Anabella Tudora

Improved Los Alamos model applied to the neutron induced fission of 235U and 237Np

Abstract The Los Alamos model, with multiple fission chances, was upgraded to include the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity. A global, model-free parameterization of this relation versus the charge and mass number of fissioning nuclei is obtained. The average fission fragment kinetic energy dependence on the incident neutron energy is also taken into account. Employing this model for the n+235U and n+237Np reactions, the prompt fission neutron spectra, the average prompt neutron multiplicity and the average prompt gamma ray energy are obtained in very good agreement with the experimental data, proving a better predictive power.

SPECTRUM : A computer code for prompt fission neutron spectrum and prompt neutron multiplicity calculation

The computer code SPECTRUM generates total prompt fission neutron spectrum, total prompt multiplicity, average energy of the spectrum and equivalent Maxwellian spectrum. Theoretical background employed in the calculations is that of Los Alamos model, with multiple chance fission, upgraded to allow a neutron incident energy dependence of the input parameters. The code handles the complete set of output data with a powerful graphical interface, allowing a comparison with reference data.

Improved Los Alamos model applied to the neutron induced fission of 239Pu and 240Pu and to the spontaneous fission of Pu isotopes

Abstract The Los Alamos model with multiple fission chances upgraded with (a) the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity and (b) the dependence of the average fission fragment kinetic energy on the incident neutron energy, is used for the n + 239 Pu and n + 240 Pu reactions, and also for the spontaneous fission of 237–241 Pu isotopes. In the case of 240 Pu fissioning nucleus the variation of the average energy released versus the incident neutron energy is also taken into account. The calculated prompt fission neutron spectra and average prompt neutron multiplicity well represent the experimental data, proving a better predictive power of the improved Los Alamos model.

Prompt fission neutron spectrum calculations for n+238U reaction using the multi-modal model

The prompt fission neutron spectra for the neutron induced fission of 238U are calculated in the incident energy range where only the first fission chance is involved using the multi-modal fission theory. The partial spectra of each fission mode are calculated on the basis of Los Alamos model and the total spectrum is calculated by superposition of partial spectra weighted with the respective probability of occurrence. The total fission spectra obtained in this manner are in good agreement with the experimental data.

Prompt fission neutron multiplicity and spectrum evaluations for n+238U reaction

Abstract The prompt fission neutron multiplicity and spectra for n+ 238 U reaction are calculated using an improved Los Alamos model which includes the linear relation between the average prompt gamma ray energy and the prompt neutron multiplicity and also the average fission fragment kinetic energy dependence on the incident neutron energy. The coefficients describing the quadratic variation of the fission fragment kinetic energy versus the incident energy are obtained by extrapolation of the data and procedure used for n+ 235 U reaction. The inverse process compound nucleus cross-section of the fissioning nucleus is calculated using the coupled channel method. In the incident energy range where only the first fission chance is involved the comparison of present spectrum evaluation with spectrum calculation using multi-modal model is made too. The calculated prompt neutron multiplicity and spectra of 238 U neutron induced fission are in good agreement with the experimental data for the entire incident energy range required in evaluations, proving the validity of the used procedure.

High accuracy average prompt fission neutron multiplicity description in the improved Los Alamos model

Abstract The Los Alamos model, with multiple chance fission, was upgraded to include the dependence of the total average fission-fragment kinetic energy and of the total average prompt gamma-ray energy on neutron incident energy. Using this model the prompt fission neutron spectrum, prompt fission neutron multiplicity, average energy of the spectrum and equivalent Maxwellian spectrum for the n+235U reaction can be calculated. The evaluation of the prompt fission neutron multiplicity is in very good agreement with the experimental data providing a better predictive power of the improved Los Alamos Model.

Neutron cross-sections of 242Pu in the energy range 5–20 MeV

Abstract The evaluation of neutron cross sections and angular distributions of 242 Pu in the energy range 5–20 MeV was performed using the direct interaction treatment by coupled channel method and the compound nucleus mechanism (statistical model) taking account of pre-equilibrium effects. The calculated cross-sections are in very good agreement with the existing experimental data (total and fission cross-sections). More accurate theoretical data have been obtained by the extension of procedures and parameterizations (previously used only for the main compound nucleus) in the incident neutron energy range where more compound nuclei are involved.

Prediction of the Prompt Neutron Multiplicity Distribution ν(A) for 235U(n,f) and 239Pu(n,f) in the Incident Energy Range of Multichance Fission

Abstract Measurements of fission fragment data at incident energies (En) up to several tens of MeV require prompt neutron multiplicity distribution ν(A) to determine the preneutron fragment properties. Those ν(A) data are not readily experimentally available. Consequently, model predictions of ν(A) at En where multichance fission occurs are needed. The Point-by-Point model of prompt emission provides the individual ν(A) of compound nuclei of the main and secondary nucleus chains that are undergoing fission at any En. Total ν(A) calculations for n + 235U and n + 239Pu are presented together with systematic behaviors of individual ν(A) with increasing energy.

Prompt fission neutron multiplicity and spectrum evaluation

The prompt fission neutron multiplicity and spectrum for spontaneous fission and neutron induced reactions in the energy range of the first chance fission threshold are evaluated using the “point by point” approach that takes into account all possible fragmentations of the fissioning nucleus, the multi‐modal concept of fission and the most probable fragmentation approach. At higher incident energy when only the most probable fragmentation approach can be used, for the first time the model was extended to take into account the fission of the secondary compound nucleus chains formed by charged particle emission. The model parameters and their dependencies on the incident energy are determined by the study of the reactions where the respective nuclei are the main compounds. The linear dependence of the prompt gamma‐ray energy on prompt multiplicity is parameterized as function of the mass and charge numbers of the fissioning nucleus. The above models were successfully applied for many neutron induced reactions on actinides, giving also good results in integral benchmarks.

Calculation of Prompt Neutron Multiplicities and Spectra for Several Actinides

The prompt fission neutron multiplicity and spectra of actinides are nuclear data of crucial importance. Based on experimental fission yield and total kinetic energy data, new calculations for the prompt neutron multiplicity and spectra for 238U(n,f), 237Np(n,f) in the incident neutron energy range up to the second chance fission threshold, for 238U(n,f), up to 50 MeV, and for 252Cf(SF) have been performed.For the first time the multi‐modality of the fission process was taken into account up to the second fission chance (about 6 MeV). Additionally, for some isotopes a more realistic fission fragment residual temperature distribution as well as an anisotropy of the prompt neutron emission led to improved agreement between the calculation and experimental results. Also, the range of fission fragment pairs entering in the multiplicity and spectrum model was extended over the entire experimental fission‐fragment mass range. This led to an improved version of the Los Alamos (LA) model and especially to an impr...