Spin-isospin Excitations Research Articles

Nuclear structure of the spin-isospin excited states inN13studied via the(He3,t)and(He3,tp)reactions at450MeV

Spin-isospin excitations in $^{13}\mathrm{N}$ have been studied by means of the $(^{3}\mathrm{He},t)$ and $(^{3}\mathrm{He},tp)$ reactions at a bombarding energy of $E(^{3}\mathrm{He})=450\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The zero-degree $(^{3}\mathrm{He},t)$ spectrum is found to be similar to those from the $(p,n)$ reactions at intermediate energies, suggesting a simple direct reaction mechanism at $E(^{3}\mathrm{He})=450\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The Gamow-Teller $(\mathrm{GT})$ states with ${J}^{\ensuremath{\pi}}=1∕{2}^{\ensuremath{-}}$ and $3∕{2}^{\ensuremath{-}}$ have been strongly excited at ${\ensuremath{\theta}}_{\mathrm{lab}}=0\ifmmode^\circ\else\textdegree\fi{}$. The angular distribution of cross sections for the $^{13}\mathrm{C}(^{3}\mathrm{He},t)^{13}\mathrm{N}$ reaction and the elastic $^{3}\mathrm{He}$ scattering off $^{13}\mathrm{C}$ were obtained at ${\ensuremath{\theta}}_{\mathrm{lab}}=0\ifmmode^\circ\else\textdegree\fi{}--17\ifmmode^\circ\else\textdegree\fi{}$ and at ${\ensuremath{\theta}}_{\mathrm{lab}}=2\ifmmode^\circ\else\textdegree\fi{}--22\ifmmode^\circ\else\textdegree\fi{}$, respectively. The obtained experimental results were compared with calculations performed in distorted-wave Born approximation. Microscopic structures of the states in $^{13}\mathrm{N}$ have further been studied by observing decay protons in coincidence with high-energy tritons measured at ${\ensuremath{\theta}}_{\mathrm{lab}}=0\ifmmode^\circ\else\textdegree\fi{}$. The branching ratios for proton decay from the $\mathrm{GT}$ states in $^{13}\mathrm{N}$ to the final low-lying $T=0$ states in $^{12}\mathrm{C}$ were obtained. The wave functions of the excited states in $^{13}\mathrm{N}$ are discussed on the basis of obtained experimental data and relevant theoretical calculations.

High-resolution determination of GT strength distributions relevant to the presupernova evolution using the (d,2He) reaction

Spin–isospin excitations of the medium-mass nucleus 58Co were studied using the (n,p)-type (d,2He) charge-exchange reaction at an incident energy Ed=170 MeV and small scattering angles. The achieved energy resolution of 130 keV, full width at half maximum, allowed the extraction of Gamow–Teller strength to 13 peaks at excitation energies lower than 4.1 MeV in the residual nucleus 58Co and is consistent with the results of a multipole-decomposition analysis. The identification of Gamow–Teller transitions is possible by comparison of the measured cross sections with DWBA calculations. The results are used to discuss the relevance of the Gamow–Teller strength distribution at low excitation energy with regard to electron-capture rates used in calculations for the final stage of a massive star.

Relativistic mean field theory with the pion for finite nuclei

We study the possible occurrence of finite pion mean field in finite nuclei in the relativistic mean field (RMF) theory. We calculate explicitly various N = Z closed-shell nuclei with finite pion mean field in the RMF theory with the standard parameter set and the pion-nucleon coupling in free space. The finite pion mean field is introduced by breaking the parity symmetry of intrinsic single-particle states. We demonstrate the actual occurrence and the property of the finite pion mean field.

Gamow-Teller Resonance of 90Zr in a Relativistic Approach

We establish the formalism of nuclear spin-isospin excitations,especially the Gamow-Teller resonance in a fully consistent relativistic random-phase approximation. A relativistic form of the Landau-Migdal parameter g' isadopted as a residual spin-isospin correlation force. In thenon-relativistic limit it reproduces the excitation energy of the giantGamow-Teller resonance state obtained in the non-relativistic model. TheGamow-Teller resonance for finite nuclei is investigated in arelativistic approach for the first time. It is found that the Ikedasum rule of 90Zr is quenched about 8% in the Hartree as well asthe correlated strengths due to the poles of the negative Dirac statesat energies above 1 GeV.

