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Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 790–796
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation V. Yakovlev a,, L. Trefilova b, A. Meleshko a a b
Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634034, Russian Federation Institute for Scintillation Materials, 60 Lenin Avenue, Kharkov 61001, Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 July 2008 Received in revised form 9 February 2009 Accepted 12 February 2009 Available online 24 February 2009
Properties of the color and emission centers induced with an electron pulse beam at temperature within 80–300 K have been studied in CsI(Tl) crystals. It has been established by optical spectrometry with time resolution that initial color centers in this crystal are only Tl0 and Vk centers, which spontaneously recombine emitting visible light at 2.25 and 2.55 eV. It has been shown that the emission decay kinetics at 80 K include two fast exponential components with decay constants 3 and 14 ms as well as slow hyperbolic component with the power index depending on the wavelength of the emitting light. The temperature effect on the emission kinetics has been studied and it has been directly proved that the emission rise stage at the temperature above 170 K is caused by the recombination of electrons, which are thermally released from single Tl0 centers, with VkA centers. The origin of scintillations in CsI(Tl) crystal is discussed in terms of the tunnel electron transitions from ground state of Tl0 centers to ground state of Vk centers at different distances from each other. & 2009 Elsevier B.V. All rights reserved.
Keywords: Thallium-doped cesium iodide Transient optical absorption Luminescence Color center
I. Introduction CsI(Tl) crystal has been widely applied as scintillation detector in different fields of science and engineering for more than 30 years. There have been different explanations of the origin of its activator emission in the visible spectral region. The emission was thought to be caused by: intracenter transitions in Tl+ ion [1,2], donor–acceptor recombination between an electron Tl0 center and a hole Vk center [3] and the radiative annihilation of two halide self-trapped excitons perturbed by Tl+ ions [4–6]. These excitons are expected to be revealed in the short-lived optical absorption spectra, like self-trapped excitons in non-doped alkali halide crystals [7]. Unfortunately, there have not been any experiments to prove this statement. The transient absorption and luminescence of CsI(Tl) crystal under pulsed electron irradiation have been studied in this work.
2. Experiment Samples of CsI(Tl) crystals with 8 10–2 mol% thallium concentration have been grown at the Institute for Scintillation Materials of NAS of Ukraine by seeding in inert atmosphere. Rectangular plate samples with polished faces and with dimen Corresponding author. Tel.: +7 382 242 0567; fax: +7 38 2241 9831.
E-mail address: [email protected] (V. Yakovlev). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.02.012
sions 10 8 2 mm3 have been irradiated in vacuum with a pulse electron beam (Ee ¼ 0.25 MeV, j ¼ 8 A/cm2, t1/2 ¼ 15 ns). The color and emission centers properties have been studied with an optical spectrometer [8] with MDR-3 monochromator, FEU-83 photomultiplier and GDS-2204 oscilloscope at temperature within 80–350 K in the spectral range 300–1100 nm with a time resolution of 7 ns.
3. Results and discussion 3.1. Spectra of transient optical absorption at T ¼ 80 K Optical absorption spectra of CsI(Tl) crystal, which were measured at different time delay in relation to an electron irradiation pulse at 80 K, are shown in Fig. 1. The spectra are non-elementary. There are two pronounced bands with maxima at 3.0 and 1.35 eV, as well as less pronounced bands in the region of 2.5–1.5 eV. It should be noted that the optical absorption spectrum measured after X-irradiation of CsI(Tl) crystal at 1.6 K by Spaeth et al. [9] has almost identical contour. A band with P P maximum at 3.0 eV is regarded to be caused by 2 u+-2 g+ transition in Vk center [9]. The origin of 1.36 eV is not discussed in mentioned paper, however the authors basing on the results of ESR study conclude that centers, which cause this band, carry an opposite charge to Vk centers. More detailed information concerning the structure and properties of color centers induced in Tl-doped alkali halide crystals with X-irradiation as well as UV
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molecular ion causes the band of Vk center with a maximum at 3.0 eV. The Vk concentration is proportional to the value of optical density at 3.0 eV. According to our approximate estimation, a single electron pulse generates about 1016 cm–3 of Vk centers (as well as Tl0 centers) in the crystal.
0.6
3.2. Transient optical absorption and luminescence relaxation kinetics
1
optical density
0.4
2
0.2
0 3
2 photon energy, eV
1
Fig. 1. Optical absorption spectra of CsI(Tl) crystal measured at different time delay in relation to an electron irradiation pulse at 80 K. Time delay is 100 ns (curve 1) and 50 ms (curve 2).
Table 1 Absorption bands parameters of Tl0 and Vk centers in alkali halide crystals containing Tl [6,9–11]. Center
Transition
Emax, (eV)
FHWM, (eV)
Crystal
Tl0
62P1/2-72S1/2 62P1/2-62P3/2
1.32 1.31 1.36 1.35* 2.27 2.14 2.25*
0.26 0.25
RbI(Tl) [10] CsI(Tl) [6] CsI(Tl) [9] CsI(Tl) RbI(Tl) [10] CsI(Tl) [6] CsI(Tl)
62P1/2-62D3/2
Vk
2P + 2P + u g 2P
u
Unknown
-2Pg+
+
Unknown
3.0 3.0* 1.97 1.9*
2.5 2.5*
0.26* 0.455 0.5 0.455*
0.47
CsI(Tl) [11] CsI(Tl) CsI(Tl) [11] CsI(Tl)
0.37*
CsI(Tl) [9] CsI(Tl)
0.47*
Values marked with * are obtained by the authors.
