A low background X-ray fluorescence system for microsphere quantitation

An automated low-background X-ray fluorescence detection system was developed for measuring blood flow rates by the microsphere technique. An X-ray beam was polarized to lower Compton scattering. A Si(Li) detector was used for efficient characteristic X-ray detection with preferentially decreased sensitivity for Compton scattering, a CaF₃ scintillation detector was engaged in the anticoincidence method to lower the background, and risetime rejection was used for the same purpose. Solutions of 1 cm³ each of Rb, Mo, Ag, I, La and Gd with concentration of 0.2-2.0 p.p.m. were measured for 80 min and those of 2.0-20 p.p.m. were measured for 10 min. For Ag the net counting rate was approximately 75 counts/min/p.p.m. The anticoincidence detector lowered the background by 10% in the region of Ag X-rays. The linear relation between net counts and actual element concentration held up to 1000 p.p.m. for Ag solutions.