Excitation Of Giant Resonances Research Articles

On activation studies of photonuclear reactions on γ-beams from backward Compton scattering at E ≤ 40 MeV

For photonuclear research with γ-quanta having energies Eγ ≤ 40 MeV at the source under developing with backward Compton scattering of laser radiation on accelerated electrons, activation experiments are proposed as initial ones. These experiments are important both for developing techniques for tuning and monitoring such γ-beams, and for studying the excitation of pygmy and giant resonances in nuclei at Eγ near the threshold of the (γ, n) reaction, as well as the multiplicity of photoneutrons during deexcitation of giant resonances at Eγ above the threshold of (γ, 2n)-reaction.

Excitation of the isoscalar giant monopole resonance and incompressibility of nuclear matter: resolution of a long-standing puzzle

Excitation of the isoscalar giant monopole resonance and incompressibility of nuclear matter: resolution of a long-standing puzzle

Measurement of the photoabsorption cross section of 24Mg.

Accurate nuclear data is a key factor in determining the suitability and reliability of many theoretical nuclear models and large-scale calculations. One of the main ingredients of these calculations is how nuclei respond to an electromagnetic field. The excitation of the isovector giant dipole resonance (GDR) is of particular importance in both nuclear structure studies as well as being the main mode of interaction of ultra-high-energy cosmic rays with the extra-galactic medium. This study investigates the photoabsorption cross section in the region of the GDR in 24Mg through the use of proton inelastic scattering and the equivalent virtual photon method. The K600 spectrometer at the iThemba LABS facility was used to obtain high resolution, low background 24Mg(p,p’)24Mg* inelastic scattering data. The virtual photon absorption method is described and the result of applying the method is presented and compared to a previous real photon absorption cross section.

Опис фотопоглинання фотонними силовими функціями із збудженням двох резонансних станів

The results of the use of photon strength function (PSF) of the model of two coupled damped oscillators (Two State Excitations, TSE) for the description of nuclear data for photoabsorption of electric dipole gamma-rays by atomic nuclei are presented. The response function widths dependent on gamma-ray energy were used instead of constant widths in the initial TSE model. It was demonstrated that such a phenomenological approach is a simple method for the description and prediction of the PSF with excitation of both low-energy (pigmy) dipole resonance (PDR) and high-energy giant dipole resonance (GDR). The best description of the experimental photoabsorption cross-sections is obtained at constant width in the PDR range and linear or quadratic energy-dependent width in the GDR range.

Self-consistent description of high-spin states in doubly magic Pb208

We analyze recent data on a long series of high-spin states in $^{208}$Pb with a self-consistent phonon-coupling model for nuclear excitations based on the Skyrme functionals. The model is the renormalized time-blocking approximation (RenTBA) which takes the coherent one-particle-one-hole (1p1h) states of the random-phase approximation (RPA) as starting point and develops from that more complex configurations beyond RPA. To the best of our knowledge, this is the first investigation of high spin states in $^{208}$Pb using self-consistent nuclear models. The interesting point here is that complex configurations are compulsory to describe the upper end of the long spin series at all. The data thus provide an ideal testing ground for phonon-coupling models as they give direct access to complex configurations. We find that standard Skyrme functionals which perform well in ground state properties and giant resonance excitations deliver at once an agreeable description of these high spin states.

The electric dipole response of even-even 154–164Dy isotopes

The excitation of pygmy dipole resonance (PDR) and giant dipole resonance (GDR) in even–even 154–164Dy isotopes is examined through quasiparticle random-phase approximation (QRPA) with the effective interactions that restore the broken translational and Galilean invariances. In each isotope, an electric response emerges by showing ample distribution at energies below and above 10 MeV. We, therefore, study the transition cross-sections and probabilities, photon strength functions, transition strengths, isospin character, and collectivity of the predicted E1 responses.