light at 80 K is discussed in studies [6,10] of photo-stimulated luminescence spectra. Spectral parameters of investigated bands [6,9–11] and electron transitions corresponding to these bands are given in Table 1. The absorption spectra (Fig. 1) are fitted by a set of elementary Gaussians. Their parameters are given in Table 1. Fitting curves and Gaussians are shown in Fig. 1 by a solid and a dash line, correspondingly. The set of Gaussians consists of absorption bands of Vk centers at 3.0 and 1.9 eV, Tl0 centers at 1.35 and 2.25 eV, as well as a band of an unknown origin with a maximum at 2.5 eV P P and FHWM of 0.37 eV. The allowed transition 2 u+-2 g+ in I–2
After the depletion of an irradiation pulse, the optical density decreases in the whole spectrum. Two pronounced stages are observed in the decay kinetics of optical density. Some centers (about 25–30% from initially formed) disintegrate for about 50 ms. The rest of the color centers disintegrate for several seconds. The typical kinetic curve of the optical density decay D(t) after the depletion of an irradiation is shown in Fig. 2(a). The optical density decay curves in the whole spectral region are well fitted P by the exponential time dependence D(t) ¼ ii¼¼13Di exp(t/ti), with decay constants: t1 ¼ 3 ms, t2 ¼ 14 ms and t3 ¼ N. Exponential decay components and the result of fitting (a smooth line) are shown in Fig. 2(a). The same decay constants are characteristic for the cathode luminescence decay at 80 K. The spectrum of CsI(Tl) cathode luminescence in visible region consists of two bands with maxima at 2.25 n 2.55 eV [5]. Fig. 2(b) shows that 2.55 eV emission decays exponentially with t ¼ 3 ms, and 2.25 eV emission decays mainly with t ¼ 14 ms. The observed match is an evidence of the correlation between both processes: the first is the destruction of color centers responsible for the observed absorption (see Fig. 1) and the second is the appearance of luminescence. It was established that the numerical values of the decay constants do not depend on the irradiation power varied from 0.003 to 0.16 J/cm2. It means that the exponential dependences of the absorption and emission decay are caused by the monomolecular process of the color center radiative annihilation. The origin of the discussed emission bands is regarded to be caused by the radiative annihilation of near-thallium excitons * + (I 2 e ) of two types, which differ in the distance of Tl ion and the exciton hole nucleus [4–6]. Due to the different values of Stokes shift, the emission bands with maxima 2.55 and 2.25 eV are attributed, respectively, to weak and strong off-center excitons perturbed by Tl+ ions [5]. According to another conception [3,9], the slow decay of CsI(Tl) emission can be explained by electron tunneling to Vk center levels. We found out by direct experimental methods that the optical absorption induced by electron irradiation in CsI(Tl) is caused by Tl0 and Vk color centers, which recombine emitting photons. The obtained results do not allow us to explain the emission at 2.55 and 2.25 eV in terms of radiative annihilation of near-thallium excitons, in this case we should have expected the appearance of near-activator excitons (I 2 e ) in the lowest triplet excited states [5]. The structure of optical transitions of a self-trapped exciton (I 2 e ) in the higher excited states is well studied for non-doped alkali halide crystals [7,8]. In particular, in pure CsI crystal the intensive photon absorption in a region of 2.0–1.0 eV is due to the transition in electron subsystem of self-trapped exciton [8]. However, such absorption is not observed in the short-living spectra of the CsI(Tl) crystal (see Fig. 1). Therefore, the phenomenon of the optical absorption and emission decay observed for microsecond time scale we can explain by radiative tunneling of electron from a Tl0 center to the ground state of a close Vk center as it occurs for donor-acceptor pair of a semiconductor. According to the model of inter-impurity radiative recombination [12], the shorter the distance between a donor and acceptor,
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1
intensity of lum., arb.u.
optical density
1
0.1
2
3
0.01 0
20 time, s
40
0
20 time, s
40
Fig. 2. Kinetic curves of CsI(Tl) after pulse electron irradiation measured at 80 K. (a) optical density decay at 1.4 eV (curve1), (b) emission at 2.5 eV(curve 2) and 2.2 eV (curve 3). Exponential decay components are shown by dashed line.
and hole wave functions, exponentially decreases with distance p ¼ t21 expðg=aB Þ,
Type II - I- Cs+ - Tl+ TypeI
Fig. 3. Model of the two types of close pairs [Tl0Vk] in CsI lattice.
the higher the photon energy ho and the probability of radiative transitions (frequency) p ¼ 1/t. Photon energy is expressed as
_o ¼ Eg ðED EA Þ þ
q2 , 4p0 r
(1)
where Eg is the band gap energy, ED and EA is the levels of the neutral donor and acceptor, correspondingly; q is the electron charge, e is the relative static permittivity of matter. As one can see from (1), the dependence of _o on r is determined by the energy of Coulomb interaction, which arises at the electron transition from a donor to an acceptor. The probability of such transition, which is determined by overlapping both the electron
(2)
where aB is Bohr radius of a charge carrier. There are two types of [Tl0Vk] donor–acceptor pairs with different geometrical configurations in a simple cubic lattice of CsI (see Fig. 3). The distances between Tl0 and Vk center are rI ¼ aO2/2 and rII ¼ aO3/2 for I and II type pairs, respectively, where a—lattice constant. We believe that the origin of two bands of the exponentially decaying emission is due to discontinuity of rI and rII values. Taking into consideration the difference between the numerical value of rI and rII, the origin of 2.55 eV emission band with t ¼ 3 ms can be explained by the radiative recombination in I type pairs of [Tl0Vk], and the band at 2.55 eV with t ¼ 14 ms in II type pairs of [Tl0Vk]. The absorption and emission spectra of CsI(Tl), which are measured with millisecond and second delays, behave in the following way. When measurement time delay gets longer the optical density D decreases evenly in all spectrum. The dependence D(t) at 1.4 eV is shown in log–log scale in Fig. 4 where the decline of the optical density is observed within 0.1–10 s. This dependence cannot be described by any elementary function. However, it is similar to time dependences of defect concentration which are calculated according to a phenomenological theory of many-particle processes for a case of static pair tunnel recombination [13,14]. We consider our conclusion to be quite sound, for Tl0 and Vk are deep capture centers immobile at 80 K. Unlike transient optical absorption, the luminescence decays unevenly along the whole spectrum. The emission in a short wavelength region decays faster than in a long one and therefore the maximum gradually shift towards a low energy region. Figs. 5 and 6 show the emission decay curves at 2.6 and 1.9 eV and a set of emission spectra measured with different measurement time delay after depletion of electron irradiation. As one can see in Fig. 5, the emission decays according to hyperbolic law with power indexes from 1 (1.9 eV) to 2 (2.6 eV),
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100
1
intensity of lum., arb.u.
optical density
2
0.1
0.01 0.0001
0.001
0.01
0.1
1
10
1
10
time, s Fig. 4. Decay curve of CsI(Tl) optical density at 1.4 eV after pulsed electron irradiation at 80 K.
which are typical for tunnel luminescence. As it is seen in Fig. 6, there is a pronounced red shift of the spectra with measurement time delay within 10–7–10–3 s. The spectra with longer measurement time delays differ slightly from each other. The effect of different measurement time delay on the emission spectrum change we can explain by different distance distribution of [Tl0Vk] recombining pairs according to (1) and (2). The similarity of spectra 4 and 5 in Fig. 6 can be explained by the fact that the emission registered in more than 1 ms is caused by recombination of Tl0 and Vk centers, which are positioned at a distance where Coulomb interaction [see (1)], affects slightly the emission process. Thus, the conclusion about the tunnel origin of luminescence in CsI(Tl) crystal agrees well with our data on spectral-kinetics properties of the transient optical absorption and the luminescence, which is registered at 80 K not only in a gated time window with width Dt ¼ 5 10–5 s, as well as with Dt varying from 1 10–5 to 1 101 s. It would be reasonable to analyze the effect of temperature on the above mentioned properties of the sample exposed to electron pulsed irradiation.
