Giant M1 Resonance Research Articles

GIANT M1 RESONANCE ON THE GROUND AND EXCITED STATES IN 2s1d-SHELL NUCLEI

The gamma decay of the resonance-like structures observed in the reaction of radiative capture of protons by the nuclei 22Ne, 26Mg, 30Si, 34,36S, and 38Ar in the region of excitation energies of 7…12 MeV was studied. The excitation functions of this reaction were measured. The resonance strengths in the energy range of the accelerated protons Ep = 1.0…3.0 MeV were determined. The obtained discrete distributions of the magnetic dipole γ-transitions on the ground and excited states for the nuclei of the 2s1d-shell have resonance character. Giant M1 resonance on the ground and excited states in the 23Na, 27Al, 31P, 35.37Cl, and 39K nuclei has been identified. The position of the M1 resonance on excited states coincides with that predicted by the Brink-Axel hypothesis for nuclei that are at the beginning of the subshell.

Microscopic investigation of the low-lying magnetic dipole transitions in the odd-mass 155–169Ho isotopes

The low-lying magnetic dipole (M1) strength in deformed odd-mass 155–169Ho nuclei is investigated using rotational invariant (RI-) Quasiparticle Phonon Nuclear Model (QPNM). The gross features and fragmentation of the scissors mode in 165Ho is well reproduced by RI-QPNM calculations. The systematics of the low-energy M1 excitation in Ho isotopic chain is discussed with respect to summed strength. Besides, the results for M1 excitations in odd-mass Ho isotopes are compared with the systematics of the scissors mode in the neighbouring even–even nuclei. The obtained results generally match the systematic and trends typical for the scissors motion. In addition to the low-lying M1 excitations, a M1 giant resonance in the 7–15 MeV energy region is predicted for 155–169Ho nuclei in the present study.

Fine structure of the giant M1 resonance in 90Zr

M1 excitations in the 90Zr nucleus have been studied in a photon-scattering experiment with monoenergetic and 100% linearly-polarized beams from 7 to 11 MeV. More than 40 Jπ = 1+ states were identified for the first time from the observed ground-state transitions revealing the fine structure of the giant M1 resonance with center of gravity of 9 MeV and sum strength of 3.8(3) μ2N. The results of the present work are compared with predictions from the shell model and the quasi-particle random-phase approximation.

Spin-flip M1 giant resonance as a challenge for Skyrme forces

Despite a great success of the Skyrme mean-field approach in the exploration of nuclear dynamics, it seems to fail in the description of the spin-flip M1 giant resonance. The results for different Skyrme parameterizations are contradictory and poorly agree with experiment. In particular, there is no parameterization which simultaneously describes the one-peak gross structure of M1 strength in doubly magic nuclei and two-peak structure in heavy deformed nuclei. The reason of this mismatch could lie in an unsatisfactory treatment of spin correlations and spin–orbit interaction. We discuss the present status of the problem and possible ways of its solution. In particular, we inspect (i) the interplay of the collective shift and spin–orbit splitting, (ii) the isovector M1 response versus isospin-mixed responses and (iii) the role of tensor and isovector spin–orbit interaction.

SKYRME-RANDOM-PHASE-APPROXIMATION DESCRIPTION OF SPIN-FLIP AND ORBITALM1GIANT RESONANCES

The self-consistent separable random-phase approximation (SRPA) with Skyrme forces is extended to the case of magnetic excitations and applied to the description of spin-flip and orbital M1 giant resonances in the isotopic chain142-152Nd . The Skyrme forces SkT6, SkM*, SLy6 and SkI3 are used. The calculations show an onset of the scissors mode with increasing deformation. A specific three-peak structure of the spin-flip response is found and explained by particular neutron and proton spin-flip transitions. Although the employed forces provide an acceptable qualitative description, the Skyrme functional still needs further improvement to reproduce quantitatively the experiment for spin modes.

Elastodynamic Features of Nuclear Matter from Macroscopic Model of Giant Magnetic Resonances

The collective model is presented providing a descriptive treatment of the magnetic resonant response of spherical nuclei. The model is based on macroscopic equations assuming elastodynamic behavior of the nuclear Fermi-continuum. Modelling a heavy nucleus by a spherical piece of an elastic continuous substance made up of a degenerate Fermi-system of nucleons, it is argued that nuclear resonant magnetization may be interpreted as the resultant of torsional wavelike vibrations excited inside a nuclear macroparticle. The emphasis is placed on the description of the giant magnetic dipole resonance. This resonance is associated with long wavelength vibrations of the magnetization current induced in the peripheral layer of finite depth, whereas the internal spherical region presumably unaffected by external perturbations is considered as an unperturbed core. The excited collective motion is found to behave like shear non-radial vibrations of a massive peripheral layer against a rotationally invariant core. The Extended Thomas-Fermi method is employed to generate a bulk density profile on the basis of Skyrme forces which is used as an input parameter in calculations of torsional inertia and stiffness of the collective Hamiltonian. Systematic calculations for the energy and total excitation probability of the giant M1 resonance are compared with data obtained both by nuclear resonance fluorescence measurements and by means of backward (e,e′) scattering.

