E1 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.

Investigating the E2 nuclear resonance effect in kaonic atoms

The nuclear E2 resonance effect occurs when an atomic de-excitation energy is closely matched by a nuclear excitation energy. It produces an attenuation of some of the atomic X-ray lines in the resonant isotope target. Investigating the nuclear E2 resonance effect in kaonic atoms, important information about kaon-nucleus strong interaction can be provided. The only nuclear resonance effect was measured by G. L. Goldfrey, G- K. Lum and C. E. Wiegand at Lawrence Berkeley Laboratory, in 1975. The nuclear E2 resonance effect was observed in 25 hours of data taking, not enough to provide a conclusive result. In four kaonic Molybdenum isotopes (, , and ), the nuclear E2 resonance effect is expected at the same transition, with similar energy values. The KAMEO (Kaonic Atoms Measuring nuclear resonance Effects Observables) experiment plans to study the E2 nuclear resonance effect in kaonic Molybdenum isotopes at the DAΦNE e+e− collider, during the SIDDHARTA-2 experiment. The experimental strategy consists of exposing four solid strip targets, each enriched with one Molybdenum isotope, to negatively charged kaons, using a germanium detector for X-ray transition measurements. A further exposure of a non-resonant isotope solid strip target will be used as reference for standard non-resonant transitions.

Investigating the E2 Nuclear Resonance Effects in Kaonic Atoms: The KAMEO Proposal

The E2 nuclear resonance effect in kaonic atoms occurs when the energy of atomic de-excitation closely matches the energy of nuclear excitation, leading to the attenuation of some X-ray lines in the resonant isotope target. This phenomenon provides crucial information on the strong interaction between kaons and nuclei. The only nuclear E2 resonance effect observed so far was in the K− −9842Mo isotope, measured by G. L. Goldfrey, G-K. Lum, and C. E. Wiegand at Lawrence Berkeley Laboratory in 1975. However, the 25 hours of data taking were not sufficient to yield conclusive results. In four kaonic Molybdenum isotopes (9442Mo, 9642Mo, 9842and Mo, and 10042Mo), the nuclear E2 resonance effect is expected to occur at the same transition with similar energy values. To investigate this, the KAMEO (Kaonic Atoms Measuring Nuclear Resonance Effects Observables) experiment plans to conduct research on kaonic Molybdenum isotopes at the DAΦNE e+e− collider during the SIDDHARTA-2 experiment. The experimental strategy involves exposing four solid strip targets, each enriched with one Molybdenum isotope, to negatively charged kaons and using a germanium detector to measure X-ray transitions. In addition, a non-resonant 9242Mo isotope solid strip target will be used as a reference for standard non-resonant transitions.

Translational and Galilean Invariant Quasiparticle Phonon Nuclear Model (TGI-QPNM) for electric dipole (E1) transitions in odd-mass deformed nuclei

The method of Pyatov, proposed for the exclusion of the spurious states associated with the spontaneous symmetry breaking due to the mean field approximation, is applied to restore broken translation and Galilean invariance of the Quasiparticle Phonon Nuclear Model (QPNM) Hamiltonian of deformed odd-mass nuclei for the first time. The point of the formulated translation and Galilean invariant (TGI-) QPNM is to study electric dipole (E1) response of odd-mass deformed nuclei. The proposed TGI-QPNM is tested by the calculations for fine and gross features of E1 resonance in 165Ho, 169Tm, and 235U in a wide range of excitation energy from 2 to 20 MeV. Predictions of the TGI-QPNM are shown to be in good agreement with the corresponding experimental data.

Effect of the Splitting of Deep Nuclear Shells on the Structure of Multipole Resonances

The effect of the splitting of nuclear shells lying substantially lower than the Fermi energy on the fragmentation of multipole resonances in cross sections for the excitation of nuclei is studied by considering the example of M6 and E1 resonances in self-conjugate sd-shell nuclei.

