Decay Of Giant Resonances Research Articles

Studies of the Giant Dipole Resonance in 27Al , 40Ca , 56Fe , 58Ni and 208Pb with high energy-resolution inelastic proton scattering under 0°

A survey of the fine structure of the Isovector Giant Dipole Resonance (IVGDR) was performed, using the recently commissioned zero-degree facility of the K600 magnetic spectrometer at iThemba LABS. Inelastic proton scattering at an incident energy of 200 MeV was measured on $^{27}$Al, $^{40}$Ca, $^{56}$Fe, $^{58}$Ni and $^{208}$Pb. A high energy resolution ($\rm{\Delta}\it{E} \simeq$ 40 keV FWHM) could be achieved after utilising faint-beam and dispersion-matching techniques. Considerable fine structure is observed in the energy region of the IVGDR and characteristic energy scales are extracted from the experimental data by means of a wavelet analysis. The comparison with Quasiparticle-Phonon Model (QPM) calculations provides insight into the relevance of different giant resonance decay mechanisms. Photoabsorption cross sections derived from the data assuming dominance of relativistic Coulomb excitation are in fair agreement with previous work using real photons.

A microscopic particle-vibration coupling approach for atomic nuclei. Giant resonance properties and the renormalization of the effective interaction

The self-consistent mean-field (SCMF) theory describes many properties of the ground state and excited states of the atomic nucleus, such as masses, radii, deformations and giant resonance energies. SCMF models are based on the independent particle picture where nucleons are assumed to move in a self-generated average potential. In the first part of this work, we apply a state-of-the-art SCMF approach, based on the Skyrme effective interaction, to two different excitations (viz. the pygmy dipole resonance and the isovector giant quadrupole resonance), investigating their relation with the nuclear matter symmetry energy, which corresponds to the energy cost for changing protons into neutrons and is a key parameter for the nuclear equation of state. However, SCMF models present well known limitations which require the inclusion of further dynamical correlations, e.g. the ones coming from the interweaving between single-particle and collective degrees of freedom (particle-vibration coupling - PVC). In the second part of this work, we report on the application to inclusive (namely, the strength function of giant resonances) and exclusive (the gamma decay of giant resonances) observables of a new self-consistent model based on the PVC idea in the Skyrme framework. In our model we use an effective interaction fitted at mean-field level to some selected experimental data. In principle, when these interactions are used in beyond mean-field theories, one would need to re-determine the parameters of the interaction at the same level of approximation. Moreover, due to the zero-range nature of the employed interaction, divergences arise. In the last and most innovative part of this thesis, we develop, for the first time in finite nuclei, a possible way to cure the divergences, paving the way to the possibility of producing an effective interaction fitted at PVC level.

Fine structure of the isovector giant dipole resonance inPb208: Characteristic scales and level densities

The IVGDR in 208Pb has been measured with high energy resolution with the (p,p') reaction under extreme forward angles and shows considerable fine structure. Characteristic scales are extracted from the spectra with a wavelet analysis based on continuous wavelet transforms. Comparison with corresponding analyses of B(E1) strength distributions from microscopic model calculations in the framework of the QPM and relativistic RPA allow to identify giant resonance decay mechanisms responsible for the fine structure. The level density of 1- states can be related to local fluctuations of the cross sections in the energy region of the IVGDR. The magnitude of the fluctuations is determined by the autocorrelation function. Scales in the fine structure of the IVGDR in 208Pb are found at 80, 130, 220, 430, 640, 960 keV, and at 1.75 MeV. The values of the most prominent scales can be reasonably well reproduced by the microscopic calculations although they generally yield a smaller number of scales. In both models the major scales are already present at the one-particle one-hole level indicating Landau damping as a dominant mechanism responsible for the fine structure of the IVGDR in contrast to the isoscalar giant quadrupole resonance, where fine structure arises from the coupling to low-lying surface vibrations. The inclusion of complex configurations in the calculations changes the E1 strength distributions but the impact on the wavelet power spectra and characteristic scales is limited. The level density of 1- states is extracted in the excitation energy range 9-12.5 MeV and compared to a variety of phenomenological and microscopic models. The back-shifted Fermi gas model parameterization of Rauscher et al., Phys. Rev. C 56, 1613 (1997) describes the level-density data well, while other phenomeological and microscopic approaches fail to reproduce absolute values or the energy dependence or both.

