Avalanche Detector Arrays for in Vivo Measurement of Plutonium and Other Low Activity, Low Energy Emitters

Arrays of silicon avalanche detectors are being used for Ti vivo detection and measurement of low energy radiation in very small activities at room and body temperatures. Plutonium may be measured (via the 13.5-20 keV daughter uranium L x-rays) with an array of three 1 cm2 avalanche detectors in the esophagus, examining the thoracic lymph nodes which accumulate inhaled PuO2. The three detectors used in this array together provide uniformity of sensitivity to a source at any angle around the longitudinal axis of the array; using three independent signal processing systems, the array can also provide localization information. Plutonium scratches and puncture wounds may be inspected for contamination and the plutonium may be localized using a specially fabricated 1/8 inch diameter avalanche detector. With this configuration, the Minimum Detectable Amount (MDA) of Plutonium-238 (84% confidence level and four minute count) measured through 2 mm of tissue is 1 nCi. Early results of improved uniformity and resolution with the avalanche detector have shown promise of increased efficiency per unit area of detector face. A step toward unambiguous detection by making better use of the nanosecond speed of avalanche is described.