Integral characteristic parameters of the giant M1 resonance.

The dipole magnetization of a heavy spherical nucleus is studied with macroscopic standpoint. The semiclassical model under consideration focuses on the giant M1 resonance as a result of long wavelength oscillations of the collective magnetization current induced in the surface massive layer of finite depth. The macroscopic picture of the excited collective flow is found to be like that for the torsional elastic vibrations of the peripheral layer against the central spherical region inert with respect to external perturbation. The emphasis is placed on calculation of scaling behavior of integral characteristic parameters of magnetic dipole resonance.

Microscopic approach to M1 radiative capture of nucleons

The microscopic analysis of the excitation mechanism of M1 giant resonance in the 208Pb(n, γ) reaction emphasizes the fundamental role of the nuclear shell structure in determining an effective surface form factor for the nucleon-nucleus interaction. Results of calculations of transition amplitudes obtained with a microscopic radial form factor in the frame of the direct-semidirect model suggest a possible connection between the excitation of the giant M1 resonance and the asymmetries in angular distributions of emitted photons, in spite of the strong statistical contribution to the capture cross section.

Strength functions of primary transitions following thermal neutron capture in strontium

The primaryE1,M1 andE2γ-radiation in87,88,89Sr observed after thermal neutron capture was compared with the predictions of single particle and giant resonance models. The nuclei feature a wide range of neutron binding energies between 6.3 and 11.1 MeV, which makes a 5.5 MeV spectrum of primary transition energies available for investigation. The (n, γ) reaction was used to estimate the parameters of the spin-flip M1 giant resonance in strontium. The total energy weightedM1 strength of this resonance exceeds the results of shell model and random phase approximation calculations for90Zr by a factor of 3–4. TheE1 strengths were found to agree with the established giant dipole resonance model. The few data on primaryE2 transitions do not allow to differentiate between the giant quadrupole resonance and the single particle models.

E1 and M1 transitions near and below particle threshold

Elastic photon scattering experiments are discussed which have been used to investigate the average distribution of E1 and M1 strengths below particle threshold at energy regions of high nuclear level density. An experiment with quasimonochromatic tagged photons has confirmed the smooth energy dependence of the average radiative strength in 238U below threshold. The inferred total photoabsorption cross sections are in agreement with the extrapolated tail of the giant dipole resonance. At energies below 5.3 MeV, however, local concentrations of E1 strength have been observed which probably can be attributed to strong isolated resonances seen in previous NRF experiments using neutron capture gamma rays. Large amounts of strongly fragmented M1 strength have been found in 90Zr, 140Ce and 206Pb using linearly polarized tagged photons. The total M1 transition strength is in agreement with recent microscopic calculations and can account for the giant M1 resonance in these nuclei.

GiantM1 resonance inZr90

The distribution of magnetic dipole transition strength in $^{90}\mathrm{Zr}$ has been measured at excitations between 8.1 and 10.5 MeV with highly polarized tagged photons. A total M1 strength of Jg${\mathrm{\ensuremath{\Gamma}}}_{0}^{2}$(M1)/ \ensuremath{\Gamma}${=37.7}_{\mathrm{\ensuremath{-}}4.0}^{+5.1}$ eV was found to be broadly distributed throughout the region. This strength can account for the giant M1 resonance predicted in $^{90}\mathrm{Zr}$. A substantial amount of E1 transition strength was also identified. This E1 strength has important implications for the interpretation of forward-angle (p,p') scattering and its use in identifying M1 transitions in heavier nuclei.

Observation of the M1 giant resonance by resonance averaging in 106Pd

An investigation of capture of 2 keV and 24 keV neutrons in a 105Pd target resulted in resonance-averaged intensities of primary gamma rays with energies between 5.2 and 9.5 MeV. From these intensities the gamma ray strength functions have been evaluated for E1, M1 and E2 radiation and compared with predictions of the giant resonance theory. The inclusion of an energy dependent spreading width for the E1 giant resonance is necessary. The energy distribution of M1 reduced strength is consistent with an interpretation of a broad resonance around 8.8 MeV. E2 data agrees satisfactorily with the giant resonance extrapolation.

