Gamow-Teller Research Articles

Total Gamow-Teller strength, ground-state correlations and sum rules in QRPA with neutron-proton pairing

Abstract The sum of the Gamow-Teller (GT) strength in 1p0f shell nuclei is investigated by the Quasiparticle Random Phase Approximation (QRPA), where BCS and RPA ground-state correlations are included with proton-proton and neutron-neutron but in addition also with neutron-proton (np) pairing correlations. Two sums of GT strength with and without closure for the intermediate excited states are derived including different approaches for the ground-state correlations. The sum of the GT strength is compared to the available experimental data and other theoretical calculations. The total GT strength in the usual BCS scheme is reduced by additional neutron-proton (np) pairing correlations. The usual RPA ground-state correlations reduce the total GT strength estimated from the BCS approximation while RPA with np pairing correlations yields an appreciably smaller reduction. Finally, ground-state correlations beyond RPA are also shown to reduce the total GT strength.

Gamow-Teller strength in 60,62,64Ni(n,p) reactions at 198 MeV.

Measurements of the (n,p) reaction cross section on $^{60,62,64}\mathrm{Ni}$ have been made at 198 MeV for ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=0--20\ifmmode^\circ\else\textdegree\fi{} in 4\ifmmode^\circ\else\textdegree\fi{} steps. Multipole analyses have been performed on the resulting spectra and Gamow-Teller (GT) strength distributions have been obtained. These GT distributions have been found to occur as strong peaks at low excitation energy in the final Co nuclei and extend weakly upwards to 30 MeV of excitation. Comparisons of the data with renormalized shell-model calculations show reasonable agreement at low excitation, though the calculations predict very little strength at excitation energies above about 6 MeV.

Fragmentation and splitting of Gamow-Teller resonances in Sn(3He,t)Sb charge-exchange reactions, A=112-124.

Fragmentation and splitting of the Gamow-Teller (GT) strength has been observed in a systematic study of the ${(}^{3}$He,t) charge-exchange reaction at E${(}^{3}$He)=200 MeV over the entire range of stable Sn isotopes. Triton energy spectra were observed with a high-resolution magnetic spectrometer at angles near \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} where \ensuremath{\Delta}L=0 transitions are enhanced. Excitation energies, widths, 0\ifmmode^\circ\else\textdegree\fi{} cross sections, and strengths B(GT) were determined. A theoretically predicted configuration splitting of the main Gamow-Teller component into two components, expected to be dominant near A=118 at the onset of the filling of the 1${\mathit{h}}_{11/2}$ neutron orbital, could not be observed. This may be due to the fact that the total widths of the resonances of 5--6 MeV exceed the predicted splitting. A comparison of the 0\ifmmode^\circ\else\textdegree\fi{} cross sections for the transitions to the Gamow-Teller resonances and the isobaric analog states leads to strengths B(GT) for the main Gamow-Teller components of typically 65% of the sum-rule value of 3(N-Z). Three to four additional Gamow-Teller fragments (``pygmy resonances'') were observed in all final nuclei at lower excitation energies. The excellent energy resolution of the experiment made it possible to observe a pronounced fine structure in these low-lying resonances which is believed to be due to coupling to two-particle--two-hole doorway states. Also seen with all target nuclei was a systematic sequence of strong ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${1}^{+}$ states near the ground states in all Sb isotopes (${\mathit{E}}_{\mathit{x}}$=0 to 220 keV). In addition, strong \ensuremath{\Delta}L=1 resonances were observed in all nuclei at excitation energies of typically 20 MeV. Furthermore, nonresonant background from quasifree charge exchange was observed. An average of \ensuremath{\sim}85% of all excess neutrons seems to contribute to this background in approximate agreement with results from (e,e'p) experiments.

