Variability of procedural blanks leads to greater uncertainty in assessing detection limits for the measurement of polonium-210

Abstract

Low-level measurements of 210 Po using alpha spectrometry with a 209 Po tracer have been conducted at the Environmental Research Institute of the Supervising Scientist (eriss) for many years, on samples with a range of activity concentrations spanning several orders of magni- tude. These samples originated from a wide range of research and monitoring projects, and included a wide variety of traditional foods consumed by the indigenous population in Australia's Northern Territory. Assessment of instrument blank and analyte blank data from these measurements collected over a period of 8 years was conducted. Instrumental blank data for 210 Po and 209 Po generally followed a normal distribution, whereas analyte blank data for 210 Po followed a lognormal distribution. Instrumental blank data for 209 Po indicated an increasing trend, indicative of a low level of polonium volatilisation from prepared sources. Lower limits of detection, including the critical limit, detection limit and quantification limit have been calculated. The critical limit ranges from 12 to 37 counts per day. Detection and quantification limits range from 0.18 to 0.33 mBq and 2.3 to 3.6 mBq for a 4-day count, with an assumed mean chemical recovery of 53 %. These limits are relatively high for alpha spectrometric techniques due to the high variability of the analyte blank signal and non-normal distribution for 210 Po. Native plant species have relatively low activity concentrations of 210 Po in their edible fruits and the amount of sample that should be used for analysis to ensure 90 % of fruit samples ana- lysed reach the specified quantification limit was 11 g.