Gamow-Teller Response Research Articles

Coupling charge-exchange vibrations to nucleons in a relativistic framework: Effect on Gamow-Teller transitions and β-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 $^{48}$Ca, $^{78}$Ni, $^{132}$Sn and $^{208}$Pb. It is found that the coupling to isospin-flip vibrations can have a non negligible impact on the strength distribution and quenching of the giant resonance, globally improving the agreement with the experimental data. The corresponding beta-decay half-lives of $^{78}$Ni and $^{132}$Sn are also calculated, and found to be decreased by a factor $\sim 2$ by the inclusion of the new phonons.

Gamow-Teller response in the configuration space of a density-functional-theory–rooted no-core configuration-interaction model

The atomic nucleus is a unique laboratory to study fundamental aspects of the electroweak interaction. This includes a question concerning in medium renormalization of the axial-vector current, which still lacks satisfactory explanation. Study of spin-isospin or Gamow-Teller (GT) response may provide valuable information on both the quenching of the axial-vector coupling constant as well as on nuclear structure and nuclear astrophysics. We have performed a seminal calculation of the GT response by using the no-core-configuration-interaction approach rooted on multi-reference density functional theory (DFT-NCCI). The model treats properly isospin and rotational symmetries and can be applied to calculate both the nuclear spectra and transition rates in atomic nuclei, irrespectively of their mass and particle-number parity. The method is applied to compute the GT strength distribution in selected $N\approx Z$ nuclei including the $p$-shell $^8$Li and $^8$Be nuclei and the $sd$-shell well-deformed nucleus $^{24}$Mg. In order to demonstrate a flexibility of the approach we present also a calculation of the superallowed GT beta decay in doubly-magic spherical $^{100}$Sn and the low-spin spectrum in $^{100}$In. It is demonstrated that the DFT-NCCI model is capable to capture the GT response satisfactorily well by using relatively small configuration space exhausting simultaneously the GT sum rule. The model, due to its flexibility and broad range of applicability, may either serve as a complement or even as an alternative to other theoretical approaches including the conventional nuclear shell model.

Theoretical investigation of two-particle two-hole effects on spin-isospin excitations through charge-exchange reactions

Coherent one-particle one-hole (1p1h) excitations have given us effective insights into general nuclear excitations. However, the two-particle two-hole (2p2h) excitation beyond 1p1h is now recognized as critical for the proper description of experimental data of various nuclear responses. The spin-flip charge-exchange reactions $^{48}{\rm Ca}(p,n)^{48}{\rm Sc}$ are investigated to clarify the role of the 2p2h effect on their cross sections. The Fermi transition of $^{48}{\rm Ca}$ via the $(p,n)$ reaction is also investigated in order to demonstrate our framework. The transition density is calculated microscopically with the second Tamm-Dancoff approximation, and the distorted-wave Born approximation is employed to describe the reaction process. A phenomenological one-range Gaussian interaction is used to prepare the form factor. For the Fermi transition, our approach describes the experimental behavior of the cross section better than the Lane model, which is the conventional method. For spin-flip excitations including the GT transition, the 2p2h effect decreases the magnitude of the cross section and does not change the shape of the angular distribution. The $\Delta l=2$ transition of the present reaction is found to play a negligible role. The 2p2h effect will not change the angular-distributed cross section of spin-flip responses. This is because the transition density of the Gamow-Teller response, the leading contribution to the cross section, is not significantly varied by the 2p2h effect.

Electromagnetic dipole and Gamow-Teller responses of even and odd 90-94 40Zr isotopes in QRPA calculations with the D1M Gogny force

In this paper we present theoretical results on the dipole response in the proton spin-saturated 90-94Zr isotopes. The electric and magnetic dipole excitations are obtained in Hartree-Fock-Bogolyubov plus Quasi-particle Random Phase Approximation (QRPA) calculations performed with the D1M Gogny force. A pnQRPA charge exchange code is used to study the Gamow-Teller response. The results on the pygmy, the giant dipole resonances as well as those on the magnetic nuclear spin-flip excitation and the Gamow-Teller transitions are compared with available experimental or theoretical information. In our approach, the proton pairing plays a role in the phonon excitations, in particular in the M1 nuclear spin-flip resonance.

Gamow-Teller response and its spreading mechanism in doubly magic nuclei

The scope of the paper is to apply a state-of-the-art beyond mean-field model to the description of the Gamow-Teller response in atomic nuclei. This topic recently attracted considerable renewed interest, due, in particular, to the possibility of performing experiments in unstable nuclei. We study the cases of $^{48}\mathrm{Ca},\phantom{\rule{0.16em}{0ex}}^{78}\mathrm{Ni},\phantom{\rule{0.16em}{0ex}}^{132}\mathrm{Sn}$, and $^{208}\mathrm{Pb}$. Our model is based on a fully self-consistent Skyrme Hartree-Fock plus random phase approximation. The same Skyrme interaction is used to calculate the coupling between particles and vibrations, which leads to the mixing of the Gamow-Teller resonance with a set of doorway states and to its fragmentation. We compare our results with available experimental data. The microscopic coupling mechanism is also discussed in some detail.

