E1 Giant Resonance Research Articles

К программе фотоядерных исследований на пучке обратных комптоновских квазимонохроматических 𝛾–квантов с перестраиваемой энергией E𝛾 ≲ 40 МэВ

Based on the analysis of development of photonuclear reaction studding at 𝐸𝛾 ≲ 40 MeV and the unique parameters of the Compton source of 𝛾–quanta under projecting, a program of priority photonuclear research using this source is proposed. The program includes research on: structures of giant E1 resonances and pygmy resonances; cross sections for partial photonucleon reactions; nuclear resonant fluorescence; photofission; problems of astrophysics and nucleosynthesis.

Study of the Cross-Section for the 122Te(γ, n)121Te Reaction by the Activation Method

Relevance. The study of photonuclear reactions plays a defining role in the formation of modern ideas about highlocalized collective excitations in nuclei of the giant resonance type. The giant dipole E1-resonance (GR) is the main feature in γ-quantum absorption cross-sections of nuclei. The presence of isotopes with multiple isotopes makes it possible to trace the evolution of GR characteristics from changes in the number of neutrons in the nuclei. One such element is tellurium. Purpose. The purpose of this research is an experimental and theoretical study of the cross-section of 122Te(γ, n)121Te reaction in the region of giant E1-resonance energies. Methods. The study of the cross-section of (γ, n) reaction on 122Te isotope was performed on the bremsstrahlung γ-beam of the M-30 microtron of the Institute of Electronic Physics of the National Academy of Sciences of Ukraine. The yield curves were measured in the range of maximum gamma-quantum energies Eγmax=10-18 MeV. The reaction cross-section was calculated by the inverse matrix method. When registering the reaction products, an activation technique was used, while the gamma activity of the products was recorded by the spectrometric method. The theoretical calculations of the cross-section for the 122Te(γ, n)121Te reaction were performed using the TALYS-1.9 software package. Results. The measured experimental yield ratios of the 122Te(γ, n)121Te і 130Te(γ, n)129Te d= Yn121 /Yn129 = f (E max) reactions allowed the experimental cross-section for the 122Te(γ, n)121Te reaction to be calculated. The cross-section has a single hump shape with a maximum at an energy of ~ 15.3 MeV. It was approximated by the Lorentz curve with the parameters δ0 =(274.1±2.1) MB, Е0 (15.27±0.10) MeV, Г0 =(4.76±0.08) MeV. The cross-section obtained was compared with similar cross-sections for 126Te and 130Te isotopes, as well as with theoretical calculations according to the TALYS-1.9 program. Conclusions. The obtained cross-section for the 122Te(γ, n)121Te reaction is consistent with the evolution of the parameters of (γ, n) reaction cross-section on tellurium isotopes 130Te(γ, n)129Te with a change in the number of neutrons. The theoretical approaches used are satisfactorily consistent with the experimental data. This agreement indicates the main contribution of the statistical mechanism to the cross-section of the considered (γ, n) reaction and a relatively small contribution of semi-direct processes

Fine Structure of Beta Decay Strength Function and Anisotropy of Isovector Nuclear Dencity Component Oscillations in Deformed Nuclei

The experimental measurement data on the fine structure of beta-decay strength function Sβ(E) in spherical, transitional, and deformed nuclei are analyzed. Modern high-resolution nuclear spectroscopy methods made it possible to identify the splitting of peaks in Sβ(E) for deformed nuclei. By analogy with splitting of the peak of E1 giant dipole resonance (GDR) in deformed nuclei, the peaks in Sβ(E) are split into two components from the axial nuclear deformation. In this report, the fine structure of Sβ(E) is discussed. Splitting of the peaks connected with the oscillations of neutrons against protons (E1GDR), of proton holes against neutrons (peaks in Sβ(E) of β+/EC-decay), and of protons against neutron holes (peaks in Sβ(E) of β–-decay) is discussed.

Дослідження збудження ізомерного стану 11/2– ядра 139Се в реакції (gamma,n)m в області гігантського Е1-резонансу

Дослідження збудження ізомерного стану 11/2– ядра 139Се в реакції (gamma,n)m в області гігантського Е1-резонансу

Separation of contributions of isovector E2 and E1 giant resonances in direct and inverse reactions with real and virtual photons

A brief overview of the methods for separating the contributions of isovector electric quadrupole (E2) and dominant dipole (E1) giant resonances in atomic nuclei, which are excited in direct and inverse reactions with photons (real and virtual), is given. The basic parameters of isovector giant resonance E2, which were declared to date by applying some of these methods to the results with 208Pb, are also presented.

The possibility of separating isovector E1 and E2 giant resonances in measuring neutron emission asymmetry with threshold detectors

The possibility of identifying the contribution from the isovector electric quadrupole giant resonance against the background of the predominant isovector electric dipole giant resonance in measuring the forward–backward asymmetry of the emission of fast neutrons in (γ, n) reactions with threshold neutron detectors is discussed.

