M1 resonances in unstable magic nuclei

Within a microscopic approach which takes into account RPA configurations, the single-particle continuum and more complex 1p 1 h?phonon configurations isoscalar and isovector M1 excitations for the unstable nuclei^56,78Ni and^100,132Sn are calculated. For comparison, the experimentally known Ml excitations in⁴⁰Ca and²⁰⁸Pb have also been calculated. In the latter nuclei good agreement in the centroid energy, the total transition strength and the resonance width is obtained. With the same parameters we predict the magnetic excitations for the unstable nuclei. The strength is sufficiently concentrated to be measurable in radioactive beam experiments. New features are found for the very neutron rich nucleus⁷⁸Ni and the neutron deficient nucleus¹⁰⁰Sn.     

Giant dipole resonances of nonmagic nuclei

A method is described for calculating giant dipole resonances of nonmagic nuclei by constructing the nuclear wave functions on the basis of deformed one-particle orbitale. The residual-interaction potential, which is taken into account in the calculation of the dipole states, includes the dependence on the quantum numbers K, which are integrals of motion in this method. Oscillator strengths are calculated as functions of the energy of the giant resonance.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 1, pp. 7–13, January, 1970.     

Electric spin resonances in light nuclei

Very recent pion inelastic scattering experiments at LAMPF have revealed the existence of strong spin-flip E1 resonances in the vicinity of the GDR in several light nuclei. We present here the results of shell-model calculations of S = 0 and S = 1 E1 strength distributions which offer a broad theoretical context for the discussion of electric spin excitations. Our results for ¹⁶O and ⁴⁰Ca corroborate the LAMPF data and indicate that a major fraction of the spin-flip strength still lies above the GDR.     

Narrow resonances at thresholds in light nuclei

Scattering states of light nuclei are discussed qualitatively in the supermultiplet scheme. It is shown that in channels with loosely bound fragments having minimal isospin, maximal channel spin and zero relative angular momentum narrow “potential” resonances may occur close to threshold.     

E1 resonances in neutron-rich nuclei within the phonon damping model

The quasiparticle representation of the phonon darnping model (PDM) is developed to include the superfluid pairing correlations microscopically. The formalism is applied to calculate the photoabsorption and the electromagnetic (EM) differential cross sections of E1 excitations in neutron-rich oxygen and calcium isotopes. The calculated photoabsorption cross sections agree reasonably well with the available data for ^16,18O and ^40,48Ca. The results of calculations show that the change of the fraction of the E1 integrated strength in the region of pygmy dipole resonance (PDR) as a function of mass number A with increasing neutron number N is in agreement with the recent experimental data, and does not follow the prediction by the simple cluster model. The EM differential cross sections obtained within PDM in this work show prominent PDR peaks below 15 MeV for ^20,22O in agreement with the recent experimental observation. It is also shown that, using low-energy RI beams at around 50–60 MeV/nucleon, one can observe clean and even enhanced PDR peaks without the admixture with the GDR in the EM differential cross sections of neutron-rich nuclei.     

E1 and E2 giant resonances in 22O

To understand the response of very neutron rich nuclei to external fields, I study the E1 and E2 giant resonances in ²²O using the time-dependent density-matrix theory (TDDM). TDDM is an extension of the time-dependent Hartree-Fock theory to include two-body correlations and gives us a framework to study both the energies and widths of the giant resonances. It is found that the E1 and E2 strength distributions quite differ from those in stable nuclei due to the existence of excess neutrons.     

Core polarization for l-forbidden M1 transitions in light nuclei

The experimental systematics of $\text{2}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?}\text{ld}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{1-}\text{forbidden}$ M1 transition rates in light nuclei with N, Z ? 20 are studied. Some surprising deviations from the expected behavior are qualitatively discussed in terms of an interplay between first- and second-order core polarization contributions. The B(M1) values in mirror pairs turn out to be close to each other within the experimental errors, the proton transition strengths being slightly larger. Isovector dominance is shown to account for this phenomenon.     

