Giant Dipole Resonance Energy Research Articles

Prompt neutron emission calculations for the photon induced reactions of 238U and 232Th in the giant dipole resonance energy region

Prompt emission calculations for the photon induced fission of 238U and 232Th in the GDR energy region were performed by using (a) a systematic of input parameters for the most probable fragmentation approach (obtained on the basis of only spontaneous and neutron induced fission of actinides) and (b) photo-fission cross section ratios (expressing the fission probabilities at incident energies where multiple fission chances are involved) provided by nuclear reaction calculations performed with the EMPIRE code. The prompt neutron emission results describe well the experimental data recently measured at the laser Compton-scattering γ-ray source of the NewSUBARU facility, validating the use of this input parameter systematic for photon induced fission, too. The modelings and procedures described in this paper can constitute an useful tool for the prediction of prompt neutron emission data in photon induced reactions without any experimental information.

Compendium on Monte Carlo simulation of photoneutrons in the Giant Dipole Resonance energy range: The first five elements

Neutrons generated by photonuclear reactions, “photoneutrons”, are encountered in various applications involving high-energy gamma sources, electron accelerators or nuclear reactors. Monte Carlo particle-transport codes are generally used to simulate the emission of photoneutrons, characterize their field or assess their impact on nuclear systems. The aim of this work is to create a compendium on the simulation of photoneutrons using several Monte Carlo codes, i.e., MCNP6, PHITS and TRIPOLI-4, each code being run successively with ENDF/B-VIII.0 and JENDL-5 nuclear data libraries. We study the photoneutron fields produced by 50 elements with their natural isotopic composition from the reaction energy threshold up to 30 MeV, i.e., in the regime of the Giant Dipole Resonance (GDR). The photoneutron fields are characterized according to three observables, i.e., photoneutron current, energy spectrum and angular distribution. This paper presents the results obtained for the first five elements in order of increasing atomic number, i.e., deuterium, beryllium, carbon, nitrogen and oxygen. The compendium could serve as a handbook for users to master the current strengths and limitations of the codes, for code developers to make progress in the sampling of neutron-emitting photonuclear reactions, and more broadly for all researchers working on photoneutrons, whether they are evaluators of nuclear data libraries or experimental nuclear physicists.

Nuclear reactions as a tool to study the microscopic structure of pygmy and giant resonances

An advanced microscopic approach based on energy-density functional theory and the quasiparticle-phonon model has been used to study nuclear excitations up to giant dipole resonance energies. In addition, our nuclear structure model has been extended to include reaction theory. The theoretical spectral distributions compared with experimental data performed with different probes and techniques are able to provide us with information about the microscopic structure of the pygmy dipole resonance as well as its collectivity. Evaluations of electric dipole (E1) transition strengths and branching ratios at low energy reveal different properties of the nuclear excitations and the effect of quasi-continuum coupling. The present studies will support above-neutronthreshold γ experiments at ELI-NP targeting γ decays of giant and pygmy resonances, as well as studies on multi-step γ decays by low-lying states.

Photofission and photoneutron cross sections for 238U and 232Th

A specific objective of the recent IAEA Coordinated Research Project on Photonuclear Data and Photon Strength Functions (Code F41032; Duration 2016-2019) has been to measure photonuclear cross-section data where needed, for unexplored nuclei and cases of discrepant existing data. A dedicated experimental campaign has been conducted at the laser Compton-scattering γ-ray source of the NewSUBARU synchrotron radiation facility of SPring8, Japan, where photoneutron reactions for 11 nuclei from 9Be to 209Bi have been investigated in the Giant Dipole Resonance energy region. The measurements followed the development of a flat-effi ciency moderated neutron detection array and the associated neutron-multiplicity sorting techniques. The IAEA CRP campaign has been followed at NewSUBARU by new measurements of photofission and photoneutron reactions on 238U and 232Th in the energy range of 5.87 MeV – 20.14 MeV. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events has been performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron (γ, xn) and photofission (γ, fxn) reactions have been discriminated by considering a Gaussian distribution of prompt-fission-neutrons (PFN) multiplicities predicted by the evaporation theory. We here provide preliminary experimental (γ, n), (γ, 2n) and (γ, F) cross sections, average energies of PFN and of photoneutrons emitted in (γ, n) and (γ, 2n) reactions, as well as the mean number of PFN per fission act. The new 238U cross sections are compared with recent statistical-model calculations performed with the EMPIRE code on existing data.