Cross section for the reaction 115In(γ, γ′)115m In in the region of the E1 giant resonance

The cross section for the reaction 115In(γ, γ′)115mIn was measured for photon energies in the range of Eγ ≅ 4–46 MeV. The parameters of the peak in this cross section near the threshold for the reaction 115In(γ, n), (Eγ)(γ,n)thr, were refined. It is shown that, in the cross section for the reaction 115In(γ, γ′)115mIn at Eγ ~ 27 MeV, there is no second peak for which δIIint would exceed about 0.2δIint for the peak at Eγ ~ (Eγ)(γ,n)thr. The possibility of employing this reaction both in studying photonuclearreaction physics and in monitoring bremsstrahlung photons in gamma-activation studies was examined.

Experimental parameters of the isovector E1 giant resonance, depending on the correctness of bremsstrahlung photon spectra calculations

An approximate analytical representation of the ratio between the Schiff spectrum widely used when studying photonuclear reactions earlier and the newly recognized Seltzer and Berger spectrum is proposed for bremsstrahlung of electrons. Based on this representation, the differences observed in the parameters of E1 giant resonance when processing experimental data with these two spectra are analyzed, depending on the shape of the structures in the cross sections of investigated reactions and the type of beam monitoring.

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.

Comparing real bremsstrahlung photons from electrons and positrons and virtual photons in electron-nuclear and positron-nuclear reactions for studies of giant resonances

The validity of determining the contributions attributable to bremsstrahlung photons from positrons using data with bremsstrahlung photons from electrons for identical radiators when studying E1 giant resonance by means of quasimonochromatic annihilation photons is confirmed. Photonuclear reactions induced by bremsstrahlung photons and electrons or positrons are compared with one another to determine the possibility of isolating the contributions from isovector E1 and E2 giant resonances.

Isovector giant E2 resonance and overtone of the isovector giant E1 resonance in photonucleon reactions

The effect of the isovector giant quadrupole resonance and an overtone of the isovector giant dipole resonance on the photodisintegration of medium-mass and heavy nuclei in the energy range of 20–40 MeV is studied on the basis of a combined model of photonucleon reactions. Basic features of these resonances are evaluated within a semimicroscopic approach. Their deformation spitting is described on the basis of the model of polarization vibrations of a two-component nuclear liquid.

Measurements of the48Ca(γ,n)reaction

The 48Ca({\gamma},n) cross section was measured using {\gamma}-ray beams of energies between 9.5 and 15.3 MeV generated at the Triangle Universities Nuclear Laboratory (TUNL) high-intensity {\gamma}-ray source (HI{\gamma}S). Prior to this experiment, no direct measurements had been made with {\gamma}-ray beams of sufficiently low energy spread to observe structure in this energy range. The cross sections were measured at thirty-four different {\gamma}-ray energies with an enriched 48Ca target. Neutron emission is the dominant decay mechanism in the measured energy range that spans from threshold, across the previously identified M1 strength, and up the low-energy edge of the E1 giant dipole resonance (GDR). This work found B(M 1) = 6.8 \pm 0.5 {\mu}N2 for the 10.23 MeV resonance, a value greater than previously measured. Structures in the cross section commensurate with extended random-phase approximation (ERPA) calculations have also been observed whose magnitudes are in agreement with existing data.

The (??SF) Method - Determination of Radiative Neutron Capture Cross Sections for Unstable Nuclei

An indirect method of determining radiative neutron-capture cross sections for unstable nuclei which we refer to as the γ-ray strength function (γSF) method is outlined and applied to Zr and Sn isotopes. Photoneutron cross sections for 91,92,94,96Zr and 117,116Sn near neutron threshold show the presence of extra γ-ray strengths known as giant M1 and pygmy E1 resonances on top of the low- energy γSF of GDR. Based on the γSF method, we present (n,γ) cross sections for two radioactive nuclei, a long-lived fission product, 93Zr with T1/2 = 1.5 × 106 y and an s-process branching point nucleus, 95Zr with T1/2 = 64.0 d.

Resonant Photonuclear Reactions for Isotope Transmutation

Coherent photonuclear isotope transmutation (CPIT) produces exclusively radioactive isotopes (RIs) by coherent photonuclear reactions via E1 giant resonances. Photons to be used are medium energy photons produced by laser photons backscattered off GeV electrons. The cross sections are as large as 0.2 - 0.6 b, being independent of individual nuclides. A large fraction of photons is effectively used for the photonuclear reactions, while the scattered GeV electrons remain in the storage ring to be re-used. CPIT with medium energy photons provides specific/desired RIs with the high rate and the high density for nuclear science, molecular biology and for nuclear medicines.