The Gamow-Teller decay of 105Sn to three-particle states in 105In

The EC/ β + decay of 105Sn was reinvestigated by using 58 Ni(5 MeV/u) + 50Cr reactions, chemically selective on-line mass separation and γ-ray spectroscopy. The half-life of 105Sn has been determined as 34 ± 1 s. Out of 104 γ transitions ascribed to the 105Sn decay, 89 have been placed in the decay scheme including 52 excited states of 105In. From the EC/ β + feeding of individual states, the distribution of the Gamow-Teller (GT) strength has been derived. It is shown that the main part of the GT strength is associated with the feeding of 105In levels having excitation energies above 3 MeV. This observation can be interpreted as a sign of dominant feeding of three-quasiparticle states in 105In, which correspond to the π( g 9 2 ) −1νg 7 2 νd 5 2 shell-model configuration spread over many levels. The sum of the GT strength deduced from the present gamma-ray data of B Σ (GT) = 1.46 provides a lower limit to the total GT strength. Many weak transitions, mainly from high-energy levels, may not have been detected in this study and therefore part of the strength may be missed. An indirect support for this conclusion has been obtained from the analysis of the indium KX-rays intensity. This analysis indicates about 50% contribution of the electron-capture to the beta decay of 105Sn, which is interpreted as a sign of a predominant feeding of the high-energy 105In states with B Σ (GT) ⩾ 3. The observed GT distribution and strength will be compared to results obtained from a finite-Fermi-system theory and a large-basis shell-model calculation. The core-polarization and higher-order hindrance factors will be discussed.

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.

Beta decay to Gamow-Teller giant resonances in dripline nuclei

Performing Hartree-Fock plus RPA (or TDA) calculations, it is shown that the “superallowed” beta decays to Gamow-Teller (GT) giant resonances (GR) are possible both in very light neutron-drip-line nuclei (as β − decay) and in proton drip-line nuclei with Z ⪆ 40 (as β + decay), without calling for the phenomena related to a halo. The result calculated by using a schematic model with simple separable interactions is also given, in order to illustrate a general structure of the beta decay to GT GR states.

Configuration of the two-neutron halo of 11Li and Gamow-Teller transition.

Effects of the halo as well as meson exchange currents (MEC) are studied for the Gamow-Teller (GT) \ensuremath{\beta} transition $^{11}\mathrm{Li}$(3/${2}^{\mathrm{\ensuremath{-}}}$)${\ensuremath{\rightarrow}}^{11}$Be(1/${2}^{\mathrm{\ensuremath{-}}}$,0.32 MeV). They are found to be important to reduce transition matrix elements and remedy deviations from the observed logft value of the GT transition. This GT transition is shown to play an indispensable role in identifying the structure of two-neutron halo of $^{11}\mathrm{Li}$. It is indicated that the two neutrons forming the halo consist of the ${\mathit{p}}_{1/2}^{2}$ configuration with 60--70 % probability.

Spin-isospin strength distributions forfpshell nuclei: Results for theMn55(n,p),Fe56(n,p), andNi58(n,p) reactions at 198 MeV

Cross sections for the reactions [sup 55]Mn([ital n],[ital p]), [sup 56]Fe([ital n],[ital p]), and [sup 58]Ni([ital n],[ital p]) have been measured at an incident energy of 198 MeV, with protons observed over a range of energies corresponding to excitations of up to about 35 meV in the residual nuclei [sup 55]Cr, [sup 56]Mn, and [sup 58]Co, respectively. Measurements were carried out at center-of-mass angles between 1.6[degree] and 19.9[degree]. A multipole analysis of the results yielded the distribution of Gamow-Teller (GT) and spin-dipole strength for each target. The total GT strength below an excitation energy of 8.5 MeV was 1.7 units for [sup 55]Mn, 2.8 units for [sup 56]Fe, and 3.8 units for [sup 58]Ni. Shell model calculations of the GT strength distribution, carried out in a restricted vector space, show fair to good agreement with the data up to an excitation energy of 8.5 MeV, but overestimate the total strength by a factor of between 3 and 4.

Systematics of Gamow-Teller strengths in mid- fp-shell nuclei

We show that the presently available data on the Gamow-Teller (GT) strength in mid- fp-shell nuclei are proportional to the product of the numbers of valence protons and neutron holes in the full fp-shell. This observation leads to important insights into the mechanism for GT quenching and to a simple parametrization of the Gamow-Teller strengths important for electron capture by fp-shell nuclei in the early stage of supernovae.

