AMS measurements of 36Cl at Chalk River

Abstract The AMS program at Chalk River continues to be devoted to 36 Cl/Cl determinations. The primary scientific investigations focus on the dispersion of 36 Cl in the Chalk River area, 36 Cl abundances in the Great Lake basin and the residence time of water trapped in the Cigar Lake uranium ore body. Recent technical advances include: the use of a multi-sample ion source equipped with a spherical ionizer; rapid sequential injection of the 35,36,37 Cl isotopes; upgrade to the velocity filter; and detector alterations to match better the spatial extent of the beam exiting the gas-filled magnet. This magnet is used in all measurements because it reduces the need for repeated chemical processing to reduce sulphur concentrations in the samples with the inherent risk of 36 Cl contamination from the local environment. In addition, ion-source memory effects, instrument background and the degree of misidentification of unwanted isotopes have been determined and do not limit isotopic ratio determinations at the 10 −15 level. Preparation techniques for water, air and rock samples have evolved to the point where backgrounds of 10 −15 , 10 −15 and 10 −14 36 Cl/Cl, respectively, can be achieved in spite of ambient 36 Cl/Cl values at this reactor-based research laboratory that exceed 10 −11 .