Relativistic Mean-Field Theory with the Pion in Finite Nuclei

We study the possibility of the existence of a finite pion mean field in finite nuclei in the relativistic mean-field (RMF) theory. We are led to conjecture this existence by findings of the essential role of the pion in few-body systems and (p,n) experiments for spin-isospin excitations of medium and heavy nuclei. We carry out explicit calculations for various N=Z closed shell nuclei with a finite pion mean field in the RMF theory with the standard parameter set and pion-nucleon coupling in free space. We find that the finite pion mean field breaks the parity symmetry of the intrinsic single-particle states. We demonstrate the actual existence and elucidate the properties of the finite pion mean field, whose qualitative consequences for various observables are also briefly discussed.

Spin–isospin excitations and [formula omitted] half-lives of medium-mass deformed nuclei

A selfconsistent approach based on a deformed HF +BCS +QRPA method with density-dependent Skyrme forces is used to describe β + -decay properties in even–even deformed proton-rich nuclei. Residual spin–isospin forces are included in the particle–hole and particle–particle channels. The quasiparticle basis contains neutron–neutron and proton–proton pairing correlations in the BCS approach, while neutron–proton pairing interaction is treated as a residual force in QRPA. We discuss the sensitivity of Gamow–Teller strength distributions and β +/ EC half-lives to deformation, pairing and the strength of the particle–particle interaction. The dependence on deformation is also compared to that of spin M1 strength distributions.

Spin–isospin excitations and beta-decay of proton rich nuclei

Spin–isospin excitations and beta-decay of proton rich nuclei

Study of spin–isospin excitations in 11Be via the ( d, 2He) reaction at 270 MeV

Study of spin–isospin excitations in 11Be via the ( d, 2He) reaction at 270 MeV

Electron scattering and nuclear structure at the S-DALINAC

Results on ongoing research of elementary electric and magnetic nuclear excitations at the superconducting Darmstadt electron linear accelerator (S-DALINAC) are presented. In the first part of this lecture electric excitation modes in the continuum are discussed by way of two examples employing the coincident detection of inelastically scattered electrons in an (e,e′x) reaction. The first example is the reaction 48Ca(e,e′n) where the excitation and decay of isoscalar electric monopole and quadrupole giant resonances has been studied. The results on the strength distribution and the partial exhaustion of energy weighted sum rules are compared to those from RPA and second-RPA (SRPA) predictions as well as to those from hadronic reactions of the type (p,p′) and (p,p′n). The second example, a discussion of the 90Zr(e,e′p) reaction and its results, can be viewed as a direct proof that in coincidence reactions of the form (e,e′x) one is able to isolate and identify narrow weakly excited levels — here isobaric analog resonances — buried in the continuum and determine their excitation and decay properties and thus subtle nuclear structure properties. The second part of the lecture is concerned with high-resolution inelastic electron scattering under 180° which is selective with respect to magnetic excitations. Of those, first systematic studies on the hitherto scarcely explored magnetic quadrupole giant resonance which is mainly a spin-isospin excitation are presented. As an example, results for 48Ca and 90Zr, in which a strongly fragmented and quenched M2 strength has been detected, are compared to SRPA and sum rules approaches. Evidence is presented for a new excitation mode, the so called nuclear twist mode, and its parameters are determined. In the third part of the lecture, the possible use of transverse electron scattering form factors at low momentum transfer for an advocated test of in-medium modifications of vector mesons is critically examined, and finally an outlook is given on the nuclear physics research program at the S-DALINAC.

Spin-isospin responses of 71Ga for solar neutrinos studied by 71Ga( 3He,t γ) 71Ge reaction

The charge-exchange 71Ga( 3He,t γ) 71Ge reaction was used to study the spin-isospin (Gamow Teller: GT) excitations relevant to the axial-vector charged-weak response for solar neutrinos. High-lying GT states in 71Ge with excitation energies up to one MeV above the neutron threshold were found to have finite γ-decay branches to low-lying states. The GT strengths for relevant states and absorption rates of solar neutrinos through the 71Ga( ν, e) 71Ge and 71Ga( ν,e γ) 71Ge reactions were derived.

(t, 3He) Reaction; New tool for studying spin-isospin excitations in neutron rich nuclei

Initial experiments were carried out to study spin-isospin transitions in nuclei using (t, 3He) measurements. A triton beam was produced from the breakup reaction ( 4He,t-p) on a 9.25 g/cm 2 Be production target using a 4He beam of 620 MeV. With a beam of 70 enA a yield of 381 MeV tritons with 4.5 × 10 6 particles/s was achieved. The first half of the A1200 beam transport system at NSCL was used to analyze the triton beam. The energy spread at the target position was estimated to be about 23 MeV. The second half of the A1200 was used to analyze the energy of 3He particles from the (t, 3He) reaction. The dispersion matching method was applied to obtain good energy resolution despite the significant energy spread of the incident tritons. The achromatic properties of the A1200 system made it possible to obtain high-resolution (t, 3He) spectra at θ = 0° with a resolution of ΔE E = 0.002 ( ΔE = 780 keV).