3.3. Temperature dependences The contour of transient optical absorption spectrum changes slightly at the increase of temperature up to 295 K, and we believe that only thermal broadening of elementary bands might cause it. Optical density of CsI(Tl), which was measured at different time delay in relation to an electron irradiation pulse at 295 K, decreases in the whole spectrum as evenly as at 80 K. The dependence D(t) in a maximum of Vk band and an oscillogram of the emission flash at 2.3 eV are shown in Fig. 7, where the decay
1 0.0001
0.001 time, s
Fig. 5. Decay curves of CsI(Tl) emission at 2.6 eV (curve 1) and 1.9 eV (curve 2) after pulse electron irradiation at 80 K.
kinetics of both D(t) and I(t) at 295 K is characterized by the same decay constants. As it was mentioned above, D(t) and I(t) have the same decay kinetics at 80 K. But among all color centers, at 80 K about 25–30% disintegrates emitting visible light and at 295 K more than 90%. The study of the slow declining of optical density shows that a small number of centers (according to our estimation, about 0.3%) absorbing photons within 3.8–2.8 eV are stable for 1 s. Because of extremely low optical density we failed to determine the contour of the spectrum. Using the following experimental data we will try to explain the origin of these absorption centers. We know that the absorption bands of I 3 center family reveal exactly in this spectral region [15], and I 3 centers with (I3 )aca structure are formed in CsI(Tl) under irradiation at 295 K and reveal as a peak of thermal stimulated luminescence at 340 K [16]. Based on these facts we believe that I 3 color centers cause the absorption that decays within seconds. Fig. 8 shows the temperature dependences of the optical density amplitude value in Tl0 band at 1.4 eV (curve 1) and the decay constants of both optical density at 1.4 eV (curves 2, 4 in Fig. 8) and emission at 2.25 eV (curves 3, 5 in Fig. 8). The irradiation temperature has little effect on color center concentration in CsI(Tl), since conduction electrons capture by Tl+ ions forming Tl0 centers, and self-trapping of holes from valence band forming Vk centers do not depend on temperature. There are two pronounced regions in dependences 2–5. The decay processes do not depend on temperature from 80 to 170 K that agrees with their tunnel origin. The dramatic decrease of the center lifetime until their radiative recombination is observed at T4170 K (curves 2, 3 in Fig. 8). An inertial rise of emission
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10000
1
1000 intensity of luminescence, arb.u.
2 3 100
4
10
5 1
0.1 3
2.5 2 photon energy, eV
1.5
Fig. 6. Emission spectra of CsI(Tl) after pulse electron irradiation at 80 K, measured with different time delay: 1 10–7, 2 10–5, 3 10–4, 4 10–3, 5 10–2 s.
Fig. 8. Temperature dependences of optical density amplitude value (curve 1) and decay constants (curves 2, 4) at 1.4 eV; temperature dependences of characteristic times of decay (curves 3, 5) and rise (curve 6) of emission at 2.25 eV.
1000
at 2.25 eV appears within the same temperature interval (170–300 K). As an example the kinetics curve of the emission rise at T ¼ 192 K is shown in Fig. 9. According to common conceptions, the rise of the scintillation pulse in CsI(Tl) crystal is due to the bimolecular recombination of Vk centers, which become mobile at T490 K, with Tl0 centers. This recombination causes the appearance of near-thallium excitons [17–19]. An appropriate description for this process should be the second order kinetics reaction where the reaction rate is proportional to the quadrate of the reagent concentrations. The rise curve of the scintillation pulse is approximated not by an exponent, but by a hyperbola as we might expect according to the model [17–19]
100 intensity of lum., arb.u.
1
10
1
IðtÞ ¼ Ið0Þ ð1 et=trise Þ et=tdecay
optical density
2 0.1
(3)
The fitting curve with the set parameters of I(0) ¼ 205,
trise ¼ 500 ns and tdecay ¼ 2.6 ms is shown in Fig. 9 by a solid line.
0.01
0.001 1000
2000
3000
4000
5000
time, ns Fig. 7. Decay kinetics of emission intensity at 2.3 eV (curve 1) and optical density at 3.0 eV (curve 2) after irradiation at 295 K. Solid line for experimental data; dashed line for time components.
It is very unusual that the concentration of color centers, which are generated under pulsed irradiation, does not affect the rate of the emission pulse rise I(t). Kinetics curves normalized to the maximum are equal regardless the beam energy density, which varied from 4 to 32 mJ/cm2 at the same irradiation temperature. It is a very significant experimental result which unambiguously proves that the process responsible for the scintillation pulse rise should be monomolecular and therefore [Tl0Vk] pairs of I and II types are not formed by jump diffusion of Vk centers towards single Tl0 centers as it was supposed before in [17–19]. Only the thermal release of an electron from a single Tl0 center with its subsequent capture by [Tl+Vk] complex (VkA center) can be an alternative mechanism of [Tl0Vk] pair formation.
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localized in the nearest halide surroundings of Tl0 and Tl+ centers with formation of I and II types pairs. Thus there is a set of immobile centers in the crystal at 80 K: 0
0
þ
þ
intensity of lum., arb. u.
½Tl V k þ þ Tl þ þ ½Tl V k þ þ Tl þ þ V k
(5)
Radiative tunnel transition of electrons between the ground states of Tl0 and Vk centers causes the recombination of Tl0 and Vk centers after the irradiation pulse depletion. The recombination in [Tl0Vk] pairs of I and II type results in appearance of the exponentially decaying emission at 2.55 and 2.25 eV correspondingly, whereas the recombination of spatially separated Tl0 and Vk centers causes the hyperbolically decaying emission. The more the registration delay time, the more the red shift of the emission spectrum [see Section 3.2.]. (ii) Vk centers become mobile at temperature above 90 K [11,20] and can recombine with single activator atoms and ions as
100
0
T90 K
0
þ
T90 K
þ
þ
Tl þ þ V k ! ½Tl V k ! Tl þ hnlum
0 0
1 time, s
(6)
2
Fig. 9. Kinetics curve of CsI(Tl) emission at 2.3 eV measured at 192 K. Dots for experimental data; solid curve for fitting.
