Double Giant Dipole Resonance Research Articles

Transitions between complex configurations in the excitation of the double giant resonance

The transitions between complex configurations, to which the giant dipole resonance (GDR) and the double giant dipole resonance (DGDR) doorway states are coupled, are taken into account in second order perturbation theory for the reaction amplitude. It is proved that only transitions between GDR and DGDR doorway states play an essential role in the Coulomb excitation of the DGDR.

On the two modes of cluster emission accompanying nuclear fission

We have shown that the “equatorial” α-emission of ternary fission can be considered as the α-decay of the double giant dipole resonance. We show now that the equatorial10Be emission can be interpreted as the10Be-decay of the DGDR. But how can the “polar” α-emission in ternary fission, or the isotropic emission of Be in incomplete fusion reactions such as232Th+4He at 200 MeV, be explained? We recall that the formation of a14C cluster in146Ba fragments is probably responsible for the recently discovered second mode of fission of252Cf, and we assume, in analogy with the146Ba case, that less fragile clusters than carbon clusters are formed in other fission fragments,e.g. O-, Ne- (etc.) clusters in Ce-, Nd- (etc.) fragments, from the valence nucleons of their132Sn core, and that these clusters can be ejected in internal energy-rich collisions of vibrating complex fragment configurations. Several arguments are presented in favour of this new hypothesis. Both modes of cluster emission are new disintegration modes of the fissionfragments themselves.

Sum Rules for Double Giant Dipole and Quadrupole Resonance States

Sum Rules for Double Giant Dipole and Quadrupole Resonance States

On the alpha- and cluster-decays of the double giant dipole resonance of fissioning nuclei

Alpha- and light charged particle-accompanied fissions can be understood as decay modes of a double giant dipole resonance (DGDR). We show that the extra energy needed by a fissioning nucleus for emitting an α-particle is equal to the energy of the DGDR. An attempt is made to interpret observations of Hongyinet al. on the prompt neutron emission in ternary fission as showing the existence of a coincidence between the DGDRs of complementary fragments and the role of a DGDR in the emission of prompt neutrons.

Possible deviation of the sum of strengths for the double giant dipole resonance from the harmonic oscillator limit

It is shown that the part of the nuclear Hamiltonian which contains the products of $\mathrm{pp}$ and $\mathrm{hh}$ pair operators (the scattering terms) and which has been usually neglected in the calculations of the giant dipole resonance turns out to be definitely important in the study of the double giant dipole resonance (DGDR). The complete energy-weighted sum of strengths (EWSS) for the DGDR and a possible deviation from its value in the harmonic oscillators' limit are derived for the first time with the full nuclear Hamiltonian taken into account. The numerical calculations within a schematic model show an example where this deviation turns to a strong enhancement of the EWSS for the DGDR at various particle numbers.

Possible enhancement of theE1decay rates of double giant dipole states

The E1 transitional rates between the double giant dipole resonance (DGDR) states and giant dipole resonance (GDR) ones are studied using the wave functions, which include one-, two-, and many-phonon components. It is shown that there is a possibility for the enhancement of the E1 transition rates from the DGDR states to the GDR ones as compared to the harmonic limit. This enhancement arises from the matrix elements for the E1 transitions between many-phonon terms of the wave functions, which differ by one E1 phonon. {copyright} {ital 1997} {ital The American Physical Society}

Probing the quantum-mechanical equivalent-photon spectrum for electromagnetic dissociation of relativistic uranium projectiles

Electromagnetic fission cross sections for the reactions U + (Be, C, Al, Cu, In, Au, U) at $E/A$ = 0.6 and 1.0 GeV are compared to theoretical calculations using recently proposed quantum-mechanical equivalent-photon spectra. In contrast to semiclassical calculations, systematically lower cross sections are obtained that cannot reproduce the experimental results. Since electromagnetic fission cross sections are virtually independent of the excitation of the double giant dipole resonance (DGDR), this conclusion is not influenced by the strength of the DGDR.

