International Conference on Nuclear Data for Science and Technology 2007
DOI: 10.1051/ndata:07680

Dynamical approach to scission-neutron emission in low energy fission

N. Carjan¹, H. Goutte², M. Rizea³, O. Serot⁴ and P. Talou<sup>4, 5

1</sup>  Centre d'Études Nucléaires de Bordeaux-Gradignan, CNRS/IN2P3 - Université Bordeaux I, BP. 120, 33175 Gradignan Cedex, France
²  DPTA/Service de Physique Nucléaire, CEA/DAM Île-de-France, BP. 12, 91680 Bruyères-le-Châtel, France
³  National Institut of Physics and Nuclear Engineering, P.O. Box MG-6, Bucharest, Romania
⁴  CEA Cadarache, DEN/DER/SPRC/LEPh, Bât. 230, 13108 Saint-Paul-lez-Durance, France
⁵  T-16, Nuclear Physics Group, Los Alamos National Laboratory, USA

carjan@in2p3.fr

Published online: 21 May 2008

Abstract
At a certain neck radius during the descent of a fissioning nucleus from the saddle to the scission point, the attractive nuclear forces can no more withstand the repulsive Coulomb forces producing the neck rupture and the sudden absorbtion of the neck stubs by the fragments. At that moment, the neutrons, although still characterized by their pre-scission wave functions, find themselves in the newly created potential of their interaction with the separated fragments. Their wave functions become wave packets with components in the continuum. The probability to populate such states gives evidently the emission probability of neutrons at scission. In this way we have studied scission neutrons for ²³⁶U, using two-dimensional realistic nuclear shapes. We have first calculated symmetric fission. Both the emission probability and the distribution of the emission points relative to the fission fragments strongly depend on the quantum numbers of the pre-scission state from which the neutron is emitted. In particular it was found that states with ω π = 1/2+ dominate the emission. Depending on the assumed pre- and post-scission configurations and on the emission barrier height, 30% to 50% of the total scission neutrons are emitted from 1/2+ states. Their emission points are concentrated in the region between the newly separated fragments. The upper limit for the total number of neutrons per scission event is predicted to lie between 0.16 and 1.73 (depending on the computational assumptions). A sudden transition not only produces scission neutrons but also leaves the fission fragments in an excited state. The corresponding excitation energy was estimated. Then we have investigated the dependence on the mass ratio of the fragments in asymmetric fission. The magnitude of the variation of the scission-neutron multiplicities with the mass of the heavy fragment was compared with that for prompt neutrons. Finally we have studied the effect of the isospin by calculating a series of plutonium isotopes. We have found a 90% increase of the scission-neutron multiplicities from ²³⁶Pu (0.318 n/fission) to ²⁵⁶Pu (0.597 n/fission) reflecting the increase of the amount of available neutrons in the system.

© CEA 2008