Remote Sensing of Plutonium by the Low-Energy Scattered Flux

Detection of plutonium and other gamma-ray emitters at penetrations of a few mean-free-paths in air or earth is improved by counting the scattered component below ∼100 keV in a low-background detector such as 5-mm-thick lithium-drifted germanium. The uncollided and scattered fluxes are calculated for point 1-MeV, 130- and 60-keV, and ²³⁹Pu spectrum sources in effectively infinite air with discrete-ordinates, Monte Carlo, and analytical methods. Count rates were estimated by summing the efficiency-weighted fluxes and multiplying by the area. Minimum detectable activities were evaluated from a signal count equal to three times the standard deviation in the background count, obtained from experimental data. The performance of the low-background Ge(Li) detector, per cm² of detector area, is shown to be considerably better than that for a thick sodium-iodide scintillation detector traditionally used for remote sensing of plutonium and other gamma-ray sources. A calculation for a 5-cm-radius plutonium ball embedded in earth shows that total-flux counting in a thin low-background detector provides good sensitivity while traditional photopeak counting of uncollided photons is impossible.