Ambiguity in gamma-ray tracking of ‘two-interaction’ events

Tracking of gamma-rays in germanium detectors can allow the full reconstruction of interactions, a feature that is useful in many applications. Scrutiny of the kinematics and geometry of gamma-rays which are Compton scattered only once prior to full absorption reveals that there are special cases where even perfect spatial and energy resolution cannot resolve the true interaction sequence and consequently the gamma-ray tracks cannot be unambiguously reconstructed. The photon energy range where this ambiguity exists is from 255 keV to about 700 keV. This is an energy region of importance for nuclear structure research and a domain where two-point interactions are probable.