Spin-isospin resonances in nuclei

Spin-isospin excitations in nuclei have been investigated via the ( 3He,t) reaction at 450 MeV. The volume integrals of the effective interactions J στ and J τ for the ( 3He,t) reaction at 450 MeV have been empirically determined to be 172±17 MeV-fm 3 and 53±5 MeV-fm 3, respectively. These values are consistent with those obtained for the (p,n) reaction at the same bombarding energy per nucleon. The Gamow-Teller strength can be deduced with the same reliability as the (p,n) reaction at the bombarding energy above 100 MeV. The neutrino absorption cross sections by 71Ga is obtained on basis of the present ( 3He,t) measurement. The new SNU value obtained for 71Ga is 130.3±1.6(stat.)±16.8(syst.). The spin-isospin responses for the Sn isotopes, 116Cd and 100Mo are presented and are discussed in relation with double β-decay. A measurement of decay protons from the spin-isospin resonances in 13N is presented. A new possibility of studying the spin-isospin responses on the β + side via the (t, 3He) reaction using a dispersion matching technique is discussed.

Spin-Isospin excitations studied by polarized beams

The spin-parity J π of the spin dipole resonances (SDR) in 12N and 12B are investigated via the measurements of polarization observables, the transverse polarization transfer conefficient D NN for the ( p , n ) reaction at 197 and 295 MeV and the tensor analyzing power A xx for the ( d, 2 He ) reaction at 270 MeV. The polarization observables, D NN and A xx for the peak at 4.5 MeV are consistent with the DWIA prediction with 2 − but those for the peak at 7.5 MeV contradict the predictions with an expected J π = 1 −. Neither polarization observables could detect any concentration of 0 − strength. The usefulness of these spin observables in identifying J π is shown.

Studies of spin-isospin excitations with the (n,p) reaction

Double-differential cross sections of the 12C, 54,56Fe, 90Zr and 208Pb(n,p) reactions have been measured at 100 MeV. The proton spectra have been interpreted in terms of direct and multistep reactions. The direct part has been decomposed into multipole components, using a technique based on sample angular distributions calculated within the DWBA. Calculations of the full spectra have also been performed with a statistical multistep direct reaction model.

Direct proton decay from the Gamow-Teller resonance in 208Bi.

Spin-isospin excitations in {sup 208}Bi have been investigated using the {sup 208}Pb ({sup 3}He,{ital t}){sup 208}Bi reaction at near {theta}{approx}0{degree} at {ital E}({sup 3}He)=450 MeV. The microscopic structure of the Gamow-Teller resonance (GTR), the isobaric analog state (IAS), and the spin-flip dipole ({Delta}{ital L}=1) resonance (SDR) in {sup 208}Bi has been studied by observing their direct proton decays to the low-lying neutron-hole states in {sup 207}Pb. Decay protons were measured at backward angles in coincidence with tritons detected at and near 0{degree}. The total branching ratio for proton decay from the GTR is determined to be only 4.9{plus_minus}1.3%. The total branching ratio for proton decay from the SDR amounts to 14.1{plus_minus}4.2%. The deduced total widths as well as the total and partial proton escape widths of the GRR and IAS are found to be in reasonable agreement with recent theoretical estimates obtained in the framework of the continuum Tamm-Dancoff approximation.

Spin-isospin excitations in nuclei via ( 3He,t) reactions at 450 MeV

Spin-isospin excitations in nuclei have been studied by means of the ( 3He,t) reactions at 450 MeV. The Gamow-Teller (GT) resonances are found to be strongly excited. A fine structure of the giant Gamow Teller resonances in medium-heavy nuclei is observed with an energy resolution of 130 keV. The volume integrals of the effective interactions J στ and J τ have been determined experimentally. The importance of high resolution observation of the ( 3He,t) reactions at intermediate energies, both, to study the double β decay processes and to measure the detection sensitivities for solar neutrino experiments is discussed.