Providing physics guidance in Bayesian neural networks from the input layer: The case of giant dipole resonance predictions

The Bayesian neural network (BNN) has been applied to evaluate and predict the nuclear data. However, how to provide physics guides in BNN is a key but an open question. In this work, the case study on giant dipole resonance (GDR) energy is presented to illustrate the effectiveness and maneuverability of the method to provide physics guides in BNN from input layer. The Spearman's correlation coefficients are applied to assess the statistical dependence between nuclear properties in the ground state and the GDR energies. Then the optimal ground-state properties are employed as the input layer in the BNN for evaluating and predicting the GDR energies. Those selected ground-state properties actively contributes to reduce the predicted errors and avoid the risk of the non-physics divergence. This work gives a demonstration to find effects of the GDR energy by using the BNN without the physics motivated model, which may be helpful for discovering physics effects from the complex nuclear data.

The description of giant dipole resonance key parameters with multitask neural networks

Giant dipole resonance (GDR) is one of the fundamental collective excitation modes in nucleus. Continuous efforts have been made to the evaluation of GDR key parameters in different nuclear data libraries. We introduced multitask learning (MTL) approach to learn and reproduce the evaluated experimental data of GDR key parameters, including both GDR energies and widths. Compared to the theoretical GDR parameters in RIPL-3 library, the accuracies of MTL approach are almost doubled for 129 nuclei with experimental data. The significant improvement is largely due to the right classification of unimodal nuclei and bimodal nuclei by the classification neural network. Based on the good performance of the neural network approach, an extrapolation to 79 nuclei around the β-stability line without experimental data is made, which provides an important reference to future experiments and data evaluations. The successful application of MTL approach in this work further proofs the feasibility of studying multi-output physical problems with multitask neural network in nuclear physics domain.

Measurement of theAm241(γ,n)Am240reaction in the giant dipole resonance region

The photodisintegration cross section of the radioactive nucleus $^{241}\mathrm{Am}$ has been obtained using activation techniques and monoenergetic $\ensuremath{\gamma}$-ray beams from the HI$\ensuremath{\gamma}$S facility. The induced activity of $^{240}\mathrm{Am}$ produced via the $^{241}\mathrm{Am}$$(\ensuremath{\gamma},n)$ reaction was measured in the energy interval from 9 to 16 MeV utilizing high-resolution $\ensuremath{\gamma}$-ray spectroscopy. The experimental data for the $^{241}\mathrm{Am}$$(\ensuremath{\gamma},n)$ reaction in the giant dipole resonance energy region are compared with statistical nuclear-model calculations.

Photonuclear reactions of actinides in the giant dipole resonance region

Photonuclear reactions at energies covering the giant dipole resonance (GDR) region are analyzed with an approach based on nuclear photoabsorption followed by the process of competition between light particle evaporation and fission for the excited nucleus. The photoabsorption cross section at energies covering the GDR region is contributed by both the Lorentz type GDR cross section and the quasideuteron cross section. The evaporation-fission process of the compound nucleus is simulated in a Monte-Carlo framework. Photofission reaction cross sections are analyzed in a systematic manner in the energy range of $\sim$ 10-20 MeV for the actinides $^{232}$Th, $^{238}$U and $^{237}$Np. Photonuclear cross sections for the medium-mass nuclei $^{63}$Cu and $^{64}$Zn, for which there are no fission events, are also presented. The study reproduces satisfactorily the available experimental data of photofission cross sections at GDR energy region and the increasing trend of nuclear fissility with the fissility parameter $Z^2/A$ for the actinides.

Bremsstrahlung target for studying photonuclear reactions in the giant dipole resonance energy region

Studying the interaction mechanism between gamma-quanta and atomic nuclei in the giant dipole resonance energy region gives us significant information about atomic nuclei. A large number of experiments for studying giant dipole resonance structure were accomplished using bremsstrahlung gamma-quanta. In such experiments cross sections of interactions are retrieved from reaction yields. To retrieve cross sections one needs to know the shape of the bremsstrahlung spectrum and to choose the optimal characteristics of the bremsstrahlung target. The purpose of this article is to choose the optimal characteristics of the bremsstrahlung target for experiments in the energy region E < 30 MeV. The analysis that was carried out based on numerical simulation showed that the optimal bremsstrahlung target consists of two parts. The first part is a tungsten plate 2.5 mm thick. The second part is aluminum plate 3 cm thick.

