Hauser-Feshbach Theory Research Articles

Double differential cross sections of light charged particle emission of n + 27Al reaction

All cross sections, angular distributions, energy spectra and double differential cross sections are consistently calculated and analyzed for n+27Al reaction at incident neutron energies below 200MeV based on the nuclear theoretical models. The optical model, the unified Hauser–Feshbach theory and the exciton model which includes the improved Iwamoto–Harada model, and the distorted wave Born approximation theory are used. Theoretical calculated results of the energy spectra and double differential cross sections of proton, deuteron, triton, helium and alpha-particle emissions are compared with the experimental data.

Intermediate Structures in Fission and Consequences on Average Partial Cross Sections

This paper is willing to illustrate and quantify the impact of a double fission barrier in terms of average fission cross section and in particular, to status on both the additional class-II width correction factor and the degree of freedom of the overall double fission barrier which is a key parameter in the calculation of the standard Wn, f factor of Hauser-Feshbach theory. 1 Problematic related to average fission cross section simulation The purpose of this communication is to both recall and emphasize the existence of the class-II state width fluctuation factor, WII , whose treatment is usually disregarded in standard average cross section evaluation codes. The impact of fission channel Intermediate Structures (IS) is well pictured by Fig. 1 which displayed the observed and current evaluated fission cross sections within the neutron-incident Unresolved Resonance energy Range (URR) for a fissile and a fertile isotope borrowed from the plutonium family. Below the fission threshold energy ( En < 500keV), we observe large manifestations of the IS (Fig. 1- right side), relatively to the fission cross section magnitude, although IS e ffects show up as well for fissile isotopes (Fig. 1- left side). We guess that more complexity in the classically (1) used URR fission cross section model will be required for a fertile isotope but are we allowed to make some approximations for a fissile isotope without collateral damage? This is the question raised by this paper. In the next sections, the impact of both the barrier tunneling and WII factor for fissile and fertile isotopes is clarified using a new way of calculating the inextricably admixed IS and WII effects based on accurate formal R-matrix Monte-Carlo-fashion calculations which have been recently applied to the whole plutonium family in a consistent and macro-microscopic approach (2).

Theoretical Calculations and Analysis of n + 27Al Reaction

All cross sections of neutron-induced reactions, angular distributions, energy spectra, and double-differential cross sections are consistently calculated and analyzed for n + 27Al reactions at incident neutron energies below 200 MeV based on the nuclear theoretical models. The optical model; the unified Hauser-Feshbach theory and exciton model, which includes the improved Iwamoto-Harada model; and the distorted wave Born approximation theory are used. Theoretical calculated results are compared with the experimental data and the evaluated results in ENDF/B-VII and JENDL-3. The optical model potential parameters are obtained according to the experimental data of total and nonelastic scattering cross sections and elastic scattering angular distributions.

The analysis of n+237Np reactions for energies up to 200 MeV

All cross sections, angular distributions, energy spectra and double differential cross sections of neutron, proton, deuteron, triton, alpha-particle emission and the prompt fission neutron spectra for n+237Np reactions are consistently calculated and analyzed. The optical model, the intra-nuclear cascade model, the unified Hauser–Feshbach theory and the exciton model which includes the improved Iwamoto–Harada model, the fission model, the linear angular momentum dependent exciton density model and the distorted wave Born approximation theory are used in the energy range of En⩽200MeV. Theoretical calculations are compared with recent experimental data and other evaluated data from ENDF/B-VII and JENDL-3.

Astrophysical(α,γ) reaction in inverse kinematics; Electron screening effect in the beta-decay