Dipole excitation of Li6 and Be9 studied with an extended quantum molecular dynamics model

The $\alpha$ ($^4$He) - clustering structure is a common phenomenon in light nuclei due to the decreasing contribution of mean field in few-body system. In this work we presented calculations of giant dipole resonance (GDR) excitations for two non $\alpha$-conjugate light nuclei, namely $^{6}$Li and $^{9}$Be, within a framework of an extended quantum molecular dynamics model. For $^{6}$Li, we investigated the GDR spectra from the two-body clustering structure with $\alpha$ + deuteron as well as the three-body structure with $\alpha$ + $n$ + $p$, and found that the major $\alpha$-clustering contribution on the GDR peak is located at around 31 MeV, while the resonance contributions between clusters, namely $\alpha$ and deuteron or ($n$ + $p$), are located on the lower energy side, which can be regarded as pygmy dipole resonance (PDR). For $^{9}$Be, a mixture configuration contribution for the Chain-like structure and Borromean-like structure of the $\alpha$ + n + $\alpha$ configuration can somehow explain its GDR results.

Jacques Raynal: memories of a physicist

This text is dedicated to the memory of Jacques Raynal. I recall some of the interactions I had with Jacques from 1975 up until his death. I first met Jacques as a young student at Saclay in the mid 1970s and from then on we collaborated frequently. Jacques initiated me into elastic and inelastic scattering calculations in the coupled channels formalism. Within this formalism we calculated the excitation of Giant Resonance by light- or heavy-ion projectiles. We collaborated on the Coulomb corrections implemented in ECIS and later we tested numerically the Incoming Wave Boundary Condition, which is an important assumption when calculating sub-Coulomb fusion cross sections. He was among the first to understand the origin of what we would call the 3- excitation hindrance and suggest solutions.

Fission of Relativistic Nuclei with Fragment Excitation and Reorientation.

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ∼2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2-10 for a large number of individual yields. Here, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the ^{238}U+^{208}Pb reaction at 1 GeV/nucleon (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

Optimizing the relativistic energy density functional with nuclear ground state and collective excitation properties

We introduce a new relativistic energy density functional constrained by the ground state properties of atomic nuclei along with the isoscalar giant monopole resonance energy and dipole polarizability in $^{208}$Pb. A unified framework of the relativistic Hartree-Bogoliubov model and random phase approximation based on the relativistic density-dependent point coupling interaction is established in order to determine the DD-PCX parameterization by $\chi^2$ minimization. This procedure is supplemented with the co-variance analysis in order to estimate statistical uncertainties in the model parameters and observables. The effective interaction DD-PCX accurately describes the nuclear ground state properties including the neutron-skin thickness, as well as the isoscalar giant monopole resonance excitation energies and dipole polarizabilities. The implementation of the experimental data on nuclear excitations allows constraining the symmetry energy close to the saturation density, and the incompressibility of nuclear matter by using genuine observables on finite nuclei in the $\chi^2$ minimization protocol, rather than using pseudo-observables on the nuclear matter, or by relying on the ground state properties only, as it has been customary in the previous studies.

Pygmy and core polarization dipole modes in 206Pb: Connecting nuclear structure to stellar nucleosynthesis

A high-resolution study of the electromagnetic response of 206Pb below the neutron separation energy is performed using a (γ→,γ′) experiment at the HIγ→S facility. Nuclear resonance fluorescence with 100% linearly polarized photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in 206Pb from 4.9 to 8.1 MeV. The extracted ΣB(E1)↑ and ΣB(M1)↑ values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9±0.2 e2fm2 and 8.3±2.0μN2, respectively. These measurements are found to be in very good agreement with the predictions from an energy-density functional (EDF) plus quasiparticle phonon model (QPM). Such a detailed theoretical analysis allows to separate the pygmy dipole resonance from both the tail of the giant dipole resonance and multi-phonon excitations. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of 206Pb of αD=122±10 mb/MeV. When compared to predictions from both the EDF+QPM and accurately calibrated relativistic EDFs, one deduces a range for the neutron-skin thickness of Rskin206=0.12–0.19 fm and a corresponding range for the slope of the symmetry energy of L=48–60 MeV. This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross section Pb205(n,γ)Pb206 at 30 keV to be σ=130±25 mb. The astrophysical impact of this measurement—on both the s-process in stellar nucleosynthesis and on the equation of state of neutron-rich matter—is discussed.

