Excitation Of Giant Resonances Research Articles

Two-phonon giant dipole resonance in 208Pb

Excitation of the two-phonon isovector giant dipole resonance was observed in 208Pb projectiles incident on different target nuclei (C, Sn, Ho, Pb, U) at high energy (640 A·MeV). Evidence is found for a two-step electromagnetic excitation mechanism. An enhancement of 1.33(16) for the excitation cross section is observed relative to the harmonic approximation. Neutron- and γ-decay probabilities are derived as well.

Electronic structure of the valence band of GdS x and Gd 3S 4

Electronic structure of clean surfaces of single crystals of magnetic compounds of sulfides of gadolinium GdS x ( x = 0.8−1.25) with NaCl and Th 3P 4 structures has been investigated by ultra-violet and X-ray photoelectron, Auger and electron energy loss spectroscopy. Clean surfaces of GdS x were obtained by cleavage in situ in ultra-high vacuum and by ion bombardment followed by annealing in vacuum. Photoelectron spectra of the valence band of GdS x family have the similar structure with a maxima at 9 eV and 5 eV related to a photoemission from the 4f-states and valence band formed by 3p-states of S anions respectively and features in the region 0–2 eV associated with 6s-5d conduction band of GdS x . An analysis of electron energy loss spectra has shown that the main features in loss spectra at high beam energy are associated with the 4d-4f giant resonance, 5p-5d resonance and plasmon excitation. At low incident energy a prominent multiplet structure is observed and interpreted in terms of the dipole-forbidden 4f → 4f electron transitions.

Double-phonon giant dipole resonances in relativistic heavy ion collisions

We report on Coulomb dissociation experiments utilizing a 208Pb-beam (641 MeV/nucleon) impinging on C, Sn, Ho, Pb and U targets. Excitation energy distributions for 208Pb are extracted from an exclusive measurement of the projectile decay products. The measured cross section is dominated for heavy targets by Coulomb excitation of giant resonance states in lead. A thorough analysis yields evidence for the excitation of the double-phonon giant dipole resonance. In contrast to earlier results obtained for other nuclei the extracted cross section for 208Pb exceeds that of theoretical predictions on the basis of a harmonic oscillator scenario only slightly by about a factor 1.3.

Excitation of the two-phonon giant dipole resonance in 238U studied via inclusive measurements of neutron-removal cross sections

As part of a comprehensive study of uranium fragmentation at relativistic energies at the GSI projectile fragment separator, FRS, inclusive neutron-removal cross sections have been measured for several xn channels at projectile energies of 600 and 950 A MeV. The electromagnetic cross sections agree surprisingly well with a semiclassical calculation based on measured photonuclear cross sections and do not require an extra enhancement of the two-phonon giant dipole excitation as concluded from similar measurements with 197Au.

Are giant resonances harmonic vibrations?

We investigate the non-linear response of a quantum anharmonic oscillator as a model for the excitation of giant resonances in heavy ion collisions. We show that the introduction of small anharmonicities and non-linearities can double the predicted cross section for the excitation of the two-phonon states.

