Isoscalar Probes Research Articles

Studying the Pygmy Dipole Resonances with isoscalar and isovector probes

We present a short review of the properties of the so-called Pygmy Dipole Resonance (PDR) which can be studied with both isovector and isoscalar probes. This is possible due to the particular property of this new mode where the isoscalar and isovector characters are mixed. The use of both probes unveils new features which otherwise will remain hidden. It is of paramount importance the use of a proper description of these modes, taking into account their main features, in order to have a good description of the experimental data.

First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy Dipole Resonance in 68Ni

The excitation of the Pygmy Dipole Resonance (PDR) in the 68Ni nucleus, above the neutron emission threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a 68Ni beam, obtained by the fragmentation of a 70Zn primary beam at INFN-LNS, impinged on a 12C target. The γ-ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The 68Ni isotope as well as other heavy ion fragments were detected using the FARCOS array. The population of the PDR was evidenced by comparing the detected γ-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68Ni channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative γ-ray angular distribution. The measured γ-ray angular distribution confirms the E1 character of the transition. The γ decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error.

Observation of isoscalar and isovector dipole excitations in neutron-rich 20O

The isospin characters of low-energy dipole excitations in neutron-rich unstable nucleus 20O were investigated, for the first time in unstable nuclei. Two spectra obtained from a dominant isovector probe (O20+Au) and a dominant isoscalar probe (O20+α) were compared and analyzed by the distorted-wave Born approximation to extract independently the isovector and isoscalar dipole strengths. Two known 1− states with large isovector dipole strengths at energies of 5.36(5) MeV (11−) and 6.84(7) MeV (12−) were also excited by the isoscalar probe. These two states were found to have different isoscalar dipole strengths, 2.70(32)% (11−) and 0.67(12)% (12−), respectively, in exhaustion of the isoscalar dipole-energy-weighted sum rule. The difference in isoscalar strength indicated that they have different underlying structures.

Pygmy dipole resonance inCe140via inelastic scattering ofO17

The γ decay from the high-lying states of ^140Ce excited via inelastic scattering of ^17O at a bombarding energy of 340 MeV was measured using the high-resolution AGATA-demonstrator array in coincidence with scattered ions detected in two segmented Δ E-E silicon detectors. Angular distributions of scattered ions and emitted γ rays were measured, as well as their differential cross sections. The excitation of 1^− states below the neutron separation energy is similar to the one obtained in reactions with the α isoscalar probe. The comparison between the experimental differential cross sections and the corresponding predictions using the distorted-wave Born approximation allowed us to extract the isoscalar component of identified 1^− pygmy states. For this analysis the form factor obtained by folding microscopically calculated transition densities and optical potentials was used.

Microscopic nuclear form factor for the pygmy dipole resonance

Pygmy dipole resonance can be populated via isoscalar probes due to the strong mixing of isoscalar and isovector components. The physical quantities are usually extracted by the use of reaction theory calculations. There, the most important ingredient is the form factor which contains all the relevant properties of the states we are dealing with. We show that the use of microscopic form factor is required for a better analysis of experimental results.

Nuclear excitation of Pygmy Dipole Resonance

Excitations of Pygmy Dipole Resonance (PDR), generated by a nuclear field, are studied within a semiclassical RPA based model. This is possible since the PDR, present in almost all nuclei with neutron excess, has a strong isoscalar character. Our results show that the employ of isoscalar probes finds its best use at incident energies below 50 MeV/u. The structural splitting of the low-lying E1 strength, found in recent (α,α'γ) experiments, is corroborated by our cross section calculations.

Isospin properties of electric dipole excitations in 48Ca

Two different experimental approaches were combined to study the electric dipole strength in the doubly-magic nucleus 48Ca below the neutron threshold. Real-photon scattering experiments using bremsstrahlung up to 9.9 MeV and nearly mono-energetic linearly polarized photons with energies between 6.6 and 9.51 MeV provided strength distribution and parities, and an (α,α′γ) experiment at Eα=136MeV gave cross sections for an isoscalar probe. The unexpected difference observed in the dipole response is compared to calculations using the first-order random-phase approximation and points to an energy-dependent isospin character. A strong isoscalar state at 7.6 MeV was identified for the first time supporting a recent theoretical prediction.

Effect of large neutron excess in the region of the giant dipole and quadrupole resonance

We study the dipole and quadrupole modes of neutron rich nuclei within the selfconsistent HF + RPA. The presence of neutron skin enhances the mixing of isoscalar and isovector modes. Then it is possible to excite modes of isovector character by an isoscalar probe. In particular we analize the excitation of dipole modes by alpha scattering. The excitation of compressional isoscalar mode is also studied.

Excitation of isovector modes in very neutron-rich nuclei via heavy-ion isoscalar probes

The existence of a neutron skin in neutron-rich nuclei is discussed in connection with the excitation of isovector dipole and quadrupole giant modes via isoscalar nuclear probes. In the case of large neutron excess, important contributions are obtained from the nuclear excitation, which may even become predominant according to proper kinematical conditions. At variance with the usual situation encountered in inelastic processes, constructive interference can be found between nuclear and Coulomb contributions.

GDR excitation by isoscalar probes: a means to determine the neutron-skin thickness

Inelastic scattering of a-particles can excite the isovector giant dipole resonance (GDR) via the Coulomb interaction. In spite of their isoscalar nature alpha-particles carl also excite the GDR via the nuclear interaction due to the difference in the radii of the neutron and proton density distributions. The absolute cross section to excite the GDR in inelastic alpha-scattering is therefore a measure of this radial difference, the so-called neutron-skin thickness. Furthermore, since the GDR strength distribution has a centroid energy which depends on the nuclear radius, these studies, when performed in deformed nuclei, can measure the neutron-skin thickness along both the short and the long axes independently. Results of an experiment performed at KVI at E-alpha = 120 MeV and small scattering angles, including 0 degrees, to determine the neutron-skin thickness in Pb-208, Sn-116, Sn-124, and the deformed Nd-150 are discussed and compared to earlier measurements and theoretical predictions. Future improvements in the experimental set-up are also discussed.

Isospin and macroscopic models for the excitation of giant resonances and other collective states

Recently there has been some confusion over the excitation of “isovector” giant resonances by isoscalar probes. We present a unified theoretical description of macroscopic models that may be used to calculate the excitation of giant resonances by inelastic scattering so as to clarify this and related questions. Particular attention is given to the relative phase of the Coulombic and hadronic contributions. After describing models for the transition densities that are based upon certain sum rules, a folding model and a deformed optical potential model for the transition potentials are discussed. Applications are made to some examples of inelastic α-particle scattering and the two models are compared. Folding yields cross sections for the breathing-mode monopole resonance that are smaller than those from the potential model by almost a factor of two, so that the measured values appear to require more than 100% depletion of the sum rule. It is suggested that this might be due to neglect of density dependence in the effective interaction.