Within the framework of the nuclear tests, we have studied the realization of a neutron diagnostic by magnetic spectrometry. We examined the possibility of an establishment near the future laser sources, L.I.L. and L.M.J.. The potential use of this spectrometer is the measurement of the ionic temperature of plasma and would come in complement of the spectrometry per time of flight. This device would be thus interesting on the condition of obtaining a sufficient resolution in energy. One will estimate with which precision this temperature may be given. We will present the codes allowing to entirely simulate the diagnostic from the source to the detector. These codes are organized around a principal code named SPECTRO entirely simulating the magnetic sector. This whole of codes allows the optimization of the characteristics of the apparatus.