Quantifying Uncertainties in Drinking Water Samples by Total Alpha and Beta Determination

This paper describes main aspects related to determination of total alpha and beta in drinking water samples. A short description of the procedure is presented, emphasizing possible sources of uncertainty. A Gas Proportional Counter from Canberra Tennelec Series 5 Extra Low Background (S5XLB) equipped with a sample changer was employed for sample counting. Proper alpha self absorption corrections were applied as needed, using calibration curves prepared with 241 Am standard solution. The calibration depends on layer thickness, composition of the source, and sample distribution on the plate. Corrections for decay, reference date, decay during counting, and recovery were applied. Since samples are counted in the simultaneous Alpha + Beta mode of operation it is appropriate to minimize the spillover or crosstalk contribution of both alpha pulses into the beta channel and beta pulses into the alpha channel. These adjustments to make crosstalk corrections were accomplished through the alpha-beta amplifier setup. Characteristic efficiencies determined through 90 Sr + 90 Y and 241 Am, were 0.46 and 0.26, respectively. The simultaneous count mode sets the system high voltage to the beta plateau voltage. At this voltage, the proportional counter is sensitive to all ionizing radiation and the separation of alpha and beta particles is accomplished using pulse height. In this paper, basic mathematical considerations were discussed regarding the measurement of radioactive alpha and beta in water samples. Each parameter whose uncertainty was quantified was carefully described, with appropriate numerical value and unit. The uncertainty associated with a certain parameter is employed to determine its partial contribution to the combined total uncertainty. In order to facilitate this procedure, the uncertainty budget was expressed in a clear and simple manner by using an available software.