- Email: [email protected]
Optical investigations of TlBr detector crystals
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
Optical investigations of TlBr detector crystals L. Grigorjevaa, D. Millersa, M. Shorohova,*, I.S. Lisitskiib, M.S. Kuznetsovb, S. Zatolokac, V. Gostiloc a
Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia b ’’GIREDMET’’, B. Tolmachevskiy per. 5, Moscow, Russia c Baltic Scientific Instruments, Ganibu Dambis 26, P.O. Box 33, LV-1005 Riga, Latvia Available online 22 June 2004
Abstract Shift of fundamental absorption edge, the position of main luminescence bands, the luminescence decay and transient absorption spectra in three TlBr crystals were studied. The g-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties. The iodine impurity in TlBr was detected by optical methods. The role of impurities and crystal defects in g-quanta detectors manufactured is discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.Nc; 81.05.Dz; 81.05.Ea; 81.05.Hd Keywords: TlBr crystals; Semiconductor detectors; X-rays
1. Introduction The promising use of TlBr crystal as a material for detectors is beyond doubt to specialists. The properties of material (high atomic number Tl=81 and Br=35), high density (7.5 g/cm3) and large band gap (B2.7 eV) promise to obtain X-ray and g-quanta detectors with high spectrometric parameters without deep cooling [1,2]. The validity of these prospective was confirmed by achieving the level of spectroscopic parameters in single and pixel detectors [3–6]. However, the wide use of TlBr detectors is restrained due to large scattering *Corresponding author. E-mail address: michail.shorohov [email protected] (M. Shorohov).
of properties for different TlBr ingots. The main problem is the low time stability of detector parameters obtained for a major part of ingots. Just as in CdTe and HgI2 detectors, the instability of parameters in time manifests as the degradation of the spectroscopic characteristics (energy resolution and efficiency) during the registration process. The reason for this instability is not clear and processes responsible can differ from these in another materials. So, for the understanding of the instability nature, it is necessary to investigate the physical processes in TlBr crystals grown under different conditions. The result of such investigations would give the recommendations for developing of TlBr crystal growth technology and obtaining the detectors with stable spectroscopy performance.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.06.005
ARTICLE IN PRESS 198
L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
The present paper is devoted to the study of TlBr single crystals by optical methods and comparison of these results with parameters of detectors made from the same single crystals. The goal of optical investigations was to find the differences in electronic properties in TlBr ingots for understanding the nature of instability, detectors.
2. Experimental procedure Three TlBr ingots (M-12, M-13 and M-14) were grown by Bridgman–Stockbarger method under different conditions which allowed to obtain the ingots with the least structural disruptions and low thermal stress. As a raw material 99.99% pure TlBr salts were used, obtained by deposition from nitrate solutions. For further deep purification before the ingot growth the methods of vacuum distillation and directed crystallization were employed. After at least four such combinations the material was used for ingot growth. The samples for investigation and detector manufacturing were produced by cutting rectangular-shaped plates from the ingots with a wire saw. The TlBr detector crystal growth technique is described in detail in Ref. [7]. Part of each ingot was used for ionizing radiation detector manufacturing, another for optical studies. For manufacturing of detectors, the crystals were lapped and polished (mechanically and chemically) after cutting. Crystal dimensions for detectors were typically 3 3 mm2 with thickness 0.8–1.0 mm. The contacts to the detectors were produced by the vacuum deposition of the gold through special masks. The gold film ( The detectors were anthickness was B800 A. nealed during the same vacuum cycle at a
temperature of about 100 C for 1.5 h. The full detector-manufacturing technology is described in Ref. [5]. The spectrometric measurements show that detectors produced from M12 crystal ingot do not reveal the charge collection and have no peaks in energy spectrum. The spectroscopy performance for detectors produced from M13 and M14 ingots was roughly the same. The energetical resolution was B5% with energy 662 keV and the degradation of spectrometric characteristics was observed after prolonged operation. Some detector parameters, content of anion impurities in ingots as well as characteristics obtained from optical investigations and discussed below, are shown in Table 1. The luminescence, transient absorption spectra and decay kinetic were obtained at B85 K. The excitation source was a pulsed electron accelerator with an electron energy B250 keV; a pulse duration 10 ns and a current amplitude IB200 A. The details of the experimental techniques of transient absorption measurement are described in Ref. [8].