Gamow-Teller transitions in theA=40isoquintet of relevance for neutrino captures inAr40

Background: The Gamow-Teller response of $^{40}\mathrm{Ar}$ is important for the use of liquid argon as a medium for neutrino detection. An ambiguity about the Gamow-Teller strength for the excitation of ${1}^{+}$ states at 2290 and 2730 keV in $^{40}\mathrm{K}$ results in a significant uncertainty for neutrino capture rates. This ambiguity is caused by the large discrepancy observed between strengths extracted from $^{40}\mathrm{Ar}$($p$, $n$) charge-exchange data and the transition strengths for the analog transitions studied in the $\ensuremath{\beta}$ decay of $^{40}\mathrm{Ti}$.Purpose: This study was aimed at resolving the ambiguity between the results from the $^{40}\mathrm{Ar}$($p$, $n$) charge-exchange and $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data.Method: Shell-model calculations in the $sd\text{\ensuremath{-}}pf$ shell with a new interaction (WBMB-C) were used to study differences between the structure of the transitions from $^{40}\mathrm{Ar}$ and $^{40}\mathrm{Ti}$. Distorted-wave Born approximation reaction calculations were used to investigate uncertainties in the extraction of Gamow-Teller strength from the $^{40}\mathrm{Ar}$($p$, $n$) data. New high-resolution data for the $^{40}\mathrm{Ar}$($^{3}\mathrm{He}$,$t$) reaction were used to gain further insight into the charge-exchange reaction mechanism and to provide more information to test the validity of the shell-model calculations.Results: The shell-model calculations showed that interference between amplitudes associated with $pf$ and $sd$ components to the low-lying Gamow-Teller transitions, in combination with a difference in Coulomb energy shifts for $^{40}\mathrm{Ar}$ and $^{40}\mathrm{Ti}$, can produce the differences on the scale of those observed between the $^{40}\mathrm{Ar}$ charge-exchange and $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data. In combination with the difference in nuclear penetrability of the ($p$, $n$) and ($^{3}\mathrm{He}$,$t$) probes, the different contributions from amplitudes associated with transitions in the $pf$ and $sd$ shells are likely also responsible for the observed discrepancy between the ratio of the cross sections for the low-lying ${1}^{+}$ states in the $^{40}\mathrm{Ar}$($p$, $n$) and $^{40}\mathrm{Ar}$($^{3}\mathrm{He}$,$t$) data.Conclusions: On the basis of this study, it is recommended to use Gamow-Teller strengths extracted from the $^{40}\mathrm{Ar}$($p$, $n$) data, not the $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data, for the calculation of neutrino capture rates. Further theoretical studies are required to achieve a consistent quantitative description for the energy differences between low-lying ${1}^{+}$ states in $^{40}\mathrm{K}$ and $^{40}\mathrm{Sc}$ and the experimentally observed Gamow-Teller strengths.

Gamow-Teller response in deformed even and odd neutron-rich Zr and Mo isotopes

Beta-decay properties of neutron-rich Zr and Mo isotopes are investigated within a microscopic theoretical approach based on the proton-neutron quasiparticle random-phase approximation. The underlying mean field is described self-consistently from deformed Skyrme Hartree-Fock calculations with pairing correlations. Residual separable particle-hole and particle-particle forces are also included in the formalism. The structural evolution in these isotopic chains including both even and odd isotopes is analyzed in terms of the equilibrium deformed shapes. Gamow-Teller strength distributions, beta-decay half-lives, and beta-delayed neutron-emission probabilities are studied, stressing their relevance to describe the path of the nucleosynthesis rapid neutron capture process.

Benchmarking nuclear models for Gamow–Teller response

A comparative study of the nuclear Gamow–Teller response (GTR) within conceptually different state-of-the-art approaches is presented. Three nuclear microscopic models are considered: (i) the recently developed charge-exchange relativistic time blocking approximation (RTBA) based on the covariant density functional theory, (ii) the shell model (SM) with an extended “jj77” model space and (iii) the non-relativistic quasiparticle random-phase approximation (QRPA) with a Brueckner G-matrix effective interaction. We study the physics cases where two or all three of these models can be applied. The Gamow–Teller response functions are calculated for 208Pb, 132Sn and 78Ni within both RTBA and QRPA. The strengths obtained for 208Pb are compared to data that enable a firm model benchmarking. For the nucleus 132Sn, also SM calculations are performed within the model space truncated at the level of a particle–hole (ph) coupled to vibration configurations. This allows a consistent comparison to the RTBA where ph⊗phonon coupling is responsible for the spreading width and considerable quenching of the GTR. Differences between the models and perspectives of their future developments are discussed.