Dynamical dipole mode in fusion reactions at 16 MeV/nucleon and beam energy dependence

High-energy {gamma} rays and light charged particles from the {sup 36}Ar+{sup 96}Zr and {sup 40}Ar+{sup 92}Zr reactions at E{sub lab}=16 and 15.1 MeV/nucleon, respectively, were measured in coincidence with evaporation residues by means of the MEDEA multidetector array coupled to four parallel plate avalanche counters. The aim of this experiment was to investigate the prompt {gamma} radiation, emitted in the decay of the dynamical dipole mode, in the {approx}16 MeV/nucleon energy range and to map its beam energy dependence, comparing the present results with our previous ones obtained at lower energies. The studied reactions populate, through entrance channels having different charge asymmetries, a compound nucleus in the region of Ce under the same conditions of excitation energy and spin. Light charged particle energy spectra were used to pin down the average excitation energy and the average mass of the system. By studying the {gamma}-ray spectra of the charge symmetric reaction {sup 40}Ar+{sup 92}Zr, the statistical giant dipole resonance (GDR) parameters and angular distribution were extracted, and a comparison of the linearized 90 deg. {gamma}-ray spectra of the two reactions revealed a 12% extra yield in the GDR energy region for the more charge asymmetric system. The center-of-mass angular distribution datamore » of this extra {gamma} yield, compatible with a dipole oscillating along the symmetry axis of the dinuclear system, support its dynamical nature. The experimental findings are compared with theoretical predictions performed within a Boltzmann-Nordheim-Vlasov transport model and based on a collective bremsstrahlung analysis of the entrance channel reaction dynamics. An interesting sensitivity to the symmetry term of the equation of state and to in-medium effects on nucleon-nucleon (nn) cross sections is finally discussed.« less

Fusion process studied with a preequilibrium giant dipole resonance in time-dependent Hartree-Fock theory

The equilibration of macroscopic degrees of freedom during the fusion of heavy nuclei, such as the charge and the shape, are studied in the time-dependent Hartree-Fock theory. The preequilibrium giant dipole resonance (GDR) is used to probe the fusion path. It is shown that such an isovector collective state is excited in $N/Z$ asymmetric fusion and to a lesser extent in mass asymmetric systems. The characteristics of this GDR, such as its deformation, rotation, and vibration, are governed by the structure of the fused system in its preequilibrium phase. In particular, we show that a loweringq of the preequilibrium GDR energy is expected as compared to the statistical one. Revisiting experimental data, we extract evidence of this lowering for the first time. We also quantify the fusion-evaporation enhancement resulting from \ensuremath{\gamma}-ray emission from the preequilibrium GDR. This cooling mechanism along the fusion path may be suitable for synthesizing in the future superheavy elements using radioactive beams with strong $N/Z$ asymmetries in the entrance channel.

On the neutron radiative capture in the vicinity of the giant dipole resonance

A semimicroscopical approach is applied for describing partial photonucleon reactions accompanying the excitation of the giant dipole resonance (GDR). The approach is based on continuum-RPA and the phenomenological description of doorway-state coupling to many-quasiparticle configurations. Apart from the phenomenological mean field, the separable isovector momentum-dependent forces are taken into consideration together with the momentum-independent Landau–Migdal interaction. It allows us to describe satisfactorily the experimental values of the GDR energy and the photoabsorption cross section integrated over the GDR in 208Pb along with some partial 208Pb( n, γ)-reaction cross sections in the vicinity of the GDR. An attempt to establish a correspondence between the approach and the well-known DSD-model was also undertaken.

Deformation on isomeric excitation of Eu isotopes in(γ,n)and(n,γ)reactions

Isomeric ratios in $(n,\ensuremath{\gamma})$ and $(\ensuremath{\gamma},n)$ reactions in Eu isotopes were measured. Thermal neutrons and bremsstrahlung photons in the giant dipole resonance energy range of 13--22 MeV were used to excite the isomeric states. The isomeric levels ${0}^{\ensuremath{-}}$ and ${8}^{\ensuremath{-}}$ were excited in the isotopes ${}^{150}\mathrm{Eu},{}^{152}\mathrm{Eu},$ and ${}^{154}\mathrm{Eu}.$ The cross section for the reaction $(\ensuremath{\gamma},n)$ leading to the high-spin isomeric state ${8}^{\ensuremath{-}}$ on ${}^{152}{\mathrm{Eu}}^{m2}$ isotope was measured in the giant-dipole-resonance region by the activation method. Experimental values of the isomeric cross section ratios were compared with the theoretical values. The influence of the nuclei quadropole deformation on the isomeric ratio was described.

Vibrational frequencies of sodium clusters

AbstractThe vibrational frequencies of NaN clusters (2 ⩽ N ⩽ 72) are calculated by direct diagonalization of the dynamical matrix. Density functional theory with a spherically averaged pseudopotential is used to compute the total energy. The geometry is optimized by the simulated annealing technique. Contributions to the Hessian matrix due to electron relaxation following the ionic displacements are calculated in linear response theory. The frequencies are in the range 0‐220 cm−1 and the electron relaxation strongly modifies those of the modes dominated by radial oscillations, particularly the breathing mode frequencies that are proportional to N−1/3. The filling of atomic shells produces a stepwise behavior of the highest frequencies. The giant dipole resonance energies are obtained as a byproduct of the calculation. © 1995 John Wiley &amp; Sons, Inc.

