It is shown that the photostimulated luminescence of the KBr-In crystal at room temperature preliminary irradiated in the exciton absorption band arise from three ...
Rudiution Effects and Defects in Sohds, 1995, Vol. 135. pp 125-128 Reprints available directly from the publisher Photocopying permitted by license only
0 1995 OPA (Overseas Publishen Association) Amsterdam B.V. Published under license by Gordon and Breach Science Publishers SA Printed in Malaysia
PHOTOSTIMULATED LUMINESCENCE OF KBr-In CRYSTALS I. PLAVINA,' A. I. POPOV2 and A. TALE'
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'Institute of Physics, Latvian Academy of Sciences, V-2169 Riga, Latvia; 21nstituteof Solid State Physics, University of Latvia, 8 Kengaraga str., LV-1063 Riga, Latvia It is shown that the photostimulated luminescence of the KBr-In crystal at room temperature preliminary
irradiated in the exciton absorption band arise from three types of close defect pairs. The stimulation spectra a for each pair are investigated.It is shown also that one of these three kinds of defect pairs is (F, In2+], whereas two other pairs have the electron centre of more complicated nature. It is demonstrated that the KBr-In is an effective radiation storage material for both UV- and X-irradiations. Key words: Alkali halides, photostimulated luminescence, color centers, F-center, storage phosphors.
1 INTRODUCTION
In recent years the photostimulated luminescence (PSL) has the great attention of many scientists because of their application for radiation imaging.' ,2 For this purposes, a variety of the different PSL phosphors, such as BaFBr:Eu, RbBr-Tl? KBr-In4 etc have been considered. In our recent paper^^-^ it was shown that { F, In2+]defect pairs, which are responsible for the distinctive PSL, are produced under the exciton-band excitation. Furthermore, when the number of the exciton-created photons is less than 5.0 x 1013photons/cm2 only the (F, In2+}pairs are formed. In order to excite the PSL of the { F, In2+}pairs, the crystal has to be illuminated with light into the F band. As a result, the F center electrons are excited and captured by the nearby In2+ centers. This electron transfer from F* to In2+ results in the PSL of In+ luminescence (420-530 nm).4.6 This paper concentrates on the new PSL properties of a KBr-In crystals proposed for UV-4 and X-ray radiation irnaging.41~ 2 EXPERIMENTAL
Experimental details were presented earlier for the case of UV irradiation X-rays in','.
and for
3 RESULTS AND DISCUSSION In order to investigate the PSL process after the optical creation of the anion excitons, we measured carefully the kinetics of PSL under constant F-light stimulation. As it was shown the PSL of the {F, In2+}pairs is exponential in time. This process can be written schematically as
+
{F, In2+} hv,,
-+
{v:,
(In+)*} + {v:,
In'}
+ hy,+
(1)
where v,' is the anion vacancy. The appropriate stimulation spectrum is presented in [623]/125
-16
Absorption cross section (10 cm2)
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PSL OF KBr-In CRYSTALS
Idn
[625]/127
1G8 Energy
(J/cd)
F'IGURE 2 Dose dependence of PSL of KBr-In accumulated under UV light (A = 195 nm). PSL lightsums magnitude obtained according to a squared signal-to-noise ratio $*'.
Figure 1. It is similar to the earlier published F-band absorption spectra. Next we have found that the PSL appearing after large dose (D > 5 x l O I 3 p h o t ~ n s k m - ~is) the superposition of three exponential components, { F, In2+}pairs define the middle of them. The spectral distribution of the absorption cross-section c of the slow component (Figure 1) looks like the M-centre absorption. It is well known that the F band covers the spectral region of the transition of the M and R centers. In the case of KBr, the M band includes the M2 and M2' - transitions (EM, = 1.990 eV, EM; = 2.006 eV, AE = 0.016 eV). Estimation of the the OM, gives the values of order 1.0 10-l6 cm-2. From the stimulation spectra (Figure 1) it follows that there are two most pronounced bands; El = 1.87 eV and E2 = 2.08 eV having 0 1 , 2 = 1.05 cm2, respectively, These 0 values are in a and 1.3 good correspondence with the above-mentioned estimation of values. The estimated oscillator strengths of the El and E2 bands are f l = 0.14 and f2 = 0.17 with good correspondence with data.' Thus we can conclude that the slow component of the PSL of KBr-In crystals is due to { M, In2+)pairs and this process can be represented schematically as {M, In2+} + hvst
--f
{M*, In2+} + {F;, (In')*}
-+
{F:,
In'}
+ hy,+
(2)
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128/[626]