( 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.

Grand unification, nuclear structure and the double beta-decay

Unification of the electroweak and the strong interaction prefers that the neutrino is a Majorana particle and therefore essentially identical with its own antiparticle. In such grand unified models the neutrino has also a finite mass and a slight right-handed weak interaction, since the model is left-right symmetric. These models have also left handed and right-handed vector bosons to mediate the weak interactions. If these models are correct the neutrinoless double beta-decay is feasable. Thus if one finds the neutrinoless double beta-decay one knows that the standard model can not be correct in which the neutrino is a Dirac particle and therefore different from its antiparticle. Although the neutrinoless double beta-decay has not been seen it is possible to extract from the lower limits of the lifetime against the double neutrinoless beta-decay upper limits for the effective electron-neutrino mass and for the effective mixing angle of the right-handed and the left-handed vector bosons mediating the weak interaction. One also can obtain an effective upper limit for the mass ratio of the light and the heavy vector bosons. The extraction of this physical quantities from the data is made difficult due to the fact that the weak interaction must not be diagonal in the representation of the mass matrix of the six neutrinos requested by such left-right symmetric models. A condition for obtaining reliable limits for these fundamental quantities from the measured lower limits of the half lifes of the 0νββ decay are correct calculations of the nuclear matrix elements involved. These nuclear structure calculations can be tested by calculating the two neutrino double beta decay (2νββ) for which we have experimental data and not only lower limits as for the 0νββ decay. The 2νββ decay is dominated by the Gamow Teller (GT) transitions. The intermediate 1 + states in the odd-odd mass nucleus are usually calculated within the Quasi-particle Random Phase Approcimation (QRPA). Since the proton-proton and neutron-neutron pairing correlations are treated in the BCS approach particle numbers are only conserved in average for the initial |0 i +〉 and the final vb0 f +〉 states. Particle number projection is used to improve on this point. Since QRPA with the physical particle-particle interaction from the Bonn potential is close to breakdown, where the excitation energy goes to zero, higher up to third order RPA is included, by allowing anharmonicities up to three bosons in the physical wave functions. Third order RPA which is the lowest order to allow modifications of the ground state correlations affect the results appreciably. Since the intermediate states are supperpositions of proton- neutron two quasi-particle states one expects that proton-neutron pairing affects these states. From the masses of the nuclei one can extract a proton-neutron (pn) pairing gap. Due to the finite basis and the inclusion of only T = 1 (and not also T = 0, J = 1) pn-pairing the bare Brückner matrix elements of the Bonn potential yield no pn-pairing gap in most nuclei. If one fits the experimental pn-pairing gap by renormalizing the pn J π = 0 + particle-particle Brückner matrix elements by a factor d pn ≈ 1.3 to 1.4 one obtains strong pn-pairing correlations, which reduce the GT strength. Since now part of the pn particle-particle correlations are already included in the BCS wave functions the 2νββ strength is reduced and the dependence on the 1 + particle-particle strength parameter g pp gets weaker. In heavier nuclei like 76Ge to 76Se this dependence on g pp of the second order GT matrix elements Σ α GT(0 i + → 1 α + → 0 f +) almost disappears. The upper limits derived from the neutrinoless double beta decay of 76Ge are for the effective neutrino mass 〈 m νe 〉≤ 0.88 eV, for the left-right mixing angle 〈 tgζ 〉≤ 1.55 × 10 −8 and for the mass ratio of the light over the heavy vector boson squared 〈 M 1 2 M 2 2 〉 ≤ 1.08 × 10 −6 .

Shell-model calculations of Gamow-Teller strength in 51V, 54Fe, and 59Co.