3. Results of optical investigation and discussion The absorption spectra (Fig. 1) for as-grown samples show differences. The absorption edge (absorption sharp drop down) between 2.82 and 2.75 eV was measured for samples studied (Fig. 1a). For sample M12 the absorption edge is shifted to the low-energy side. Absorption edge shift is known for TlClxBr1x crystals and the shift depends on bromine concentration [9]. A similar shift is known for TlBrxI1x; for example, in the KRS-5 crystal (x ¼ 0:45) the absorption edge is
Table 1 M12 R (O cm) mte (cm1) mth (cm1) Cl (ppm) I (ppm) Dt
M13 10
1.2 10 — — 300 6000 300 ns+slow ms component
M14 10
1.3 10 5 105 Initially Emte, followed by degradation 100 40 22 ns
1.6 1010 3 105 Initially Emte, followed by degradation — — 28 ns+slow ms component
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201 0.30
18 16
199
TlBr,RT
0.25
m ax 20 ns
TlBr M13 LNT
14 Ilum, a.u.
K, 1/cm
0.20
M14 M12 M13
12 10 8
0.15 0.10
6
0.05
4 0.00 1.2
2 0 2.700
1.4
1.6
1.8
2.0
2.725
2.750
2.775
2.800
2.825
2.850
2.4
2.6
2.8
2.2
2.4
2.6
2.8
2.2
2.4
2.6
2.8
0.30
E, eV
(a)
2.2
E,eV
(a)
0.25
m ax 20 ns
TlBr M13 LNT
2 .5
TlBr,RT
Ilum, a.u.
0.20 2 .0
M14 M12 M13
K, 1/cm
1 .5
0.15 0.10 0.05
1 .0
0.00 1.2
0 .5
1.4
1.6
1.8
0.0 1 .7 5
(b)
2.0
E,eV
(b) 0.30 2 .0 0
2 .2 5
2 .5 0
2 .75
3 .00
E, eV
0.25
Fig. 1. Absorption spectra for samples M12, M13 and M14 at 300 K: fundamental absorption edge (a) and in the spectral range of transparency of the TlBr crystal (b).
max 20 ns
TlBr M14 LNT
0.20
located at B2.34 eV [10]. The absorption edge observed in sample M12 is below that in TlBr. Chemical analysis shows that the content of iodine in the sample M12 significantly exceeds that for samples M13 and M14. Performances of crystals from investigated ingots are collected in Table 1. Therefore, it is concluded that the iodine impurity is responsible for absorption edge shift. Absorption edges for samples M13 and M14 are very close at room temperature. The tail of absorption extending from B2.72 eV down to 1.5 eV also show differences. The absorption in this spectral region is a good ‘‘identificator’’ for defect content in the crystal. The larger absorption in this range for sample M13 is possibly from small colloids or the optical losses due to light scattering from grain surfaces inside the crystal. Therefore, the crystal M13 is less perfect than the M14.
Ilum, a.u.
0.15
0.10
0.05
0.00 1.2
(c)
1.4
1.6
1.8
2.0
E,eV
Fig. 2. Luminescence spectra under electron beam excitation at 85 K in samples M12 (a), M13 (b), and M14 (c).
