Magnetic Dipole Excitations Research Articles

Magnetic dipole excitation mode according to the collective model with separate neutron and proton deformations

The collective model with separate neutron and proton deformations is developed. The collective motion of individual neutron and proton surfaces is reduced to the motion of the mass surface and the neutron skin. The strong coupling scheme of the neutron skin vibrations to the mass-surface motion is applied for well-deformed nuclei. The magnetic dipole excitation mode, which was discovered experimentally, is explained as theK = 1 one-phonon neutron skin vibration in a deformed nucleus. Some estimates of parameters of the model are performed. Finally, the model is confronted with the two-rotor model and the interacting boson model (IBM-2).

Mixing of isoscalar and isovector magnetic dipole excitation modes in nuclei

Isoscalar and isovector magnetic dipole excitation modes are discussed within the frame of a two-state model using a spin- and spin-isospin-dependent effective separable interaction and the RPA. Simple expressions for excitation energies, gyromagnetic factors and M1 transition strengths are derived, and by considering spin-flip M1 transitions in the isospin T ≠ 0 nuclei 48Ca, 90Zr and 208Pb as examples the importance of the mixing of isoscalar and isovector modes is pointed out.

Bound state M1 transitions in sd-shell nuclei

Nuclear resonance fluorescence experiments with unpolarized and linearly polarized bremsstrahlung and high-resolution Ge(Li) detectors have yielded model-independent values of magnetic dipole excitation strengths to highly excited states in 20,22Ne, 24,26Mg, 28,30Si, and 32,34S. The measured B( M1) values are quenched relative to the j- j limit of the shell model, but are in general agreement with predictions of recent shell-model calculations which treat intra-sd-shell configuration mixing completely.

Shell-model study on M1 strength distributions in N = 28 isotones

Magnetic dipole excitations in N = 28 isotones 48Ca, 50Ti, 52Cr and 54Fe are studied in terms of shell model which includes configurations with one- and two-particle excitations from f 7 2 to p 3 2 , p 1 2 and f 5 2 shell orbits. Distributions of the strength observed in (e, e') and (p, p') experiments are well reproduced. Total strength reduces by about 25% due to the mixing in the 0 + ground state, but the calculated strengths are yet 2–3 times greater than the experimental values. Contributions arising from proton excitation and the orbital magnetization are also discussed.

New magnetic dipole excitation mode studied in the heavy deformed nucleus 156Gd by inelastic electron scattering

The discovery of a new collective magnetic dipole excitation mode in high-resolution inelastic electron scattering on 64 156Gd is discussed in terms of a geometrical two-rotor model and the interacting boson model. Besides in the deformed nucleus 156Gd evidence has also been accumulated for the existence of this mode in 158Gd and for the absence in the transitional nucleus 60 146Nd in agreement with the theoretical prediction.

Direct electromagnetic excitationN→Δ(1232), final-state interactions, and the multipolesM1+(32)andE1+(32)of photopion production off nucleons in the range160≲Eγ≲800MeV

In this paper one of the classics of particle theory in the resonance region is reexamined, namely the prediction of the amplitudes M/sub 1//sub +//sup() 3/2/ and E/sub 1//sub +//sup() 3/2/ of photopion production on nucleons around the resonance ..delta..(1232). The tool employed for this purpose is the final-state-interaction theory, and the principal aim of the study is to advocate for the appearance of two distinct dynamical mechanisms of photoexcitation in such an approach. Special emphasis is put then on the naturally emerging links between the present treatment and the isobaric approach and calculations based on the static quark model. It is argued that the dynamical meaning of the troublesome arbitrary constants (known to appear in most studies of the type presented in this paper) can be very convincingly explained when the resonances formed in the reaction (..delta..(1232) in this specific case) are treated as ''elementary'' objects represented in the T-matrix elements by singularities of the Castillejo-Dalitz-Dyson kind. In the case discussed here, ''direct'' magnetic-dipole excitation N..--> delta..(1232) seems to emerge very clearly as an independent dynamical mechanism of photoproduction in the p/sub 33/ state, parallel to the usual electromagnetic excitation of nonresonant ..pi..N systems, both followed by resonant pion-nucleonmore » rescattering. It turns out that the magnitude of such direct ..gamma..N..delta.. coupling is completely determined by the related strong interactions. The necessity of allowing for both specified mechanisms in this and other similar calculations is emphasized.« less

Proton correlations and the missing M1 strength in 48Ca: A two-state model

The recently measured spectroscopic factor for proton pick-up on 48Ca is exploited to estimate the influence of proton correlations on the strength of the magnetic dipole excitation at E X = 10.23 MeV in 48Ca. A calculation within the framework of a two-state shell-model for both the 0 + ground state and the 1 + excited state indicates that the presence of proton correlations does not account for the missing M1 strength.

