Abstract: The active neutron spectrometer (1) comprises a polyhedral moderator body (2) of hydrogenated material having a first, a second and a third orthogonal main axis (X1, X2; Y1, Y2; Z1, Z2), a first series of thermal neutron detectors (3a1, 3a2, 3a3, 3a4, 3a5, 3a6, 3b1, 3b2, 3b3, 3b4, 3b5, 3b6) arranged along the first main axis (X1, X2), a second series of thermal neutron detectors (4a1, 4a2, 4a3, 4a4, 4a5, 4a6, 4b1, 4b2, 4b3, 4b4, 4b5, 4b6) arranged along the second main axis (Y1, Y2), and a third series of thermal neutron detectors (5a1, 5a2, 5a3, 5a4, 5a5, 5a6, 5b1, 5b2, 5b3, 5b4, 5b5, 5b6) arranged along the third main axis (Z1, Z2).