POINT DEFECTS AND LUMINESCENCE IN SINGLE NaCl:Cu CRYSTALS

E.Cruz-Zaragoza1,*, V.Chernov2, R.Meléndrez2, S.Ramos B.1, A.Negrón-Mendoza1, H.Murrieta S.3, J.Hernández A.3, M.Barboza-Flores2. 1Instituto de Ciencias Nucleares UNAM, A. Postal 70-543, México D.F., México. 2Centro de Investigación en Física, Universidad de Sonora, A. Postal 5-088, Hermosillo, Sonora, México 3Instituto de Física UNAM, A. Postal 20-364, México D.F. 01000, México

Abstract

Optical absorption, thermally stimulated luminescence and photoluminescence experiments were performed in NaCl:Cu(0.4%) and NaCl:Cu(0.8%) crystals blocks grown by Czochralski technique. The monocrystals NaCl:Cu were exposed to gamma rays from 60Co source up to 30 kGy as well as to UV radiation. The radiation-induced defects were mainly in the F-centers form and their optical absorption bands were found to be dependent from the Cu impurity concentration. It was observed that the gamma induced absorption band at 256.7 nm was shifted toward 236.3 nm when the crystals were irradiated with UV light. At high radiation dose (20 kGy) the F-centers were bleached with F-light. The change in the glow curve after bleaching suggests that the recombination occurs around the impurity of the doped crystal.

*Corresponding author: [email protected]

1. Introduction

The study of defects generated by ionizing radiation and non ionizing radiation, optical and thermoluminescent (TL) properties are being studied in order to elucidate the roll of the defects generated by irradiation in alkali halides, in particular NaCl doped with Cu+, that under irradiation the Cu change1 to Cu+. Some effort was made for to determining optical properties and TL emisions in NaCl:Cu, inorder to find a relationship among some defects like F, M and other centers that might be present in this single crystal subjected to ionizing radiation2,3.

This work shows the relationship between F-center generated by gamma rays and TL emission in NaCl:Cu for two different concentrations of copper. The F-centers generated were bleached with F light in order to find a relationship. In previous work4, it was shown that the F center, generated in the host single crystal NaCl with lead doped, the optical property was modified with the small among of the Pb impurity and F centers amount it was dependent by lead concentration and by dose rate also, similar behaviour occurred in mixed single crystals KCl-KBr with lead impurity7,8,9. In the present work, it is shown that increase in concentration (0.08%) of the Cu impurity change drastically its

1 TL emission, while optical absorption characteristics remained unchanged from impurity Cu impurity. But the UV irradiation in the NaCl:Cu modified the nature defects recombination, this effect then changed the optical and TL emission from NaCl doped.

2. Experimental procedures

From two NaCl:Cu blocks monocrystals with different concentration, 0.04% and 0.08% of Cu, the crystals samples were cleaved, and was measured the optical absorption as- grown crystal and after heating treatments for the experiments. The crystals were grown by Czochralski method under 300 Torr argon atmosphere and samples 5x5x0.5 mm3 in size were cleaved for optical and TL measurements.

Before irradiation crystals samples were heated at 450 ºC for 1h and then were fasted quenching at room temperature (22 ºC) in order to avoid the dipoles complex and avoid any wrong measured that can to do confusing results. Optical absorption measurements were performed using a Milton Roy model 3000 and Perkin Elmer model Lambda 19 spectrophotometers. The crystals were exposed to the dose rate 3.855 kGy/h from 60Co in Gammabeam 651 PT irradiation facility at room temperature. The TL measurements were carried out by a nitrogen atmosphere in Harshaw TLD 3500 and TL reader 4000, using a linear heating rate of 5 ºCs-1. All TL measurements and irradiation samples were performed in dark condition. The UV light deuterium lamp was guided through an optical fiber to the samples and then was made the TL measurements.

3. Results and discussions

The crystals samples were quenched at 450 ºC for one hour before any irradiation and optical absorption allowed to characterize the impurity present in the crystals. The optical absorption (Figure 1) for as grown NaCl:Cu+, with 0.04% of Cu, showed in 256 nm associated to Cu impurity, for template samples the same characteristic were observed. At 0.954 kGy the F band appear at 464 nm and the intensity of the optical density increase when the dose increase up to 0.954- 30.051 kGy with gamma rays, the M band appear at 725 nm. The presence of the Cu+ was difficult to observe in all exposed samples, it should be observed at about the 261 nm (4.88 eV) closed to the 256 nm band (Figure 1), and it was visible up to 2.385 kGy. For other concentration NaCl:Cu (0.08%) a similar behaviour was observed. Several absorption bands appear in the and vacuum ultraviolet region in the colored crystals, these bands have been attributed to Cu- ion5,6. This experimental result indicates the Cu- ion is converted to the original Cu+ ion by gamma irradiation at high dose. The Cu- ion has a position in the anion site in NaCl lattice, whereas in this lattice the Cu+ ion occupied a cation site.

The formation of the F-centers and Cu0 centers, can be visualized by the electron released to Cu- under irradiation and then trapped by a negative-ion vacancy. The electron is released from the F center and reatrapping by Cu0 center, resulting in a

2 formation of the Cu-. Then was interesting analysed the behaviour of the F center under irradiation.

