Optimization of the Sample Preparation Procedure for the Conversion of Enriched 28Si Single Crystal to Enriched Gaseous 28SiF4

This work is a detailed study of the investigation of the optimization of the procedure for the conversion of highly enriched 28 Si single crystal into gaseous silicon tetrafluoride ( 28 SiF 4 ) with the highest purity ever achieved. The gaseous 28 Si-F 4 of highest possible purity is required for the determination of silicon molar mass via performing silicon isotope ratio measurements on highly enriched 28 Si (> 99.99% of 28 Si) using electron impact gas mass spectrometry. The problem is in avoiding the contamination of enriched 28 Si samples with silicon of natural isotopic composition. The maximal allowed contamination is in order of 10 -9 g of silicon of natural isotopic composition. The sample preparation of highly enriched 28 Si samples is the largest contributor to the relative combined uncertainty of silicon isotope ratio measure-ments, which should not exceed 1 x 10 -4 . Accordingly this contribution affects the relative combined uncertainty of silicon molar mass. The experiments necessary to achieve this uncertainty in the procedure were carefully designed to avoid superfluous work and to reduce the costs which might be high due to the required quantities of very expensive enriched 28 Si samples. The optimal sample preparation conditions (and execution of these measurements) obtained within this work are part of the large-scale experiments designed for the calibration of Si isotope ratio measurements of highly enriched 28 Si, needed for the final determination of Si molar mass as one of the input quantities needed for determination of the Avogadro constant.