Determination of the isotensor symmetry energy for the giant dipole resonance.

Comparison of giant dipole resonance energies measured in photonuclear reactions and in pion-induced double charge exchange allows determination of the isotensor component of the symmetry energy. Different models for the structure of the giant dipole and different assumptions for the mass dependence of the isotensor energy all produce the same qualitative results, with the isotensor energy about 1% of the isovector energy, and of the same sign.

Statistical gamma decay of the giant dipole resonance in highly excited 46Ti and 52Cr.

We have measured inclusive \ensuremath{\gamma}-ray spectra from the reactions \ensuremath{\alpha}${+}^{48}$Ti${\ensuremath{\rightarrow}}^{52}$Cr and $^{19}\mathrm{F}$${+}^{27}$Al${\ensuremath{\rightarrow}}^{46}$Ti, populating final states at moderate temperature and spin. These spectra were analyzed using a statistical model including giant resonance strength functions in the \ensuremath{\gamma}-decay width. The parameters of the giant dipole resonance (GDR) strength function were varied to fit the data. Two different level-density formulations were utilized---one (P\uhlhofer) in which the level-density parameters were adjusted to obtain the best overall fit to the data, and the other (Reisdorf) which contained no free parameters. While the magnitudes of the extracted GDR parameters differed in the two cases, their dependence on final-state energy was similar. The GDR energy was observed to be nearly constant with the final-state energy, and the GDR strength exhausted nearly a full E1 sum rule. In the \ensuremath{\alpha}${+}^{48}$Ti reaction, the GDR width was observed to increase slightly with increasing bombarding energy. In the $^{19}\mathrm{F}$${+}^{27}$Al reaction, a pronounced GDR width increase was observed with increasing bombarding energy and was attributed to an increase in compound nucleus spin. These results are consistent with current calculations based on the rotating liquid-drop model and with recent experimental reports.

NUCLEAR PHOTOABSORPTION IN 208Pb BETWEEN THRESHOLD AND 140 MeV

We have investigated photonuclear reactions in 208 Pb from the reaction threshold to pion production threshold. In the Giant Dipole Resonance energy region we have considered the results of a recent 1− 1p-1h continuum calculation and multiplied them, heuristically, by a factor, which includes the effect of neglected nucleon-nucleon correlations, through the occupation number probabilities of the single particle and hole energy levels. We have thus obtained a much better agreement with experiment. In the energy region above the Giant Dipole Resonance we have considered a quasi deuteron model, which contains the effect of nucleon swelling. Both this model and corresponding integrated sum rules yield a satisfactory agreement with experiment.

Out-of-Plane Measurement of the65Cu(e, e'p0) Reaction in the Giant Resonance Region

The 65 Cu(e, e'p 0 ) coincidence cross section at θ e' =30° and φ p =-90° is measured at θ p =0°–180° in the giant dipole resonance energy region. The present geometrical setup is selected to enhance the Coulomb multipole strength. The angular distribution obtained is decomposed into a Legendre series and analyzed by the theory of Kleppinger and Walecka. The result shows the success in geometrically obtaining the contribution of electric quadrupole excitation.

Skyrme-force parametrization: Least-squares fit to nuclear ground-state properties.

We investigate systematically the possibilities and the limits of the Skyrme force for reproducing nuclear ground-state properties in a spherical Hartree-Fock calculation. This investigation is performed by means of least-squares fits of the force parameters to the measured binding energy, diffraction radius, and surface width of eight selected nuclei. Particular emphasis is put on the density dependence of the interaction, which turns out to be determined mainly by the surface width. The least-squares fitting procedure yields the best-fit parameters together with uncertainties on them, and it also allows one to estimate the uncertainties of an extrapolation to other fields, e.g., nuclear matter properties. We also study the contribution of random-phase-approximation correlations to the ground-state properties and their influence on the parameters of the effective interaction. Here, we also compare to giant dipole resonance energies.

Threshold pion photoproduction from the trinucleon

Neglecting pion rescattering and certain exchange currents, we perform a microscopic calculation of threshold pion photoproduction in order to ascertain the effect of binding. We show that there are two opposite limits determined by a single parameter: the ratio of the giant dipole resonance energy to the pion mass. For large values of this parameter, the usual low energy theorems can be obtained, while the opposite, physical, regime leads to the one-body approach favored in most calculations. A discussion of binding effects in the Thomson (threshold photon scattering) cross section is also presented. Our conclusion is that the use of PCAC and soft pion theorems for a nucleus can be dangerous in some circumtances.