The abundance calculations of the p-nuclei produced in explosive stellar sites rely on the Hauser-Feshbach (HF) theory with the alpha-article optical model potential (α-OMP) one of its major ingredients. To date, most of the (α, γ) cross sections measured show that HF calculations can be wrong by a factor of ten or more especially when phenomenological α-OMP are employed. To investigate the relevant uncertainties entering the HF calculations and furthermore develop global microscopic α-OMPs, systematic (α, γ) cross-section measurements are necessary. This led us to perform a feasibility study of (α, γ) measurements in inverse kinematics that will allow us to employ also radioactive beams in the future. Hence, the 4He(78Kr,γ)82Sr reaction was studied using the LISE3 spectrometer to separate the 82Sr recoils from the primary 78Kr beam. Although an excellent rejection factor > 1010 was achieved, the position of the ions of interest was unexpectedly masked by a secondary beam of high intensity. Given these, new setup improvements are proposed to remove the pollutant ions.Recently, many experiments were conducted in order to study the influence of the environment (especially in a metallic material) on the decay probability of radioactive nuclei. Additionally, hydrogen-like fusion reactions were performed indicating a change in the cross-section due to the influence of the Coulomb field screening induced by quasi-free electrons in metals. This was explained by the Debye screening model which treats metallic electrons within Maxwell-Boltzmann statistics. We measured the decay rate of 19O in metallic, insulating and superconducting environments whereas the electrons in the superconductors should obey the Bose-Einstein statistics. The decay rate measurement was supported by a branching ratios measurement. We found that the effect on the decay rate, if any, is less than the 0.1%, far below the theoretical predictions.

Double differential cross sections of proton emission in neutron induced reaction on27Al

Double differential sections for proton emission on aluminium are presented for 40 MeV incident neutron energies. Angular distributions, energy differential and total production cross sections are obtained. The results are compared to existing data and to nuclear model calculations which include preequilibrium and compound decay mechanisms. The EMPIRE code treats these mechanisms in the framework of TUL model for multistep direct and NVWY model for multistep compound. The Hauser Feshbach theory is used to calculate emission from the compound nucleus, with a full conservation of spin and parity.

Monte Carlo Simulation for Statistical Decay of Compound Nucleus

We perform Monte Carlo simulations for neutron and γ-ray emissions from a compound nucleus based on the Hauser-Feshbach statistical theory. This Monte Carlo Hauser-Feshbach (MCHF) method calculation, which gives us correlated information between emitted particles and γ-rays. It will be a powerful tool in many applications, as nuclear reactions can be probed in a more microscopic way. We have been developing the MCHF code, CGM, which solves the Hauser-Feshbach theory with the Monte Carlo method. The code includes all the standard models that used in a standard Hauser-Feshbach code, namely the particle transmission generator, the level density module, interface to the discrete level database, and so on. CGM can emit multiple neutrons, as long as the excitation energy of the compound nucleus is larger than the neutron separation energy. The γ-ray competition is always included at each compound decay stage, and the angular momentum and parity are conserved. Some calculations for a fission fragment 140Xe are shown as examples of the MCHF method, and the correlation between the neutron and γ-ray is discussed.

Inclusive Proton Energy Spectra of the Deuteron Induced Reaction

The continuum-discritized coupled channel method and the glauber model are applied for the description of deuteron elastic breakup and the stripping processes, respectively. Combined with the conventional two-component exciton model for pre-equilibrium processes and the Hauser-Feshbach theory for compound process, an approach based on models is proposed to analyze the inclusive proton energy spectra of a deuteron-induced reaction. The contributions from each process to the energy spectra of the 58Ni(d,xp) reaction are quantitatively given. The results show that this approach is able to reasonably reproduce the experimental data of the double differential cross sections, energy spectra and cross sections, although further improvements are needed.

Applications of the Hauser-Feshbach Theory to Advanced Nuclear Sciences

Unique applications of the Hauser-Feshbach (HF) statistical theory in the nuclear science eld, as well as the HF model code development at LANL are discussed. Our applied eld in which HF is applicable is not necessarily limited to the particle-induced reactions. We combine the HF model with a theory of -decay to calculate the -delayed neutron and -ray emissions. These calculations can be performed for each -decay precursor to estimate aggregate neutron and -ray energy release at ssion. Another relatively new example at LANL is the ability to compute HF predictions with a Monte Carlo technique (MCHF), which gives all the correlated information.

Nuclear level densities in 47V, 48V, 49V, 53Mn, and 54Mn from neutron evaporation spectra

The spectra of neutrons from the (p, n) reactions on 47Ti, 48Ti, 49Ti, 53Cr, and 54Cr nuclei were measured in the proton-energy range 7–11 MeV. The measurements were performed with the aid of a fast-neutron spectrometer by the time-of-flight method over the base of the EGP-15 tandem accelerator of the Institute for Physics and Power Engineering (IPPE, Obninsk). Owing to a high resolution and a high stability of the time-of-flight spectrometer used, low-lying discrete levels could be identified reliably along with a continuum section of neutron spectra. An analysis of measured data was performed within the statistical equilibrium and preequilibrium models of nuclear reactions. The relevant calculations were performed by using the exact formalism of Hauser-Feshbach statistical theory supplemented with the generalized model of a superfluid nucleus, the back-shifted Fermi gas model, and the Gilbert-Cameron composite formula for the nuclear level density. The nuclear level densities for 47V, 48V, 49V, 53Mn, and 54Mn were determined along with their energy dependences and model parameters. The results are discussed together with available experimental data and recommendations of model systematics.

