In order to counteract this, the expensive and small high resolution detector is surrounded by larger and cheaper low resolution detectors, usually a scintillator (NaI and BGO are the most common) The suppression detector is shielded from the source by a thick collimator, and it is operated in anti-coincidence with the main detector: if they both detect a gamma ray, it must have scattered out of the main detector before depositing all of its energy, so the Ge reading is ignored. The cross section for interaction of gamma rays in the suppression detector is larger than that of the main detector, as is its size, thus it is highly unlikely that a gamma ray will escape both devices.