Science News

Coupling charge-exchange vibrations to nucleons in a relativistic framework: effect on Gamow-Teller transitions and beta-decay half-lives

The nuclear response theory for isospin-transfer modes in the relativistic particle-vibration coupling framework is extended to include coupling of single nucleons to isospin-flip (charge-exchange) phonons, in addition to the usual neutral vibrations. This new coupling introduces dynamical pion and rho-meson exchange, beyond the Hartree-Fock approximation, up to infinite order. We investigate the impact of this new mechanism on the Gamow-Teller response of a few doubly-magic neutron-rich nuclei, namely 48Ca, 78Ni, 132Sn and 208Pb. It is found that the coupling to isospin- flip vibrations can have a non negligible impact on the strength distribution and quenching of the Gamow-Teller resonance, globally improving the agreement with the experimental data. The corresponding beta-decay half-lives of 78Ni and 132Sn are also calculated, and found to be decreased by the inclusion of the new phonons. Overall the lifetimes are very close to the experimental data using unquenched value of the weak axial coupling constant gA

Finite-temperature relativistic nuclear field theory: an application to the dipole response

Nuclear response theory beyond the one-loop approximation is formulated for the case of finite temperature. For this purpose, the time blocking approximation to the time-dependent part of the in-medium nucleon-nucleon interaction amplitude is adopted for the thermal (imaginary-time) Green's function formalism. We found that introducing a soft blocking, instead of a sharp blocking at zero temperature, brings the Bethe-Salpeter equation to a single frequency variable equation also at finite temperatures. The method is implemented self-consistently in the framework of Quantum Hadrodynamics and designed to connect the high-energy scale of heavy mesons and the low-energy domain of nuclear medium polarization effects in a parameter-free way.  In this framework, we investigate the temperature dependence of dipole spectra in the even-even nuclei 48Ca, 120Sn and 132Sn with a special focus on the giant dipole resonance's width problem and on the low-energy dipole strength distribution.

Soft modes in the proton-neutron pairing channel as precursors of deuteron condensate in N=Z nuclei

Relativistic nuclear response theory is formulated for the proton-neutron pairing, or deuteron transfer, channel. The approach is based on the meson-nucleon Lagrangian of Quantum Hadrodynamics (QHD) and advances the relativistic field theory to connect consistently the high-energy scale of heavy mesons, the medium-energy range of the pion and the low-energy domain of emergent collective vibrations (phonons) in a parameter-free way. Mesons and phonons build up the in-medium nucleon-nucleon interaction in spin-isospin transfer channels, in particular, the phonon-exchange part takes care of the leading-order retardation effects. In this framework, we explore Jπ = 0and Jπ = 1channels of the nuclear response to the proton-neutron pair removal and addition in 56Ni and 100Sn with a special focus on the lowest (soft) modes as precursors of deuteron condensate and candidates for being the mediators of the proton-neutron pairing interaction. 

E. Litvinova, C. Robin, and I.A. Egorova, Phys. Lett. B 776, 72 (2018). 

Observation of the First Isovector Giant Monopole Resonance via the 28-Si(10-Be,10-B[1.74 MeV]) Reaction at 100 AMeV

The charge-exchange reaction (10-Be,10-B[1.74 MeV]) at 100 AMeV was presented as a new probe, which is capable of isolating the isovector ( T = 1) non-spin-transfer ( S = 0) nuclear response. The N=Z neutron deficient nucleus 28-Si was chosen for this study at the National Superconducting Cyclotron Laboratory (NSCL). A secondary 10-Be beam produced by fast fragmentation of 18-O nuclei at the NSCL Coupled Cyclotron Facility, the dispersion-matching technique with the S800 magnetic spectrometer, and the high-precision gamma-ray tracking with the Gamma Ray Energy Tracking Array (GRETINA) were used to obtain a clean S = 0 excitation-energy spectrum in 28-Al. Monopole and dipole contributions were extracted through a multipole decomposition analysis, and, thereby, the isovector giant dipole (IVGDR) and the isovector giant monopole (IVGMR) resonances were identified. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing a stringent test of theoretical approaches at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. Fig. reprinted from [PRL118, 172501 (2017)] shows the extracted distributions compared with theoretical calculations based on the normal-modes (NM) formalism and on our strength functions computed within the proton-neutron relativistic time blocking approximation (pn-RTBA). One can see that the latter describes very reasonably the shapes of the experimental cross sections, which can not be achieved within the proton-neutron relativistic random phase approximation (pn-RRPA). This emphasizes the importance of long-range correlations for isospin-flip resonances.