Giant resonances in exotic spherical nuclei within the RPA approach with the Gogny force

Theoretical results for giant resonances in the three doubly magic exotic nuclei ⁷⁸Ni, ¹⁰⁰Sn and ¹³²Sn are obtained from Hartree-Fock (HF) plus Random Phase Approximation (RPA) calculations using the D1S parameterization of the Gogny two-body effective interaction. Special attention is paid to full consistency between the HF field and the RPA particle-hole residual interaction. The results for the exotic nuclei, on average, appear similar to those of stable ones, especially for quadrupole and octupole states. More exotic systems have to be studied in order to confirm such a trend. The low energy of the monopole resonance in ⁷⁸Ni suggests that the compression modulus in this neutron-rich nucleus is lower than the one of stable ones.     

Decoupling and coherence in E1 and E2 moments in drip line nuclei

I discuss first the effect of decoupling of extended wave functions and the coherence in the low-energy E1 strength in drip line nuclei ¹²Be and ¹³O, which are studied by large-scale shell model calculations including 3 ?ω configuration space. The calculated results are compared to recent experimental data of Coulomb excitations. The quenching of the core polarization charges in drip line nuclei is also discussed in relation to recent observations of quadrupole moments in B-isotopes. Received: 1 May 2001 / Accepted: 4 December 2001     

Two-photon E1M1 and E1E2 transitions between 2p and 1s levels in hydrogen

Two-photon transitions in the hydrogen atom are analytically evaluated within the nonrelativistic limit utilizing the Coulomb Green function method. The two-photon emission probability for the transition process 2s→2γ(E1)+1s serves as a test for the other calculations and was compared with the results of previous analytical and numerical calculations. The two-photon emission probabilities for the processes 2p→ γ(E1)+γ(M1)+1s and 2p→γ(E1)+γ(E2)+1s are also evaluated and compared with previous numerical calculations. Different nonrelativistic “forms" for the decay probabilities in combination with different gauge choices are considered.     

A theory of intermediate structure,overlapping resonances and photonuclear reactions in light nuclei

Starting from a many body Hamiltonian a general theory involving intermediate structures and overlapping resonances in nuclear reaction has been worked out. This treatment, based on Trefftz's work on dielectronic recombination in atomic physics, avoids explicit use of projection operators and all relevant quantities like decay widths and energy shifts are explicitly expressed in terms of two body matrix elements. In particular, attention has been focused on the interaction of bound states among themselves and then on the coupling of the continuum with these interacting bound states. For the case of overlapping resonances, it is shown that in general one cannot take a simple energy average of the resonant amplitudes, and explicit equations for this case have been obtained. This microscopic theory also provides a justification of the model of Duke, Malik, and Firk in explaining the intermediate structure in giant dipole resonance region of ¹⁶O and ²⁸Si. However, the formalism is a general one and is suitable for the study of intermediate structure involving isolated and overlapping resonances for many types of reactions.     

Double giant resonances in nuclei

A brief review of the results of microscopic calculations aimed at describing the characteristics of double giant dipole resonances (DGDR) is presented. Special attention is paid to a systematic microscopic study of the anharmonic properties of DGDRs for nuclei with mass numbers 40≤A≤208. It is found that the corrections of the energy centroid of a DGDR from its harmonic limit are negative, have a value on the order of a few hundred keV, and follow an A ^?1 dependence.     

IBM-2 description of M1 properties in deformed nuclei

Results of schematic calculations are presented in which various terms breaking F-spin symmetry are considered in the hamiltonian of the neutron-proton interacting boson model (IBM-2). Specific attention is paid to the effect of F-spin symmetry breaking on γ → ground and γ → γ M1 transitions in deformed nuclei. A comparison with available M1 data in the rare-earth nuclei is presented. The constraints implied by these data on the form of the IBM-2 hamiltonian in well-deformed nuclei are discussed.     

Decay of giant resonances in light and medium-mass nuclei from coincidence experiments with charged particles

In recent years, the physics of electromagnetic interactions has made considerable advances in studying giant multipole resonances, which are highly excited collective states of nuclei. After the advent of new-generation continuous-beam accelerators, (e, e′x) coincidence experiments were performed, and new precision data on giant multipole resonances were obtained from these experiments. A number of examples of cross sections for decay accompanied by proton and alpha-particle emission in p-, sd-, and pf-shell nuclei of mass number in the range 16 ? A ? 64 are given. The relative contributions of these reactions are compared, their general and special features are revealed, and evaluations for semidirect and statistical decays are presented.