The theoretical calculation of p+ 232Th reaction for energies up to 250 MeV

All reaction cross sections, energy spectra and double differential cross sections of neutron, proton, deuteron, triton, and alpha-particle emission for p+ 232Th reaction are consistent calculated and analyzed with the optical model, the intranuclear cascade model, the unified Hauser–Feshbach theory and the exciton model which included the improved Iwamoto–Harada model, the fission model and the distorted wave Born approximation theory in the energy range of E p ⩽ 250 MeV . Theoretical calculations are compared with existing experimental data.

Calculation and analysis of n+ 90,91,92,94,96,natZr reactions in the En ⩽ 250 MeV energy range

All of reaction cross sections, angular distributions, energy spectra, γ-ray production cross sections, and the double differential cross section for neutron, proton, deuteron, triton, helium and alpha emission are calculated and analyzed for n+ 90,91,92,94,96,natZr at incident neutron energies from 0.1 to 250 MeV. The optical model, intranuclear cascade model, the unified Hauser–Feshbach theory and the exciton model which included the improved Iwamoto–Harada model are used. Theoretical calculated results are compared with existing experimental data and other evaluated data from ENDF/B-VI.8, ENDF/B-VII.0 and JENDL-3.3. The optical model potential parameters are obtained according to the experimental data of total, nonelastic cross sections and elastic scattering angular distributions.

Nuclear level densities in 208Bi and 209Po from the neutron spectra in the (p, n) reactions on 208Pb and 209Bi nuclei

The spectra of neutrons from the (p, n) reactions on the 208Pb and 209Bi nuclei were measured in the proton-energy range 8–11 MeV. These measurements were performed by using a time-of-flight spectrometer of fast neutrons on the basis of the pulsed tandem accelerator EGP-15 of the Institute of Physics and Power Engineering (Obninsk, Russian Federation). A high resolution and stability of the time-of-flight spectrometermade it possible to identify reliably low-lying discrete levels alongwith the continuum section of the neutron spectra. The measured data were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed by using the precise formalism of Hauser-Feshbach statistical theory together with the generalizedmodel of a superfluid nucleus and the back-shifted Fermi gas model for the nuclear-level density. The nuclear-level densities in 208Bi and 209Po were determined along with their energy dependences and model parameters. Our results are discussed together with available experimental data and recommendations of model systematics.

Monte Carlo Simulation for Particle and γ-Ray Emissions in Statistical Hauser-Feshbach Model

Monte Carlo simulations for particle and γ-ray emissions from a compound nucleus based on the Hauser-Feshbach statistical theory with pre-equilibrium emission are performed. The simulation yields reliable nuclear-reaction-wise energy spectra, or so-called exclusive spectra, for emitted neutrons and γ-rays, which are required in particle transport calculations for nuclear applications. The Monte Carlo method is applied to neutron-induced nuclear reactions on 56Fe, and the results are compared with a traditional deterministic method. The neutron and γ-ray emission correlation is examined by gating on an 847 keV γ-ray that is produced by an inelastic scattering process. The partial γ-ray energy spectra for different γ-ray multiplicities are inferred using this Monte Carlo method. In addition, we investigate a correlation between two neutrons in the (n,2n) reaction.

Monte Carlo simulation for statistical Hauser-Feshbach theory

Monte Carlo simulations for particle andγ-ray emissions from a compound nucleus based on the Hauser-Feshbach statistical theory are performed. The Monte Carlo method is applied to the neutron induced nuclear reactions on 56 Fe, and the results are compared with a traditional deterministic method. The neutron and γ-ray emission correlation is examined by gating on an 847 keVγ-ray that is produced by an inelastic scattering process. The partial γ-ray energy spectra for differentγ-ray multiplicities is inferred using this Monte Carlo method.

