shaped radiography detector array may be positioned beside the sodium iodide detectors or in front of them preferably the array of neutron detectors is located immediately in front of the sodium iodide detectors the neutron detectors 88 yield a coarse image of the transparency of the item to neutrons the image contains both transmitted and reflected neutrons blurring can be reduced by arranging collimation around each detector so that it responds only to near axis neutrons the purpose of this detector array is to check for regions of the item which neutrons do not penetrate thereby revealing that neutron shielding has been employed the use of neutron absorbers would be recognised immediately as suspicious and requiring further investigation gamma rays from the neutron absorbing materials are also detected by the germanium detector 86 fig6 illustrates a general signal processing system for the number of detector output signals produced in the system basically all signals are digitised and then processed by a computer power supplies and high voltage detector bias circuits are not illustrated in the drawing for clarity referring first to the sodium iodide detectors of arrays 80 82 84 the output of each detector is passed via a pre amp 90 to a sample and hold circuit 92 for clarity not all detector output signal leads are shown only the first and nth are indicated the signal values held in circuits 92 are periodically read and digitised by analogue to digital convertor 94 and passed to a computer 96 as previously discussed the amplitude of these signals is indicative of gamma.