The luminescence spectra at 85 K are shown in Fig. 2a–c. Two luminescence bands (B1.8 and 2.2–2.4 eV) was observed under cw excitation. The main luminescence band for samples M13 and M14 peaks at B2.45 eV, whereas for sample M12 at B2.25 eV. The shape of the luminescence band at the long-wave side (B1.7–1.8 eV) shows that overlapping of several luminescence bands takes
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
200
place. Therefore, the luminescence spectra indicate that a number of recombination centers were involved and concentrations of these centers are different in samples studied. The iodine impurity is suggested to be responsible for the main luminescence band shift to the lower photon energy in sample M12. The luminescence decay kinetics (Fig. 3) were not the same over all spectra for the samples studied. Since the decay kinetics are complicated, we use as a general parameter the decay time Dt during which the intensity of luminescence was reduced three-fold. These facts indicate that ‘‘as-grown’’ crystals contain defects. These defects seem to be similar in samples M13 and M14, whereas in sample M12 they differ. The luminescence kinetics for sample M12 shows: (a) the rise of 2.3 eV luminescence intensity is delayed relative to the excitation pulse; (b) decay of luminescence has no fast initial stage at 2.3 eV; (c) luminescence decay extends to the microsecond range; (d) decay kinetics was nonexponential and approximation by the sum from three exponents is possible. The listed results indicate a quite complicated recombination process. The observed experimental facts could be explained in suggestions of migration of charges (possibly diffusion controlled) before the luminescence center excited state formation. The hole in TlBr must be Tl2+ [11]. It is suggested that the electron recombination with Tl2+ is responsible for the main luminescence
band in all samples, possibly associated with some defect. The model of primary defect formation and luminescence center model is described also in Ref. [12]. The shift of luminescence band observed in sample M12 arises due to perturbation of luminescence center (Tl+Vc) by a near-placed iodine impurity. Therefore, the cw absorption as well as luminescence study show that samples M12, M13 and M14 have different properties. The transient absorption was induced by pulsed electron beam. The spectra of transient absorption reveal two main bands peaking at B1.1 and 2.2 eV for all samples studied. Therefore, the transient absorption spectra shape is the same as that described in Ref. [13]. Spectra for samples M13 and M14 match well (Fig. 4), whereas optical density for sample M12 is twice that of M13 and M14 (it can be pointed out that the sizes of all samples are the same and thus the comparison of optical density was possible). This indicates that in the formation of centers responsible for transient absorption the pre-irradiation defects were involved. The absorption band at B1.1 eV can be due to Tl2+ being possibly stabilized by some defect [13], whereas the electron center can be responsible for the band at B2.2 eV. It is possible that during the recombination process the fraction of electron–hole pairs creates excitons (Tl+) and nonradiative decay of some excitons results in formation of Frenkel 0 pairs: Tl2+ V c and Tl . A similar process is known and studied in detail for silver halides [14]. Since the relaxation kinetics of transient absorption
1.1
1.0
1.0 0.9 0.8
TlBr Optical density
Ilum,a. u.
0.8
#12
0.7 0.6 0.5
#14
0.4 0.3 0.2
M12 M13 M14
TlBr RT
0.9
0.7 0.6 0.5 0.4 0.3
#13
0.2
0.1 0.0 0
50
100
150
200
250
300
t,ns Fig. 3. Luminescence kinetic at 2.3 eV spectral region.
0.1 1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
E,eV
Fig. 4. Transient absorption spectra measured at 20 ns delay after excitation pulse starts.
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
differs from luminescence decay kinetics, it is concluded that electrons and holes created by irradiation have a number of possible recombination channels. The fraction of charge carriers are trapped at pre-irradiation defects and it reduces the efficiency of charge collection in detectors. 4. Conclusions The iodine in TlBr can be detected from the shift of fundamental absorption edge and position of the main luminescence band, whereas specific resistance is not sensitive to this impurity. The transient absorption is much larger in iodine containing TlBr. The g-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties also. The crystals showing fast recombination (DtB30 ns) seem to be better candidates for detector manufacturing. High-purity TlBr with low intrinsic defect concentration is necessary for high quality g-quanta detectors. Acknowledgements This work has been supported by LME (grant TPP 00-57) and the European Commission Center of Excellence (CAMART).
201
References [1] K. Shah, J. Lund, F. Olschner, L. Moy, M. Squillante, IEEE Trans. Nucl. Sci. NS-36 (1) (1989) 199. [2] F. Olschner, K. Shah, J. Lund, J. Zhang, K. Daley, S. Medrick, M. Squillante, Nucl. Instr. and Meth. A 322 (1992) 504. [3] K. Hitomi, O. Muroi, T. Shoji, T. Suehiro, Y. Hiratate, Nucl. Instr. Meth. A 436 (1999) 160. [4] A. Owens, M. Bavdaz, I. Lisjutin, A. Peacock, S. Zatoloka, Nucl. Instr. and Meth. A 458 (2001) 413. [5] V. Gostilo, A. Owens, M. Bavdaz, I. Lisjutin, A. Peacock, H. Sipila, S. Zatoloka, IEEE Trans. Nucl. Sci. NS-49 (5) (2002) 2513. [6] S. Zatoloka, I. Lisjutin, V. Gostilo, H. Sipila, IEEE Trans. Nucl. Sci., (2004) in press. [7] M.S. Kouznetsov, I.S. Lisitsky, S.I. Zatoloka, V.V. Gostilo, Nucl. Instr. and Meth. (to be published). [8] L. Grigorjeva, V. Pankratov, D. Millers, G. Corradi, K. Polgar, Integrated Ferroelectr. 35 (2001) 137. [9] L.G. Grigorjeva, D.K. Millers, I.S. Lisitskii, T.L. Liholetova, Izvestija Latvian Acad. Sci. Fiz. Techn. Ser. 6 (1985) 37 (in Russian). [10] A.A. Blistanov, V.S. Bondarenko, N.V. Perelomova, F.N. Strizevskaja, V.V. Chkalova, M.P. Shaskolskaja, Acoustic Crystals, Nauka, Moscow, 1982, p. 632. [11] L. Grigorjeva, D. Millers, Excitonic Processes in Condensed Matter, Electrochemical Society Inc., 98(25), 1998, p. 438. [12] L. Grigorjeva, D. Millers, Nucl. Instr. and Meth. 131 (2002) 131. [13] D.K. Millers, A.V. Nomoev, L.G. Grigorjeva, Izvestija Latvian Acad. Sci. Fiz. Techn. Ser. 3 (1989) 60 (in Russian). [14] L. Grigorjeva, E.A. Kotomin, D.K. Millers, R. Eglitis, J. Phys. Condens. Matter 7 (1995) 1483.
Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
Optical investigations of TlBr detector crystals L. Grigorjevaa, D. Millersa, M. Shorohova,*, I.S. Lisitskiib, M.S. Kuznetsovb, S. Zatolokac, V. Gostiloc a
Institute of Solid State Physics, University of Latvia, Kengaraga 8, LV-1063 Riga, Latvia b ’’GIREDMET’’, B. Tolmachevskiy per. 5, Moscow, Russia c Baltic Scientific Instruments, Ganibu Dambis 26, P.O. Box 33, LV-1005 Riga, Latvia Available online 22 June 2004
Abstract Shift of fundamental absorption edge, the position of main luminescence bands, the luminescence decay and transient absorption spectra in three TlBr crystals were studied. The g-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties. The iodine impurity in TlBr was detected by optical methods. The role of impurities and crystal defects in g-quanta detectors manufactured is discussed. r 2004 Elsevier B.V. All rights reserved. PACS: 07.85.Nc; 81.05.Dz; 81.05.Ea; 81.05.Hd Keywords: TlBr crystals; Semiconductor detectors; X-rays
1. Introduction The promising use of TlBr crystal as a material for detectors is beyond doubt to specialists. The properties of material (high atomic number Tl=81 and Br=35), high density (7.5 g/cm3) and large band gap (B2.7 eV) promise to obtain X-ray and g-quanta detectors with high spectrometric parameters without deep cooling [1,2]. The validity of these prospective was confirmed by achieving the level of spectroscopic parameters in single and pixel detectors [3–6]. However, the wide use of TlBr detectors is restrained due to large scattering *Corresponding author. E-mail address: michail.shorohov [email protected] (M. Shorohov).
of properties for different TlBr ingots. The main problem is the low time stability of detector parameters obtained for a major part of ingots. Just as in CdTe and HgI2 detectors, the instability of parameters in time manifests as the degradation of the spectroscopic characteristics (energy resolution and efficiency) during the registration process. The reason for this instability is not clear and processes responsible can differ from these in another materials. So, for the understanding of the instability nature, it is necessary to investigate the physical processes in TlBr crystals grown under different conditions. The result of such investigations would give the recommendations for developing of TlBr crystal growth technology and obtaining the detectors with stable spectroscopy performance.
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.06.005
ARTICLE IN PRESS 198
L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
The present paper is devoted to the study of TlBr single crystals by optical methods and comparison of these results with parameters of detectors made from the same single crystals. The goal of optical investigations was to find the differences in electronic properties in TlBr ingots for understanding the nature of instability, detectors.
