Resonances In Heavy Nuclei Research Articles

Nature of Isoscalar Dipole Resonances in Heavy Nuclei

The isoscalar dipole nuclear response reveals low- and high-energy resonances. The nature of isoscalar dipole resonances in heavy spherical nuclei is studied, by using a translation-invariant kinetic model of small oscillations of finite Fermi systems. Calculations of the velocity field at the centroid energy show a pure vortex character of the low-energy isoscalar dipole resonance in spherical nuclei and confirm the anisotropic compression character of the high-energy one. The evolution of the velocity field as a function of the excitation energy of the nucleus within the resonance width is studied. It is found that the low-energy isoscalar dipole resonance retains a vortex character, while with this collective excitation also involves a compression, as the energy increases. The high-energy resonance keeps the compression character with a change in the excitation energy within the resonance width, but the compression-expansion region of the velocity field related to this resonance shifts inside the nucleus.

ElectromagneticM1transition strengths from inelastic proton scattering: The cases ofCa48andPb208

Inelastic proton scattering at energies of a few hundred MeV and extreme forward angles selectively excites the isovector spin-flip M1 (IVSM1) resonance. A method based on isospin symmetry is presented to extract its electromagnetic transition strength from the (p,p') cross sections. It is applied to 48Ca, a key case for an interpretation of the quenching phenomenon of the spin-isospin response, and leads to a M1 strength consistent with an older (e,e') experiment excluding the almost two times larger value from a recent (\gamma,n) experiment. Good agreement with electromagnetic probes is observed in 208Pb suggesting the possibility to extract systematic information on the IVSM1 resonance in heavy nuclei.

Description of dipole strength in heavy nuclei in conformity with their quadrupole degrees of freedom

In conformity to new findings about the widespread occurrence of triaxiality arguments are given in favor of a description of the giant dipole resonance in heavy nuclei by the sum of three Lorentzians. This TLO parameterization allows a strict use of resonance widths {\Gamma} in accordance to the theoretically founded power law relation to the resonance energy. No additional variation of {\Gamma} with the photon energy and no violation of the sum rule are necessary to obtain a good agreement to nuclear photo-effect, photon scattering and radiative capture data. Photon strength other than E1 has a small effect, but the influence of the level density on photon emission probabilities needs further investigation.

Interaction of slowJ/ψandψ′with nucleons

The interaction of the charmonium resonances J/psi and psi' with nucleons at low energies is considered using the multipole expansion and low-energy theorems in QCD. A lower bound is established for the relevant gluonic operator average over the nucleon. As a result we find the discussed interaction to be significantly stronger than previously estimated in the literature. In particular we conclude that the cross section of the J/psi - nucleon elastic scattering at the threshold is very likely to exceed 17 millibarn and that existence of bound states of the J/psi in light nuclei is possible. For the psi' resonance we estimate even larger elastic scattering cross section and also a very large cross section of the process psi'+N -> psi+N giving rise to the decay width of tens of MeV for the psi' resonance in heavy nuclei.

Total photoabsorption cross sections for 1H, 2H, and 3He from 200 to 800 MeV.

The total photoabsorption cross sections for $^{1}\mathrm{H}$, $^{2}\mathrm{H}$, and $^{3}\mathrm{He}$ have been measured for incident photon energies ranging from 200 to 800 MeV. The $^{3}\mathrm{He}$ data are the first for this nucleus. By using the large acceptance detector DAPHNE in conjunction with the tagged photon beam facility of the MAMI accelerator in Mainz, cross sections of high precision have been obtained. The results show clearly the changes in the nucleon resonances in going from $^{1}\mathrm{H}$ to $^{3}\mathrm{He}$. In particular, for the ${\mathit{D}}_{13}$ region the behavior for $^{3}\mathrm{He}$ is intermediate between that for $^{1}\mathrm{H}$, $^{2}\mathrm{H}$, and heavier nuclei. This will provide a strong constraint to the theories that are presently being developed with a view to explaining the apparent ``damping'' of higher resonances in heavy nuclei. \textcopyright{} 1996 The American Physical Society.

Collective isospin excitations in nuclear matter droplets.

The isovector giant resonances in heavy nuclei are considered. These excitations correspond to a gapless Goldstone mode which comes along due to the broken symmetry in the isospin space, and can exhibit themselves in a nucleus where the numbers of neutrons and protons are not equal. These modes can be interpreted as weakly damped fluctuations of the transverse isospin components, which propagate through the nuclear matter. The dispersion law of such isospin waves is calculated on the basis of the Fermi-liquid theory as well as by means of the semiclassical variational method. The finite size of the nuclei leads to quantization of the spectra and results in a series of excited states. The number of long-living excited states is strongly affected by the Landau damping. The orthogonality relations and energy-weighted sum rule are formulated. The collective isospin modes of [sup 208]Pb and other heavy nuclei are computed.

