Radio and thermoluminescence studies in CsI doped with F centers

Radioluminescence, thermoluminescence, and u.v. excitation measurements in CsI additively colored crystals showed a new luminescent component at 2.5 eV. This is attributed to the radiative recombination of Vk centers with F centers. Another new component at 2.72 eV was observed only under uv excitation of 5.14 eV. This is related to the radiative recombination of localized excitons at F+ center sites (α band)

Electric field effect on the luminescence of KI:Tl

Thermoluminescence of KI:Tl, x- or &#946;-irradiated at T<77°K showed 2 main peaks at 105 and 170°K. They are resp. attributed to the recombination of mobile VK centers with Tl0 centers and to the recombination of thermally released electrons from Tl0 centers with Tl2+ centres. Similar experiments performed under static electric fields (E<40 kV cm-1) show that the intensity of the 2nd glow peak is strongly reduced. The relative intensity variation is anticorrelated with the intensity of glow peaks occurring at >230°K. We suggestthat in the temperature range in which Tl0 centres are thermally ionised, the effect of the electric field favour the retrapping of these electrons on other traps (still unknown). Irradiation doses also play an important role and their effects are studied at 77°K and T=200°K

Study of Vk centers in CsI crystals

Vk centers were observed in CsI doped with Na+ and Tl+ after x-ray irradiation at liquid He temperature by using optical and EPR techniques. They are oriented along [100] directions. By studying thermoluminescence, 2 types of thermal migration were found, one due to linear displacement of the centers along the cubic axis and the other due to 90° rotations. They correspond to 2 glow peaks at 60 and 90°K, respectively

Electric field effect on the luminescence of KI:Tl

Thermoluminescence of KI:Tl, x- or &#946;-irradiated at T230°K. We suggestthat in the temperature range in which Tl0 centres are thermally ionised, the effect of the electric field favour the retrapping of these electrons on other traps (still unknown). Irradiation doses also play an important role and their effects are studied at 77°K and T=200°K

Limiting processes for the defect accumulation under electron irradiation in KBr at 4 K

Les cinétiques de croissance de tous les défauts (F, F+, H, H2, I, Vk) créés dans KBr par irradiation électronique ont été étudiées dans un large domaine de concentration (1016-5 x 1019 cm-3), à 4 K, température à laquelle aucun des défauts n'est thermiquement mobile. Les lois de croissance sont interprétées en considérant la limitation du rendement de création des nouvelles paires de Frenkel par les interstitiels stabilisés. A très haute concentration, la possibilité de recombinaison des centres H, lors de leur déplacement sous forme de crowdion avec les centres F présents, entraîne une nouvelle diminution du rendement d'accumulation des centres F. Nos résultats suggèrent, en outre, une limitation indépendante de la croissance des centres F et F+, donc des processus de formation indépendants.The growth kinetic in a wide range of concentrations (1016-5 x 1019 cm-3) of all defects (F, F+, H, H2, I, Vk) created in pure KBr by electron irradiation have been studied at 4 K, temperature at which no defect is thermally mobile. The kinetic shapes are explained by considering a local action of the stabilized interstitiel centers decreasing the creation yield of new Frenkel pairs. At concentration higher than 1019 cm-3, the decrease of the F center accumulation yield is due to the possibility of uncorrelated recombinations of the F centers and the H centers moving as dynamical crowdions. Our results suggest also an independent limitation of F and F+ center growth, thus independent formation processes

INTERSTITIAL STABILIZATION IN IRRADIATED ALKALI HALIDES AT 77 K

Dans les halogénures alcalins irradiés à 77 K, la cinétique de croissance des centres F est fonction des réactions secondaires ayant lieu après le processus primaire de création de la paire de Frenkel. Le taux de création des centres F est fixé par la compétition entre divers processus secondaires : recombinaison des interstitiels libres avec les centres F et piégeage des interstitiels par des impuretés. La forme de la cinétique est reliée au nombre d'impuretés capables de stabiliser les interstitiels et de donner lieu à la nucléation d'amas. Dans les cristaux les plus purs, la loi en t0,8 de la cinétique des centres F est interprétée par la croissance de larges agglomérats (jusqu'à quelques centaines d'interstitiels) nucléés sur des impuretés résiduelles. Ces amas ont un rayon de capture qui croît avec leur taille. Dans les cristaux impurs contenant en particulier du sodium et du lithium (environ 50 ppm), ces impuretés agissent comme points de nucléation pour des agrégats de petites tailles (environ 10 interstitiels) qui constituent des pièges non saturables. Les rapports des rayons de capture des amas au rayon de piégeage des centres F sont déterminés.In alkali halides irradiated at 77 K the growth kinetics of F centers is a function of the secondary reactions taking place after the primary Frenkel pair creation process. The F center creation rate is fixed by the competition between various secondary thermally activated processes : free interstitial recombination with F centers and the trapping of interstitials by impurities. The kinetics shape is related to the number of impurities able to stabilize the interstitials and to give rise to cluster nucleation. In the purest crystals, the t0,8 law for the F center kinetics is accounted for by the growing of large clusters (to a few hundred of interstitials) nucleated at some residual impurities. These clusters have an increasing trapping radius with the size. In the impure crystals, containing specially sodium and lithium (typically 50 ppm), these impurities act as nucleation sites for small size clusters (up to about 10 interstitials) constituting non saturable traps. The ratios of the cluster capture radii to the F center capture radius are determined

Piégeage des interstitiels par des impuretés divalentes dans KBr : Sr

Studies on the création kinetics of the Hayes Nichols (HN) and D3 band in Sr2+ doped KBr by electron irradiation at 77 K show that these two bands obey a square root relation before HN centre saturation. This is in agreement with a model in which the impurity vacancy dipoles trap H centres and the D3 centre is a di-interstitial halogen centre formed by capture of a second free interstitial at the HN centre. The results show that a trapped H centre has a rather high cross section for a free one. It is found that the saturation value of the created HN centres is proportional to the square of the divalent impurity concentration. A possible model for the complex formed by the halogen interstitials and the dipole is suggested.Des études sur les cinétiques de croissance des bandes Hayes Nichols (H N) et D3 dans KBr dopé strontium irradié aux électrons à 77 K montrent que ces deux bandes satisfont une loi quadratique avant la saturation des centres HN. Ceci est en accord avec un modèle où les dipoles impureté-lacune piègent des centres H et où le centre D3 est un di-interstitiel formé par la capture d'un second interstitiel libre, par le centre HN. Les résultats montrent qu'un centre H piégé a une très forte section de capture pour un interstitiel halogène libre. La valeur à saturation des centres H N varie comme la racine carrée de la concentration en impuretés divalentes. Un modèle possible pour le complexe formé des interstitiels halogènes et du dipôle est suggéré