In fact, the existence of [Tl+Vk] complexes in irradiated CsI(Tl) crystals was established by ODEPR and MCDA methods [9]. The peak of thermo-glow curve at 128 K is regarded to be due to the thermal release of electrons from Tl0 centers [20]. According to the suggested alternative mechanism, the exponential rise time of I(t) can be interpreted as electron lifetime on P level before its thermal release from Tl0 center. Lifetime (trise) depends on temperature in the following way: 1 1 E trise ¼ ¼ o exp a ; (4) kT p where p is the probability of thermal ionization of Tl0 center, o is the frequency factor, Ea is the thermal activation energy of the process Fig. 8 (curve 6) shows the temperature dependence of time constant trise(T), which straightens in Arrenius’s coordinates very well. The energy of thermal activation Ea ¼ 0.1370.01 eV, estimated by the slope angle of trise(T), corresponds to the height of energy barrier dividing 62P1/2 state of Tl0 from the conduction band bottom. We believe that exactly the thallium 6P subband has all the properties of the conduction band. It is known that the band gap in CsI(Tl) is 6.2 eV [21], and the electron transitions between states of Tl+ ion in CsI(Tl) cause the band with a maximum at 4.27 eV [5]. According to data [22], Tl0Va+ color centers appear in CsI(Tl) crystal under exposure to the 4.27 eV band light. So, the energy sufficient for color center formation is substantially less than the energy of the electron transition from the valence band to the proper S conduction band. These experimental facts show that the ground state of Tl0 center is separated from the proper conduction band bottom of the crystal by an energy gap about 2 eV which is more than 50 times exceeds the obtained values of Ea ¼ 0.13 eV.
4. Formation of the color and emission centers in CsI(Tl) crystal The results discussed in this study can be described in the following way. (i) Vk and Tl0 centers arise at different distances from each other under electron pulsed irradiation at 80 K. Some Vk centers are
Tl þ þ V k ! ½Tl V k
(7)
As it follows from (6), the rise stage in emission kinetics is expected to appear at T490 K, however it is not experimentally proved. The absence of the rise stage can be explained by the dominating capture of Vk centers by Tl+ centers, forming VkA centers [see reaction (7)], as the number of Tl+ centers about 1000 times bigger than of Tl0 centers which arise under irradiation. (iii) According to Section 3.3, the appearance of exponential rise stage of emission intensity at temperature above 130 K is caused by thermal ionization of single Tl0 centers with subsequent capture of conduction electron by [Tl+Vk] complexes, which are formed under irradiation and also as the result of Vk centers diffusion according to reaction (7). 0
T130K
þ
þ
þ
Tl þ þ ½Tl V k ! Tl þ þ e þ þ ½Tl V k þ
0
þ
þ
! Tl þ þ ½Tl V k ! Tl þ þ Tl þ þ hn
(8)
(iv) At increase temperature up to 295 K, the duration of reactions (6)–(8) gets shorter than the lifetime of [Tl0Vk] pairs before their radiative recombination (at 295 K trise equals to about 35 ns) and the phosphorescence component has an insignificant share in the emission pulse in comparison to the scintillation component. However, the fact that phosphorescence is observed at room temperature can be explained by the presence of V2 color centers. Further study is necessary to confirm such supposition. Explanation needs to be investigated.
References [1] W.B. Fowler, Physics of Color Centres, Akademische, Leipzig, 1969 (Chapter 2). [2] S. Masunaga, I. Morita, M. Ishiguro, J. Phys. Soc. Jpn. 21 (1966) 638. [3] V.B. Gutan, L.M. Shamovskii, A.A. Dunina, B.S. Gorobets, Opt. Spectrosc. 37 (1974) 407. [4] L.P. Smol’skaya, T.A. Kolesnikova, Opt. Spectrosc. 47 (1979) 292. [5] V. Nagirnyi, A. Stolovich, S. Zazubovich, V. Zepelin, E. Mihokova, M. Nikl, G.P. Pazzi, L. Salvini, J. Phys.: Condens. Matter 7 (1995) 3637. [6] S.A. Chernov, L. Trinkler, A.I. Popov, Radiat. Eff. Defects Solids 143 (1998) 345. [7] K.S. Song, R.T. Williams, Self Trapped Excitons, Springer, Berlin, 1993 (Springer Series of Solid State Sciences 105). [8] V.A. Kravchenko, V.M. Lisitsyn, V.Yu. Yakovlev, Fiz. Tverd. Tela 27 (7) (1985) 2181 (Opt. Spectrosc. 60 (1986) 404). [9] J.M. Spaeth, W. Meise, K.S. Song, J. Phys. C 6 (1994) 3999. [10] M. Thoms, H. Von Seggern, A. Winnaker, J. Appl. Phys. 76 (3) (1994) 1800.
ARTICLE IN PRESS 796
[11] [12] [13] [14] [15] [16] [17]
V. Yakovlev et al. / Journal of Luminescence 129 (2009) 790–796
J.-J. Pilloud, C. Jaccard, Phys. Status Solidi B 92 (1979) 233. F.E. Williams, J. Phys. Chem. Solids 12 (3–4) (1960) 265. V.N. Kuzovkov, Chem. Phys. 6 (8-C) (1987) 1146. H. Schnorer, V.N. Kuzovkov, A. Blumen, Phys. Rev. Lett. 63 (1989) 805. A.M.T. Allen, J.D. Comins, P.J. Ford, J. Phys. C 18 (1985) 5783. V.V. Kolotilin, V.I. Shtan’ko, Fiz. Tverd. Tela 26 (1) (1984) 236. S.A. Chernov, A.I. Popov, L.E. Trinkler, Opt. Spectrosc. 80 (4) (1996) 661.
[18] A.I. Popov, S.A. Chernov, L.E. Trinkler, Nucl. Inst. & Meth. B 122 (1997) 602. [19] J.D. Valentine, W.W. Moses, S.E. Derenzo, D.K. Wehe, G.F. Knoll, Nucl. Inst. & Meth. A 325 (1993) 147. [20] V. Babin, K. Kalder, A. Krasnikov, S. Zazubovich, J. Lumin. 96 (2002) 75. [21] F.S. Eby, K.J. Teegarden, D.B. Dutton, Phys. Rev. 116 (5) (1959) 1099. [22] L. Trefilova, T. Charkina, A. Kudin, N. Kosinov, L. Kovaleva, A. Mitichkin, J. Lumin. 102–103 (2003) 543.
