Precise isotope ratio measurements by MC-ICP-MS and application of the isotope dilution technique for the determination of germanium in the Gallium Neutrino Observatory

摘要

In the Gallium Neutrino Observatory (GNO) experiment solar neutrinos are studied. Low energy neutrinos interact with Ga-71 producing radioactive Ge-71, which decays with a half-life of 11.43 days back to Ga-71. This beta(+)-decay is used for quantifying the number of solar neutrinos observed. Stable Ge isotopes are used as carriers when Ge-71 atoms are extracted from the GaCl3 target for measuring their decay. The carrier serves at the same time for quantification of the chemical yield of the extraction process. Ge-removal from the GaCl3 detector solution proceeds exponentially and a small amount of Ge carrier remains in the Ga-tank, which is removed only during the following extraction(s). The use of different stable enriched Ge isotopes (Ge-70, Ge-72, Ge-74 and Ge-76) in alternating sequence allows one to determine the extraction yield of the carrier by high precision isotope ratio measurements. The measurements are carried out using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The exact quantification of any residual Ge carrier from the preceding run is achieved through isotope dilution calculations and isotope mixture calculations. In this paper we describe the evaluation of the results using the two-isotopes and two-components approach, as well as the multi-isotope and three-components approaches. We could demonstrate that the measured isotopic composition of the carrier after extraction can be fully modelled by a mixture of the initially used Ge carrier, of the Ge carrier from the previous run and of a small, but non-negligible contribution of a natural Ge contamination.