Modelling of the total excitation energy partition including fragment deformation and excitation energies at scission

Abstract

In the frame of refined prompt neutron emission models used for evaluation purposes, the partition of the total excitation energy (TXE) between fully accelerated fission fragments plays an important role. In this work the TXE partition between complementary fission fragments is obtained by taking into account the energy components at scission. The method consists of two steps: the calculation of additional deformation energies of nascent fragments (which are relaxed into excitation energy at full acceleration) and the partition of the available excitation energy at scission (obtained by subtracting the calculated deformation energies from TXE) assuming statistical equilibrium. The obtained fragment excitation energies, as the sum of deformation and excitation energy components at scission, are then used in the frame of the Point-by-Point model to describe experimental data of prompt fission quantities. This TXE partition method was applied to the following fissioning systems: 233,235 U(n th ,f), 239 Pu(n th ,f), 237 Np(n,f) at 0.8 and 5.5 MeV incident energies and 252 Cf(SF). The presented procedure of TXE partition is based exclusively on models and straightforward assumptions practically without the need for adjustable parameters (the parameters have been adjusted a priori and to global systematics). It allows the prediction of prompt neutron emission data and, therefore, may be successfully used for evaluation purposes.

Properties of prompt-fissionγrays

“…This includes the initial excitation energy, initial spin and parity of each fragment. We simply assume that the parity distribution is equiprobable for positive and negative parities, but the problem of determining the partitioning of excitation energy sharing is more complicated and phenomenological models have been developed [4,21,[25][26][27]. Similarly, for the initial spin distribution we use a simple model [28].…”

Comparing different energy partitions at scission used in prompt emission model codes GEF and Point-by-Point

“…[2], is currently used in the deterministic treatment of prompt emission by the PbP model. This modeling stars from the energy conservation at scission for a pair of nascent fragments.…”

“…This intrinsic energy is at the same level of magnitude as the available excitation energy at scission that is shared in the TXE partition method described in Ref. [2] and used in the PbP model.…”

Parameterisation of the residual temperature distribution based on the modelling of successive emission of prompt neutrons

“…<ε> k is the average energy of the k-th emitted neutron in the center-of-mass frame, a k and T k are the level density parameter and the temperature of the k-th residual nucleus, respectively. For the first emitted prompt neutron, k=1, E r (0) = E * is the excitation energy of the initial fragment (before the first neutron is emitted) resulting from the TXE partition by modelling at scission [5,6]. To solve the residual temperature equations (1) the following approximations are needed: (a) non-energy dependent level density parameters of initial and residual fragments, e.g.…”