Passive detection of gamma ray shadows from small-scale soil surface anomalies

Abstract

We exhibit theoretical and experimental evidence that some surface anomalies can be passively detected from their shadowing of low-level gamma rays emitted from natural soil. Surface objects on the order of decimeter size with high electron density cause variable attenuation in several bands of gamma ray energies, while some surface holes cause other detectable changes in the gamma background. Using a broadly collimated portable 7.6 x 7.6 cm cylindrical NaI (T1) scintillation detector, we characterize the small-scale (on the order of meter scale or less) homogeneity of gamma rays at the soil surface in the context of detectability of objects and holes. We suggest that passive detection of anomalies below the surface may have impractically low count rates due to the greater collimation needed.