The Gamow-Teller (GT) strength functions of $^{51}\mathrm{V}$, $^{54}\mathrm{Fe}$, and $^{59}\mathrm{Co}$ measured by recent charge-exchange experiments have been studied using shell-model calculations employing two model spaces. The shell model is fairly successful at reproducing GT strength in the (p,n) direction, but the agreement is not as good in the (n,p) direction. It is found that both directions can be fit consistently by shifting single particle energies in a systematic manner. It is found that roughly 75% of the sum-rule strength can be seen in the experiments for both $^{51}\mathrm{V}$ and $^{54}\mathrm{Fe}$. It is also found that roughly 35% of the GT strength predicted by the shell model is seen in the (n,p) direction for all three nuclei, but that the quenching in the (p,n) direction varies with nucleus. The consequences of these findings for improving the calculation of weak interaction rates are discussed.

Gamow-Teller beta decay and isospin impurity in nuclei near the proton drip line.

It is shown that ``superallowed'' Gamow-Teller (GT) beta decays are possible for N=Z nuclei heavier than $_{28}^{56}\mathrm{Ni}_{28}$, performing Hartree-Fock plus random phase approximation (Tamm-Dancoff approximation) calculations. Since Fermi-type beta decays are forbidden in N=Z even-even nuclei, it is pointed out that the main decay mode of the nuclei such as $_{50}^{100}\mathrm{Sn}_{50}$ may be ``superallowed'' GT beta decay, which is a beta decay to the GT giant resonance state. The amplitude of isospin T=1 admixed to the T=0 ground state in N=Z nuclei is also calculated and discussed.

Beta decay of 44V.

We have calculated the [beta][sup +] decay of the proton-rich nuclei [sup 44]V in the full [ital fp]-shell valence space. Using the standard quenching factor 0.77 for the Gamow-Teller (GT) operator we obtain a theoretical half-life of 71 ms in good agreement with the experimental value [ital t][sub 1/2]=90[plus minus]25 ms. We also compute the Gamow-Teller strength functions corresponding to the decays of the 2[sup +] ground state and 6[sup +] isomer state. Our results indicate that 80% of the total GT strength falls within the [beta][sup +] energy window.

Gamow-Teller states in light nuclei near the neutron drip line

Performing Hartree-Fock plus RPA (TDA) calculations, it is shown that “super-allowed” Gamow-Teller (GT) beta-decays are possible for light neutron-drip-line without calling for the phenomena related to a neutron halo. The calculated result is in agreement with the feature of the observed GT strength in 8He, if we use the effective GT operator, which is 60% of the free operator. The result calculated by using a schematic model with simple separable interactions is also presented, in order to illustrate a general view.

Gamow-Teller strength in ß-decay of 25Si

The β-rays and β-delayed protons following the decay of 25Si have been studied using a recoil mass separator and highly efficient ß- and proton-detector systems. A new proton transition of 0.403 MeV has been assigned to the decay of 25Al. The proton spectra have given the relative ß-feeds of the proton-unbound levels in 25Al and the ß-ray measurements the summed feed of the proton-bound states. From these results, the ß-decay branching ratios of 25Si to the state of up to 9.42 MeV in 25Al are derived exclusively on the experimental basis. The branching ratios are then converted to the Gamow-Teller (GT) strengths and compared, together with the level structure of 25Al, with the large-scale shell-model calculation. The results have shown an excellent predictability of the shell-model calculation both in level energies and in relative variation of the GT strengths below 6 MeV of excitation in 25Al. For high-lying states, the disagreements between the experimental results and the calculation become large, and consequently the reasonable basis for deriving the quenching factor of the GT strength by using the shell-model calculation seems to be lost.

Gamow-Teller and dipole strength distribution in 40Ca(n,p)40K reaction.

Double differential cross-section angular distributions covering 0\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$\ensuremath{\le}40\ifmmode^\circ\else\textdegree\fi{} have been measured for the $^{40}\mathrm{Ca}$(n,p${)}^{40}$K charge exchange reaction. With the white neutron source at LAMPF-WNR, we were able to study not only the angular distribution but also the energy dependence of this charge exchange reaction covering simultaneously incident neutron energies between 60 and 260 MeV. The identification of Gamow-Teller (GT) transitions and L=1 dipole resonance was carried out using a multipole decomposition technique. The empirically deduced GT strength in the $^{40}\mathrm{Ca}$(n,p${)}^{40}$K reaction evaluated up to 15 MeV excitation energy is compared to theoretical values. We have used a simple one-particle--one-hole shell model calculation to study the excitation of the giant dipole resonance and the giant spin-dipole resonance, which are compared with empirical results.

