Reactions parasites rencontrees lors du dosage de l’oxygene dans le silicium au moyen des particules alpha

Abstract

Activation analysis by charged particles may encounter great difficulties when ‘parasitic’ reactions induce in the matrix itself the same radionuclide as the one requested by the reaction for analytical aim. In this investigation we study reactions inducing¹⁸F activity in silicon and we show the importance of these reactions for determining low levels of oxygen in silicon by the reactions$O\mathop \to \limits^\alpha {}^{18}F$. For irradiations carried out with 54 MeV α-particles, the profile of¹⁸F production in silicon presents an activity decreasing very rapidly from the surface to depths of 500 μm without any connection with the$O\mathop \to \limits^\alpha {}^{18}F$ activation curve. The diffusion of¹⁸F nuclides created at the surface from the oxide bed does not at all explain that result. The differences between these curves are attributed to parasitic reactions inducing¹⁸F activity in silicon. In order to confirm the reaction$Si\mathop \to \limits^\alpha {}^{18}F$ we also investigated the profile of²⁴Na and²⁹Al production by another parasitic reaction in silicon. It is pointed out that the choice of incident energy is quite primordial, for analyses of oxygen in silicon with 54 MeV α-particles are not quite suitable. The best energy range appears to be around 35 MeV.