Isospin Mixing Research Articles

Production of E 1 Radiation in the 2 H( d , Υ) 4 He Reaction at Very Low Energies

An argument is given that the mechanism of charge polarization cannot give rise to E1 radiation in the 2H(d, Υ)4He reaction. It then follows that E1 radiation produced in this reaction probably arises from the spin-dependent part of the E1 operator, although small E1 admixtures have been shown to arise in coupled-channels calculations as a result of Coulomb-induced isospin mixing in 3H and 3He. A proper calculation of this reaction should consider mesonexchange currents and remains to be performed.

Isospin mixing and the sum rule of super-allowed fermi β decay

The amplitudes of isospin mixing to the ground states of 14O and several odd-odd N  Z nuclei are studied in the Hartree-Fock approximation taking into account charge symmetry and charge independence breaking forces. We find that isospin mixing increases the sum rule of super-allowed Fermi transitions by 0.1 % in 14O and by more than 1 % in 54Co because of core effects, while valence particles tend to decrease the sum rule. It is shown that the sum rule increases due to the CSB and CIB interactions amounts to about 40% of that due to the Coulomb interaction alone.

Neutron-proton halo structure of the 3.563-MeV 0(+) state in 6Li.

We explore the possibility that a stable nucleus has halolike structure in high isospin excited states. As an example the structure of the 3.563 MeV ${0}^{+}$ state of $^{6}\mathrm{Li}$ is studied in a fully microscopic \ensuremath{\alpha}+p+n three-cluster model, because it is an isobaric analog state of $^{6}\mathrm{He}$ with two-neutron halo structure. An extensive calculation with isospin mixing has shown that the state has spatially extended halolike structure formed by the neutron and the proton outside the \ensuremath{\alpha} particle. Electron scattering form factors related with the state are reproduced fairly well.

Isospin mixing in the electroexcitation of the Ex = 10.84 MeV, Jπ; T = 1 −; 0 state in 12C at low momentum transfers

Electroexcitation of the isospin-forbidden transition to the broad J π ; T = 1 −; 0 state at 10.84 MeV in 12C has been studied for momentum transfers q < 0.6 fm −1. The longitudinal form factor exhibits a sensitivity to the interference of small T = 1 admixtures. The isospin breaking Coulomb matrix element is determined in a two-state model to 〈 H C〉 = 145(80) keV. A shell-model calculation using a new effective psd-shell interaction qualitatively agrees with the shape of the form factor but underpredicts its magnitude by a factor of about two.

Isospin impurities in ground states of N = Z nuclei near the proton-drip line

Isospin mixing probabilities in the N = Z nuclei towards the proton drip-line nucleus 100Sn are evaluated using the deformed Hartree-Fock solutions with the Skyrme interaction. The mixing probability is proportional to only slightly lower power in Z than Z 8 3 obtained from the hydrodynamical estimate by Bohr and Mottelson, however, it is larger by a factor of about 2.

Isospin character of transitions to the 21+ and 31- states of 90,92,94,96Zr.

The elastic and inelastic scattering of 35.4 MeV alpha particles by $^{90,92,94,96}\mathrm{Zr}$ have been measured to investigate the isospin mixing of transitions to the ${2}_{1}^{+}$ and ${3}_{1}^{\mathrm{\ensuremath{-}}}$ states. The data have been analyzed using a deformed optical model potential and a folding model assuming transition densities of either the standard collective model type or resulting from random-phase approximation calculations. For the ${2}_{1}^{+}$ states of $^{90,92,94}\mathrm{Zr}$, both models give B(E2)\ensuremath{\uparrow} values which are in excellent agreement with those determined from Coulomb excitation or lifetime measurements. Both models of analysis suggest a sharp drop in the B(E2)\ensuremath{\uparrow} at $^{96}\mathrm{Zr}$ relative to the value at $^{90,92,94}\mathrm{Zr}$, in agreement with an earlier measurement of alpha-particle scattering. Deduced ${\mathit{M}}_{\mathit{n}}$/${\mathit{M}}_{\mathit{p}}$ ratios for $^{92,94,96}\mathrm{Zr}$ are considerably larger than their respective ratios of N/Z. General agreement is found between folding model calculations for alpha-particle scattering and $^{6}\mathrm{Li}$ scattering when using the same random-phase approximation (RPA) transition densities. A small shift in phase is observed between the oscillations in the calculated inelastic angular distributions and the measured ones. A possible means to reproduce this shift is presented, but the changes required appear to be too large to be physically meaningful.

