Electromagnetic Observables Research Articles

Manifestation of quadrupole collectivity in the magnetic dipole strength

We discuss the systematics of E2 and M1 transition strengths as obtained from experimental data in the rare-earth region. The similarity is studied in the light of the observed dependence of E2 and M1 transition strength on quadrupole deformation. Especially the “saturation” effect for these electromagnetic observables is discussed, starting from the Nilsson deformed-shell model. The data are compared to the present calculations, using a QTDA approach. The particular E2-M1 correlation is investigated within the IBM-2 using a sum-rule approach.

Strangeness matrix elements in the nucleon

We investigate the strangeness “content” of the proton in three nucleonic models: a simple relativistic meson-loop picture, a naive nonrelativistic quark model with mesons, and an extended cloudy bag model. In each case, pseudoscalar mesons are included. Nucleon matrix elements involving strange quarks are generated through the formation of a kaon cloud. We concentrate on the last two models, with the one free parameter chosen to reproduce the electromagnetic nucleon observables. We give theoretical predictions for F 2 s(0), the strange quark contribution to the weak magnetic form factor, 〈 r 2〉 s, the strangeness radius of the nucleon, and g A s, the strangeness axial vector matrix element. We observe a strong dependence of the strange quark matrix elements on the choice of the momentum cutoff or bag size.

Electromagnetic properties of the pion as a composite Nambu-Goldstone boson.

Motivated by the Nambu--Jona-Lasinio model of light mesons, we introduce a covariant separable interaction to model the structure of relativistic quark-antiquark systems. The Schwinger-Dyson equation for the quark self-energy is solved analytically, generating a dynamical quark mass through spontaneous breaking of chiral symmetry, and yielding a pion which has zero mass in the chiral limit. The Bethe-Salpeter vertex function for this qq\ifmmode\bar\else\textasciimacron\fi{} pion, which has a momentum distribution and composite structure associated with the interaction, is obtained analytically. Using this vertex function, and a similar one for the \ensuremath{\rho} meson, we calculate the electromagnetic observables of this composite Nambu-Goldstone boson, including effects from \ensuremath{\rho}-meson dominance processes. Our calculation takes the composite structure of the mesons into account. The \ensuremath{\rho}-meson effects are found to be very small in the pion charge form factor, but substantial in the charge radius. Using the model, predictions are made for ${\ensuremath{\gamma}}^{\mathrm{*}}$${\mathrm{\ensuremath{\pi}}}^{0}$\ensuremath{\rightarrow}\ensuremath{\gamma} and \ensuremath{\rho}\ensuremath{\pi}\ensuremath{\gamma} transition form factors.

Using model-based parameter estimation to increase the physical interpretability and numerical efficiency of computational electromagnetics

All of electromagnetics, whether measurement, analysis, or computation, may be regarded as activities of information acquisition, processing, and presentation. It is relevant to mention the information-intensive nature of electromagnetics, because this property can provide a useful, unifying perspective from which might be developed efficiency improvements in these various areas. If, for example, redundant information can be reduced or avoided in numerical calculations, the result should be a possibly substantial decrease in the amount of computer time that is needed for computer modeling. In this paper, we consider the use of a generalized signal-processing approach called “model-based parameter estimation” (MBPE) for making various improvements in either the efficiency of electromagnetic computer modeling or the efficiency of representing electromagnetic observables. One is to increase the efficiency of the basic model computation itself, a typical example being that of replacing the rigorous Green's function for the antenna-interface problem by a simpler, more easily computable, approximation. Another is to make more efficient use of the results that are computed, for example, by reducing the number of samples needed to construct a transfer function over some frequency band. A third arises because the electromagnetic fields/sources in various transformed-pair domains, such as exhibited by the far fields and locations of a linear-source array, are described by exponential and pole series from which physically relevant parameters can be extracted, which are useful for such purposes as data compression and physical interpretation. The discussion below deals primarily with MBPE based on exponential- and poles-series models which yield generalized waveform- and spectral-domain response functions of various transform-pair variables. We demonstrate how sampling such observables in terms of the appropriate variable or derivatives thereof leads to a data matrix from which the model parameters can be computed. Various kinds of data and models are used to illustrate the breadth of potential applications, with an emphasis on estimating frequency responses from frequency-sampled data. Some concluding remarks are addressed to use of MBPE for the Green's-function application.

