Abstract: Fission chamber with a wide measuring range and apparatus for measuring the neutron flux density using said fission chamber. The chamber comprises an enclosure (2) filled with a gaseous medium and, in the latter, electrodes (8,10) adjacent the periphery of the enclosure, whereof at least one is covered with a fissile material having a large surface density or mass, as well as other electrodes (10,12) adjacent the center of the enclosure, whereof, at least one is covered with a fissile material having a smaller surface density, chosen so as to optimize (a) the ratio of the electric currents respectively due to the neutrons and to spurious gamma radiation detected by the other electrodes and (b) the linearity of the signal supplied by the latter for a given flux density range.