Electric dipole strength distribution below the E1 giant resonance in N = 82 nuclei

AbstractIn this study quasiparticle random-phase approximation with the translational invariant Hamiltonian using deformed mean field potential has been conducted to describe electric dipole excitations in 136Xe, 138Ba, 140Ce, 142Nd, 144Sm and 146Gd isotones. The distribution of the calculated E1 strength shows a resonance like structure at energies between 6–8 MeV exhausting up to 1% of the isovector electric dipole Energy Weighted Sum Rule and in some aspects nicely confirms the experimental data. It has been shown that the main part of E1 strength, observed below the threshold in these nuclei may be interpreted as main fragments of the Pygmy Dipole resonance. The agreement between calculated mean excitation energies as well as summed B(E1) value of the 1− excitations and the available experimental data is quite good. The calculations indicate the presence of a few prominent positive parity 1+ States in heavy N = 82 isotones in the energy interval 6–8 MeV which shows not all dipole excitations were of electric character in this energy range.

SKYRME-RANDOM-PHASE-APPROXIMATION DESCRIPTION OF E1 STRENGTH IN 92–100Mo

The isovector dipole E1 strength in 92,94,96,98,100 Mo is analyzed within the self-consistent separable random-phase approximation (SRPA) model with Skyrme forces SkT6, SkM*, SLy6, and SkI3. The special attention is paid to the low-energy region near the particle thresholds (4-12 MeV), which is important for understanding of astrophysical processes. We show that, due to a compensation effect, the influence of nuclear deformation on E1 strength below 10-12 MeV is quite modest. At the same time, in agreement with previous predictions, the deformation increases the strength at higher energy. At 4-8 MeV the strength is mainly determined by the tail of E1 giant resonance. The four Skyrme forces differ in description of the whole giant resonance but give rather similar results below 12 MeV.

Cross section of metastable state population in the photoneutron decay of the E1 giant resonance for 110Pd, 112Cd, and 113In nuclei

The cross sections of isomeric state excitation in the 110Pd(γ, n)109mPd, 112Cd(γ, n)111mCd, and 113In(γ, n)112mIn reactions have been investigated. The measurements were performed in the γ-ray energy range 9–18 MeV with a step ΔE = 0.5 MeV. The dependences of the isomeric ratios on the γ-ray energy are obtained. The experimental data are compared with the results of calculations within the cascade-evaporation model.

TDDFT WITH SKYRME FORCES: EFFECT OF TIME-ODD DENSITIES ON ELECTRIC GIANT RESONANCES

Time-odd densities and their effect on electric giant resonances are investigated within the self-consistent separable random-phase-approximation (SRPA) for a variety of Skyrme forces (SkT6, SkO, SkM*, SIII, SGII, SLy4, SLy6, SkI3). Time-odd densities are essential for maintaining the Galilean invariance of the Skyrme functional. Their contribution is determined by the values and signs of the isovector and isoscalar effective-mass parameters of the force. In even-even nuclei these densities are not active in the ground state but can affect the dynamics. As a particular case, we explore the role of the current density in the description of isovector E1 and isoscalar E2 giant resonances in a chain of spherical and deformed Nd isotopes with A=134-158. The relation of the current to the effective masses and relevant parameters of the Skyrme functional is analyzed. It is shown that the current contributes substantially to E1 and E2 and the contribution is the same for all the isotopes along the chain, i.e. for both standard and exotic nuclei.

Spin- and Parity-Resolved Level Densities from the Fine Structure of Giant Resonances

Level densities of J pi=2+ and 2- states extracted from high-resolution studies of E2 and M2 giant resonances in 58Ni and 90Zr are used to test recent predictions of a possible parity dependence. The experimental results are compared to a combinatorial approach based on the Hartree-Fock-Bogoliubov model and to shell-model Monte Carlo calculations including both spin and parity projection. No parity dependence is observed experimentally, which is in agreement for 90Zr but in contrast with the model predictions for 58Ni.

Self-consistent separable random-phase approximation for Skyrme forces: Giant resonances in axial nuclei

We formulate the self-consistent separable random-phase-approximation (SRPA) method and specify it for Skyrme forces with pairing for the case of axially symmetric deformed nuclei. The factorization of the residual interaction allows to avoid diagonalization of high-rank RPA matrices, which dramatically reduces the computational expense. This advantage is crucial for the systems with a huge configuration space, first of all for deformed nuclei. SRPA takes self-consistently into account the contributions of both time-even and time-odd Skyrme terms as well as of the Coulomb force and pairing. The method is implemented to description of isovector E1 and isoscalar E2 giant resonances in a representative set of deformed nuclei: $^{154}$Sm, $^{238}$U, and $^{254}$No. Four different Skyrme parameterizations (SkT6, SkM*, SLy6, and SkI3) are employed to explore dependence of the strength distributions on some basic characteristics of the Skyrme functional and nuclear matter. In particular, we discuss the role of isoscalar and isovector effective masses and their relation to time-odd contributions. High sensitivity of the right flank of E1 resonance to different Skyrme forces and the related artificial structure effects are analyzed.