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Time-resolved optical spectroscopy of CsI(Tl) crystals by pulsed electron beam irradiation V. Yakovlev a,, L. Trefilova b, A. Meleshko a a b
Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634034, Russian Federation Institute for Scintillation Materials, 60 Lenin Avenue, Kharkov 61001, Ukraine
a r t i c l e in f o
a b s t r a c t
Article history: Received 15 July 2008 Received in revised form 9 February 2009 Accepted 12 February 2009 Available online 24 February 2009
Properties of the color and emission centers induced with an electron pulse beam at temperature within 80–300 K have been studied in CsI(Tl) crystals. It has been established by optical spectrometry with time resolution that initial color centers in this crystal are only Tl0 and Vk centers, which spontaneously recombine emitting visible light at 2.25 and 2.55 eV. It has been shown that the emission decay kinetics at 80 K include two fast exponential components with decay constants 3 and 14 ms as well as slow hyperbolic component with the power index depending on the wavelength of the emitting light. The temperature effect on the emission kinetics has been studied and it has been directly proved that the emission rise stage at the temperature above 170 K is caused by the recombination of electrons, which are thermally released from single Tl0 centers, with VkA centers. The origin of scintillations in CsI(Tl) crystal is discussed in terms of the tunnel electron transitions from ground state of Tl0 centers to ground state of Vk centers at different distances from each other. & 2009 Elsevier B.V. All rights reserved.
Keywords: Thallium-doped cesium iodide Transient optical absorption Luminescence Color center
I. Introduction CsI(Tl) crystal has been widely applied as scintillation detector in different fields of science and engineering for more than 30 years. There have been different explanations of the origin of its activator emission in the visible spectral region. The emission was thought to be caused by: intracenter transitions in Tl+ ion [1,2], donor–acceptor recombination between an electron Tl0 center and a hole Vk center [3] and the radiative annihilation of two halide self-trapped excitons perturbed by Tl+ ions [4–6]. These excitons are expected to be revealed in the short-lived optical absorption spectra, like self-trapped excitons in non-doped alkali halide crystals [7]. Unfortunately, there have not been any experiments to prove this statement. The transient absorption and luminescence of CsI(Tl) crystal under pulsed electron irradiation have been studied in this work.
2. Experiment Samples of CsI(Tl) crystals with 8 10–2 mol% thallium concentration have been grown at the Institute for Scintillation Materials of NAS of Ukraine by seeding in inert atmosphere. Rectangular plate samples with polished faces and with dimen Corresponding author. Tel.: +7 382 242 0567; fax: +7 38 2241 9831.
E-mail address: [email protected] (V. Yakovlev). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.02.012
sions 10 8 2 mm3 have been irradiated in vacuum with a pulse electron beam (Ee ¼ 0.25 MeV, j ¼ 8 A/cm2, t1/2 ¼ 15 ns). The color and emission centers properties have been studied with an optical spectrometer [8] with MDR-3 monochromator, FEU-83 photomultiplier and GDS-2204 oscilloscope at temperature within 80–350 K in the spectral range 300–1100 nm with a time resolution of 7 ns.
3. Results and discussion 3.1. Spectra of transient optical absorption at T ¼ 80 K Optical absorption spectra of CsI(Tl) crystal, which were measured at different time delay in relation to an electron irradiation pulse at 80 K, are shown in Fig. 1. The spectra are non-elementary. There are two pronounced bands with maxima at 3.0 and 1.35 eV, as well as less pronounced bands in the region of 2.5–1.5 eV. It should be noted that the optical absorption spectrum measured after X-irradiation of CsI(Tl) crystal at 1.6 K by Spaeth et al. [9] has almost identical contour. A band with P P maximum at 3.0 eV is regarded to be caused by 2 u+-2 g+ transition in Vk center [9]. The origin of 1.36 eV is not discussed in mentioned paper, however the authors basing on the results of ESR study conclude that centers, which cause this band, carry an opposite charge to Vk centers. More detailed information concerning the structure and properties of color centers induced in Tl-doped alkali halide crystals with X-irradiation as well as UV
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molecular ion causes the band of Vk center with a maximum at 3.0 eV. The Vk concentration is proportional to the value of optical density at 3.0 eV. According to our approximate estimation, a single electron pulse generates about 1016 cm–3 of Vk centers (as well as Tl0 centers) in the crystal.
0.6
3.2. Transient optical absorption and luminescence relaxation kinetics
1
optical density
0.4
2
0.2
0 3
2 photon energy, eV
1
Fig. 1. Optical absorption spectra of CsI(Tl) crystal measured at different time delay in relation to an electron irradiation pulse at 80 K. Time delay is 100 ns (curve 1) and 50 ms (curve 2).
Table 1 Absorption bands parameters of Tl0 and Vk centers in alkali halide crystals containing Tl [6,9–11]. Center
Transition
Emax, (eV)
FHWM, (eV)
Crystal
Tl0
62P1/2-72S1/2 62P1/2-62P3/2
1.32 1.31 1.36 1.35* 2.27 2.14 2.25*
0.26 0.25
RbI(Tl) [10] CsI(Tl) [6] CsI(Tl) [9] CsI(Tl) RbI(Tl) [10] CsI(Tl) [6] CsI(Tl)
62P1/2-62D3/2
Vk
2P + 2P + u g 2P
u
Unknown
-2Pg+
+
Unknown
3.0 3.0* 1.97 1.9*
2.5 2.5*
0.26* 0.455 0.5 0.455*
0.47
CsI(Tl) [11] CsI(Tl) CsI(Tl) [11] CsI(Tl)
0.37*
CsI(Tl) [9] CsI(Tl)
0.47*
Values marked with * are obtained by the authors.