2. Experimental procedure Three TlBr ingots (M-12, M-13 and M-14) were grown by Bridgman–Stockbarger method under different conditions which allowed to obtain the ingots with the least structural disruptions and low thermal stress. As a raw material 99.99% pure TlBr salts were used, obtained by deposition from nitrate solutions. For further deep purification before the ingot growth the methods of vacuum distillation and directed crystallization were employed. After at least four such combinations the material was used for ingot growth. The samples for investigation and detector manufacturing were produced by cutting rectangular-shaped plates from the ingots with a wire saw. The TlBr detector crystal growth technique is described in detail in Ref. [7]. Part of each ingot was used for ionizing radiation detector manufacturing, another for optical studies. For manufacturing of detectors, the crystals were lapped and polished (mechanically and chemically) after cutting. Crystal dimensions for detectors were typically 3 3 mm2 with thickness 0.8–1.0 mm. The contacts to the detectors were produced by the vacuum deposition of the gold through special masks. The gold film ( The detectors were anthickness was B800 A. nealed during the same vacuum cycle at a
temperature of about 100 C for 1.5 h. The full detector-manufacturing technology is described in Ref. [5]. The spectrometric measurements show that detectors produced from M12 crystal ingot do not reveal the charge collection and have no peaks in energy spectrum. The spectroscopy performance for detectors produced from M13 and M14 ingots was roughly the same. The energetical resolution was B5% with energy 662 keV and the degradation of spectrometric characteristics was observed after prolonged operation. Some detector parameters, content of anion impurities in ingots as well as characteristics obtained from optical investigations and discussed below, are shown in Table 1. The luminescence, transient absorption spectra and decay kinetic were obtained at B85 K. The excitation source was a pulsed electron accelerator with an electron energy B250 keV; a pulse duration 10 ns and a current amplitude IB200 A. The details of the experimental techniques of transient absorption measurement are described in Ref. [8].
3. Results of optical investigation and discussion The absorption spectra (Fig. 1) for as-grown samples show differences. The absorption edge (absorption sharp drop down) between 2.82 and 2.75 eV was measured for samples studied (Fig. 1a). For sample M12 the absorption edge is shifted to the low-energy side. Absorption edge shift is known for TlClxBr1x crystals and the shift depends on bromine concentration [9]. A similar shift is known for TlBrxI1x; for example, in the KRS-5 crystal (x ¼ 0:45) the absorption edge is
Table 1 M12 R (O cm) mte (cm1) mth (cm1) Cl (ppm) I (ppm) Dt
M13 10
1.2 10 — — 300 6000 300 ns+slow ms component
M14 10
1.3 10 5 105 Initially Emte, followed by degradation 100 40 22 ns
1.6 1010 3 105 Initially Emte, followed by degradation — — 28 ns+slow ms component
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201 0.30
18 16
199
TlBr,RT
0.25
m ax 20 ns
TlBr M13 LNT
14 Ilum, a.u.
K, 1/cm
0.20
M14 M12 M13
12 10 8
0.15 0.10
6
0.05
4 0.00 1.2
2 0 2.700
1.4
1.6
1.8
2.0
2.725
2.750
2.775
2.800
2.825
2.850
2.4
2.6
2.8
2.2
2.4
2.6
2.8
2.2
2.4
2.6
2.8
0.30
E, eV
(a)
2.2
E,eV
(a)
0.25
m ax 20 ns
TlBr M13 LNT
2 .5
TlBr,RT
Ilum, a.u.
0.20 2 .0
M14 M12 M13
K, 1/cm
1 .5
0.15 0.10 0.05
1 .0
0.00 1.2
0 .5
1.4
1.6
1.8
0.0 1 .7 5
(b)
2.0
E,eV
(b) 0.30 2 .0 0
2 .2 5
2 .5 0
2 .75
3 .00
E, eV
0.25
Fig. 1. Absorption spectra for samples M12, M13 and M14 at 300 K: fundamental absorption edge (a) and in the spectral range of transparency of the TlBr crystal (b).
max 20 ns
TlBr M14 LNT
0.20
located at B2.34 eV [10]. The absorption edge observed in sample M12 is below that in TlBr. Chemical analysis shows that the content of iodine in the sample M12 significantly exceeds that for samples M13 and M14. Performances of crystals from investigated ingots are collected in Table 1. Therefore, it is concluded that the iodine impurity is responsible for absorption edge shift. Absorption edges for samples M13 and M14 are very close at room temperature. The tail of absorption extending from B2.72 eV down to 1.5 eV also show differences. The absorption in this spectral region is a good ‘‘identificator’’ for defect content in the crystal. The larger absorption in this range for sample M13 is possibly from small colloids or the optical losses due to light scattering from grain surfaces inside the crystal. Therefore, the crystal M13 is less perfect than the M14.
Ilum, a.u.
0.15
0.10
0.05
0.00 1.2
(c)
1.4
1.6
1.8
2.0
E,eV
Fig. 2. Luminescence spectra under electron beam excitation at 85 K in samples M12 (a), M13 (b), and M14 (c).