Measurement of 67Zn(n, p)67Cu, 92Mo(n, p)92mNb, and 98Mo(n,γ)99Mo Reaction Cross Sections at Incident Neutron Energies of En = 1.6 and 3.7 MeV

The cross sections for the reactions 92Mo(n,p)92mNb, 98Mo(n,γ)99Mo, and 67Zn(n,p)67Cu have been measured using an activation technique and an off-line gamma counting method at neutron energies of 1.6 and 3.7 MeV. The 7Li(p,n)7Be reaction was used as the neutron source with the proton beam from the 14-MV Bhabha Atomic Research Centre–Tata Institute of Fundamental Research Pelletron accelerator. The reaction 115In(n,n′)115mIn was used as the standard for the cross-section measurement. The measured data were compared with the predictions of the EMPIRE-2.19 statistical model code based on the Hauser-Feshbach theory and good agreement, after some adjustment of the level density parameter, is obtained for all the systems.

Dynamics of nuclear interaction in the reaction 56Fe(α, xn)

The spectra and angular distributions of neutrons from the reaction 56Fe(α, xn) were measured at the alpha-particle energies of 12.3, 16.3, 18.3, 26.8, and 45.2 MeV. The measurements were performed with time-of-flight fast-neutron spectrometers at pulsed accelerators of charged particles. The measured data were analyzed within models of equilibrium, preequilibrium, and direct reaction mechanisms. The calculations were performed by using an exact formalism of Hauser-Feshbach statistical theory. The required nuclear-level densities in nickel isotopes excited in this reaction were determined on the basis of new experimental data on low-lying levels, neutron resonances, and spectra of evaporated neutrons. The contributions of equilibrium, preequilibrium, and direct neutron emission were studied over a broad range of alpha-particle energies.

Neutron capture cross section ofAm241

The neutron capture cross section of $^{241}\mathrm{Am}$ for incident neutrons from 0.02 eV to 320 keV has been measured with the detector for advanced neutron capture experiments (DANCE) at the Los Alamos Neutron Science Center. The thermal neutron capture cross section was determined to be $665\ifmmode\pm\else\textpm\fi{}33$ b. Our result is in good agreement with other recent measurements. Resonance parameters for ${E}_{n}l12$ eV were obtained using an $R$-matrix fit to the measured cross section. The results are compared with values from the ENDF/B-VII.0, Mughabghab, JENDL-3.3, and JEFF-3.1 evaluations. ${\ensuremath{\Gamma}}_{n}$ neutron widths for the first three resonances are systematically larger by 5--15% than the ENDF/B-VII.0 values. The resonance integral above 0.5 eV was determined to be 1553\ifmmode\pm\else\textpm\fi{}7 b. Cross sections in the resolved and unresolved energy regions above 12 eV were calculated using the Hauser-Feshbach theory incorporating the width-fluctuation correction of Moldauer. The calculated results agree well with the measured data, and the extracted averaged resonance parameters in the unresolved resonance region are consistent with those for the resolved resonances.

Theoretical Calculations of n + 232,234,236,238,240U Reaction Cross Sections

Consistent calculation and analysis of neutron scattering data of 232,234,236,238,240U with a spherical optical model, the unified Hauser-Feshbach theory and the exciton model, the linear angular momentum–dependent exciton state density model, and the coupled channel theory are carried above the resolved resonance range and below 20 MeV based on the experimental data of total, nonelastic-scattering, fission, and other reaction cross sections and elastic-scattering angular distributions. Theoretical calculations are compared with experimental data and other evaluated data from ENDF/B-VII and JENDL-3.3.

Neutron-emission mechanisms in the 115In(α, xn) reactions

Spectra and angular distributions of neutrons from the 115In(α, xn) reactions were measured at α-particle energies of 16, 18, 27, and 45 MeV. The measurements were performed with time-of-flight neutron spectrometers at pulsed accelerators of charged particles. The data obtained in this way were analyzed within the models of equilibrium, preequilibrium, and direct reaction mechanisms. The exact formalism of Hauser-Feshbach statistical theory was used in the calculations, level densities in residual nuclei excited in the reactions under consideration being found from the neutron evaporation spectra in the (p, n) reactions on tin isotopes. Contributions from equilibrium, preequilibrium, and direct neutron emission were studied in a wide range of α-particle energies.