The optical absorption (Figure 1) for as grown NaCl:Cu+, with 0.04% of Cu, showed a band in 256 nm associated to Cu impurity, and the same characteristics for templated samples. At 0.954 kGy the F band appear at 464 nm and the intensity of the optical density increase when the doses increase up to 30.051 kGy, and additional band appear at 725 nm it is associated to M center. The presence of the Cu+ was difficult to observe in all exposed samples, it was about at 261 nm (4.88 eV) closed to the 256 nm band that showed in Figure 1, and it was visible up to 2.385 kGy. When the impurity concentration was increased, 0.08% of Cu, similar position for optical absorption was observed but the band at 256 nm increased up to 3.339 kGy and for 7.155 kGy this band decrease and the other band at 236 nm increased .Finally for high dose, 30.051 kGy, this band was easier to observe when UV light was used. The F bands formed at 5 kGy in the samples. The thermal bleaching was done heating from room temperature (RT) up to 650 K, then F bands was decrease faster than Cu band. The band located in 5.3 eV moved to 4.88 eV when the temperature was increased for thermal bleaching. This behaviour is associated to dissociation of the dipoles impurity-vacancy.

The participation of the defects generated by irradiation is related to luminescence emission, then it is interesting to analyse the thermoluminescence behaviour in this crystal exposed to gamma rays at high doses for both impurity Cu concentration in the NaCl. TL measurements showed a broad emission with three peaks at 99, 185 and 225 °C for NaCl:Cu at 0.08%, while for 0.04% of Cu the crystal shows four peaks at 99, 149, 183 and 220°C. However five TL peaks were created under monochromatic UV light (Figure 2) in the 200-300 nm range, for 0.04% of Cu, these peaks were located at 98, - 130, 150, 185 and 245 °C. These emissions are related to F-H and Vk-Cu recombinations at the temperature maxima of the glow peak. Delgado et al2 attributed 3.40 eV and 2.78 eV emission at the glow peak due to these pairs, respectively. When the concentration was 0.08% of the impurity the crystals shows more intensity for the TL signal and it was similar behaviour for TL emission. Our measurements showed that F center has a important contribution in the glow curve, then the F band at 5 kGy was bleaching with F light, for 0.08%, up to diminish and the band in 4.88 eV is associated with dipolar structure and it was remaining (Figure 3). Finally, the effect of preheat on integrated OSL and TL, at 0.04% of Cu, was OSL fastest decay than TL for 2.5 kGy.

Conclusions

When the F-centers of the sample are optically bleached, from the originally five peaks of the glow curve, only one remains. Therefore it is possible to associate those peaks of the glow curve, with the color centers generating with ionizing radiation. However, this remainig peaks has shown some dependency of the color centers due that after optical bleaching its intensity by about 40%.

3 On the other hand if the thermal bleaching is made the band associated with the copper is also affected. All this suggest that the F-centers are resposibles for the TL emission. Even if the F-band is bleached the copper bands do not change their conditions.

It is possible to suggest that the electron release from F centers by optical or thermal excitations are eventually captured by complexes centers: 1)complexes of Cu0, causing an increase in Cu- absorption5; 2) the intermediate stage in the conversion from Cu- to Cu+; 3) complexes centers like neutral copper atoms with cation vacancies as nearest neighbors.

Acknowledgements

This work has been supported by the Oficina de Colaboración Interinstitucional UNAM.

References

1. Nanto, H., Usuda, T., Murayama, K., Inabel, K. and Takeuchi, N. Radiat. Prot. Dosim. 47 (1/4), 293-296 (1993). 2. Delgado, L. and Alvarez Rivas, J.L. Phys. Rev. B., 23 (12), 6699-6710 (1981). 3. Fussgaenger, K. Physica Status Solidi 36, 157-169 (1968). 4. Ramos Bernal S., Cruz E., Hernandez J.M. and Murrieta, H. Radiat. Phys. Chem. 52 (1-6), 643-648 (1998). 5. Tsuboi Taiju and Jacobs P.W.M. Journal of Physics and Chemistry of Solids 52 (1), 69-80 (1991). 6. Nagasaka, S., Ikeya, M. and Ueta, M. J. Phys. Soc. Jpn. 20, 1540-1541 (1965). 7. E.Cruz Zaragoza. Doctoral Thesis (2003), unpublish. 8. E.Cruz Z., A.Negrón, A.Ramos A., S.Ramos B., J.Hernández A., F.Jaque, H.Murrieta S. Radiat. Phys. Chem 61, 443-444 (2001). 9. E.Cruz-Zaragoza, R.Meléndrez, V.Chernov, M.Barboza-Flores, T.M.Piters, J.A.Hernández, H.S.Murrieta. Radiation Protection Dosimetry 100 (1-4), 455-458 (2002).

Figures captions

Figure 1. The NaCl:Cu optical absorption at 0.9-30 kGy. a) as grown crystals and b) templated at RT.

Figure 2. TL created with monochromatic UV light for NaCl:Cu (0.04%).

Figure 3. F-bleaching effect at 460 nm on optical absorption of NaCl:Cu at 5 kGy.

4

5