The luminescence spectra at 85 K are shown in Fig. 2a–c. Two luminescence bands (B1.8 and 2.2–2.4 eV) was observed under cw excitation. The main luminescence band for samples M13 and M14 peaks at B2.45 eV, whereas for sample M12 at B2.25 eV. The shape of the luminescence band at the long-wave side (B1.7–1.8 eV) shows that overlapping of several luminescence bands takes
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
200
place. Therefore, the luminescence spectra indicate that a number of recombination centers were involved and concentrations of these centers are different in samples studied. The iodine impurity is suggested to be responsible for the main luminescence band shift to the lower photon energy in sample M12. The luminescence decay kinetics (Fig. 3) were not the same over all spectra for the samples studied. Since the decay kinetics are complicated, we use as a general parameter the decay time Dt during which the intensity of luminescence was reduced three-fold. These facts indicate that ‘‘as-grown’’ crystals contain defects. These defects seem to be similar in samples M13 and M14, whereas in sample M12 they differ. The luminescence kinetics for sample M12 shows: (a) the rise of 2.3 eV luminescence intensity is delayed relative to the excitation pulse; (b) decay of luminescence has no fast initial stage at 2.3 eV; (c) luminescence decay extends to the microsecond range; (d) decay kinetics was nonexponential and approximation by the sum from three exponents is possible. The listed results indicate a quite complicated recombination process. The observed experimental facts could be explained in suggestions of migration of charges (possibly diffusion controlled) before the luminescence center excited state formation. The hole in TlBr must be Tl2+ [11]. It is suggested that the electron recombination with Tl2+ is responsible for the main luminescence
band in all samples, possibly associated with some defect. The model of primary defect formation and luminescence center model is described also in Ref. [12]. The shift of luminescence band observed in sample M12 arises due to perturbation of luminescence center (Tl+Vc) by a near-placed iodine impurity. Therefore, the cw absorption as well as luminescence study show that samples M12, M13 and M14 have different properties. The transient absorption was induced by pulsed electron beam. The spectra of transient absorption reveal two main bands peaking at B1.1 and 2.2 eV for all samples studied. Therefore, the transient absorption spectra shape is the same as that described in Ref. [13]. Spectra for samples M13 and M14 match well (Fig. 4), whereas optical density for sample M12 is twice that of M13 and M14 (it can be pointed out that the sizes of all samples are the same and thus the comparison of optical density was possible). This indicates that in the formation of centers responsible for transient absorption the pre-irradiation defects were involved. The absorption band at B1.1 eV can be due to Tl2+ being possibly stabilized by some defect [13], whereas the electron center can be responsible for the band at B2.2 eV. It is possible that during the recombination process the fraction of electron–hole pairs creates excitons (Tl+) and nonradiative decay of some excitons results in formation of Frenkel 0 pairs: Tl2+ V c and Tl . A similar process is known and studied in detail for silver halides [14]. Since the relaxation kinetics of transient absorption
1.1
1.0
1.0 0.9 0.8
TlBr Optical density
Ilum,a. u.
0.8
#12
0.7 0.6 0.5
#14
0.4 0.3 0.2
M12 M13 M14
TlBr RT
0.9
0.7 0.6 0.5 0.4 0.3
#13
0.2
0.1 0.0 0
50
100
150
200
250
300
t,ns Fig. 3. Luminescence kinetic at 2.3 eV spectral region.
0.1 1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
E,eV
Fig. 4. Transient absorption spectra measured at 20 ns delay after excitation pulse starts.
ARTICLE IN PRESS L. Grigorjeva et al. / Nuclear Instruments and Methods in Physics Research A 531 (2004) 197–201
differs from luminescence decay kinetics, it is concluded that electrons and holes created by irradiation have a number of possible recombination channels. The fraction of charge carriers are trapped at pre-irradiation defects and it reduces the efficiency of charge collection in detectors. 4. Conclusions The iodine in TlBr can be detected from the shift of fundamental absorption edge and position of the main luminescence band, whereas specific resistance is not sensitive to this impurity. The transient absorption is much larger in iodine containing TlBr. The g-quanta detector made from TlBr crystals with similar transient absorption and luminescence parameters shows similar detector properties also. The crystals showing fast recombination (DtB30 ns) seem to be better candidates for detector manufacturing. High-purity TlBr with low intrinsic defect concentration is necessary for high quality g-quanta detectors. Acknowledgements This work has been supported by LME (grant TPP 00-57) and the European Commission Center of Excellence (CAMART).
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