F.C. Khanna

Corrections to the single-particle M1 and Gamow-Teller matrix elements

Abstract Consideration of second-order core-polarisation, isobar-current and meson-exchange-current processes gives a satisfactory understanding of the ground-state magnetic moments and mirror β-decay transition probabilities in closed-shell-plus- (or minus) one nuclei, A = 3, 15, 17, 39 and 41. Perturbation contributions from high-lying excited states (tensor correlations) from one-body and two-body meson-exchange operators cancel strongly in diagonal matrix elements, but in off-diagonal matrix elements between spin-orbit partners the cancellation is less. Similarly, with isobar currents, there is a strong cancellation between direct and exchange contributions in diagonal matrix elements, while the exchange term is suppressed in off-diagonal cases. Of the two contributions, tensor correlations are found to be as important as, if not more important than, isobar currents. The calculated lowest-order meson-exchange-current contributions to magnetic moments are close to the soft-pion limit predictions.

Magnetic moments and half-lives of isomeric states in polonium isotopes

Abstract We have observed small violations of the additivity rule for isomeric two-proton states in 202, 208–210Po. The g-factors of the 110 ns 8+ state in 202Po and of the 24 ns 13 2 − state in 209Po were measured for the first time. The accuracy of relative g-factors which were previously measured has been considerably improved. The static quadrupole moment of 209Bi g.s. is deduced from three E2 transition probabilities in 210Po. The absolute value of Q is ≈ 15% larger than obtained by optical spectroscopy and possible reasons for the discrepancy are discussed.

Role of 2p-2h states in weak 0+−0− transitions in A = 16 nuclei

Abstract Muon-capture and β-decay rates for the transitions 16 O(0 + , ground) (a3) 16 N(0 − , 120 keV) are calculated to second order in perturbation theory. With a one-boson-exchange residual interaction, reasonable agreement to the decay rates Λ μ and Λ β can be achieved, but only if meson-exchange corrections are incorporated in the transition operator. These corrections strongly influence the time-like part of the axial-vector current.

Magnetic moments in N = 126 isotones and core polarisation blocking

Abstract Core polarisation calculations taken beyond first order of perturbation theory provide a mechanism to explain the breakdown in the additivity rule for magnetic moments in high spin isomers of configuration, [( h 9 2 ) n ; J] . Taking the meson exchange current correction from Hyuga and Arima, our calculated moment for the ground state of 209Bi is 0.25 n.m. short of the experimental value.

Precise magnetic moments of (h92)n proton states

Abstract The induced microscopic magnetic fields (Knight shifts) for Fr and Ra recoils in metallic hosts have been determined by several independent methods. The observed accurate g-factors of ( h 9 2 ) n proton states and the magnetic moments of single-particle states are difficult to explain by a consistent theoretical approach.

Magnetic properties of single particle states in the Pb-region

Abstract The reaction matrices of Kahana, Lee and Scott are used to calculate magnetic moments in the Pb region. Excellent agreement with experiment is obtained if contributions from mesonic exchange currents are included.

The incoherent neutron scattering length of 3He

Abstract The complex incoherent neutron scattering length of 3 He is estimated on the basis of effective range theory and also by means of a Breit-Wigner analysis. Both methods indicate that the incoherent scattering is a factor 4 weaker than the coherent scattering.

Doubly radiative thermal neutron capture in 2H and 16O: Experiment and theory

Abstract Measurements and calculations are presented for the cross sections for two-photon emission following thermal neutron capture in 2H and 16O. Upper limits for σ2γ were measured in both cases. For 2H, σ2λ = 8 ± 15 μb, for γ-rays in the energy region 700

Quenching of axial-vector coupling constant in the β-decay of finite nuclei

Abstract The meson-exchange-current contributions to the Gamow-Teller matrix element in closed-shell-plus- (or minus) one nuclei are calculated and expressed in terms of effective coupling constants multiplying standard rank-one spherical tensors. The dependence of the quenching of g A on the orbit and mass of the nucleus is determined.

DOUBLY RADIATIVE np CAPTURE AND THE ELECTRIC DIPOLE OPERATOR

Abstract The (E1, E1) doubly radiative np capture cross section is calculated using several methods. Upper and lower limits are obtained. Significant discrepancies in the results obtained using respectively the correct dipole operator and the approximate operator e v′ϵ are critically analyzed.