Ambiguity of gamma-ray tracking of "two-interaction" events
  Tracking of gamma-ray interactions in germanium detectors can allow reconstruction of the photon paths, and is useful for 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 cases where even perfect spatial and energy resolution cannot resolve the true interaction sequence and consequently gamma-ray tracks cannot be reconstructed. The photon energy range where this ambiguity exists is from 255 keV to around 700 keV. This is a region of importance for nuclear structure research where two-point interactions are probable. 
N.J. Hammond, T. Duguet, C.J. Lister