Field enhancement at silicon surfaces by gold ellipsoids probed by optical second-harmonic generation spectroscopy

Optical second-harmonic generation (SHG) spectroscopy has been used to determine the field enhancements from Au nanoparticles on a silicon substrate. Au particles with diameters from 30 to 250 nm have been deposited on a Si substrate passivated by a 1 nm thick surface oxide. The linear optical spectra are dominated by a horizontal plasmon resonance near 1.0 eV, and the experimental spectra are modelled by the island film model in order to extract the linear properties of the metal particles. SHG spectroscopy from this system shows resonances from the metal particles and from the silicon/oxide substrate. By following the evolution of these Si resonances with the size of the Au particles, the field enhancement in the Si surface has been modelled. The effect of the Au particles on SHG at the Si E1 resonance is a combination of charge transfer through the thin oxide that changes the space charge region and an enhancement of the optical field in a thin surface layer of the Si substrate.

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.

Features of E1 resonances in 28Si and 30Si nuclei

The structure of the E1 resonance in 28Si and 30Si isotopes is calculated within the particle–core coupling (PCC) version of the multiparticle shell model using experimental data on direct pickup reactions.

Isovector and isoscalar dipole excitations inBe9andBe10studied with antisymmetrized molecular dynamics

Isovector and isoscalar dipole excitations in $^9$Be and $^{10}$Be are investigated in the framework of antisymmetrized molecular dynamics, in which angular-momentum and parity projections are performed. In the present method, 1p-1h excitations on the ground state and large amplitude $\alpha$-cluster mode are incorporated. The isovector giant dipole resonance (GDR) in $E>20$ MeV shows the two peak structure which is understood by the dipole excitation in the 2$\alpha$ core part with the prolate deformation. Because of valence neutron modes against the $2\alpha$ core, low-energy E1 resonances appear in $E<20$ MeV exhausting about $20\%$ of the Thomas-Reiche-Kuhn sum rule and $10\%$ of the calculated energy-weighted sum. The dipole resonance at $E\sim 15$ MeV in $^{10}$Be can be interpreted as the parity partner of the ground state having a $^6$He+$\alpha$ structure and has the remarkable E1 strength because of coherent contribution of two valence neutrons. The ISD strength for some low-energy resonances are significantly enhanced by the coupling with the $\alpha$-cluster mode. The calculated E1 strength of $^9$Be reasonably describes the global feature of experimental photonuclear cross sections consisting of the low-energy strength in $E<20$ MeV and the GDR in $E>20$ MeV.

Effects of orbital and spin current interference in E1 and M2 nuclear excitations

The interference of contributions from the orbital and spin currents to the E1 and M2 resonances is investigated. The results of the current interference analysis within the shell model are compared with the experimental data.

Photoneutron reactions in astrophysics

Among key problems in nuclear astrophysics, that of obtaining deeper insight into the mechanism of synthesis of chemical elements is of paramount importance. The majority of heavy elements existing in nature are produced in stars via radiative neutron capture in so-called s- and r processes, which are, respectively, slow and fast, in relation to competing β−-decay processes. At the same time, we know 35 neutron-deficient so-called bypassed p-nuclei that lie between 74Se and 196Hg and which cannot originate from the aforementioned s- and r-processes. Their production is possible in (γ, n), (γ, p), or (γ, α) photonuclear reactions. In view of this, data on photoneutron reactions play an important role in predicting and describing processes leading to the production of p-nuclei. Interest in determining cross sections for photoneutron reactions in the threshold energy region, which is of particular importance for astrophysics, has grown substantially in recent years. The use of modern sources of quasimonoenergetic photons obtained in processes of inverse Compton laser-radiation scattering on relativistic electronsmakes it possible to reveal rather interesting special features of respective cross sections, manifestations of pygmy E1 and M1 resonances, or the production of nuclei in isomeric states, on one hand, and to revisit the problem of systematic discrepancies between data on reaction cross sections from experiments of different types, on the other hand. Data obtained on the basis of our new experimental-theoretical approach to evaluating cross sections for partial photoneutron reactions are invoked in considering these problems.