Role of anharmonicities and nonlinearities in heavy ion collisions A microscopic approach

Using a microscopic approach beyond RPA to treat anharmonicities, we mix two-phonon states among themselves and with one-phonon states. We also introduce nonlinear terms in the external field. These nonlinear terms and the anharmonicities are not taken into account in the “standard” multiphonon picture. Within this framework we calculate Coulomb excitation of 208Pb and 40Ca by a 208Pb nucleus at 641 and 1000 MeV/ A. We show with different examples the importance of the nonlinearities and anharmonicities for the excitation cross section. We find an increase of 10% for 208Pb and 20% for 40Ca of the excitation cross section corresponding to the energy region of the double giant dipole resonance with respect to the “standard” calculation. We also find important effects in the low-energy region. The predicted cross section in the DGDR region is found to be rather close to the experimental observation.

On single and double resonances in spherical nuclei

Microscopic calculations of the fine structure of single and double giant dipole resonances (GDR) in 136Xe are presented. The single GDR is treated by a wave function that takes into account a coupling of one-phonon with two-phonon configurations while for the double GDR we mix two-phonon and three-phonon configurations.

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of238U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsacker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.

Generator coordinate description of the giant dipole and double giant dipole resonance of 16O and 40Ca

In the framework of the generator coordinate method (GCM), and utilizing an appropriate ansatz for the generator states and an effective many-body Skyrme interaction, we derive analytically the collective Hamiltonians for the giant dipole resonance mode of 16O and 40Ca. The eigenfunctions, eigenvalues and relevant physical quantities are obtained. The collective spectra are analysed and the GCM results for 40Ca are compared with pion double-charge-exchange data for the double giant dipole resonance.

Relativistic Coulomb excitation of the 2-phonon GDR in 208Pb

The photon decay of the relativistic Coulomb excitation of the double Giant Dipole Resonance (GDR) in the target has been observed in 1 AGeV 209Bi on 208Pb. The photon spectrometer TAPS[1] has been used in conjunction with the FOPI[2] detector system at GSI (Darmstadt). For peripheral events which are dominated by relativistic Coulomb excitation, the sum energy of coincident γ — γ pairs shows a broad Lorentzian shaped feature at 25.6±1.0 MeV and width of 5.8±1.6 MeV which we assign to the Double GDR observed via the 2-γ decay channel.

Microscopic origin of the giant resonance structure

Microscopic calculations of the fine structure of giant resonances for spherical nuclei are presented. Excited states are described by wave functions which take into account the coupling of simple one-phonon configurations with two-phonon states. Few examples are presented: the γ-decay of resonances to the ground and low-lying excited states and the relativistic Coulomb excitation of the double giant dipole resonance states.

Electromagnetic excitation of the two-phonon giant dipole resonance

The nuclear response of 136Xe and 208Pb to electromagnetic excitation was studied in peripheral, near-relativistic heavy-ion collisions. Large cross sections were observed for the one-phonon and two-phonon isovector giant dipole resonance. The results, in particular the unexpectedly large strength found for the double giant dipole resonance, are discussed and compared to that of other recent measurements.

DOUBLE GIANT RESONANCES

A long-standing issue in nuclear structure is whether double giant resonances do exist. Exotic excitations like the double giant-dipole were predicted for many years but not observed experimentally until recently. Several experiments have been carried out at Los Alamos National Laboratory and other laboratories to search for these new collective modes of the nucleus. The results discover the existence of two previously unobserved types of double giant resonances. This work presents a review of the pion double charge exchange data and the analysis which support the existence of two such exotic vibrational nuclear modes: the giant dipole resonance built on the isobaric analog state and the double isovector giant-dipole resonance. Excitation of multiphonon states also becomes a very likely process in relativistic heavy-ion reactions where a direct identification of the double giant-dipole resonance has been reported recently.

Multiple excitation of giant dipole resonances in relativistic heavy ion collisions.

The relativistic excitation of the double giant dipole resonance in $^{136}\mathrm{Xe}$ has been calculated, describing the resonance as a harmonic vibration of the mean field, damped through its coupling to doorway states. While the predictions associated with the one-phonon states provide an overall account of the experimental findings, the calculated cross section for the two-phonon states is much smaller than that extracted from the involved analysis of the data. The experimental elucidation of this discrepancy seems essential for assessing the validity of the standard picture of giant resonances.

