Effects of measurement distance and source activity on the precision of X-ray fluorescence measurements of lead in a bone phantom

The effects of measurement distance and source activity on the measurement of lead in a plaster-of-Paris bone phantom were investigated for a 109Cd source K-shell X-ray fluorescence system. Measurement precision was assessed from analyses of average measurement uncertainty and reproducibility. For a source activity of 1000 MBq, a source to sample distance of 9 mm and sample to detector distance of 25 mm were identified as yielding superior measurement precision. Source activities of 710 and 320 MBq produced superior precision results with a source to sample distance of 4 mm and sample to detector distance of 20 mm. Significant improvements of 18% in average measurement uncertainty and 39% in reproducibility were observed when compared with the previously standard distance protocol. Individual estimates of measurement uncertainty tended to underestimate measurement reproducibility. An effect on measured phantom lead concentration as a function of measurement distance was observed, with closer range trials producing slightly lower concentrations of lead when using the same bone phantom. Reductions in 109Cd source activity affected measurement precision adversely, but not to the degree expected from a consideration of radiation intensity alone.