Possible overestimation of the external standard quench parameter on Wallac 1220 QuantulusTM with high energetic beta-emitters

The 1220 Quantulus is equipped with an 152Eu external source used to determine an external standard quench parameter SQP(E). The relationship between 90Sr/90Y counting efficiency and SQP(E) was found linear in a defined range of SQP(E) values. This function was fixed after many counting experiences made with 90Sr/90Y standards. The same equation is used for the measurement of 90Sr in environmental samples. The first goal of this paper is to objectify an overestimation of the SQP(E) when high energetic beta-emitters like 90Y are present (1 to 4% in routine conditions). The second one is to show how this artefact could induce a bias in the calculation of 90Sr activity in environmental samples. The median of this overestimation is estimated around 6%. Another approach using 85Sr standard is suggested to avoid this overestimation. Provided the chemical composition - and thus the quenching - is similar for the two standards, the counting efficiency measured with the 90Sr/90Y standard could be related to the SQP(E) value of the 85Sr standard. Indeed this one appears to be more robust regarding to the range of activity and to the counting time of the external source. (author)