Excitation of M1 states in 58Ni and 60Ni by 65 MeV polarized proton inelastic scattering

We studied the 1 + states in 58Ni and 60Ni by using the 58 Ni( p , p′) 58 Ni and 60 Ni( p , p′) 60 Ni reactions at 65 MeV with high resolution, in order to search for the fragmentation of the 1 + states at excitation energies of E x = 7–11 MeV. From the similarity in experimental shapes of both cross sections and analyzing powers for the well-known 1 + states and from comparison with microscopic DWBA calculations under the assumptions of pure (f 7 2 −1f 5 2 ) and ν(p 3 2 P 1 2 ) configurations, four states in 58Ni and one state in 60Ni have been identified as the candidates of the 1 + states of a (f 7 2 −1f 5 2 ) or ν(p 3 2 P 1 2 ) configuration in the M1 giant resonance region of E x = 7–10 MeV, though the strengths are very weak.

Doorways in the reaction207Pb + n

Abstract High Resolution neutron TOF spectroscopy on 207Pb at the Geel LINAC facility is described. The aim of these measurements is the search for the expected M1 giant resonance and investigation of the mechanisms in compound nucleus formation. Different doorways, partly common to neighbouring isotopes, are observed and we try to explain them on the base of the Independent Particle Model (IPM). The total M1-strength has been measured with better accuracy and first results are presented.

Giant M1 resonance in Pb.

Highly polarized tagged photons were used to measure the distribution of M1 transition strength in /sup 206/Pb at excitations between 6.7 and 8.1 MeV. The observed B(up-arrowM1) of about 19..mu../sub 0//sup 2/ can account for most of the isovector M1 strength that is expected in the Pb nucleus. This result in /sup 206/Pb is compared with the current experimental situation in /sup 208/Pb. The discrepancy between predicted and observed M1 strengths in /sup 208/Pb can probably be attributed to local fragmentation of the strength into states that are too weak to have yet all been identified.

Radiative strength functions in odd- A spherical nuclei

The radiative strength functions for the partial γ-transitions from neutron resonances to the ground and low-lying states of odd- A spherical nuclei are calculated within the quasiparticle-phonon nuclear model. The fragmentation of one-quasiparticle and quasiparticle-plus-phonon states is calculated. This allowed one to calculate γ-transitions between the one-quasi-particle components (valence transitions) and γ-transitions between the quasiparticle-plus-phonon and one-quasiparticle components of the wave functions. The energy dependence of the strength functions C (E1, η) and C (M1, η) is calculated near the neutron binding energy B n for 55Fe and 59, 61Ni. The corresponding experimental data are described qualitatively. The contribution of the valence E1 transitions to the strength function is shown to be from 20% to 90%, and M1 transitions about 1%. The influence of the M1 giant resonance is important for M1 transition probabilities.

Study of the 208Pb( n, γ0) 209Pb reaction between 0.8 and 7.7 MeV

The reaction 208 Pb( n, γ 0) 209Pb was studied from 0.8 to 7.7 MeV to investigate relative contributions of the compound-nucleus and direct and semidirect processes in this energy range. Compound-nucleus reactions dominate below about 5 MeV and semidirect processes above 6 MeV. The direct-semidirect (DSD) model with a complex particle-vibration coupling describes the experimental data in the giant resonance region. A relatively large imaginary term is necessary to obtain a good fit to the data indicating either that the reactions proceed to a large extent in more complicated ways than the simple two-step semidirect reaction or that the model has a serious defect in its present formulation. A second objective was to search for a possible excitation of the isoscalar E2 and the M1 giant resonances by measuring asymmetries around 90° in the angular distribution of the γ-rays. The results indicate no (or very weak) asymmetry effects.

Search for M1 and M2 giant resonances in 140Ce with high-resolution electron scattering and comments on a recent theoretical prediction

A broad structure at E X ≈ 9 MeV in 140Ce which hitherto had been interpreted as the M1 giant resonance is shown to consist of an accumulation of J π = 2 − states with a total strength of about 6 × 10 3 μ K 2fm 2. There is no indication of an M1 giant resonance in the energy region E X ≈ 7.5–10 MeV, contrary to a recent theoretical prediction.

Search for isobaric analogues of M1 states and giant spinflip resonances in the 208Pb(p, n) reaction

The 208Pb(p, n) 208Bi reaction has been studied at E p=120 and 160 MeV. The GT resonance is found to be concentrated in disagreement with a recent theoretical suggestion that it is highly fragmented in heavy nuclei, but in good agreement with an earlier calculation. A giant Δl = 1, ΔS = 1 resonance is also observed.

Highly excited spin-1 states in 88Sr by the (γ, γ) reaction

The resonant scattering of bremsstrahlung γ-rays by a SrCO 3 target has been studied for γ-ray energies of 5–11 MeV. Six γ-transitions of energies between 6–8 MeV, which indicate six resonant states in 88Sr, were observed. The relative intensities of the resonantly scattered γ-rays at 125 and 150° were found to be compatible only with the assignment of spin 1 to the six states. Radiative widths of the resonant states were deduced. The possibility that these states are components of the giant M1 resonance in 88Sr is discussed.