The 70,72Ge(n,p)70,72Ga reactions: Suppression of Gamow-Teller strength near N=40.

The {sup 70,72}Ge({ital n},{ital p}){sup 70,72}Ga reactions have been measured at a beam energy of 200 MeV in order to study the behavior of Gamow-Teller (GT) strength near the {ital N}=40 subshell closure. The GT quenching factor extracted for {sup 70}Ge is consistent with results for {sup 54}Fe and with calculations done on the quasiparticle random-phase-approximation model if they are renormalized by a factor of about 0.6. The quasiparticle random-phase approximation predicts a drastic reduction in concentrated GT strength from {sup 70}Ge to {sup 72}Ge which is also seen in the data. These results are relevant to models of supernovae.

Gamow-Teller strength from the 20Ne(n,p)20F reaction at En=198 MeV.

Cross sections for the $^{20}\mathrm{Ne}$(n,p${)}^{20}$F reaction at ${\mathit{E}}_{\mathit{n}}$=198 MeV were measured at five angles using the TRIUMF charge-exchange facility and a novel high-pressure (20 atm, or 2.03 MPa) gas target. The L=0, L=1, and L\ensuremath{\ge}2 contributions to the cross sections were obtained from a multipole decomposition. Using an empirical proportionality between the cross section and Gamow-Teller (GT) strength from \ensuremath{\beta} decay, GT strength was deduced up to an excitation of 10 MeV in $^{20}\mathrm{F}$. Most of the strength is concentrated in a discrete state at ${\mathit{E}}_{\mathit{x}}$=1.0 MeV, with the extracted strength of the \ensuremath{\Delta}L=0 continuum at high excitation having a large error. These results are compared with measurements of GT strength using other probes. Comparison with the measured magnetic dipole (M1) strength in $^{20}\mathrm{Ne}$ shows that there is constructive interference between the spin part and a large orbital part of the M1 transition to the 11.26 MeV state in $^{20}\mathrm{Ne}$ (analog of the 1.0 MeV state in $^{20}\mathrm{F}$). If all the strength from the multipole decomposition is used, we find agreement of the total GT strength compared to untruncated sd shell-model calculations which use free-nucleon operators, although experimental and theoretical distributions differ in detail. The confirmation of strong orbital M1 contributions at the beginning of the sd shell together with systematics of GT quenching in heavier sd shell nuclei may provide evidence for meson exchange currents in M1 transitions.

Ground-state Gamow-Teller strength in 64Ni(n,p)64Co cross sections at 90-240 MeV.

Cross sections have been measured for the reaction $^{64}\mathrm{Ni}$(n,p${)}^{64}$Co at laboratory angles between 0\ifmmode^\circ\else\textdegree\fi{} and 10\ifmmode^\circ\else\textdegree\fi{} for incident neutron energies from 90 to 240 MeV. The ground-state cross sections together with the ${\mathrm{\ensuremath{\beta}}}^{\mathrm{\ensuremath{-}}}$ decay ft value for the transition $^{64}\mathrm{Co}$(g.s.)${\ensuremath{\rightarrow}}^{64}$Ni(g.s.) are used to normalize the q=0 differential cross section in units of mb/sr per unit Gamow-Teller (GT) strength. This is the first absolute measurement of the A(n,p) unit cross section for a nucleus in the (fp) shell, and it may be used to calibrate the GT strength measured in other (n,p) reactions of similar mass nuclei. Since the (${\mathit{e}}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\nu}}_{\mathit{e}}$) channel involves the same nuclear matrix element as the (n,p) channel, knowledge of GT strength in these nuclei is important for supernova modeling codes which depend on knowledge of ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ capture rates of (fp)-shell nuclei to determine parameters of stellar core collapse.