Get The Lead Out

Among the technical talks, of particular note at the 43rd Annual Denver X-ray Conference in August 1994 were two involving calculations of the amount of crystalline and amorphous materials present in polymer samples, and a very interesting, almost spectacular, neutron diffraction talk describing in situ (nondestructive) characterization of residual stress in engine components in a COMPLETE Pratt & Whitney airplane engine.

A particular entertaining aspect of the meeting concerned the controversy surrounding in vivo use of Pb L- X-ray fluorescence measurements. Quantitative in vivo measurement of Pb L- fluorescence from bones of children who have ingested zinc is apparently quite the rage in the medical profession. Talks by clinician and academic experts on X-ray in vivo X-ray fluorescence measurements, clinical application of X-ray fluorescence and basic research in the field raised interesting questions. In the studies  described, the sample (human leg or plaster of Paris, etc.) is irradiated and consequently, emits bremsstrahlung and characteristic X-rays. The radiation emitted by the "sample" (Pb L- X-rays) has an energy of about 10-12 keV (mid-way between Cu K- and Mo K- ) These fairly "soft" X-rays are readily absorbed by relatively low thicknesses of materials. The academic expert demonstrated that a few mm of lucite "absorber" in front of a plaster-of-Paris "source" would eliminate the detectability of low levels of Pb. He showed that Pb could not be detected from goat bones containing 40 ppm Pb if more than 3-4 mm of pig skin was on top of the bone!! If you require examination for lead "poisoning" remember that if the absorber is more than a few mm thick, assume that the doctor may be looking at random detector noise. Of course, Pb K- X-ray fluorescence is more penetrating and more easily detected.