In vivo measurements of lead in bone using a 109Cd 'spot' source.

Previous logCd-based K XRF bone lead measurement systems have used an annular source of “‘Cd mounted coaxially with, and in front of, a 16 mm diameter hyperpure germanium detector. To take advantage of the advent of larger volume detectors, the IogCd fluorescing source was redesigned to allow its positioning at the center of the I detector face. The redesigned source is a cylinder of external diameter 5.8 mm, length t 5.0 mm and has an entrance window of 0.25 mm stainless steel. The “‘Cd itself is in a 3:I: I cylindrical cavity, within the tungsten/steel outer, of diameter 3.0 mm and length ~2 mm. The activity of the source at delivery was 1.85 GBq. The source is housed in a cylindrical collimator/detector shield of wall thickness 3 mm and length 11 mm. The material of the collimator/shield is 90% tungsten, 6% nickel and 4% copper and is of density 17 g cmm3. The silver X-rays, associated with the decay of “‘Cd, are filtered by 0.25 mm of 99.999% purity copper. The source is mounted at the center of a 2000 mm’, 20.nun-thick hyperpure germanium detector (Canberra-Packard GL2020R). The resolution of 88.035 keV ““Cd