light at 80 K is discussed in studies [6,10] of photo-stimulated luminescence spectra. Spectral parameters of investigated bands [6,9–11] and electron transitions corresponding to these bands are given in Table 1. The absorption spectra (Fig. 1) are fitted by a set of elementary Gaussians. Their parameters are given in Table 1. Fitting curves and Gaussians are shown in Fig. 1 by a solid and a dash line, correspondingly. The set of Gaussians consists of absorption bands of Vk centers at 3.0 and 1.9 eV, Tl0 centers at 1.35 and 2.25 eV, as well as a band of an unknown origin with a maximum at 2.5 eV P P and FHWM of 0.37 eV. The allowed transition 2 u+-2 g+ in I–2
After the depletion of an irradiation pulse, the optical density decreases in the whole spectrum. Two pronounced stages are observed in the decay kinetics of optical density. Some centers (about 25–30% from initially formed) disintegrate for about 50 ms. The rest of the color centers disintegrate for several seconds. The typical kinetic curve of the optical density decay D(t) after the depletion of an irradiation is shown in Fig. 2(a). The optical density decay curves in the whole spectral region are well fitted P by the exponential time dependence D(t) ¼ ii¼¼13Di exp(t/ti), with decay constants: t1 ¼ 3 ms, t2 ¼ 14 ms and t3 ¼ N. Exponential decay components and the result of fitting (a smooth line) are shown in Fig. 2(a). The same decay constants are characteristic for the cathode luminescence decay at 80 K. The spectrum of CsI(Tl) cathode luminescence in visible region consists of two bands with maxima at 2.25 n 2.55 eV [5]. Fig. 2(b) shows that 2.55 eV emission decays exponentially with t ¼ 3 ms, and 2.25 eV emission decays mainly with t ¼ 14 ms. The observed match is an evidence of the correlation between both processes: the first is the destruction of color centers responsible for the observed absorption (see Fig. 1) and the second is the appearance of luminescence. It was established that the numerical values of the decay constants do not depend on the irradiation power varied from 0.003 to 0.16 J/cm2. It means that the exponential dependences of the absorption and emission decay are caused by the monomolecular process of the color center radiative annihilation. The origin of the discussed emission bands is regarded to be caused by the radiative annihilation of near-thallium excitons * + (I 2 e ) of two types, which differ in the distance of Tl ion and the exciton hole nucleus [4–6]. Due to the different values of Stokes shift, the emission bands with maxima 2.55 and 2.25 eV are attributed, respectively, to weak and strong off-center excitons perturbed by Tl+ ions [5]. According to another conception [3,9], the slow decay of CsI(Tl) emission can be explained by electron tunneling to Vk center levels. We found out by direct experimental methods that the optical absorption induced by electron irradiation in CsI(Tl) is caused by Tl0 and Vk color centers, which recombine emitting photons. The obtained results do not allow us to explain the emission at 2.55 and 2.25 eV in terms of radiative annihilation of near-thallium excitons, in this case we should have expected the appearance of near-activator excitons (I 2 e ) in the lowest triplet excited states [5]. The structure of optical transitions of a self-trapped exciton (I 2 e ) in the higher excited states is well studied for non-doped alkali halide crystals [7,8]. In particular, in pure CsI crystal the intensive photon absorption in a region of 2.0–1.0 eV is due to the transition in electron subsystem of self-trapped exciton [8]. However, such absorption is not observed in the short-living spectra of the CsI(Tl) crystal (see Fig. 1). Therefore, the phenomenon of the optical absorption and emission decay observed for microsecond time scale we can explain by radiative tunneling of electron from a Tl0 center to the ground state of a close Vk center as it occurs for donor-acceptor pair of a semiconductor. According to the model of inter-impurity radiative recombination [12], the shorter the distance between a donor and acceptor,
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intensity of lum., arb.u.
optical density
1
0.1
2
3
0.01 0
20 time, s
40
0
20 time, s
40
Fig. 2. Kinetic curves of CsI(Tl) after pulse electron irradiation measured at 80 K. (a) optical density decay at 1.4 eV (curve1), (b) emission at 2.5 eV(curve 2) and 2.2 eV (curve 3). Exponential decay components are shown by dashed line.
and hole wave functions, exponentially decreases with distance p ¼ t21 expðg=aB Þ,
Type II - I- Cs+ - Tl+ TypeI
Fig. 3. Model of the two types of close pairs [Tl0Vk] in CsI lattice.
the higher the photon energy ho and the probability of radiative transitions (frequency) p ¼ 1/t. Photon energy is expressed as
_o ¼ Eg ðED EA Þ þ
q2 , 4p0 r
(1)
where Eg is the band gap energy, ED and EA is the levels of the neutral donor and acceptor, correspondingly; q is the electron charge, e is the relative static permittivity of matter. As one can see from (1), the dependence of _o on r is determined by the energy of Coulomb interaction, which arises at the electron transition from a donor to an acceptor. The probability of such transition, which is determined by overlapping both the electron
(2)
where aB is Bohr radius of a charge carrier. There are two types of [Tl0Vk] donor–acceptor pairs with different geometrical configurations in a simple cubic lattice of CsI (see Fig. 3). The distances between Tl0 and Vk center are rI ¼ aO2/2 and rII ¼ aO3/2 for I and II type pairs, respectively, where a—lattice constant. We believe that the origin of two bands of the exponentially decaying emission is due to discontinuity of rI and rII values. Taking into consideration the difference between the numerical value of rI and rII, the origin of 2.55 eV emission band with t ¼ 3 ms can be explained by the radiative recombination in I type pairs of [Tl0Vk], and the band at 2.55 eV with t ¼ 14 ms in II type pairs of [Tl0Vk]. The absorption and emission spectra of CsI(Tl), which are measured with millisecond and second delays, behave in the following way. When measurement time delay gets longer the optical density D decreases evenly in all spectrum. The dependence D(t) at 1.4 eV is shown in log–log scale in Fig. 4 where the decline of the optical density is observed within 0.1–10 s. This dependence cannot be described by any elementary function. However, it is similar to time dependences of defect concentration which are calculated according to a phenomenological theory of many-particle processes for a case of static pair tunnel recombination [13,14]. We consider our conclusion to be quite sound, for Tl0 and Vk are deep capture centers immobile at 80 K. Unlike transient optical absorption, the luminescence decays unevenly along the whole spectrum. The emission in a short wavelength region decays faster than in a long one and therefore the maximum gradually shift towards a low energy region. Figs. 5 and 6 show the emission decay curves at 2.6 and 1.9 eV and a set of emission spectra measured with different measurement time delay after depletion of electron irradiation. As one can see in Fig. 5, the emission decays according to hyperbolic law with power indexes from 1 (1.9 eV) to 2 (2.6 eV),
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100
1
intensity of lum., arb.u.
optical density
2
0.1
0.01 0.0001
0.001
0.01
0.1
1
10
1
10
time, s Fig. 4. Decay curve of CsI(Tl) optical density at 1.4 eV after pulsed electron irradiation at 80 K.
which are typical for tunnel luminescence. As it is seen in Fig. 6, there is a pronounced red shift of the spectra with measurement time delay within 10–7–10–3 s. The spectra with longer measurement time delays differ slightly from each other. The effect of different measurement time delay on the emission spectrum change we can explain by different distance distribution of [Tl0Vk] recombining pairs according to (1) and (2). The similarity of spectra 4 and 5 in Fig. 6 can be explained by the fact that the emission registered in more than 1 ms is caused by recombination of Tl0 and Vk centers, which are positioned at a distance where Coulomb interaction [see (1)], affects slightly the emission process. Thus, the conclusion about the tunnel origin of luminescence in CsI(Tl) crystal agrees well with our data on spectral-kinetics properties of the transient optical absorption and the luminescence, which is registered at 80 K not only in a gated time window with width Dt ¼ 5 10–5 s, as well as with Dt varying from 1 10–5 to 1 101 s. It would be reasonable to analyze the effect of temperature on the above mentioned properties of the sample exposed to electron pulsed irradiation.
