MONTECARLO CALCULATION OF THE ISOMERIC CROSS SECTIONS RATIO FOR THE REACTION ^{237}Np(n,2n)^{236}NpE.N.E.A., Dipartimento Reattori Veloci, C.R.E. Casaccia P.O. BOX 2400, I-00100 Roma A.D. (Italy) A Montecarlo calculation of the isomeric cross section ratio for the (n,2n) reaction on ^{237}Np has been carried out based on the Hauser-Feshbach formulation. A standard energy-dependent optical model potential was used, with zero deformation parameters and no spin-orbit coupling. Investigation was made about the role of the energy cut-off value, of the higher multipole (E2) transition, of the gamma-ray versus second neutron emission, of the value of the spin cut-off parameter. The results give the correct qualitative energy dependence of the branching ratio, with the assumption that the 1^{−} level is the ground state. The spin cut-off value obtained indicates a less pronounced deviation of the nuclear moment of inertia from the rigid-body value, with respect to older evaluations for high-mass nuclei. KEYWORDS: Montecarlo method, coupled-channel statistical model, (n,2n) reactions, isomeric ratios, ^{237}Np |