1. In a hodoscope for forming an image of a target irradiated by neutrons from a source to produce fast neutrons and gamma rays, the hodoscope including a collimator with channels to produce a plurality of discrete beams of fast neutrons and gamma rays, a detector of gamma rays and a detector of fast neutrons at each channel generating signals responsive to the beams, and means for reconstructing an image from the signals from the gamma and fast-neutron detectors, the improvement comprising: means for scanning the collimator with respect to the target; control means connected to the scanning means and to the means for reconstructing an image to associate the image with position of the collimator.