Methode experimentale pour la mesure des sections efficaces de reactions nucleaires par spectrometrie de masse

Abstract

Resume Most experiments concerning the determination of nuclear reaction cross sections by the use of mass-spectrometric methods have dealt with reactions induced either by neutrons or by relatively low energy charged particles so that with the exception of fission, the reaction products were restricted to elements in the neighbourhood of the target nucleus and hence the scope of the mass-spectrometric studies somewhat limited. The development of powerful accelerators and the increased interest in the study of high enrgy nuclear reactions have changed this situation quite substantially: products of spallation or fragmentation reactions from a single target may cover a very wide range in Z and A and systematic studies involving the measurement of both stable and radioactive isotopes can now be performed by the mass-spectrometric method. Moreover, in all cases, when targets can be bombarded by the circulating beam of high energy accelerators, the amount of nuclear reaction products formed is in a large measure independant of the target thickness due to the multiple traversals so that the use of thin foils makes it possible for new methods to be used. What we describe in the present report is a very sensitive technique which has been in use for about three years and which applies to the study of stable or radioactive isotopes produced in high energy nuclear reactions. Its main feature consists in isolating the nuclear reaction products through their vapor pressure differences with that of the target material, thus reducing contamination to a minimum. The isotope dilution method is emproved for absolute cross-section determinations and in this case the spike is produced in situ by the nuclear reaction itself. A simple high efficiency ion source with three accurately adjusted demountable filaments has been especially designed for this work. Some examples of the results of nuclear reaction studies are given.