Relativistic RPA plus phonon-coupling analysis of pygmy dipole resonances

The relativistic random-phase approximation (RRPA) plus phonon-coupling (PC) model is applied in the analysis of E1 strength distributions in $^{208}$Pb and $^{132}$Sn, for which data on pygmy dipole resonances (PDR) have recently been reported. The covariant response theory is fully consistent: the effective nuclear interaction NL3 is used both to determine the spectrum of single-nucleon Dirac states, and as the residual interaction which determines the collective phonon states in the relativistic RPA. It is shown that the picture of the PDR as a resonant oscillation of the neutron skin against the isospin saturated proton-neutron core, and with the corresponding RRPA state characterized by a coherent superposition of many neutron particle-hole configurations, remains essentially unchanged when particle-vibration coupling is included. The effect of two-phonon admixtures is a weak fragmentation and a small shift of PDR states to lower excitation energy. Even though the PDR calculated in the extended model space of $ph \otimes$phonon configurations contains sizeable two-phonon admixtures, it basically retains a one-phonon character and its dynamics is not modified by the coupling to low-lying surface vibrations.Comment: 17 pages, 3 figures, 4 table

Nuclear vorticity and the low-energy nuclear response - Towards the neutron drip line

The transition density and current provide valuable insight into the nature of nuclear vibrations. Nuclear vorticity is a quantity related to the transverse transition current. In this work, we study the evolution of the strength distribution, related to density fluctuations, and the vorticity strength distribution, as the neutron drip line is approached. Our results on the isoscalar, natural-parity multipole response of Ni isotopes, obtained by using a self-consistent Skyrme-Hartree-Fock + Continuum RPA model, indicate that, close to the drip line, the low-energy response is dominated by L>1 vortical transitions.Comment: 8 pages, incl. 4 figures; to appear in Phys.Lett.

Spectroscopy of high-lying resonances in 9 Be by the measurement of ( p , p ), ( p , d ), and ( p , α ) reactions with a radioactive 8 Li beam

We measured the 8 Li ( p , p ) 8 Li , 8 Li ( p , d ) 7 Li , and 8 Li ( p , α ) 5 He reactions at low energies using the thick target inverse kinematics method, with a polyethylene [ CH 2 ] n target and a radioactive 8 Li beam available at the Radioactive Ion Beams in Brazil facility of São Paulo. By measuring simultaneously several reaction channels ( p , p ) , ( p , d ) , and ( p , α ) , the still uncertain high-lying resonances of 9 Be , close to the proton threshold, can be studied and their parameters, such as energy, width, and spin parity can be more reliably determined. The experimental excitation functions of the reactions 8 Li ( p , p ) 8 Li , 8 Li ( p , d ) 7 Li , and 8 Li ( p , α ) 5 He were analyzed using the R -matrix theory, which allows us to infer the properties of the resonances. Multichannel R -matrix analysis provides evidence for a significant clustering in the ( p , d ) channel. The experimental data and the multichannel R -matrix analysis will be presented.Fundação de Amparo à Pesquisa do Estado de São Paulo. Brasil. (FAPESP) 2011/15904-7, 2013/22100-7 y 2016/21434-7VI Plan Propio de Investigación de la Universidad de Sevilla. España. (2017–2018

Fragmentation of exotic oxygen isotopes

Abrasion-ablation models and the empirical EPAX parametrization of projectile fragmentation are described. Their cross section predictions are compared to recent data of the fragmentation of secondary beams of neutron-rich, unstable 19,20,21O isotopes at beam energies near 600 MeV/nucleon as well as data for stable 17,18O beams

Search for low lying dipole strength in the neutron rich nucleus 26^{26}Ne

Coulomb excitation of the exotic neutron-rich nucleus $^{26}$Ne on a $^{nat}$Pb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an $^{nat}$Al target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV $^{40}$Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the $^{25}$Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV. The reconstructed $^{26}$Ne angular distribution confirms its E1 nature. A reduced dipole transition probability of B(E1)=0.49$\pm$0.16 $e^2fm^2$ is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is obtained. The results are discussed in terms of a pygmy resonance centered around 9 MeV

A novel transparent charged particle detector for the CPET upgrade at TITAN

The detection of an electron bunch exiting a strong magnetic field can prove challenging due to the small mass of the electron. If placed too far from a solenoid's entrance, a detector outside the magnetic field will be too small to reliably intersect with the exiting electron beam because the light electrons will follow the diverging magnetic field outside the solenoid. The TITAN group at TRIUMF in Vancouver, Canada, has made use of advances in the practice and precision of photochemical machining (PCM) to create a new kind of charge collecting detector called the "mesh detector." The TITAN mesh detector was used to solve the problem of trapped electron detection in the new Cooler PEnning Trap (CPET) currently under development at TITAN. This thin array of wires etched out of a copper plate is a novel, low profile, charge agnostic detector that can be made effectively transparent or opaque at the user's discretion.Comment: 6 Pages. 6 Figures. Submitted to Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipmen

Relativistic Random-Phase Approximation with density-dependent meson-nucleon couplings

The matrix equations of the relativistic random-phase approximation (RRPA) are derived for an effective Lagrangian characterized by density-dependent meson-nucleon vertex functions. The explicit density dependence of the meson-nucleon couplings introduces rearrangement terms in the residual two-body interaction, that are essential for a quantitative description of excited states. Illustrative calculations of the isoscalar monopole, isovector dipole and isoscalar quadrupole response of $^{208}$Pb, are performed in the fully self-consistent RRPA framework, based on effective interactions with a phenomenological density dependence adjusted to nuclear matter and ground-state properties of spherical nuclei. The comparison of the RRPA results on multipole giant resonances with experimental data constrains the parameters that characterize the isoscalar and isovector channel of the density-dependent effective interactions.Comment: RevTeX, 8 eps figures, submitted to Phys. Rev.

Linear Responses in Time-dependent Hartree-Fock-Bogoliubov Method with Gogny Interaction

A numerical method to integrate the time-dependent Hartree-Fock Bogoliubov (TDHFB) equations with Gogny interaction is proposed. The feasibility of the TDHFB code is illustrated by the conservation of the energy, particle numbers, and center-of-mass in the small amplitude vibrations of oxygen 20. The TDHFB code is applied to the isoscalar quadrupole and/or isovector dipole vibrations in the linear (small amplitude) region in oxygen isotopes (masses A = 18,20,22 and 24), titanium isotopes (A = 44,50,52 and 54), neon isotope (A = 26), and magnesium isotopes (A = 24 and 34). The isoscalar quadrupole and isovector dipole strength functions are calculated from the expectation values of the isoscalar quadrupole and isovector dipole moments.Comment: 10 pages, 13 figure