NEUTRINO NEUTRAL CURRENT EXCITATION OF 12C*(1+; T=1; 15.1 MeV)

We report here the total cross-sections for [Formula: see text][Formula: see text], for incident neutrino energies from 16 MeV to 54 MeV, so as to provide theoretical values for comparison with the recent Karlsruhe-Rutherford Medium Energy Neutrino Experiment (KARMEN). The effects of the isospin mixing from the 1+; T=0; 12.7 MeV level of 12C are taken into account. The effect of the only strong interaction induced weak magnetism form factor is studied, as well as that of the isovector axial coupling. The nuclear models used are the combination of general 1p-shell wave functions of Cohen-Kurath for the 15.1 MeV level, and Gillet-VinhMau particle-hole wave functions with 2ħω excitations for 12.7 MeV level. The neutrino (antineutrino) flux averaged sum [Formula: see text] is found to be 9.86×10−42 cm2without isospin mixing and 9.83×10−42 cm2with isospin mixing (δ=0.05) to be compared with the KARMEN result for the sum as [10.8±1.1(stat)±1.2(syst)]×10−42 cm2.

Investigation of the isospin-forbidden reaction6Li(6Li,d 2)10 B 0+,1

Angular distributions of the isospin-forbidden reaction6Li(6Li, d2)10B0+,1 were measured at six beam energies in the range from 3 to 8 MeV. The contribution of two-step transfer mechanism to the reaction under consideration was found to be negligible. Hauser-Feshbach model cross sections, obtained with parameters derived from a previous analysis of isospin-allowed reactions in the6Li +6Li system are consistent with the data, provided a 2% reduction factor reflecting isospin mixing of compound nucleus levels. From this factor an isospin-breaking matrix element of 〈H c 2 〉1/2≈13 keV was deduced. The presence of one broad or several resonances with (J P ,T)=(1−, 1) or (2−,1) concentrated at an excitation energy of about 30.3 MeV in12C seems to be responsible for the observed shape of angular distributions and their energy dependence.

Restoration of isospin symmetry in highly excited compound nuclei.

We have deduced compound nuclear isospin mixing in $^{28}\mathrm{Si}^{\mathrm{*}}$ and $^{26}\mathrm{Al}^{\mathrm{*}}$ at excitation energies between 33 and 65 MeV. The \ensuremath{\gamma}-ray yield from the decay of giant dipole resonances built on excited states is found to be suppressed, implying that isospin mixing is small. By comparison with data from the literature at lower excitation energy, we conclude that isospin is a better symmetry in these compound nuclei at high excitation energy.

Deformed Gaussian orthogonal ensemble description of isospin mixing and spectral fluctuation properties.

An analysis of isospin symmetry breaking in the case of pure spectral observables is made within the recently developed deformed Gaussian orthogonal ensemble. The low-energy spectrum of [sup 26]Al recently studied by Mitchell [ital et] [ital al]. is used to perform the analysis. Both the spacing and eigenvector distributions are considered. The value of the Coulomb matrix elements that causes the isospin breaking is extracted and found to be consistent with other measurements.

Current correlators in QCD at finite temperature.

Current correlators in QCD at a finite temperature $T$ are considered from the viewpoint of operator product expansion. It is stressed that at low $T$ the heat bath must be represented by hadronic, and not quark-gluon states. A possibility to express the results in terms of $T$-dependent resonance masses is discussed. It is demonstrated that in order $T^2$ the masses do not move and the only phenomenon which occurs is a parity and isospin mixing.

Positron-electron decay of 28Si at an excitation energy of 50 MeV

The electron-position pair decay of 28Si at 50 MeV excitation produced by the isospin T=0 (α + 24Mg) and the mixed isospin T=0,1 (3He + 25Mg) reactions has been studied using a special designed Positron-Electron pair spectrometer PEPSI.

Systematics of the double isobaric analog state cross section at 50 MeV.

Double charge exchange cross sections for 50 MeV pions to double isobaric analog states in $^{44}\mathrm{Ca}$, $^{56}\mathrm{Fe}$, and $^{93}\mathrm{Nb}$ were measured. The results indicate that the effective double charge exchange operator at this energy has a strong short-range component. In addition, evidence is presented tending to implicate isospin mixing as the mechanism for excitation of the analog of the antianalog in $^{93}\mathrm{Nb}$.

Effective isospin non-conserving interaction and isospin-forbidden beta decays in the 1 s0d shell

An effective isospin non-conserving interaction is determined for a 1s0d shell full space calculation. The interaction parameters are determined by requiring the interaction to reproduce the observed energy splittings of isobaric multiplets. We introduce a new mass dependence of the parameters, which significantly improves the quality of agreement. Applying the obtained interaction, the isospin mixing matrix elements are calculated. The calculation has reproduced existing experimental values deduced from isospin-forbidden beta decays.

