Mechanism of neutron emission in²⁵²Cf fission

The emission of instantaneous fission neutrons is closely associated with the mechanism and time of conversion of the energy released in fission to the energy of the emitted neutrons. Thus, in the well-known model of isotropic neutron emission from accelerated fragments (the isotropic model) [1, 2], the time in which some of the fission energy is converted to energy exciting the internal degrees of freedom of the fission fragments and the subsequent neutron emission occurs is assumed to exceed the time of complete fragment acceleration in the mutual Coulomb field, which is (0.5-1) . 10(-20) sec. Experimental data on the spectra and angular distribution of instantaneous neutrons from low-energy fission show that most of the emitted neutrons may be accounted for within the isotropic model. Deviation from the isotropic model was observed in [3-7] and attributed to various factors; for example, in [2, 8], it was linked to possible anisotropy of emission in the center-of-mass system on account of the angular momentum of the fragment. The possibility of the emission of nonevaporative neutrons at the instant of fission of the parent nucleus or in establishing the equilibrium form of the fragments was considered in [9]. The corresponding time intervals are approximately 10(-22)-10(-21) sec. The possibility of neutron evaporation from incompletely accelerated fragments was considered in [10, 11] as a source of deviation from the isotropic model. Thus, investigation of the deviation of fission-neutron emission from the isotropic model yields information on the early stages of fission and the energy balance in those stages. The deviations from the isotropic model observed in different experiments differ greatly, from 3% [7] to 20% [3]. In the present work, the isotropic model is evaluated again, by comparing the neutron spectra at angles of 0 and 90 degrees to the direction of fragment motion in the laboratory coordinate system.