3.3. Temperature dependences The contour of transient optical absorption spectrum changes slightly at the increase of temperature up to 295 K, and we believe that only thermal broadening of elementary bands might cause it. Optical density of CsI(Tl), which was measured at different time delay in relation to an electron irradiation pulse at 295 K, decreases in the whole spectrum as evenly as at 80 K. The dependence D(t) in a maximum of Vk band and an oscillogram of the emission flash at 2.3 eV are shown in Fig. 7, where the decay
1 0.0001
0.001 time, s
Fig. 5. Decay curves of CsI(Tl) emission at 2.6 eV (curve 1) and 1.9 eV (curve 2) after pulse electron irradiation at 80 K.
kinetics of both D(t) and I(t) at 295 K is characterized by the same decay constants. As it was mentioned above, D(t) and I(t) have the same decay kinetics at 80 K. But among all color centers, at 80 K about 25–30% disintegrates emitting visible light and at 295 K more than 90%. The study of the slow declining of optical density shows that a small number of centers (according to our estimation, about 0.3%) absorbing photons within 3.8–2.8 eV are stable for 1 s. Because of extremely low optical density we failed to determine the contour of the spectrum. Using the following experimental data we will try to explain the origin of these absorption centers. We know that the absorption bands of I 3 center family reveal exactly in this spectral region [15], and I 3 centers with (I3 )aca structure are formed in CsI(Tl) under irradiation at 295 K and reveal as a peak of thermal stimulated luminescence at 340 K [16]. Based on these facts we believe that I 3 color centers cause the absorption that decays within seconds. Fig. 8 shows the temperature dependences of the optical density amplitude value in Tl0 band at 1.4 eV (curve 1) and the decay constants of both optical density at 1.4 eV (curves 2, 4 in Fig. 8) and emission at 2.25 eV (curves 3, 5 in Fig. 8). The irradiation temperature has little effect on color center concentration in CsI(Tl), since conduction electrons capture by Tl+ ions forming Tl0 centers, and self-trapping of holes from valence band forming Vk centers do not depend on temperature. There are two pronounced regions in dependences 2–5. The decay processes do not depend on temperature from 80 to 170 K that agrees with their tunnel origin. The dramatic decrease of the center lifetime until their radiative recombination is observed at T4170 K (curves 2, 3 in Fig. 8). An inertial rise of emission
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10000
1
1000 intensity of luminescence, arb.u.
2 3 100
4
10
5 1
0.1 3
2.5 2 photon energy, eV
1.5
Fig. 6. Emission spectra of CsI(Tl) after pulse electron irradiation at 80 K, measured with different time delay: 1 10–7, 2 10–5, 3 10–4, 4 10–3, 5 10–2 s.
Fig. 8. Temperature dependences of optical density amplitude value (curve 1) and decay constants (curves 2, 4) at 1.4 eV; temperature dependences of characteristic times of decay (curves 3, 5) and rise (curve 6) of emission at 2.25 eV.
1000
at 2.25 eV appears within the same temperature interval (170–300 K). As an example the kinetics curve of the emission rise at T ¼ 192 K is shown in Fig. 9. According to common conceptions, the rise of the scintillation pulse in CsI(Tl) crystal is due to the bimolecular recombination of Vk centers, which become mobile at T490 K, with Tl0 centers. This recombination causes the appearance of near-thallium excitons [17–19]. An appropriate description for this process should be the second order kinetics reaction where the reaction rate is proportional to the quadrate of the reagent concentrations. The rise curve of the scintillation pulse is approximated not by an exponent, but by a hyperbola as we might expect according to the model [17–19]
100 intensity of lum., arb.u.
1
10
1
IðtÞ ¼ Ið0Þ ð1 et=trise Þ et=tdecay
optical density
2 0.1
(3)
The fitting curve with the set parameters of I(0) ¼ 205,
trise ¼ 500 ns and tdecay ¼ 2.6 ms is shown in Fig. 9 by a solid line.
0.01
0.001 1000
2000
3000
4000
5000
time, ns Fig. 7. Decay kinetics of emission intensity at 2.3 eV (curve 1) and optical density at 3.0 eV (curve 2) after irradiation at 295 K. Solid line for experimental data; dashed line for time components.
It is very unusual that the concentration of color centers, which are generated under pulsed irradiation, does not affect the rate of the emission pulse rise I(t). Kinetics curves normalized to the maximum are equal regardless the beam energy density, which varied from 4 to 32 mJ/cm2 at the same irradiation temperature. It is a very significant experimental result which unambiguously proves that the process responsible for the scintillation pulse rise should be monomolecular and therefore [Tl0Vk] pairs of I and II types are not formed by jump diffusion of Vk centers towards single Tl0 centers as it was supposed before in [17–19]. Only the thermal release of an electron from a single Tl0 center with its subsequent capture by [Tl+Vk] complex (VkA center) can be an alternative mechanism of [Tl0Vk] pair formation.
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localized in the nearest halide surroundings of Tl0 and Tl+ centers with formation of I and II types pairs. Thus there is a set of immobile centers in the crystal at 80 K: 0
0
þ
þ
intensity of lum., arb. u.
½Tl V k þ þ Tl þ þ ½Tl V k þ þ Tl þ þ V k
(5)
Radiative tunnel transition of electrons between the ground states of Tl0 and Vk centers causes the recombination of Tl0 and Vk centers after the irradiation pulse depletion. The recombination in [Tl0Vk] pairs of I and II type results in appearance of the exponentially decaying emission at 2.55 and 2.25 eV correspondingly, whereas the recombination of spatially separated Tl0 and Vk centers causes the hyperbolically decaying emission. The more the registration delay time, the more the red shift of the emission spectrum [see Section 3.2.]. (ii) Vk centers become mobile at temperature above 90 K [11,20] and can recombine with single activator atoms and ions as
100
0
T90 K
0
þ
T90 K
þ
þ
Tl þ þ V k ! ½Tl V k ! Tl þ hnlum
0 0
1 time, s
(6)
2
Fig. 9. Kinetics curve of CsI(Tl) emission at 2.3 eV measured at 192 K. Dots for experimental data; solid curve for fitting.
