Chemical removal of sulphur from AgCl and AgBr for 36Cl measurements at ANSTO

Abstract

Measurements of 36Cl on the 6MV tandem accelerator (SIRIUS) at ANSTO began in 2016, and since then over 1000 groundwater and rock samples have been processed and measured. The challenge with the sample preparation for 36Cl is to keep the 36S rates consistently low to minimise the impact to the 36Cl ion detection. This is generally achieved by precipitating sulphate as BaSO4 before a final precipitation as AgCl. We tested a range of methods for their efficacy, ease of use and consistency in keeping the 36S rates low. For measurement, the AgCl is backed in a bed of AgBr, but the sulphur rate of commercially available AgBr can vary significantly between batches. Preparing AgBr in house can produce very low sulphur rates [1,2]. Alternatively, we have achieved similarly low 36S rates by treating commercially available AgBr in 1M HNO3 for >24 hours.