A Si(Li)-NaI(Tl) Detector for Direct Measurement of Plutonium in Vivo

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Abstract
A Si(Li)-NaI(Tl) detector system is shown to be very promising for measurements of Pu in human lungs via the L x rays. The Si(Li) detectors are mounted on their edge to permit the front surface to view the lung and the back surface to view a NaI(Tl) anticoincidence scintillator for background suppression. Silicon has high intrinsic efficiency for detection of low-energy x rays and low sensitivity to background-producing y rays. By virtue of the high-energy resolution of the Si(Li) detectors, the UL x-ray peaks are separated from the NpL x-ray peaks thereby making it feasible to measure Pu in the presence of 241Am. The high resolution further reduces background thereby providing for high sensitivity. Measurements of Pu in lung phantoms were made with a prototype Si(Li)-NaI(Tl) detector system to simulate a pair of six - Si(Li)-detector arrays using a 2.5-cm-thick tissue-equivalent absorber. With a lung burden of 15 nCi 239Pu and 0.1% 241Am, a 2000-sec measurement gave a total UL x-ray count (ULbeta and ULxi) to square-root-of background ratio of 2.1 which corresponds to a confidence level of 85%.