Isovector excitations in 100Nb and their decays by neutron emission studied via the [formula omitted] reaction at 115 MeV/u

Spin–isospin excitations in Nb100 were studied via the Mo100(t,He3) charge-exchange reaction at 115 MeV/u with the goal to constrain theoretical models used to describe the isovector spin response of nuclei. The experiment was performed with a secondary beam of tritons, and 3He particles were analyzed in the S800 magnetic spectrometer. Decay by neutron emission from excited states in Nb100 was observed by using plastic and liquid scintillator arrays. Differential cross sections were analyzed and monopole excitations were revealed by using a multipole decomposition analysis. The Gamow–Teller transition strength observed at low excitation energies, which is important for estimating the electron-capture rate in astrophysical scenarios, was strongly fragmented and reduced compared to single-particle and spherical mean-field models. The consideration of deformation in the theoretical estimates was found to be important to better describe the fragmentation and strengths. A strong excitation of the isovector spin giant monopole resonance was observed, and well reproduced by the mean-field models. Its presence makes the extraction of Gamow–Teller strengths at high excitation energies difficult. The branches for statistical and direct decay by neutron emission were identified in the spectra. The upper limit for the branching ratio by direct decay (integrated over all observed excitations) was determined to be 20±6%. Even though the statistical uncertainties in the neutron-coincident data were too large to perform detailed studies of the decay by neutron emission from individual states and resonances, the experiment demonstrates the feasibility of the method.

Investigation of the energy-averaged double transition density of isoscalar monopole excitations in medium-heavy mass spherical nuclei

The particle–hole dispersive optical model, developed recently, is applied to study properties of high-energy isoscalar monopole excitations in medium-heavy mass spherical nuclei. The energy-averaged strength functions of the isoscalar giant monopole resonance and its overtone in 208Pb are analyzed. In particular, we analyze the energy-averaged isoscalar monopole double transition density, the key quantity in the description of the hadron–nucleus inelastic scattering, and studied the validity of the factorization approximation using semi classical and microscopic one body transition densities, respectively, in calculating the cross sections for the excitation of isoscalar giant resonances by inelastic alpha scattering.

Electromagnetic processes in silver isotopes

Peculiarities of electromagnetic interactions in silver isotopes are considered. Excited states of Ag isotopes are studied as a function of the mass number A = 95–117. Low-lying excited states reveal clear features that are specific for rotational spectra for the energies E < 4–5 MeV. In the energy domain above the nucleon separation threshold of 5–10 MeV, single-particle excited states overlap and produce a continuous spectrum. For energies E = 10–35 MeV, excitation of giant dipole resonance plays the most important role in Ag isotopes. Experimental data on the cross sections of photonuclear reactions in Ag isotopes are analyzed.

Isoscalar giant resonance studies in a stored-beam experiment within EXL

In the first campaign of the exotic nuclei studied with light-ion induced reaction in storage rings (EXL) collaboration at the existing storage ring experimental heavy-ion storage ring (ESR) at Helmholtz Center for Heavy Ion Research (GSI), we performed the first experiments using a stored beam of 58Ni and an internal helium gas-jet target aiming for the investigation of isoscalar giant resonances in inverse kinematics. In this experiment, inelastically scattered recoil particles (at very forward angles, °) were detected with a dedicated setup, including ultra-high vacuum (UHV)-compatible double-sided silicon strip detector (DSSDs). Preliminary results show evidence for the excitation of the isoscalar giant monopole resonance (ISGMR) in the 58Ni nucleus. This opens the opportunity to study in the near future giant resonances also with stored radioactive beams, like 56Ni, and extract important information about the nuclear matter incompressibility. In the present work the current status of the data analysis and results are shown and discussed.