Comparison of Gamow–Teller strengths in the random phase approximation

The Gamow–Teller response is astrophysically important for a number of nuclides, particularly around iron. The random phase approximation (RPA) is an efficient way to generate strength distributions. In order to better understand both theoretical systematics and uncertainties, we compare the Gamow–Teller strength distributions for a suite of nuclides and for a suite of interactions, including semi-realistic interactions in the 1p-0f space with the RPA and a separable multi-shell interaction in the quasi-particle RPA. We also compare with experimental results where available.

Spin-Flip Response Function of Finite Nuclei in a Fully Self-Consistent RPA Approach

The spin-flip response of finite nuclei is studied in a fully self-consistent Hartree—Fock plus random phase approximation (RPA) within the SLy5 Skyrme parameter set. The effect of J2 terms and spin-orbit interaction on the response function are discussed. The J2 terms are included/excluded at both the mean field and the excited state for self-consistency The numerical results show that the J2 terms give a strong effect on the properties of the magnetic dipole response and the Gamow—Teller response. The spin-orbit residual interaction has almost no contribution to the spin-flip response according to our present study.

Spin-isospin responses of nuclei — Gamow-Teller response functions and pionic response functions in the quasi-elastic scattering region

Recent elaborate experiments of ( p , n ) and ( n , p ) reactions provide detailed information about spin-isospin responses of nuclei at two different energy-momentum regions. One is the Gamow-Teller (GT) response function at small energy and momentum transfers ( ω , q ) , and the other is the pionic response functions in the quasi-elastic scattering (QES) region at relatively large q ( ≈ 1.7 ) fm −1 . The measured GT strength distributions and isovector spin-longitudinal cross sections ID q in the QES region are analyzed in the same theoretical framework of the distorted wave impulse approximation with the continuum random phase approximation (CRPA) including the Δ-isobar degree of freedom. As the effective interactions for the CRPA, the π + ρ + g ′ model interactions are utilized. The Landau-Migdal parameters g N N ′ and g N Δ ′ , which specify the effective interactions, are determined for the two different ( ω , q ) regions.

Determination of the Gamow-Teller strength distribution from the odd-odd nucleusV50measured throughV50(d,He2)Ti50and astrophysical implications

Cross sections for the charge-exchange reaction $(d,^{2}\mathrm{He})$ on $^{50}\mathrm{V}$, the only odd-odd stable nucleus in the $pf$ shell, have been measured at an incident energy of 171 MeV. The two correlated protons of the unbound $^{2}\mathrm{He}$ system were both momentum analyzed and detected by the same spectrometer and detector. Measurements have been carried out at forward center-of-mass angles up to 6\ifmmode^\circ\else\textdegree\fi{}. The $\mathrm{GT}{}^{+}$ strength distribution has been extracted up to about 12 MeV of excitation energy. Because charge-exchange reactions from $^{50}\mathrm{V}$ provide a unique opportunity to probe Gamow-Teller response from an odd-odd nucleus of the $pf$ shell, the present study serves as important test case for model calculations and astrophysical processes that proceed through GT-type transitions. Consequently, the experimental result is compared with a large-scale shell-model calculation, and the implications for electron-capture processes in presupernova environments are discussed.

Symmetry violations in nuclear Hamiltonians and their consequences for electroweak decays

We discuss the results of the treatment of nuclear Hamiltonians in terms of collective and intrinsic variables. The BRST method is adapted to identify spurious and physical sectors of the wave functions and operators. Counterterms are added to the Hamiltonian to enforce the symmetries broken by the single-particle field and/or by the residual two-body interactions. We focus on the study of Fermi and Gamow-Teller transitions, with reference to the nuclear double-beta-decay processes, and on the study of vector operators (λπ = 1−) with reference to (μ, e−) conversion processes. We address the following aspects of the problem: (a) Isospin symmetry and the calculation of 0+ and 1+ states; sensitivity of the Fermi and Gamow-Teller response in double-beta-decay processes; (b) Restoration of the translational and Galilean invariance of the nuclear Hamiltonians and the calculation of Iπ = 1− states; sensitivity of the nuclear response to the spurious center-of-mass motion and μ-electron lepton-flavor-violation processes.

The role of three-particle-three-hole excitations in the Gamow-Teller nuclear response

Besides exploring the Gamow-Teller response to the 2p2h excitations we also investigate the role of the 3p3h excitations in the final state, which become involved as soon as the ground-state correlations are considered. Both type of correlations in the final state are treated on the same footing. Particular emphasis is put on the conservation of the Ikeda sum rule in the framework of different approximations for the nuclear response. As an example we calculate the strength distribution in 48Ca. We find that the combined effect of the ground state correlations and the 3p3h excitations produces a relatively small quenching of the low-lying strength compared with that generated by the 1p1h-2p2h coupling.