Etude de la radioactivité de météorites métalliques par la méthode photographique

On a étudié par la méthode des émulsions photographiques nucléaires la radioactivité alpha de deux échantillons de météorites métalliques: Toluca (Hambourg) et Carbo. On décrit la procédure utilisée pour la préparation des échantillons, l'exposition et le développement photographique, le contrôle du fading et du background et l'observation microscopique. Les météorites ont été exposées 140 jours à des plaques Ilford C-2. Deux mille particules alpha réparties sur 36 cm2 des plaques témoins et des plaques exposées aux météorites ont été comptées et localisées, leurs vraies longueurs ont été mesurées. L'analyse de la distribution des parcours des alpha montre que plus de 85% des alpha émis par les météorites doivent être attribués à une contamination de la surface de l'échantillon par du polonium, malgré les précautions prises. II faut noter que cette contamination correspond à la présence de 100 atomes seulement de polonium par cm2. Pour minimiser la contribution de la contamination de la surface des plaques et des échantillons, on a calculé les concentrations en uranium et thorium en se basant seulement sur les traces d'alpha ayant un parcours résiduel dans l'émulsion inférieur à 15 microns (parcours de l'alpha du Th-232). Vu l'importance relative du background, on peut seulement donner des limites supérieures des concentrations, qui sont pour les deux échantillons: U < 0,6 · 10−8 g/g (Th = 0) Th < 2 · 10−8 g/g (U = 0). Ces limites sont très proches des valeurs trouvées par le groupe de Paneth sur d'autres échantillons de ces mêmes météorites. On a aussi déterminé une valeur maximum de 10−7 g/g pour la concentration en uranium dans la troïlite de Toluca. Dans l'état actuel de la technique, les limites de détection correspondent à des concentrations en uranium et thorium de l'ordre de 5 · 10−8 g/g. La distribution des parcours des alpha sur les plaques témoins montre que 90% du background de surface des plaques provient du radon de l'air. Il serait done possible dans les meilleures conditions d'exposition d'abaisser le background d'un facteur 10 et de détecter des concentrations en uranium et thorium de 5 · 10−9 g/g. © 1956, All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Alpha radioactivity of iron meteorites (second letter)

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Radioactivity of iron meteorites by the photographic method [2]

We wish to present the preliminary results of a study of the α-radioactivity of iron meteorites by the method of nuclear emulsions. © 1956 Nature Publishing Group.SCOPUS: le.jinfo:eu-repo/semantics/publishe

Application de la methode du RaD a la mesure de l'age "chimique" d'un minerai d'Uranium

The RaD method suggested by F. G. Houtermans for the determination of the "chemical" age of uranium minerals consists in the measurement of Pb/U, from the ratio RaD/Pb. Since RaD is an isotope of lead, this ratio can be determined on any quantity of lead from the mineral. In this way the quantitative analyses of uranium and lead are avoided. The determination can be carried out on an extremely small quantity of matter. The conditions under which the method is applicable, and the ways of measuring RaD, are discussed. A confirmation has been obtained with two samples of pitchblende from Shinkolobwe: the ratio Pb/U was determined on the one hand by chemical analysis, and on the other by the RaD method. All the measurements of the two methods were carried out independently at the second Physical Institute of the University of Gottingen, and at the Centre de Physique Nucléaire de l'Université Libre de Bruxelles. One method of lead determination by dithizone extraction and polarography is described in detail. The technique of measuring the RaD by counting the beta particles of RaE was perfected. The ratios Pb/U determined by the two methods agree within the experimental errors. The advantages of the RaD method are summarized in the conclusion. © 1953.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

On a correlation between the common lead model age and the trace-element content of galenas

It is now well known that the isotopic composition of common lead varies in a significant way with the age of the lead occurrence. Providing certain assumptions are made, this isotopic composition can be used for an estimate of the age of the mineral. This "model" or "conventional" age is often in accordance with other evidence. However there exist a certain number of leads which do not conform to the systematic pattern generally observed. The purpose of this study is to investigate certain aspects of the geology and geochemistry of those leads, the model ages of which are older than the geological age. Their isotopic composition usually appears to be perfectly normal and if the age of the host formation or of the emplacement of the lead occurrence is unknown it may be impossible to detect which leads belong to this type, so that the search for criteria enabling to recognize them is of some importance. Twenty leads from the Alps and twenty-four from North Africa have been studied. In each case the model age, derived from the isotopic composition, has been compared to the age of the host formation or that of the emplacement of the galena. The silver content (and that of some trace elements) of each of these galenas has been measured and compared to the abovementioned ages. Practically all galenas we have investigated that have model ages comparable to the age of the host formation are found in veins, often with more or less obvious magmatic connexions and they are silver rich. On the other hand the leads of those occurrences, located in sedimentary rocks, and apparently independent of igneous activity, are poor in silver and have model ages which are older than the host formation. The origin of the latter, "telethermal", type of occurrence has often been debated and it has been suggested that these occurrences are "rejuvenated" from an older source of ore. This seems to be borne out by the fact that it is precisely occurrences of this type which contain leads with model ages older than the age of the host formation. It thus seems that when the age of the host formation is known, isotope analyses of lead minerals and the model age derived therefrom, constitute objective criteria enabling us to distinguish which occurrences are emplaced for the first time and which are derived or rejuvenated. In formations of unknown age, a similar criterion which might furnish the same information appears to be the silver content of the galena. In each of the areas studied, we have found that, as regards silver content, there is a sharp distinction, with little or no overlap, between leads with model ages in agreement with the age of the host formation and those with model ages older than that of the host formation. © 1958.SCOPUS: ar.jinfo:eu-repo/semantics/publishe