Admixture of the giant dipole resonance to low lying 1−-states in heavy nuclei

An investigation of E1 transitions from low lying Jπ=1− states detected in (e,e′) and (γ,γ′) measurements is presented. The B(E1) strengths are discussed in terms of the admixture of the giant dipole resonance to the low lying octupole states. We show that the spreading widths Γ ↓ are around 20–50 keV in the examined A=142–238 nuclei and compare this result with spreading widths extracted from isospin mixing matrix elements of isobaric analog resonances.

Charge symmetry breaking in n

We report the results of a precise test of charge symmetry in the scattering of polarized neutrons from polarized protons at a laboratory bombarding energy of 183 MeV. The observable sensitive to charge symmetry is the difference between the analyzing powers associated with the neutron spin [${\mathit{A}}_{\mathit{n}}$(\ensuremath{\theta})] and with the proton spin [${\mathit{A}}_{\mathit{p}}$(\ensuremath{\theta})]. Systematic errors and experimental ambiguities in the measurement of this difference are extensively discussed. Our result for \ensuremath{\Delta}A\ensuremath{\equiv}${\mathit{A}}_{\mathit{n}}$-${\mathit{A}}_{\mathit{p}}$, averaged over the angular range 82.2\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}116.1\ifmmode^\circ\else\textdegree\fi{}, is (33.1\ifmmode\pm\else\textpm\fi{}5.9\ifmmode\pm\else\textpm\fi{}4.3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. With the statistical and systematic errors added in quadrature, this value is 3.4 standard deviations larger than the effect expected from pure photon exchange (the electromagnetic spin-orbit interaction) between the neutron and proton, and thus represents clear evidence of charge symmetry breaking in the strong interaction. We also extract information about the angular dependence of \ensuremath{\Delta}A(\ensuremath{\theta}), within limitations imposed by uncertainties in the measured beam and target polarizations. Both the angle-averaged value and the angular dependence measured for \ensuremath{\Delta}A are in excellent agreement with predictions from meson-exchange theory, when these include both the effect of the n-p mass difference on one-pion exchange and the isospin mixing of ${\mathrm{\ensuremath{\rho}}}^{0}$ and ${\mathrm{\ensuremath{\omega}}}^{0}$ mesons. In particular, the Bonn nucleon-nucleon (NN) potential model accounts for the present results utilizing \ensuremath{\rho}NN and \ensuremath{\omega}NN coupling constant values deduced previously for this potential from fits to more conventional NN scattering data.

Resonant absorption by the lowest two [formula omitted] levels of 9Be

The ground-state radiative widths of the E x = 14.393 and 16.975 MeV, T = 3 2 levels of 9Be were measured with a new γ-ray resonant absorption technique. Thin boron targets and broad resonances in the 11B(p, γ 0γ 1) 12C reaction were used to produce the required monochromatic γ-rays of variable energy. The total width of the 16.98 MeV level was obtained from measurements in a separate high-resolution 7Li(d, γ) 9Be experiment. The result leads to a revision of partial widths of the 16.98 MeV level that were reported earlier. The present results are compared to results of (e, e') scattering and, along with the revised particle widths of the 16.98 MeV level, to shell-model calculations. The isospin mixing and giant M1 character of the levels are also discussed.

One-body, collective contributions to parity mixing in compound nuclear states.

One-body contributions to parity mixing in compound nuclear states are discussed, with reference to the recent polarized neutron transmission experiments. The role of the {ital J}=0{sup {minus}} giant spin-dipole resonance as a mediator of parity mixing in complex nuclear states is pointed out, and an expression of the parity violating spreading width derived. The collective nature of the mixing mechanism is stressed, and an analogy with isospin mixing in nuclei is drawn.

Triaxiality and isospin-forbidden E1 decays in the N = Z nucleus 64Ge

The neutron-deficient isotope 32 64Ge 32 was produced in the reaction 12C( 54Fe, 2nγ) 64Ge at a beam energy of 165 MeV. The production cross section for 64Ge was measured to be 640 ± 70 μb, which represents only ∼0.1% of the total fusion cross section. The Daresbury recoil separator was operated in conjunction with a large γ-array to study recoil-γγ correlations and measure recoil-γ angular distributions. A level scheme for 64Ge was constructed containing 19 levels. The nucleus appears to have a structure consistent with a γ-soft shape. Shape changes induced by aligned g 9 2 nucleons are discussed. An overview of octupole correlations in the A ⋍ 60−80 mass region is given. Evidence for forbidden E1 transitions was found which is indicative of considerable isospin mixing.

Comment on "Isospin forbidden ( alpha,d) transitions to the low-lying states in 26Al"

The interpretation of the {sup 24}Mn({alpha},{ital d}){sup 26}Al, isospin forbidden reaction as a successive-transfer reaction is very uncertain. The data do not support the existence of significant isospin mixing in low-lying states of {sup 26}Al.