Recent Results on Elastic and Inelastic Scattering

In this review article, which corresponds to lectures given by one of us (N.A) at the third Euroschool on Exotic Beams held in Leuven in September 1995, we present experimental results and theoretical developments in heavy-ion elastic and inelastic scattering and Giant Resonance excitation. The paper contains a short review of the field with special emphasis on more recent results and problems. We start by recalling the theoretical situation concerning the description of nucleon-nucleus elastic scattering. We show that in the framework of the local density approximation, complex potentials derived from fundamental effective nucleon-nucleon interactions, describe successfully the data. However, the main part of the discussion on elastic scattering, is dedicated to the description of intermediate energy heavy-ion elastic scattering. We present different folding models for the calculation of the real part of the nucleus-nucleus optical potential, M3Y, DDM3Y, .... The theoretical predictions are compared to experimental data mainly obtained at GANIL (20-100 Mev per nucleon). We show that a new density dependent interaction which reproduces the equilibrium density and the binding energy of normal nuclear matter, leads also to a satisfactory description of heavy-ion elastic scattering angular distributions. This interaction reproduces also the density and energy dependence of the nucleon optical potential. We present a new simple effective interaction with a real and imaginary part for peripheral heavy-ion collisions at intermediate energies. Finally the effect of the isospin and spin terms of the effective nucleon-nucleon interaction on the nucleus-nucleus folded potentials is discussed. We introduce the deformed optical model potential which is the most frequently used model, to obtain inelastic scattering transition potentials. However the most direct approach to obtain transition potentials is from the folding of the transition densities with an effective nucleon-nucleon interaction and the ground state density of the nucleus which is not excited. We show that the predictions of the optical and folding model are very different, especially for transitions dominated by nuclear excitation. The difference between the cross sections estimated within the deformed optical model and the folding model increases with multipolarity. Following the theoretical work of R. Satchler, we recommend the use of a folding model to extract deformation lengths and multipole moments from inelastic scattering measurements. We present the state of the field concerning Electric Giant Resonances and multiphonon excitations. We introduce the different sum rules which can be found usually in the literature and we show the link between them. The excitation of Giant Resonances with intermediate energy or high energy heavy ions, measured at GANIL or GSI and the technical problems met in the analysis of these experiments are discussed. Recent results concerning the two-phonon excitation of the Giant Quadrupole and Dipole mode are presented. Concerning the breathing mode, macroscopic and microscopic prescriptions introduced to access the compressibility of the nuclear matter are discussed. We show, in the light of theoretical arguments developed recently by J.P. Blaizot and collaborators, that microscopic calculations remain the most reliable tool for the determination of the nuclear matter compression modulus from the energy of the monopole vibration of nuclei.

Excitation of giant monopole resonance in 24Mg using 6Li scattering.

The isoscalar giant monopole resonance in the nucleus {sup 24}Mg was investigated by inelastic {sup 6}Li scattering at {ital E}{sub Li}=156 MeV. At extreme forward angles fragmented {ital E}0 strength was observed up to {ital E}{sub {ital x}}=23 MeV. The extracted strength centered at 18.3{plus_minus}0.5 MeV excitation energy with a width of {Gamma}=4.8{plus_minus}0.5 MeV corresponds to 97.3%{plus_minus}15% of the {ital E}0 energy weighted sum rule.

Pion and neutron production by cosmic-ray muons underground.

The production of positive pions and neutrons by cosmic muons at underground sites of various depths is investigated. We first test the equivalent photon method in the particular case of \ensuremath{\Delta} excitation by the muon. We find that this method, when it neglects the momentum dependence of the transverse response, reproduces remarkably well the theoretical muon cross section. This success has lead us to apply the method to higher energies, where it has not been tested. We evaluate in this way the production of positive pions in liquid scintillator from known photo-absorption cross sections. At a shallow depth of 20 meters our estimate reproduces the measurement. As for the neutron emission, we include the obvious sources, such as the giant-resonance excitation, the quasideuteron process, the quasifree pion production as well as neutrons emitted following pion capture. Our evaluation underestimates the number of neutrons produced and finds a too weak dependence on the depth. This suggests that secondary neutron production is important at all depths.

Folded potential analysis of the excitation of giant resonances by heavy ions.

Measurements of the excitation of nuclear giant resonances by heavy-ion inelastic scattering have previously been analyzed using the deformed optical potential model. Here we reexamine these data using a folded potential model which employs a simple, but effective, nucleon-nucleon interaction that was deduced recently from heavy-ion elastic scattering measurements. The resulting estimates of the sum-rule exhaustion by the giant quadrupole resonance increase by amounts ranging from about 20% for the lighter targets to no change for $^{208}\mathrm{Pb}$. Applying the same model to data for excitation of the giant monopole resonance, we find that these transitions overexhaust the corresponding sum rule even more than previously indicated.

Anharmonicities and non-linearities in the excitation of double giant resonances

Abstract We investigate the non-linear response of a quantum anharmonic oscillator as a model for the excitation of giant resonances in heavy ion collisions. We show that the introduction of small anharmonicities and non-linearities can double the predicted cross section for the excitation of the two-phonon states. These findings suggest that such ingredients must be included in future more complete calculations in order to reduce the huge discrepancy between the previous theoretical predictions and the experimental cross section of double giant resonance states.

On the giant dipole resonance excitations in very hot nuclei

We discuss some of the ingredients necessary in the study of the excitation of the Giant Dipole Resonance in very hot nuclei.

Excitation of giant resonances in the 40Ca(e,e'n)39Ca reaction.