In fact, the existence of [Tl+Vk] complexes in irradiated CsI(Tl) crystals was established by ODEPR and MCDA methods [9]. The peak of thermo-glow curve at 128 K is regarded to be due to the thermal release of electrons from Tl0 centers [20]. According to the suggested alternative mechanism, the exponential rise time of I(t) can be interpreted as electron lifetime on P level before its thermal release from Tl0 center. Lifetime (trise) depends on temperature in the following way: 1 1 E trise ¼ ¼ o exp a ; (4) kT p where p is the probability of thermal ionization of Tl0 center, o is the frequency factor, Ea is the thermal activation energy of the process Fig. 8 (curve 6) shows the temperature dependence of time constant trise(T), which straightens in Arrenius’s coordinates very well. The energy of thermal activation Ea ¼ 0.1370.01 eV, estimated by the slope angle of trise(T), corresponds to the height of energy barrier dividing 62P1/2 state of Tl0 from the conduction band bottom. We believe that exactly the thallium 6P subband has all the properties of the conduction band. It is known that the band gap in CsI(Tl) is 6.2 eV [21], and the electron transitions between states of Tl+ ion in CsI(Tl) cause the band with a maximum at 4.27 eV [5]. According to data [22], Tl0Va+ color centers appear in CsI(Tl) crystal under exposure to the 4.27 eV band light. So, the energy sufficient for color center formation is substantially less than the energy of the electron transition from the valence band to the proper S conduction band. These experimental facts show that the ground state of Tl0 center is separated from the proper conduction band bottom of the crystal by an energy gap about 2 eV which is more than 50 times exceeds the obtained values of Ea ¼ 0.13 eV.
4. Formation of the color and emission centers in CsI(Tl) crystal The results discussed in this study can be described in the following way. (i) Vk and Tl0 centers arise at different distances from each other under electron pulsed irradiation at 80 K. Some Vk centers are
Tl þ þ V k ! ½Tl V k
(7)
As it follows from (6), the rise stage in emission kinetics is expected to appear at T490 K, however it is not experimentally proved. The absence of the rise stage can be explained by the dominating capture of Vk centers by Tl+ centers, forming VkA centers [see reaction (7)], as the number of Tl+ centers about 1000 times bigger than of Tl0 centers which arise under irradiation. (iii) According to Section 3.3, the appearance of exponential rise stage of emission intensity at temperature above 130 K is caused by thermal ionization of single Tl0 centers with subsequent capture of conduction electron by [Tl+Vk] complexes, which are formed under irradiation and also as the result of Vk centers diffusion according to reaction (7). 0
T130K
þ
þ
þ
Tl þ þ ½Tl V k ! Tl þ þ e þ þ ½Tl V k þ
0
þ
þ
! Tl þ þ ½Tl V k ! Tl þ þ Tl þ þ hn
(8)
(iv) At increase temperature up to 295 K, the duration of reactions (6)–(8) gets shorter than the lifetime of [Tl0Vk] pairs before their radiative recombination (at 295 K trise equals to about 35 ns) and the phosphorescence component has an insignificant share in the emission pulse in comparison to the scintillation component. However, the fact that phosphorescence is observed at room temperature can be explained by the presence of V2 color centers. Further study is necessary to confirm such supposition. Explanation needs to be investigated.
References [1] W.B. Fowler, Physics of Color Centres, Akademische, Leipzig, 1969 (Chapter 2). [2] S. Masunaga, I. Morita, M. Ishiguro, J. Phys. Soc. Jpn. 21 (1966) 638. [3] V.B. Gutan, L.M. Shamovskii, A.A. Dunina, B.S. Gorobets, Opt. Spectrosc. 37 (1974) 407. [4] L.P. Smol’skaya, T.A. Kolesnikova, Opt. Spectrosc. 47 (1979) 292. [5] V. Nagirnyi, A. Stolovich, S. Zazubovich, V. Zepelin, E. Mihokova, M. Nikl, G.P. Pazzi, L. Salvini, J. Phys.: Condens. Matter 7 (1995) 3637. [6] S.A. Chernov, L. Trinkler, A.I. Popov, Radiat. Eff. Defects Solids 143 (1998) 345. [7] K.S. Song, R.T. Williams, Self Trapped Excitons, Springer, Berlin, 1993 (Springer Series of Solid State Sciences 105). [8] V.A. Kravchenko, V.M. Lisitsyn, V.Yu. Yakovlev, Fiz. Tverd. Tela 27 (7) (1985) 2181 (Opt. Spectrosc. 60 (1986) 404). [9] J.M. Spaeth, W. Meise, K.S. Song, J. Phys. C 6 (1994) 3999. [10] M. Thoms, H. Von Seggern, A. Winnaker, J. Appl. Phys. 76 (3) (1994) 1800.
ARTICLE IN PRESS 796
[11] [12] [13] [14] [15] [16] [17]
V. Yakovlev et al. / Journal of Luminescence 129 (2009) 790–796
J.-J. Pilloud, C. Jaccard, Phys. Status Solidi B 92 (1979) 233. F.E. Williams, J. Phys. Chem. Solids 12 (3–4) (1960) 265. V.N. Kuzovkov, Chem. Phys. 6 (8-C) (1987) 1146. H. Schnorer, V.N. Kuzovkov, A. Blumen, Phys. Rev. Lett. 63 (1989) 805. A.M.T. Allen, J.D. Comins, P.J. Ford, J. Phys. C 18 (1985) 5783. V.V. Kolotilin, V.I. Shtan’ko, Fiz. Tverd. Tela 26 (1) (1984) 236. S.A. Chernov, A.I. Popov, L.E. Trinkler, Opt. Spectrosc. 80 (4) (1996) 661.
[18] A.I. Popov, S.A. Chernov, L.E. Trinkler, Nucl. Inst. & Meth. B 122 (1997) 602. [19] J.D. Valentine, W.W. Moses, S.E. Derenzo, D.K. Wehe, G.F. Knoll, Nucl. Inst. & Meth. A 325 (1993) 147. [20] V. Babin, K. Kalder, A. Krasnikov, S. Zazubovich, J. Lumin. 96 (2002) 75. [21] F.S. Eby, K.J. Teegarden, D.B. Dutton, Phys. Rev. 116 (5) (1959) 1099. [22] L. Trefilova, T. Charkina, A. Kudin, N. Kosinov, L. Kovaleva, A. Mitichkin, J. Lumin. 102–103 (2003) 543.