M. Scott, R. G. T. Zegers, R. Almus, Sam M. Austin , D. Bazin , B. A. Brown, C. Campbell , A. Gade, M. Bowry, S. Galès, U. Garg, M. N. Harakeh, E. Kwan, C. Langer, C. Loelius, S. Lipschutz, E. Litvinova, E. Lunderberg, C. Morse, S. Noji, G. Perdikakis, T. Redpath, C. Robin, H. Sakai, Y. Sasamoto, M. Sasano, C. Sullivan, J. A. Tostevin, T. Uesaka, D. Weisshaar, Phys. Rev. Lett. 118, 172501 (2017).

Isovector excitations in 100-Nb and their decays by neutron emission studied via the 100-Mo(t,3He + n) reaction at 115 MeV/u

Another recent application of the proton-neutron relativistic quasiparticle time blocking approximation (pn-RQTBA) is the spin-isospin excitation spectrum in 100-Nb, which was studied by the charge-exchange 100-Mo(t, 3-He) reaction at NSCL.  The neutron decays from the excited 100-Nb were also observed. The statistical and direct decay branches were both identified in the spectra. The upper limit for the direct-decay branching ratio was determined to be 20 +/- 6%, which revealed the decay predominantly happened via the statistical process. The Figure presents the isovector spin monopole (IVSM) resonance  extracted from this measurement, together with the theoretical pn-RQRPA and pn-RQTBA calculations in the form of histograms consistent with the experimental energy resolution. The IVSM resonance is the overtone of the Gamow-Teller resonance: their operators differ by the radial formfactors only. In practice, the IVSM resonance reflects the leading-order effect of the momentum transfer dependence of the spin-isospin response dominated by the GTR.  It has been shown, in particular, that the IVSM resonance can absorb a few percent of the total Ikeda sum rule and, thereby, contribute to the quenching of the GTR. Thus, understanding the formation of the IVSM resonance is of a great importance because the identification of microscopic mechanisms of the GTR's  quenching remains one of the most difficult unsolved problems in the nuclear structure physics. One can see from the Figure that both pn-RQRPA and pn-RQTBA give reasonable strength distributions, which means that for the IVSM excitations the long-range correlations are most likely less important than for the non-spin-flip isovector monopole resonance. The discrepancies between the data and calculations can be, therefore, attributed to possible deformation effects or to deficiencies of the static interaction, such as the overall simplicity of its one-boson-exchange character and the absence of the delta meson. This points to further work in those directions. 

K. Miki, R.G.T. Zegers, Sam M. Austin, D. Bazin, B.A. Brown, A.C. Dombosa, R.K. Grzywacz, M.N. Harakeh, E. Kwan, S.N. Liddick, S. Lipschitz, E. Litvinova, M. Madurga, M.T. Mustonen, W.J. Ong, S.V. Paulauskas, G. Perdikakis, J. Pereira, W.A. Peters, C. Robin, M. Scott, A. Spyrou, C. Sullivan, R. Titus, Phys. Lett. B. 769, 339 (2017).

Spin-orbit splittings of neutron states in N=20 isotones 
from covariant density functionals and their extensions

Spin-orbit splitting is an essential ingredient for our understanding of the shell structure in nuclei. One of the most important advantages of relativistic mean-field (RMF) models in nuclear physics is the fact that the large spin-orbit (SO) potential emerges automatically from the inclusion of Lorentz-scalar and -vector potentials in the Dirac equation. It is therefore of great importance to compare the results of such models with experimental data. We investigate the size of 
2p and 1f splittings for the isotone chain 40Ca38Ar36S, and 34Si in the framework of various relativistic and no-nrelativistic density functionals. They are compared with the results of nonrelativistic models and with recent experimental data.

Nuclear response theory for spin-isospin excitations in a relativistic quasiparticle-phonon coupling framework

A new theoretical approach to spin-isospin excitations in open-shell nuclei is presented. The developed method is based on the relativistic meson-exchange nuclear Lagrangian of Quantum Hadrodynamics and extends the response theory for superfluid nuclear systems beyond relativistic quasiparticle random phase approximation in the proton-neutron channel (pn-RQRPA). The coupling between quasiparticle degrees of freedom and collective vibrations (phonons) introduces a time-dependent effective interaction, in addition to the exchange of pion and  -meson taken into account without retardation. The time-dependent contributions are treated in the resonant time-blocking approximation, in analogy to the previously developed relativistic quasiparticle time-blocking approximation (RQTBA) in the neutral (non-isospin-flip) channel. The new method is called proton-neutron RQTBA (pn-RQTBA) and is applied to the Gamow-Teller resonance in a chain of neutron-rich nickel isotopes 68-78Ni. A strong fragmentation of the resonance along with quenching of the strength, as compared to pn-RQRPA, is obtained. Based on the calculated strength distribution, beta-decay half-lives of the considered isotopes are computed and compared to pn-RQRPA half-lives and to experimental data. It is shown that a considerable improvement of the half-life description is obtained in pn-RQTBA because of the spreading effects, which bring the lifetimes to a very good quantitative agreement with data.