Decay neutrons from the $^{40}\mathrm{Ca}$(e,e'n${)}^{39}$Ca reaction were studied in the giant resonance region. The cross sections and angular distributions, separated for ${\mathit{n}}_{0}$ and ${\mathit{n}}_{1}$ decays, were obtained for excitation energies between 19 and 27 MeV, at the effective momentum transfer of 0.35 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. Legendre polynomial coefficients obtained from fitting the data are compared with those from the (e,e'p) reaction. In the energy range 19--21 MeV, the interference coefficients ${\mathit{b}}_{1}$ and ${\mathit{b}}_{3}$ for the ground state transition are in agreement, but the noninterference coefficient ${\mathit{b}}_{2}$ is different. The different behavior of the angular distribution for protons and neutrons may suggest the interference of the decay from a T=0 quadrupole resonance and the T=1 giant dipole resonance. A similar tendency was also seen in comparing with the (e,e'${\mathit{p}}_{1}$) reaction. The Legendre polynomial coefficients for the ${\mathit{n}}_{0}$ decay in the (e,e'n) reaction, transformed to the photon point, agree well with those of the (\ensuremath{\gamma},${\mathit{n}}_{0}$) reaction. The reduced total cross section is consistent between the (e,e'n) and (\ensuremath{\gamma},n) reactions, but the cross section for (e,e'${\mathit{n}}_{0}$) is larger than that of (\ensuremath{\gamma},${\mathit{n}}_{0}$) near the peak of the resonance. The values of the longitudinal-transverse interference term are close to zero in the present region, which is rather small compared with the value near the resonance of the (e,e'p) reaction.

Retardation of single-particle E1 transitions from the 622 keV broad d-wave neutron resonance in 9Be

Capture gamma rays from the 622 keV d-wave neutron resonance in 9Be with large reduced neutron width have been observed by means of a time-of-flight method, employing an anti-Compton NaI(TI) detector. Partial radiative widths have been derived for the strong primary E1 transition to the 3368 keV (2 +) state in 10Be and also for the weak one to the 5958 keV (2 +) state. Those widths were compared with predictions of the Lane-Mughabghab valence-capture model. As a consequence we found certain evidence to show that the 3368 keV state transition is appreciably hindered. The discrepancy can be removed by using a renormalized neutron effective charge which takes into account the coupling between the d-state neutron single-particle motion and the E1 giant resonance excitation of the target nucleus.

The localization of unoccupied 3d states in oxidized Sc

Inverse photoemission and 3p-3d giant resonance excitation in electron energy loss spectroscopy in oxidized Sc are related to 2p to 3d X-ray absorption in Sc2O3. The observations are shown to be consistent with 3d states being localized in both the presence and the absence of a core hole. Inverse photoemission reveals 3d crystal field splitting comparable to that found in 2p to 3d excitations, while 3p-3d excitations show in addition strong hole-electron exchange coupling.

Excitation and decay of electric giant resonances in the 40Ca(e,e'x) and 40Ca(p,p'x) reactions

The excitation and decay of electric giant resonances in 40Ca in the excitation energy range E x = 8 – 26 MeV was investigated with the (e,e'x; x=p,α) reaction for momentum transfers from 0.25 to 0.66 fm −1. The angular correlations of proton decay into low-lying states of 39K are successfully described by a RPA continuum approach emphasizing the importance of multistep contributions. The dominant reaction mechanism is extracted from a comparison of the branching ratios into resolved final channels with statistical model calculations and a fluctuation analysis. First results of a 40Ca(p,p'x) experiment covering the same 40Ca excitation region permit insight into the fine structure of the GQR and its decay.

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.

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.

New results on multiple excitations of giant resonances

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 to search for these new collective modes of the nucleus. The results discover two previously unobserved types of double giant resonances. This work presents the recent pion double charge exchange data and the analysis that support the existence of two such exotic vibrational nuclear modes.

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.

Test of lepton-flavour conservation in μ → e conversion on titanium

A search for μ → e conversion in muonic atoms is being performed at PSI with the SINDRUM II spectrometer. A first measurement on Ti gives upper limits on the branching ratios for the ground-state transitions of Γ(μ − Ti → e − Ti g.s. ) Γ(μ − Ti capture) < 4.3 × 10 −12 and Γ(μ − Ti → e + Ca g.s. ) Γ(μ − Ti capture) < 4.3 × 10 −12 (90% confidence) . With the assumption of a giant resonance excitation of the Ca nucleaus the limit on the total rate for μ − → e + conversion is Γ(μ − Ti → e + Ca ∗) Γ(μ − Ti capture) < 8.9 × 10 −11 .