Measurement of ⁹⁰Sr –
In Need of Treatment

Simon Jerome and Mike Woods

Centre for Ionising Radiation Metrology,
National Physical Laboratory

The measurement of ⁹⁰Sr has been carried out for many years as an indicator of the dissemination of fission products around the globe as a result of the human use of nuclear energy for both civilian and military purposes.

As far as chemistry goes the separation and assay of ⁹⁰Sr is straightforward in that strontium exists in the Sr²⁺ oxidation state only and is not given to forming difficult chemical complexes. The measurement of ⁹⁰Sr activity is almost universally achieved by the separation of ⁹⁰Sr from the sample; this is then set aside for a period of 2-3 weeks in order to allow the ⁹⁰Y daughter to grow in. The ⁹⁰Y daughter is then separated and assayed by some form of b-measurement. Thus, the overall analysis is based on very straightforward chemistry that has the advantage of being very specific and unambiguous.

However, from our experiences in the ongoing sequence of NPL environmental intercomparison exercises (which are based on aqueous samples), we find that the measurement of ⁹⁰Sr appears to cause the participating analysts real difficulties in terms of achieving an acceptable performance in the exercise.

This paper will examine the results achieved in the series of exercises run by NPL since1989 and attempt to correlate the data we have against the performance of participating laboratories over this time period. In addition, the possible sources of bias will also be analysed to see if performance is correlated against such parameters such as:

· Analytical technique (classical, modern or other)

· Chemical yield measurement (chemical or radiometric)

· Radiometric assay (many and varied)

· Uncertainties (are they estimated)

· Quality issues (reference materials)

· Traceability (calibration standards)

The foregoing analysis may lead to the identification of the more reliable techniques that have been employed over the years, but more importantly the authors expect to comment on the uncertainties that are reported by the analyst and whether some of the more optimistic uncertainties reported by participants can be justified in the light of experience.