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.

Global R-Matrix Analysis of the 12C(α, γ)16O reaction

The 12C(α,γ)16O reaction, in combination with the triple-alpha process, determines the 12C/16O fraction at the end of stellar helium-burning. This fraction has been shown to strongly influence any subsequent stellar evolution and, due to imprecise knowledge of 12C(α, γ)16O reaction rate, complicates precision tests of stellar models. The 12C(α, γ)16O reaction rate at stellar energies is determined via the R-matrix analysis of experimental data. These data are discussed, and new measurements of the E1-E2 interference sign between the Ec.m. = 2.68-MeV E2 resonance and an underlying E1 strength are described. An E1-E2 asymmetry parameter of a = 0.07 ± 0.05 was extracted from the thick-target γ-ray yields of the narrow resonance at angles of 45° and 135°. The positive sign of a corresponded to constructive interference at forward angles and, further, allowed the interference between the resonance and an E2 background to be identified as constructive below the resonance energy. The E2-E2 interference was then used to evaluate the global SE2 data within the vicinity of the resonance 2.5 < Ec.m. ≤ 3.0 MeV. A global analysis of SE2 data that agreed with the interference scenario has determined the E2-E2 interference scheme of the 4.34-MeV resonance and background, resulting in a value of SE2(300) = 62+9−6keV b.

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.

Microscopic description of the E1 resonance in titanium isotopes

The structure of the dipole resonance in even-even isotopes of titanium is calculated within the particle-core coupling (PCC) version of the shell model using data from spectroscopy of direct (p, d) reactions. The main features of the E1 resonances in 46Ti, 48Ti, and 50Ti are reproduced.

Microscopic description of E1 resonance in 48Ca

The cross sections of photoneutron and photoproton disintegration of the 48Ca nucleus are obtained on the basis of the particle-core coupling version of the multiparticle shell model using the spectroscopy data on direct pickup reactions.

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.

Size-dependent optical properties of Si nanocrystals embedded in amorphous SiO2 measured by spectroscopic ellipsometry

Silicon nanocrystals (Si NCs) with average diameters ⟨dNC⟩=3, 5 and 8 nm embedded in amorphous SiO2 synthesized by Si ion implantation have been characterized by spectroscopic ellipsometry. The dielectric function of the Si NCs has been extracted from SE data using a Bruggeman effective medium approximation and a Gauss–Lorentz oscillator model, taking into account the size dispersion of the nanoparticles. The dielectric function is found to depend strongly on average NC size. Although electronic critical point transitions of bulk silicon are discernible in NCs down to 3 nm diameter, the E1 resonance weakens with decreasing NC size, nearly disappearing for 3 nm diameter, while the dominant E2 transition blueshifts. In addition, a non-bulk-Si-like resonance intermediate between E1 and E2 is observed.

Calculation of a narrow resonance with the LIT method

The possibility to resolve narrow structures in reaction cross sections in calculations with the Lorentz integral transform (LIT) method is studied. To this end we consider a fictitious two-nucleon problem with a low-lying and narrow resonance in the $^3P_1$ nucleon-nucleon partial wave and calculate the corresponding ``deuteron photoabsorption cross section''. In the LIT method the use of continuum wave functions is avoided and one works instead with a localized function \tilde\Psi. In this case study it is investigated how far into the asymptotic region \tilde\Psi has to be determined in order to obtain a precise resolution of the artificially introduced E1 resonance. Comparing with the results of a conventional calculation with explicit neutron-proton continuum wave functions it is shown that the LIT approach leads to an excellent reproduction of the cross section in the resonance region and of further finer cross section details at higher energies. To this end, however, for \tilde\Psi one has to take into account two-nucleon distances up to at least 30 fm.

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.