Determination of silicon in refined aluminum by thermal neutron irradiation and separation of the silica by precipitation

The reaction /sup 30/OSi(n, gamma )/sup 31/Si (2.6 hrs.) is sufficiently sensitiv e to allow the determination of the Si present in very low concentrations in a material. The fact that the usable radiation is a BETA /sup -/, a very selective chemical separation of Si, requiring a sufficiently short time lag, is necessary. The separation methods usually used for the determination of Si traces, extraction of the silicomolybdic complex and distillation of the fluosilicic acid, do not give reliable results. The precipitation method, after addition to the samples and standards of an exactly known mass of silica in solution, was used. Because the precipitation yield is established with precision by simple weighing, the reaction does not have to be quantitative. The precipitate is purified by double precipitation and its purity determined by gamma spectrometry. After redissolution in HF at a temperature of 5 deg C in order to avoid Si losses, counting is done in a mylar capsule placed on the counter. The counting reproducibility is excellent and the activity of the boundary layer is measured. The standards of quartz wool are treated in more » the same fashion as the samples. Experience showed that the chemical intederences are negligible. On the contrary a very good thermalization of the neutron flux is indispensable because of the reactions /sup 31/P(n,p)/sup 31/Si and /sup 34/S(n, alpha )/sup 3/Si. Taking into consideratio n the P and S concentrations in the refined Al and the neutron flux characteristics during the irradiation the determination limit is 3 plus or minus 0.5 mu g/g. The results made it possible to obtain precise calibration slope for emission spectrography. One person can process 6 samples and 2 standards in one 8-hr. day. (tr-auth) « less