Threshold Photoneutron Angular Distribution and Polarization Studies of Nuclei

The photoneutron method was applied to the study of (i) deuteron photodisintegration, (ii) giant magnetic dipole resonances in heavy nuclei, (iii) mechanism of radiative capture in light nuclei and (iv) isospin splitting of the giant dipole resonance in 60Ni. These studies were performed with the pulsed bremsstrahlung beam and high-resolution spectrometer available at the Argonne high-current electron linac. A threshold photoneutron polarization method was developed in order to search for the giant Ml resonance in heavy nuclei. A surprisingly small amount of Ml strength was found in 208Pb. Furthermore, the Ml strength for the 5.08-MeV excitation in 17O, the best example of a single-particle Ml resonance in nuclei, was found to be strongly quenched. In addition, the 17O(?, no) 16O reaction was found to provide an ideal example of the Lane-Lynn theory of radiative capture. The interplay among the three components of the theory, internal, channel and potential capture, were evident from the data. An electron beam transport system was developed which allows the bremsstrahlung to impinge on the photoneutron target on an axis perpendicular to the usual reaction plane. This system provides an accurate method for the measurement of relative angular distributions in (?, n) reactions. This system was applied to a high-accuracy measurement of the relative angular distribution for the D(?, n)H reaction. The question of isospin-splitting of the giant dipole resonance in 60Ni was studied by using the unique pico-pulse from the accelerator and the newly installed 25-m, neutron flight paths.

The excitation of giant multipole resonances in heavy nuclei by inelastic electron scattering

The electroexcitation of various giant multipole resonances (E1, E2, E0) is discussed. The relative strength of the cross sections has been derived in Born approximation. It is shown that the E2 giant resonance should be observed experimentally very clearly at scattering angles of the electron of about 40°–50°, if the primary energy of the electrons is about 200 MeV. At these angles the tail of the strong E1 giant resonance can be depressed nearly completely. Since the cross sections for the excitation of E2 and E0 giant multipoles behave quite similarly as functions of the scattering angles, it seems impossible to distinguish between them. The monopole excitation cross section has also been calculated exactly by a phase shift analysis and compared with the results of the Born approximation.

Electroexcitation of giant multipole resonances in the dynamic collective theory

In a previous paper the electroexcitation of various giant multipole resonances in heavy nuclei has been discussed in Born approximation. This has given only the qualitative features of the cross section, since the electron wave functions in heavy nuclei are considerably distorted by the nuclear charge. Therefore we derive in this paper the corresponding cross sections using a phase shift analysis for the electron wave functions. Moreover, the coupling between giant resonances and surface oscillations has been taken into account. This leads to transitions not only to the several giant resonances itself but also to their “satellites” (i.e. giant resonance plus surface oscillations). Since the giant resonances have rather large widths, the calculated differential cross sections have been folded using a Lorentz shape and plotted against excitation energy. It is shown that the quadrupole giant resonance levels should be observed very clearly at scattering angles of the electron of about 40° (primary energy of the electrons about 200 MeV). It seems, however, unlikely to observe the monopole giant resonance as a distinct peak of the electron cross section because of the relatively large damping to be expected.

Damping of the Giant Resonance in Heavy Nuclei

In heavy nuclei the damping of the giant resonance is due to thermalization of the energy rather than to direct emission of particles; the latter process is strongly inhibited by the angular-momentum barrier. The thermalization proceeds via inelastic collisions leading from the particle-hole state to two-particle-two-hole states. In heavy nuclei, several hundred such states are available at the energy of the giant dipole resonance. The rather large width of the giant resonance arises from the addition of many small partial widths of channels leading to the different two-particle-two-hole states. Both the density of the two-particle-two-hole states and the mean value of the interaction matrix elements between the particle-hole and two-particle-two-hole states are evaluated in a simplified square-well shell model. In a given nucleus the energy dependence of the widths is determined mainly by the density of states; the $A$ dependence is determined mainly by the size of the matrix elements. For $A\ensuremath{\approx}200$, we find $0.5 \mathrm{MeV}\ensuremath{\le}\ensuremath{\Gamma}\ensuremath{\le}2.5$ MeV. The uncertainty in this value comes mostly from the uncertainty in the strength of the interaction. Representing the energy dependence of the width by a power law we find for the exponent the value \ensuremath{\sim}1.8.

Giant resonance in heavy nuclei

The giant resonance in the dipole photoabsorption of nuclei is investigated. The main parameters of the photoproduction cross section (the position of the maximum, its width, the sum rule taking into account exchange forces, and the maximum splitting in deformed nuclei) are calculated with the aid of equations of the theory of a Fermi liquid in finite systems.

The Giant Nuclear Dipole Resonance

We have applied the collective description of nucleon interactions to the investigation of the giant dipole resonance in heavy nuclei. Our method follows closely that developed by Bohm and Pines for the treatment of electron-electron interactions in metals. We start with a system of individual nucleons, interacting via short-range two-body forces, and investigate to what extent these forces lead to collective behavior.