... formule empirique que relie la concentration des centres-F pendant la première étape de croissance à la racine carrée de la concentration d'impuretés catio-.
JOURNAL DE PHYSIQUE
Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-166
First stage F-centre production in irradiated alkali halides J. D. Comins and B. O. Carragher Department of Physics, University of the Witwatersrand, Johannesburg, South Africa
Résumé. — Dans cette communication un système d'équations cinétiques décrivent la formation de centres-F pendant l'irradiation d'halogériures d'alcoyle dopés d'impuretés cationiques divalentes est développé. Ces équations tiennent compte de la stabilisation des centres-H et des halogènes di-interstitiels aux dipôles impuretélacune isolés ou entassés, ainsi que du dépiègement d'interstitiels provenant de l'irradiation. Les solutions analytiques numériques et approximatives des équations révèlent, en accord avec l'expérience, l'existence d'une dépendance de la température et d'effets de saturation dynamique dans la première étape du processus de croissance des centres-F. Elles expliquent aussi d'une façon nouvelle la formule empirique que relie la concentration des centres-F pendant la première étape de croissance à la racine carrée de la concentration d'impuretés cationiques. Abstract. — Kinetic equations describing the formation of F-centres during irradiation of alkali halides doped with divalent cation impurities have been developed. These incorporate the stabilization of H-centres and diinterstitial halogens at impurity-vacancy dipoles or dipole aggregates as well as radiation-induced interstitial detrapping processes. Numerical and approximate analytical solutions of the equations show the first stage of F-centre growth to be a dynamic, temperature-dependent process in agreement with experiment. They also provide a new explanation of the experimentally observed relation between the first stage F-centre concentration and the square root of the divalent cation impurity concentration.
1. Introduction. — Substitutional divalent cation impurities in the alkali halides alter F-centre production rates during colouration with ionizing radiation near room temperature. There is enhancement of the first, rapid stage of defect growth, while the second, slow stage is suppressed [1, 2]. Most models have assumed the exhaustion of preexisting defects to explain the saturation of the first stage [1, 2]. An apparently successful model [3] in which isolated cation vacancies act as saturable traps for halogen interstitials was based on the experimentally observed relation To °c J~rh-
(1)
Here f0 is the concentration of F-centres in the first stage and nx is the divalent cation impurity concentration. The association reaction between isolated cation vacancies (« v ) and divalent cations to form impurityvacancy (IV) dipoles («D) shows that near room temperature « v °c \f*h provided nD p nY. Thus relation (1) might be expected if the isolated cation vacancies but not the IV dipoles are efficient interstitial traps. Recent work [4-10] has shown that although isolated cation vacancies do act as interstitial traps, it is the IV dipoles and dipole aggregates which play the major role. Furthermore, the transition from first to second stage F-centre production does not involve the exhaustion of these dipole traps [5-10]. Thus a new explanation of relation (1) is required in which the
first stage saturation is viewed as a dynamic rather than as an exhaustion process, and which incorporates the present understanding of the structure of the stabilized halogen interstitials [4, 8]. 2. Theoretical model. — For the simple situation where either IV dipoles or a dominant type of dipole aggregate exists prior to irradiation in concentration nD, the kinetic equations are
