AUTORADIOGRAPHIC HOMOGENEITY CONTROL OF HIGHLY DILUTED IMPURITIES BY EQUIDENSITOMETRY USING STRIPPING FILM

Résumé
Nous avons appliqué les techniques de l'autoradiographie au contrôle de l'homogénéité des sources Mössbauer. L'autoradiographie peut être utilisée à cette fin dans tous les cas où l'isotope Mössbauer radioactif est introduit dans une matrice non radioactive (par fusion ou par diffusion par exemple). On peut aussi utiliser l'autoradiographie pour étudier des alliages non radioactifs si par activation de neutrons on ne produit dans la matrice qu'une activité faible ou de courte durée de vie alors que l'impureté s'active fortement. En utilisant des films à négatif pelliculaire on peut résoudre des distances entre agrégats de quelques micromètres ; la détection de ces agrégats n'est limitée que par le nombre d'émissions radioactives provenant d'un agrégat pendant la durée d'exposition du film. En utilisant un microdensitomètre ou un film équidensitométrique nous avons, après avoir calibré le film (concentration d'impureté-intensité du film), tracé les courbes d'equidensité des impuretés.

Abstract
We report on the application of autoradiographic techniques to the homogenity control of Mössbauer sources. Autoradiography can be used for this purpose in all cases where the radioactive Mössbauer source isotope is introduced into a non-radioactive host, e. g. by diffusion or melting. In addition, it can also be used for the study of non-radioactive alloys in cases where a neutron activation of the sample produces only weak or short lived activities in the matrix material but causes high activities of the impurity atoms. By the use of special stripping film material cluster distances down to a few micrometers can be resolved. The minimal size of clusters needed for detection is given by the condition that a sufficient number of radio-active decays must originate from a single cluster during the exposure time of the film to produce a grain capable of development. With the use of a microdensitometer or by using equidensitometric film, fluctuations in impurity concentration throughout the matrix can be mapped by creating equidensity lines. To obtain quantitative results on such fluctuations one has to calibrate the impurity concentration versus grain density of the film. This task can be achieved without serious difficulties. An important application of these techniques arises in Mössbauer studies of properties of highly dilute alloys (e. g. Kondo system). Measurements are then usually performed as source experiments in order to keep the impurity concentration in the ppm range. The properties of such systems depend critically on the condition that the impurities are distributed uniformly throughout the bulk of the sample. No clustering of the impurity atoms should occur. In the desired concentration range the classical metallurgical techniques like microprobe analysis, electron microscopy and neutron or x-ray diffraction are not applicable. Microprobe analysis which comes closest to the autoradiographic technique, however, does not sample automatically the bulk of the alloy. Standard single line sources for Mössbauer spectroscopy in ⁵⁷Fe are also prepared by diffusing the ⁵⁷Co activity into a metallic, non-magnetic matrix. The question of homogenity of the ⁵⁷Co impurity is crucial to such questions as the minimum emission linewidth of such a source in particular if used at low temperatures. The autoradiographic technique has been applied successfully to the control of cluster precipitation of ⁵⁷Co and rare earth activities (e. g. ¹⁷⁰Tm) in Cu, Al, Au, Ag and Pd. Often these activities are preferentially attached to grain boundaries and the appropriate heat treatments had to be developed to insure homogenous sources.