Finite‐difference calculations of the transient field of an axially symmetric earth for vertical magnetic dipole excitation

A finite‐difference formulation of the coaxial‐loop or wire‐loop transient electromagnetic (EM) prospecting systems is used to model the fields from a buried cylindrical conductor whose axis is coincident with that of the field system. Solutions are obtained directly in the time domain. The formulation is implicit and two‐dimensional (2-D) in space. The variable‐directions method reduces each advance of one step in time from one 2-D problem to a large number of one‐dimensional (1-D) problems. The result is a reduction in computational effort. In order to avoid including the air in the finite‐difference grid, an integral equation approach is used to formulate the surface boundary condition. Thus, two sets of 1-D finite‐difference solutions and one Fredholm integral equation solution are required for each step forward in time. Comparison with analytical solutions shows excellent agreement in the case of a four‐layer earth. All computations were carried out for a perfectly conducting basement, but the method can be used for finitely conducting basement as well. If the basement is an insulator, an additional integral equation solution is required on the lower boundary. Results for a buried cylindrical conductor show that there is a high degree of sensitivity to conductor size. Inversion of transients to a stratified model can be useful if the effect of finite conductor size is taken into account. For cylindrical conductors with lateral extent comparable to or larger than the source‐receiver separation, the inversion results are valid. For conductors with lateral extent small compared with source‐receiver separation, the inversion will yield a stratified model which shows better agreement between actual and inverted thicknesses than resistivities. The involved resistivities are somewhat higher than those actually present in this case.

Numerical modeling of geophysical structures excited by an electromagnetic wave

The numerical solution for the electromagnetic response of inhomogeneities of any shape in the ground has been developed for a plane wave or electric or magnetic dipole excitation. Two approaches are usually used depending on the structure of the ground in which the inhomogeneity is embedded. For a horizontally stratified ground, the solution is obtained in the form of an integral equation system, while for a heterogeneous medium, the finite element or the finite difference method is used. A brief review of these techniques is made, and examples of application are given. Furthermore, the limitations of each of these models are outlined.

Magnetic dipole strength in 16O

The strength of magnetic dipole excitation from the 16O ground state is calculated. One anticipates a relatively large amount of the M1 strength between 16 MeV and 18 MeV which is about one third of that of the 15.1 MeV, 1+ state in 12C.

A strong magnetic dipole excitation in 40Ca observed in high-resolution inelastic electron scattering and coherent spin-flip transitions due to ground-state correlations

An ususually strong magnetic dipole transition from the ground state to a state at Ex = 10.319 MeV in 40Ca has been observed in high-resolution inelastic electron scattering, contrary to expectations from the pure independent particle shell model. The transition strength, however, can be accounted for by coherent spin-flip transitions due to strong ground-state correlations.

Effects of core polarization and exchange currents on the form factors of magnetic transitions in 12C

We have studied the form factor of the magnetic dipole excitation to the 15.1 MeV, 1 + state in 12C, taking into account the effects of core polarization and exchange currents. It is shown by using the Cohen-Kurath wave functions that the core polarization contributions, from both central and tensor interactions, are important and enhance significantly the second maximum of the magnetic form factor, while the effects of the exchange currents are shown to be small.

The influence of sample shape on microwave magnetoplasma and heliconlike resonances in n-type InSb

Magnetoplasma and heliconlike dimensional resonances are investigated in small samples of n-type InSb interacting with 8-mm wave radiation at 77 K, in order to determine the influence of sample geometry on the interaction, particularly the shift of the resonance condition. Special attention is paid to the disk geometry because of its practical importance. The results are compared with mathematically tractable, but experimentally less practical, geometries: the sphere, the ellipsoid of revolution, and the infinite slab. It is found that, in analogy with ellipsoids, the magnetoplasma resonance in disk samples is determined by an empirical effective depolarizing factor, which is surprisingly close to the depolarizing factor of an ellipsoid of the same axial ratio as the disk. By using very thin disks, magnetoplasma resonance can be shifted to significantly lower magnetic fields than the resonance field observed in spheres. This is of importance for determining the lattice dielectric constant in materials containing a high concentration of carriers, for which magnetoplasma resonance in spheres occurs at inaccessibly high fields. We also observe a sequence of heliconlike dimensional resonances in disks. These resonances form a series of electric dipole and magnetic dipole excitations, as in the case of spheres. However, the resonance conditions for the disk samples are close to the Fabry-Perot resonances in an infinite plate.

