L X-ray spectrometry in vivo with a Si(Li)-NaI(Tl) detector

A novel Si(Li)-NaI(Tl) spectrometer was developed for measurements of UL X-rays from ²³⁹Pu in the lungs of exposed persons. The spectrometer consists of six large, cooled Si(Li) detectors mounted on edge to provide two windows, one facing the lungs and the other a NaI(Tl) scintillator for anticoincidence background suppression. The sensitive area of the array is 54.5 cm² and the FWHM (full width at half maximum) resolution at 17 keV is 390 eV. The ambient background count rate per minute in the energy band of interest of the detector is 0.13. This is <1/F that of an equal-area HPGe detector and 1/15 that of an equal-area phoswich. The sensitivity of this detector for ²³⁹Pu approaches the maximum achievable, where the minimum detectable level is fundamentally limited by the lung geometry (signal) and the natural radioactivity in the human body (background). With this detector one can measure low-level Pu in human lungs directly, even in the presence of ²⁴¹Am. A spectrum of ZrK X-rays from ^92mNb (simulating Pu), obtained from the lungs of a human subject in vivo, demonstrates the detector performance