New ONR research initiative announced

The Office of Naval Research announces a new Accelerated Research Initiative (ARI) on the electromagnetic properties of sea ice. The ARI is sponsored by the Ocean Optics, Remote Sensing, Arctic Sciences, Systems and Electromagnetic Theory, and Mathematics Programs. Funding in the FY1993–FY1997 time frame will support innovative research on sea ice interaction with electromagnetic radiation from the microwave through the ultraviolet.The ARI's goals are: to understand the mechanisms and processes that relate the electromagnetic properties of sea ice to its physical and morphological characteristics; to exploit this understanding to develop and verify predictive models of electromagnetic radiation interaction with sea ice; and to develop and verify mathematical inverse scattering theory using electromagnetic observables to reconstruct sea ice history geophysical properties. A balanced program of theory, laboratory, and Arctic field work is anticipated. The field work will be coordinated with the ONR Arctic Sciences Geodesic Underice Tramway System.

Quark exchange effects in nuclei

The Pauli principle requires that the quark substructure of nucleons in nuclei must be accounted for in the nuclear wave function. This becomes important when one wants to calculate consistently (weak and electromagnetic) observables for which the corresponding properties for the nucleons are determined by the quark substructure, specifically the quark wave functions. This leads to exchange contributions that constitute deviations from the simple impulse approximation description. For instance, in the nuclear form factors it destroys the factorization in a body form factor and the nucleon form factor. Examples of the effects on the elastic form factors of the A ⩽ 4 systems are shown.

Structure of 39Ar.

A spherical shell-model calculation of the odd- and even-parity states of $^{39}\mathrm{Ar}$ is made in a full sdpf model space. The calculation includes absolute binding energies and energy spectra, single-neutron spectroscopic factors, and electromagnetic and beta decay observables. Agreement with experiment is fairly good. Comparison is hampered by incomplete experimental knowledge and by the presence of low-lying intruder states.

The g-factor and lifetime of the 3 − (298 keV) state in 16N

The g-factor of the 3 − (298 keV) state in 16N has been measured by observing the hyperfine modulation of the γ-ray anisotropy in one-electron ions recoiling in vacuum following the 2H( 15N, p) 16N reaction. From the observed spatial frequency the g-factor was deduced to be | g| = 0.532±0.020. Simultaneous lifetime determinations for this state and the 1 − (397 keV) state in 16N gave values of τm = 131.7±1.9 ps and τ m = 5.63±0.05 ps, respectively. These and other electromagnetic observables associated with the lowest four levels in 16N are compared to shell-model calculations.

An improved impulse approximation treatment of the “allowed” weak and analogue electromagnetic transitions in 12C

Weak and electromagnetic transitions between the 12C ground state and 1 + members of the T = 1 isomultiplet are studied in the framework of the impulse approximation. Uncertainties due to the nuclear configuration mixing and radial effects are examined, the former in the framework of the Cohen-Kurath and Migdal models, and the latter in the density-dependent Hartree-Fock approach. Within the uncertainties of the models, the weak and electromagnetic observables at low momentum transfer (≦ 0.5 fm −1) are reasonably well reproduced. The inelastic electron scattering from factor at higher momentum transfer shows strong deviations from the impulse approximation estimates.

Shell model calculations of two to four identical-“particle” systems near 208Pb

The structure of the nuclei 204–206Pb, 210–212Pb, 210Po, 211At, and 212Rn is studied in terms of conventional nuclear shell models. An inert 208Pb core is assumed, and active particles (holes) are distuibuted in the low-lying single-particle (hole) orbits. Experimental single-particle energies are used for the one-body part of the effective residual interaction. Realistic interaction matrix elements developed for this mass region by Kuo and Herling are used for the matrix elements of the two-body part of the residual interactions. As much as possible, other effective one-body operators for electromagnetic observables are derived from experimental data on the single-particle (hole) nuclei 207Pb, 209Pb, and 209Bi. Observables treated are ground state binding energies, excitation energies, strengths for one- and two-particle transfers, and E2 and M1 observables. Generally, excellent agreement is found. The configuration mixing calculations do not remove anomalies in the magnetic moments of excited states in 206Pb and 212Rn. Many states in these nuclei are predicted by the models which have not been observed as yet. It is found that a truncation scheme for doubly even nuclei treated here in which only seniority-0 and seniority-2 states are allowed is potentially very useful.