Analysis for the electromagnetic fields of a dipole located within a metal-cased borehole

A solution is obtained for the electromagnetic fields of a dipole of arbitrary orientation located within a cylindrical metal tube of infinite length. The external medium is homogeneous with finite conductivity. The expressions for the resultant fields are given explicitly for the case where the wall of the tube has a thickness small compared with the skin depth of the metal. A numerical example for magnetic dipole excitation is given for the case where the observer is in the external medium at the same level as the source. Not surprisingly, the results show that the metal casing in a borehole will have a severe shielding effect on the external fields.

Two-Body Photodisintegration ofHe3and a New Test of Time-Reversal Invariance in the Electromagnetic Interaction

We have measured the two-body photodisintegration process $\ensuremath{\gamma}^{3}\mathrm{He}\ensuremath{\rightarrow}\mathrm{pd}$ in the energy region sensitive to intermediate photoexcitation of one nucleon to the isobar $\ensuremath{\Delta}(1238)$. We present angular distributions at center-of-mass angles from 30\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} for incoming photon energies 200-600 MeV. Magnetic dipole excitation appears to be suppressed.

Multipole analysis of resonance photoproduction

The single-quark-transition language of the Melosh transformation is used to extract reduced multipole matrix elements from three recent sets of photocouplings for nonstrange baryon resonances. Magnetic dipole excitation of 56, L = 2 resonances is found to be suppressed, a result to be expected if spin-orbit effects are weak. Suggestions are made for further reduction of uncertainties in the data.

A canonical solution to the three‐dimensional electromagnetic prospecting problem

The problem of three‐dimensional scattering from anomalies buried in the earth is considered. In the general situation, this requires the fields to be evaluated using two‐dimensional numerical integration. In this or the related inverse problem, subsurface characteristics are extracted from a two‐dimensional grid of surface field measurements. This grid of points is computed with time‐saving two‐dimensional digital Fourier transforms, i.e., with plane‐wave analysis. A simple method for selection of sampling windows and increments in x and κ (wavenumber) space which bounds and minimizes aliasing errors is presented and used. The theory is applied to the canonical problem of magnetic dipole excitation of a buried cylinder.

Electroproduction of π o mesons in the region of the first pion-nucleon resonance

The reaction e −+p→e −+p+ π o has been studied in the π−N centre of mass range from 1130 to 1340 MeV/ c 2 by detecting the final proton in coincidence with the scattered electron. The four-momentum transfer to the pion-nucleon system was between 0.40 and 0.50 (GeV/ c) 2 and the polarisation of the exchanged virtual photon near unity. The proton angular distribution is analysed phenomenologically in terms of the multipoles M 1+, E 1+, S 1+ contributing to the resonant partial wave and E o+, S o+ contributing to the s-wave background. The magnetic dipole excitation M 1+ is shown to dominate the cross section but clear evidence is obtained for a significant contribution from the scalar multipole S 1+.

Ladder approximation in partial-wave dispersion relations

The left-hand cut of the 3/2, 3/2\(\pi - \mathcal{N}\) partial-wave amplitude is calculated by iterating the Bethe-Salpeter equation with nucleon exchange including a smooth cut-off. The partial-wave dispersion relation is approximately satisfied, if the cut-off is chosen properly. The magnetic-dipole excitation of the isobar as well as the transition form factor are determined through a dispersion sum rule retaining only a class of ladder diagrams.

Dipole radiation in a periodically stratified medium

A formal, exact solution is obtained for the electromagnetic field radiated by a dipole antenna of electric or magnetic type oriented normal to the striations of a periodically stratified dielectric medium of fairly arbitrary profile. The scalar potential functions appropriate to each excitation are evaluated asymptotically by the saddle-point technique. The case of magnetic dipole excitation of a sinusoidally varying medium is studied as an example. It is found that, for frequencies greater than some minimum value, the emitted radiation is characterized by the presence of caustic surfaces which confine most of the radiated power in the broadside direction. The caustics increase in number and colatitude angle with increasing frequency.