Spin-isospin excitation in sd-shell nuclei studied by the (d, 2He) reaction at E d = 270 MeV

The (d, 2He) reactions on 24Mg, 26Mg and 28Si were studied at E d = 270 MeV. The 0 o cross sections obtained for the 1 + states from the 24Mg, 28Si(d, 2He) reactions show a good correlation with those from the mirror (p,n) reactions. Four peaks were identified in the 26Mg(d, 2He) 26Ne reactions as being due to the 1 + excitation. The B(GT +) values for these transitions were estimated and compared with the shell model prediction.

( 3He,t) Charge-Exchange Reactions at [formula omitted], θ = 0°

Spin-isospin excitations in nuclei have been studied by means of the ( 3He,t) reactions at 450 MeV on the targets 9Be, nat C, 28Si, 58Ni, 62Ni, 90Zr, 118Sn, 120Sn, 124Sn, and 154Sm. The new spectrometer “Grand Raiden” was employed for the ( 3He,t) study for the first time. Observed ( 3He,t) spectra at 0° show remarkable similarity with those from the (p,n) results at intermediate energies, suggesting a simple direct reaction mechanism for the ( 3He,t) reaction at 450 MeV. The Gamow-Teller (GT) resonances and the spin-flip ΔL = 1 resonances are excited very strongly. A fine structure of the GT resonances in medium-heavy nuclei is observed with an energy resolution of 210 keV. It is demonstrated that the ( 3He,t) reaction at 450 MeV is a powerful tool to study the spin-isospin excitations in nuclei.

(d,2He) reactions at Ed=260 MeV as a possible probe to nuclear spin-isospin excitation.

The (d${,}^{2}$He) reactions on light nuclei were studied at ${\mathit{E}}_{\mathit{d}}$=260 MeV with a large solid-angle, wide momentum-acceptance spectrometer. The data indicated the direct one-step nature of the reaction at this energy. Observed excitation-energy spectra for N=Z nuclei compared well with those from the (p,n) reactions at similar momentum transfer. The peaks corresponding to the ${0}^{+}$\ensuremath{\rightarrow}${1}^{+}$ transitions showed angle dependence characteristic of \ensuremath{\Delta}L=0. The 0\ifmmode^\circ\else\textdegree\fi{} (d${,}^{2}$He) cross sections were found to be proportional to the Gamow-Teller \ensuremath{\beta}-decay strengths.

Nuclear spin and isospin excitations

A review is given of our present knowledge of collective spin-isospin excitations in nuclei. Most of this knowledge comes from intermediate-energy charge-exchange reactions and from inelastic electron- and proton-scattering experiments. The nuclear-spin dynamics is governed by the spin-isospin-dependent two-nucleon interaction in the medium. This interaction gives rise to collective spin modes such as the giant Gamow-Teller resonances. An interesting phenomenon is that the measured total Gamow-Teller transition strength in the resonance region is much less than a model-independent sum rule predicts. Two physically different mechanisms have been discussed to explain this so-called quenching of the total Gamow-Teller strength: coupling to subnuclear degrees of freedom in the form of $\ensuremath{\Delta}$-isobar excitation and ordinary nuclear configuration mixing. Both detailed nuclear structure calculations and extensive analyses of the scattering data suggest that the nuclear configuration mixing effect is the more important quenching mechanism, although subnuclear degrees of freedom cannot be ruled out. The quenching phenomenon occurs for nuclear-spin excitations at low excitation energies ($\ensuremath{\omega}\ensuremath{\sim}10\ensuremath{-}20$ MeV) and small-momentum transfers ($q\ensuremath{\le}0.5$ ${\mathrm{fm}}^{\ensuremath{-}1}$). A completely opposite effect is anticipated in the high ($\ensuremath{\omega}$, $q$)-transfer region ($0\ensuremath{\le}\ensuremath{\omega}\ensuremath{\le}500$ MeV, $0.5\ensuremath{\le}q\ensuremath{\le}3$ ${\mathrm{fm}}^{\ensuremath{-}1}$). The nuclear spin-isospin response might be enhanced due to the attractive pion field inside the nucleus. Charge-exchange reactions at GeV incident energies have been used to study the quasifree peak region and the $\ensuremath{\Delta}$-resonance region. An interesting result of these experiments is that the $\ensuremath{\Delta}$ excitation in the nucleus is shifted downwards in energy relative to the $\ensuremath{\Delta}$ excitation of the free proton. The physical origin of this shift is discussed, and it is shown that it may be related to the energy-dependent, attractive one-pion exchange interaction in the medium.