Softness of Sn isotopes in relativistic semiclassical approximation

Within the framework of relativistic Thomas–Fermi (RTF) and relativistic extended Thomas–Fermi (RETF) approximations, we calculate the giant monopole resonance (GMR) excitation energies for Sn and related nuclei. A large number of nonlinear relativistic force parameters are used in these calculations. We find a parameter set is capable to reproduce the experimental monopole energy of Sn isotopes, when its nuclear matter compressibility lies within 210–230 MeV, however, it fails to reproduce the GMR energy of other nuclei. That means, simultaneously a parameter set cannot reproduce the GMR values of Sn and other nuclei.

Fluxes and spectra of quasimonochromatic annihilation photons for studying E1 giant resonances in nuclei

The fluxes and spectra of quasimonochromatic photons originating from the in-flight annihilation of positrons interacting with electrons of targets are analyzed in the energy region characteristic of the excitation of E1 giant resonances in nuclei. Targets of small thickness and low atomic number are used. The dependences of the spectra on the energy and angle (and their scatter) for positrons incident to the target, on the collimation angle for photons, and on the target thickness are studied.

Description of the simplest photonuclear reactions within the particle-hole dispersive optical model

A recently developed particle-hole dispersive optical model is applied to describe the cross sections of photoabsorption, $\text{direct}+\text{semidirect}$ photoneutron and inverse reactions accompanied by excitation of the isovector giant dipole and quadrupole resonances in medium--heavy-mass spherical nuclei. The model is an extension of the standard and nonstandard versions of the continuum-random-phase approximation by including the spreading effect in a phenomenological way. It contains the following ingredients: the Landau--Migdal particle-hole interaction and a phenomenological mean field consistent with this interaction, isovector velocity-dependent forces taken in the simplest form and the imaginary part of an effective single-particle optical-model potential determining the corresponding dispersive part. All the model parameters are taken from the other data and from the description of photoabsorption, while the $\text{direct}+\text{semidirect}$ photoneutron and inverse reactions are described without the use of specific adjustable parameters. Calculation results obtained for a few neutron-closed-shell nuclei are compared with the corresponding experimental data.

Excitation of giant monopole resonance in 208Pb and 116Sn using inelastic deuteron scattering

The excitation of the isoscalar giant monopole resonance (ISGMR) in 208Pb and 116Sn has been investigated using small-angle (including 0°) inelastic scattering of 100 MeV/u deuteron and multipole-decomposition analysis (MDA). The extracted strength distributions agree well with those from inelastic scattering of 100 MeV/u α particles. These measurements establish deuteron inelastic scattering at Ed∼100 MeV/u as a suitable probe for extraction of the ISGMR strength with MDA, making feasible the investigation of this resonance in radioactive isotopes in inverse kinematics.

Electromagnetic excitation of nuclei and neutron evaporation in ultrarelativistic ultraperipheral heavy ion collisions

We present a new approach for calculating electromagnetic excitation of nuclei as well as probabilities of emission and distributions of neutrons from decays of excited nuclear systems for ultrarelativistic, ultraperipheral heavy ion collisions. Excitation functions for $\gamma$ + Pb $\to$ Pb$^*$ are parametrized using physics-motivated components: excitation of giant resonances, quasi-deuteron absorption mechanism, excitation of nucleon resonances as well as high-energy dissociation of protons and neutrons. Neutron emission (up to 10 neutrons) from low-energy excitations of $^{208}$Pb is modelled in terms of the Hauser-Feshbach formalism. The probabilities of a given number of neutrons are calculated as a function of excitation energy in a Monte-Carlo code GEMINI. These functions are parametrized by smooth analytical functions. The results are compared to appropriate data for the $\gamma$ Pb $\to$ Pb$^*$ $\to$ k n reaction. As an example the approach is used for calculating electromagnetic excitation in UPC processes. Both single photon and double photon excitations are included and discussed. Topological cross sections with a given number of neutrons in forward and backward directions are calculated in the calculation at the LHC energies. Excitation functions are presented. The results of calculation are compared with SPS and more recent ALICE experimental data and good agreement is achieved.