I. PLAVINA E T A L .
The origin of the shortest component of the PSL decay is not so obvious. Appearing under large irradiation doses these centers may have a complex structure. They could be the R centers as well as the F centers perturbed by some defect. The component has a spectrum shifted to the shortwavelength side to 0.1 eV compared with the F band. The pairs responsible for the shortest component will be designated as {DF, In2+]. Thus we have concluded that the PSL of KBr-In crystals preliminary irradiated in the exciton fundamental absorption is due to three types of the close defect pairs, such as { M, In2+] and { DF, In2+), in which center-to-center recombination occurs. Recently we show that the resulting {V,', Inf} pairs may be converted again into the {F, In2+] pairs under the C-absorption band irradiation accompanied with the optical ionization of In+ ion. The electron capture by the Coulomb field of the near anion vacancy leads to the restoration of the {F, In2+] pair.6 The same is true for the {DF, In2+} defects, but not for the {M, In2+) ones. The last fact can be explained by the thermal unstability of the F l centre at RT. It should be noted that in the case of C-band irradiation the main part of the PSL decays in accordance with the hyperbolic law. As such irradiation leads to the optical ionization of impurity In+ ions, the released electrons are captured by the Schottky vacancies, thus creating the F centers. Schottky vacancies are randomly distributed (at least in virgin samples !) as well as the F and In2+ centers are. The kinetics of the F-destruction of such randomly arranged F and In2+ centres differ considerably from that of { F, In2+ ] In conclusion, in Figure 2 we demonstrate that KBr-In is an effective UV radiation storage material with a wide dynamic range. A direct proportionality of the PSL intensity to X-ray exposure have been recently established over a wide dose range, viz. from 6 I O-' to 30 R at 44 kV ~ o l t a g e that , ~ is definitely the outstanding property and is much better then that, reported for BaFBr-Eu.' The comparison of the stimulation energies of numerous storage materials also showed the advantage of the KBr-In." The research described in this publication was made possible in part by Grant LB2000 from International Science Foundation.
REFERENCES 1. 1. Plavina, V. Obedkov, V. Chernyak, G . Balandin and B. Sestroretskii, Izv. Akad. Nauk Latvian SSR, Ser.
fiz. tekn. nauk, No. 5, 67 (1969). 2. G. Vlasov, R. Kalnins, L. Nagli, V. Obedkov, I. Plavina and A. Tale, Avtornetriya I, 66 (1980). 3. H. von Seggern, A. Meijerink, T. Voigt and A. Winnacker, J . Appl. Phys. 66,4418 (1989); K. Amitani, A. Kano, H. Tsuchino and F. Shimada, Konicn Technical Report 1, 120 (1988). 4. P. Bratslavets, A. Kalnins, A. Popov, B. Rapoport, A. Tale, B. Zeigurs, in: 'The Advancement of Imaging Science and Technology', 1990, Int. Acad. Pub. (A. Pergamon-CNPIEC Joint Venture) p. 474-476. 5. A. Kalnins, I. Plavina, A. I. Popov and A. Tale, J . Phys. Condens Matter 3, 1265 (1991); Proc. Intern. Conf. Luminescence, Lissabon, July 1990, (pp. 280, 281). 6. P. F. Braslavets, A. Kalnins, I. Plavina, A. I .-Popov, B. I . Rapoport and A. Tale, Phys. Status Solidi B170, 395 (1992). 7. A. Kalninsh, I. Plavina and A. Tale, Nuclear Instr. and Meth. B84 95 (1994); I. Plavina, A. Kalninsh, A. I. Popov and A. Tale, Nuclear Instr. and Meth. B (in press). 8. T. Y. Neubert, Sh. Susman, J. Chern. Phys. 43, 2819 (1965). 9. J. Miyahara, K. Takahashi, Y. Amemiya, N. Kamiya and Y. Satow, Nucl. Instr. and Mrth. A246, 572 (1986). 10. L. E. Trinkler, M. F. Trinkler and A. I. Popov, Phys. Status Solidi B180, K31 (1983).
