Gamow-Teller Strength Distributions Research Articles

Spin-isospin excitations

We discuss the spin-isospin response in 3 regions. The Gamow-Teller strength distribution is still not understood. New data from ( n, p) reactions question the conjecture that part of the missing strength was to be found in the high energy tail region. In the quasielastic region rather different sets of data suggest that the attraction in the spin longitudinal channel is gone. In the Δ-region the existing data are consistent with an attractive Δ-hole interaction in the spin longitudinal channel.

Gamow-Teller strength deduced from charge exchange reactions on 54Fe at 300 MeV.

Angular distributions of the {sup 54}Fe({ital p},{ital n}){sup 54}Co and {sup 54}Fe({ital n},{ital p}){sup 54}Mn cross sections have been measured to test the Gamow-Teller sum rule ({ital S}{sub {minus}}{minus}S{sub +}=3(N{minus}Z)) in a case where the Gamow-Teller strength is large for both channels. The results for {ital S}{sub {minus}} and {ital S}{sub +} are compared to several models which have moderate success in describing the data. Large scale shell-model and quasiparticle random-phase-approximation calculations correctly predict the distribution of Gamow-Teller strength but overestimate the total strength. A model that approximates the nuclear surface to be a semi-infinite slab describes the cross sections well in the quasielastic scattering region if 2p-2h correlations are included.

Doorway-state formation and structure in beta -delayed proton decay.

The structure in the /beta/-delayed proton spectra of the precursors /sup 1//sub 66//sup 47/Dy/sup /ital g//, /sup 1//sub 68//sup 49/Er/sup /ital g//, and /sup 1//sub 70//sup 51/Yb/sup /ital g// is analyzed within the framework of a doorway-state model. Indications from /gamma/-ray decay studies, /beta/ strength-function measurements, and calculations of state densities and Gamow-Teller strength distributions, lead us to suggest that some of the observed proton transitions arise from the preequilibrium decay of doorway states populated in /beta//sup +/ decay or electron capture.

Ground state correlation effects on the Gamow-Teller strength distribution in 48Ca

We calculate the Gamow-Teller strength distribution in 48Ca using an extended second RPA which explicitly includes ground state correlations. The following effects are taken into account: (i) the reduction of the 1p1h matrix elements due to the partial occupation of single-particle orbitals, (ii) the modification of the 1p1h Green function in the presence of ground state correlations and 2p2h correlations in the excited states, (iii) renormalization of the external field and (iv) the pure 2p2h response due to unblocking of the ground state. The first three effects largely cancel in the region of the Gamow-Teller resonance while the last two cancel at higher energies. The resulting strength distribution is therefore quite similar to the one obtained previously in second RPA.

Quasi-elastic nucleon transfer and single charge exchange in 48Ti + 42Ca collisions

Single nucleon transfer and single charge exchange have been studied for quasi-elastic collisions of 48Ti and 42Ca at E lab = 240, 300 and 385 MeV. Specific features below 12 MeV excitation energy are displayed in all of the measured particle energy spectra for each channel. These have been accounted for in the case of nucleon transfer by a one-step direct transfer mechanism in which all of the available hole and bound particle states in the donor and acceptor nucleus, respectively, have been included. This core excitation model was extended to single charge exchange (SCX) two-step transfers in which a proton and a neutron are exchanged. The absolute magnitude and distribution of strength in the SCX channel was obtained. The contributions from direct charge exchange via the isovector parts of the NN interaction has also been investigated in a microscopic model and is found to contribute of the order of 1–5% to the charge exchange cross section. The distribution of Fermi and Gamow-Teller strength is discussed.

The ΔTz = + 1 Gamow-Teller excitation of N = 28 isotones

Results of shell-model calculations of Gamow-Teller strength distributions are presented for 50Ti → 50Sc, 52Cr → 52V and 54Fe → 54Mn. The results support the previous shell-model estimates that a large fraction of Gamow-Teller strength is concentrated at low excitation energy in the daughter nuclei. The strength distributions are in fairly good agreement with those of forward-angle cross sections of T 0 + 1 isospin states of the intermediate-energy (p, p') reaction on N = 28 isotones. An estimate of quenching in the T 0 → T 0 + 1 τσ mode transition is attempted. Calculations are performed also for the first 2 + and 4 + parent states in the context of electron capture at the late stages of stellar evolution.

Giant Gamow-Teller resonances in nuclei described with realistic two-body interactions

Using effective interactions based on a Brueckner G-matrix in nuclear matter we calculate Gamow-Teller strength distributions in 48Ca, 90Zr and 208Pb in local density approximation. The conventional RPA theory is extended to include 1p1h- as well as 2p2h-excitations in a consistent way. We obtain a very reasonable description of the centroid energies as well as the spreading width of the resonances in all three nuclei. The tails of the distributions typically contain 30–40% of the 3( N − Z) sum rule and are mostly produced by central spin-isospin correlations. The tensor force only plays a minor role in shifting the στ -strength to higher energies.

