This document specifies a test method to determine the activity concentration of ²²⁷Ac in all types of waters by alpha spectrometry.

The test method is applicable to test samples of supply/drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic, and industrial wastewater after proper sampling and handling and test sample preparation (see ISO 5667-1, ISO 5667-3, ISO 5667-10). Filtration of the test sample is necessary.

The detection limit depends on the sample volume, the instrument used, the background count rate, the detection efficiency, the counting time, the chemical yield, and the progeny ingrowth. The method described in this document, using currently available alpha spectrometry apparatus, has a detection limit of approximately 0,03 Bq·l⁻¹, when directly measuring the alpha peak of ²²⁷Ac. This detection limit is lower than the WHO criteria for safe consumption of drinking water for any actinide alpha emitter (0,1 Bq·l⁻¹).^[4] This value can be achieved with a counting time of 48 h for a sample volume of 1 l.

Only a small fraction of ²²⁷Ac decays through alpha emissions (~1,42 %). An option to lower the detection limit of the method is to wait, let the progenies of ²²⁷Ac grow in, and measure an alpha progeny peak of ²²⁷Ac (e.g. ²¹⁵Po). This is a longer technique, but a lower detection limit of approximately 0,000 2 Bq·l⁻¹ can be obtained by re-counting the sample approximately 90 days after purification. The sample can be re-counted before 90 days, but with a higher detection limit.

The test method(s) described in this document can be used during planned, existing and emergency exposure situations as well as for wastewaters and liquid effluents with specific modifications that can increase the overall uncertainty, detection limit and threshold. For an emergency situation, it is preferable to reduce the counting time rather than the sample volume.

The analysis of ²²⁷Ac adsorbed to suspended matter is not covered by this document.