases. In the TL emission spectra of CsCl:Sr*‘, the observed 425 nm emission band is the one observed in pure CsCl itself. Hence it could be assigned to the recombination of thermally detrapped electrons from different traps with V
5. Experimentation a TLD phosphor
on the use of CsBr:Sr2+
Among the four glow peaks observed in the CsBr:Sr*+ system, the behaviour of the 403 K glow peak with size of irradiation dose is particularly striking from the dosimetric viewpoint (Fig. 10). The phosphor fully satisfied all the basic requirements of a good TLD material described elsewhere [15] like other alkaline earth doped cesium bromide systems reported recently [8,9]. The various features of the phosphor can be listed as follows. The phosphor has substantial TL output at 403 K. The shape of the glow curve clearly indicates that the trap is single valued, which is a basic requirement of a good TLD material. There is no change in the TL peak position with respect to size of irradiation dose. It is, therefore, inferred that the material under investigation is resistive to radiation damage. The shape of the glow curve remains the same on increasing the concentration of the strontium impurity.
10-3.
lo-'. g
6
k lo-5_ I2
10
102
crystals as
103 WUQ4R EXPOSURE
10'
105
(R)
Fig. 10. Linear dependence of TL intensity of the 403 K glow peak in CsBr:Sr*+ crystals on gamma dose.
296
S. Selvasekarapandiun et al./Nucl.
Instr. and Meth. in Phys. Res. B III (1996) 290-296
The TL emission exhibited by the material around the glow at 403 K occurs around 450 nm. This matches with the spectral sensitivity of the photomultiplier with the spectral distribution of the luminescence emission. Storage of the coloured samples at room temperature up to 20 days does not alter the TL response of the phosphor by more than 30%. It is obvious from the growth studies of the 403 K glow peak with dose that the TL intensity is having a linear dose response up to 1000 R and thereafter supralinear. The other requirements like reproducibility of the glow curves, desirable shape and size of the specimen and very low cost of the material are additional factors which strengthen the claim of CsBr:Sr’+ phosphor as a y-dosimetry TLD material. Since most of the desirable characteristics of a TLD material are satisfied by the phosphor it is felt that the CsBr:Sr’+ material can be of use in y-dosimetry in the range loo-lo3 R.
Acknowledgements One of the authors (P.C.) gratefully acknowledges UGC, New Delhi for a fellowship through a scheme.
the
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