Double giant resonances in pion double charge exchange on 51V, 115In, and 197Au.

Resonances were observed in the continuum in pion-induced double charge exchange (at [ital T][sub [pi]]=295 MeV) on [sup 51]V at [ital Q]=[minus]14.0, [minus]32.4, and [minus]48.0 MeV, on [sup 115]In at [ital Q]=[minus]25.0, [minus]37.3, and [minus]49.6 MeV, and on [sup 197]Au at [ital Q]=[minus]35.0, [minus]44.3, and [minus]54.9 MeV. The three resonances have different characteristic angular distributions. Partial angular distributions were measured on [sup 51]V and [sup 115]In and observed to have [ital J][sup [pi]]=0[sup +], 1[sup [minus]], and 2[sup +] shapes, respectively. Based on their centroid energies, angular distributions, and observed cross sections, we identify the resonances as the double isobaric analog state, the giant dipole built on the isobaric analog, and the double giant dipole resonance. The new observations are in close agreement with the gross features reported recently for the double giant resonances, and provide some of the best signals obtained to date for the double resonance states in nuclei.

Inclusive measurements of electromagnetic dissociation of 197Au targets.

Cross sections for the electromagnetic dissociation of $^{197}\mathrm{Au}$ targets by 1.7 GeV/nucleon $^{20}\mathrm{Ne}$ and 1 GeV/nucleon $^{86}\mathrm{Kr}$, $^{197}\mathrm{Au}$, and $^{209}\mathrm{Bi}$ beams have been measured by an activation method. We observe systematic discrepancies between the experimental data and a simple Weizs\"acker-Williams calculation: For the heaviest projectiles, the calculation overpredicts the 1n cross sections, whereas the 2n and 3n cross sections are systematically underpredicted. The deviations can be significantly reduced, however, by calculations including isoscalar and isovector giant quadrupole excitations as well as the possibility to excite multiphonon states. The most important experimental result of the present study is the observation of large cross sections for three-neutron removal from $^{197}\mathrm{Au}$ with $^{197}\mathrm{Au}$ and $^{209}\mathrm{Bi}$ projectiles which can only be explained by a dominant contribution from two-phonon giant dipole excitation. For an almost quantitative reproduction for all xn channels observed, we have to assume a higher excitation probability by roughly a factor of 2 for the double giant dipole resonance as compared to the harmonic approach.

Electromagnetic excitation of the double giant dipole resonance in 136Xe.

The nuclear response of a medium-mass nucleus (136Xe) to electromagnetic excitation in a near-relativistic heavy-ion collision was investigated in the reaction136Xe(0.7A GeV)+Pb. From an exclusive measurement of the neutron decay of the excited Xe136 projectiles, strong excitations of giant resonances and, in particular, of the double isovector giant dipole resonance were identified. A resonance energy of 28.3±0.7 MeV, a width of 6.3±1.6 MeV, and a total cross section of 215±50 mb were found for the double giant dipole resonance.Received 30 September 1992DOI:https://doi.org/10.1103/PhysRevLett.70.1767©1993 American Physical Society

First observation of the Coulomb-excited double giant dipole resonance in 208Pb via double- gamma decay.

The photon decay of the relativistic Coulomb excitation of the single and double giant dipole resonance (GDR) in the target has been observed in the system 1A GeV $^{209}\mathrm{Bi}$ on $^{208}\mathrm{Pb}$. For peripheral events which are dominated by relativistic Coulomb excitation, a large Lorentzian structure in the photon energy spectrum is peaked at 13.3\ifmmode\pm\else\textpm\fi{}0.1 MeV with a width of 4.1\ifmmode\pm\else\textpm\fi{}0.1 MeV, corresponding to the single GDR in the $^{208}\mathrm{Pb}$ target. The sum energy of coincident \ensuremath{\gamma}-\ensuremath{\gamma} pairs shows a broad feature at 25.6\ifmmode\pm\else\textpm\fi{}0.9 MeV with a Lorentzian width of 5.8\ifmmode\pm\else\textpm\fi{}1.1 MeV, which we assign to the double GDR observed via the two-\ensuremath{\gamma} decay channel.