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

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. The presented procedure of TXE partition is based exclusively on models and straightforward assumptions without the need of adjustable parameters. It allows predicting prompt neutron emission data and, therefore, may be successfully used for evaluation purposes.JRC.D.4-Nuclear physic

Prompt gamma-ray energy in the frame of prompt neutron emission models

As output of the Point by Point model (taking into account the entire fission fragment range) and of the most probable fragmentation approach (working with one fragmentation and average values of model parameters) not only prompt neutron quantities are provided but also the prompt gamma-ray energy as a function of fragment Egamma(A) and the total average prompt gamma-ray energy . The almost linear behaviour of the total average with the prompt neutron multiplicity was observed experimentally in the incident neutron energy range where only the first fission chance is involved. This was parameterised as a function of Z and A of the fissioning nucleus. This parameterisation was validated by calculations in the frame of the most probable fragmentation approach. The results describe well the experimental data measured by J.Frehaut from thermal up to about 15 MeV incident energy for three fissioning systems U-235(n,f), Np-237(n,f) and Th-232(n,f).The Point by Point model provides average prompt gamma-ray energy as a function of the fragment pair in very good agreement with the existing experimental data of Cf-252(SF) and U-235(nth,f). The unique experimental data of prompt gamma-ray energy as a function of fragment measured for U-235(nth,f) is very well described by the Point by Point model results obtained by using two methods of total excitation energy partition between complementary fission fragments.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Possible reference method of total excitation energy partition between complementary fission fragments

A method of total excitation energy (TXE) partition between fully-accelerated fission fragments based exclusively on the systematic behaviour of experimental ν(A) (νH /νpair as a function of AH being parameterized) is proposed. This TXE partition method has the advantage to be not dependent on models and assumptions made at scission and in this sense it can be taken as a possible reference method. From this TXE partition applied on many fissioning systems a general parameterization of the fragment residual temperature ratio RT = TL/TH as a function of AH is obtained, giving the possibility to predict prompt fission quantities as a function of fragment. Other TXE partition methods (based on models and assumptions at scission or on equal residual temperatures of complementary fragments at full acceleration) are compared to this one, revealing limits of applicability as well as the sensibility of prompt neutron emission models to the TXE partition. The behaviour of experimental ν(A) with the increase of incident energy consisting in the multiplicity increase for heavy fragments only, is argued and entirely supported by the quantitative Point-by-Point model results of ν(A) describing very well the experimental ν(A) of 237Np(n, f) at 0.8 and 5.5 MeV incident energies.JRC.D.5-Nuclear physic

Scintillators in High-Power Laser-Driven Experiments

Nowadays, it is possible to accelerate bunches of particles in the interaction of ultrahigh intensity (UHI) laser pulses with matter. Electrons, protons, ions, and high-energy photon beams can be produced in experiments and reach kinetic energies close to hundreds of megaelectronvolts for protons and gigaelectronvolts for electrons and for the associated Bremsstrahlung photons. At these energies, these beams can induce a large variety of nuclear reactions, which can be detected and studied using y-ray spectroscopy techniques. At standard accelerator facilities, scintillator detectors are commonly used to perform prompt y-ray spectrometry studies. However, during laser-matter interactions, high fluxes of X-rays (mostly soft) are generated, which lead to instantaneous huge energy deposits (~1 μJ) in these scintillators. Depending on the laser characteristics (energy and pulse duration), the detector recovery time after these X-ray flashes can reach several milliseconds, which makes any prompt or “in beam” measurement impossible. The origin of this long-duration signal is investigated in the case of a LaBr3 crystal coupled to different photodetectors. While it was impossible using standard photomultiplier tubes to detect y-ray emissions before a few milliseconds after a laser shot, we could, using a hybrid photodiode, resolve single y-ray emission a few tens of microseconds after the laser shot. Furthermore, we have also shown that the LaBr 3 scintillator presents an unexpected long-lived light emission (afterglow). Directions are suggested for future studies in order to minimize the effects of this afterglow emission