Gamow-Teller electron capture strength distributions in stars: Unblocked iron and nickel isotopes

Abstract Results of shell-model calculations of the Gamow-Teller strength distributions for the (n, p) or electron capture direction are presented for 54 Fe → 54 Mn, 56 Fe → 56 Mn, 56 Ni → 56 Co, 60 Fe → 60 Mn, and 64 Fe → 64 Mn . In the cases of 56Fe, 60Fe, and 64Fe calculations were performed on both the ground and first excited states. These and similar transitions are characterized by large amounts of Gamow-Teller strength at low excitation energy in the daughter nuclei. The question of quenching in the (n, p) direction remains open. It is expected that the late stages of presupernova stellar evolution (before neutron shell blocking) will depend on the character and distribution of Gamow-Teller strengths for electron capture transitions such as these.

Measurements of Gamow-Teller strength distributions in masses 13 and 15.

The differenctial cross section and the transverse spin-flip probability have been measured for the dominant transitions in $^{13}\mathrm{C}$(p,n${)\mathrm{}}^{13}$N and $^{15}$N(p,n${)\mathrm{}}^{15}$O at ${\mathrm{E}}_{\mathrm{p}}$=160 MeV. The Gamow-Teller transition strengths deduced from the data show that the major ${(1/2)}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${(3/2)}^{\mathrm{\ensuremath{-}}}$ transitions are strongly quenched relative to the ${(1/2)}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${(1/2)}^{\mathrm{\ensuremath{-}}}$ mirror transitions, in strong disagreement with simple shell-model expectations.

GAMOW-TELLER STRENGTH DISTRIBUTION IN THE β-DECAY OF EXOTIC NUCLEI

We describe experimental results on the Gamow-Teller strength distribution as obtained from the β-decay of exotic nuclei. The most complete results, in light nuclei, confirm the absence of strength noted in (p,n) measurements. In heavier nuclei it is the remote tail of the resonance that is established by β-decay.

The Gamow-Teller strength distribution in 208Bi — a prove of the charge distribution in 208Pb?

Comparison of the calculated Gamow-Teller (GT) strength distribution in 208Bi with the measured (p,n) spectrum suggests evidence for ground state correlations in 208Pb, which also seem to be necessary for an understanding of the charge distribution of lead.

Gamow-Teller strength in theO18(p, n)F18reaction at 135 MeV

The distribution of Gamow-Teller strength in the /sup 18/O(p,n) /sup 18/F reaction was studied at a bombarding energy of 135 MeV. Five 1/sup +/, T = 0 states are identified below E/sub x/ = 7 MeV and a concentration of 1/sup +/ states of presumed T = 1 character is observed between E/sub x/ = 9.5 and 12 MeV. Approximately 82% of the 1/sup +/ strength is concentrated into the ground-state transition and only 5.5% is seen in the T = 1 component. Normalization of the ground-state transition to the known Gamow-Teller matrix element from the analogous beta decay of /sup 18/Ne allows the (p,n) cross sections to be related to the Gamow-Teller strength. The resulting total Gamow-Teller strength observed in the (p,n) reaction is about two-thirds of the minimum value required by the sum rule for a T = 1 nucleus. This result is in reasonable agreement with the total Gamow-Teller strength predicted from a shell-model calculation which uses empirically renormalized single-particle Gamow-Teller matrix elements. The concentration of the T = 0 strength predominantly into the ground state and the observed ratio of T = 1 to T = 0 strength also are consistent with these calculations.

The 54Fe( 3He, t) 54Co reaction at 70 MeV, Gamow-Teller strength

The 54Fe( 3He, t) 54Co reaction has been studied at 70 MeV with an energy resolution around 70 keV (FWHM). The triton spectra are characterized by sharp peaks up to 10 MeV excitation energy superimposed on a continuum. Most of the sharp peaks have a forward-peaked angular distribution and 38 peaks or groups of peaks are found to have an angular distribution corresponding to an angular momentum transfer of 2. Model considerations lead to the conclusion that most of these states are 1 + states. A shell-model calculation with parameters that account for the Gamow-Teller strength distribution in 48Ca- 48Sc divides the β-strength in 54Co in a ratio 5.7:6.8:1.3 for the T = 0, 1 and 2 states. A comparison is made with the 1 + spectrum in 54Mn ( T = 2 states) and a tentative assignment of T = 2 states in 54Co is reached. The cross section has been calculated for the 0 +, 1 + and 3 + states in 54Co assuming a pure ( π f 7 2 −1ν f 7 2 −1 ) configuration finite-range DWBA is used and the conclusions are that the ( 3He, α, t) and ( 3He, d, t) processes give significant contributions to the cross sections.

Gamow-teller strength distribution in the vicinity of A = 90 : (I). Schematic calculation of GT strength in 90, 92, 94Nb

The distributions of Gamow-Teller strength in 90, 92, 94Nb have been calculated utilizing the so-called GT force in the random phase approximation. For 90Nb the calculated distribution is in striking agreement with that part of the 90Zr(p, n) excitation function leading to 1 + excitations in 90Nb and qualitative agreement is demonstrated for data from the reactions 92, 94Zr(p, n). Some 25 % of the GT strength is found to lie well below the structureless giant resonance proposed by Ikeda et al. The implications of this result to the β-decay properties of neutron rich nuclei near A = 90 are discussed.