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Absorption-Edge Transmission Technique Using Ce-139 for Measurement of Stable Iodine Concentration

University of Utah Medical Center, Salt Lake City, Utah

Correspondence: For reprints Contact: James A. Sorenson, Div. of Medical Physics, Dept. of Radiology, Univ. of Utah College of Medicine, IC 402 Medical Ctr., Salt Lake City, UT 84132.

ABSTRACT

We have investigated a technique for measuring stable iodine concentrations by absorption-edge transmission measurements using a Ce-139 radiation source. The lanthanum daughter emits characteristic x-rays whose energies just bracket the absorption edge of iodine at 33.2 keV. Relative transmission of these x-rays is sensitive to iodine concentration in the sample, but is relatively insensitive to other elements. By applying energy-selective beam filtration, it is possible to determine the relative transmission of these closely spaced x-ray energies with Nal(Tl) detectors. Optimizations of sample thickness, detector thickness, and Ce-139 source activity are discussed. Using sample volumes of about 10 ml, one can determine iodine concentration to an uncertainty (standard deviation) of ±5 µg/ml with a 5-mCi source in a measurement time of 400 sec. Potential clinical applications of the in vitro technique are discussed, along with comparative aspects of the Ce-139 technique and other absorption and fluorescence techniques for measuring stable iodine.