Gamma decay of giant resonances by using Skyrme functionals

A microscopic model, entirely based on Skyrme energy density lunctionals, has been developed to study the γ-decay ol giant resonances. In particular, it treats the ground-state decay within the fully self-consistent Random Phase Approximation (RPA) and the decay to low-lying states at the lowest order beyond RPA. This model is applied to 208Pb and the results are compared with experimental data and with previous theoretical results obtained, in the past decades, using phenomenological models.

Decay of giant resonances in light and medium-mass nuclei from coincidence experiments with charged particles

In recent years, the physics of electromagnetic interactions has made considerable advances in studying giant multipole resonances, which are highly excited collective states of nuclei. After the advent of new-generation continuous-beam accelerators, (e, e′x) coincidence experiments were performed, and new precision data on giant multipole resonances were obtained from these experiments. A number of examples of cross sections for decay accompanied by proton and alpha-particle emission in p-, sd-, and pf-shell nuclei of mass number in the range 16 ⩽ A ⩽ 64 are given. The relative contributions of these reactions are compared, their general and special features are revealed, and evaluations for semidirect and statistical decays are presented.

Dilepton decay of Giant Resonances built on excited nuclei

The dilepton yield from the statistical decay of Giant Resonances built on excited nuclei is discussed. An overview is given of the research leading to the experimental determination of the Giant Monopole Resonance on excited states. The data obtained for 28Si and 56 Ni, are presented, since they established so far the stringest experimental upper limits for the monopole strength of 0.45 and 4.3 nb, respectively. The future plans for the second generation experiments are discussed.

Feeding of hole states by proton decay of Gamow-Teller and isobaric analog state resonances.

The study of giant resonances in nuclei is attracting much attention both experimentally and theoretically @1,2#. The importance of these studies is that they may provide information about the continuum part of nuclear spectra as well as an understanding of the nuclear dynamics in the continuum. However, there is not much experimental evidence regarding partial decay widths from giant resonances—even the theoretical results are scarce. The reason for this is that the processes of formation and decay of giant resonances are time dependent and a proper treatment of such processes is a very difficult task. Experimentally, it is not easy to subtract the interesting signals from the background. Moreover, if the resonance is not isolated what one measures is not related to the resonance itself but rather to the result of the contribution of all the overlapping resonances, including their interference. The relativistic random-phase approximation ~RRPA !@ 3 # is a natural extension of the random-phase approximation ~RPA! which allows for the treatment of particle-hole states in the continuum. The eigenvalues of the RRPA can be complex due to the admixture of bound and resonant states in the unperturbed particle-hole basis ~Berggren’s representation !. The imaginary part of a complex eigenvalue is associated with the escape width of the state @4#. The formalism has been presented in detail previously and several applications of it have been given, particularly for the description of charge-conserving multipole excitations in 208 Pb @7#. As has been discussed in @4# the residues of the S matrix at a complex energy v n are the partial decay widths of the resonance n if the resonance is isolated ~nonoverlapping!. For such a resonance there exists a direct correspondence between the total escape width and the imaginary part of the energy, i.e., G n522Im(v n). This result has been confirmed for the case of the decay of the giant monopole resonance in 208 Pb @7# .I n the present case we shall show that similar results are also valid for charge-exchange modes, i.e., the isobaric analog ~IAS! and Gamow-Teller giant resonances ~GTGR’s! in 208 Bi.

On description of the direct nucleon decay of giant resonances

A semimicroscopical approach is formulated to describe the direct nucleon decay of various giant resonances in intermediate and heavy mass spherical nuclei. The approach consists in: (i) the exact continuum-RPA calculations for amplitudes of the reactions accompanied by excitation and the nucleon decay of the collective particle-hole states (doorway states); (ii) the Breit-Wigner parametrization of the RPA reaction-amplitudes in the vicinity of the giant giant-resonance energy and evaluation of the doorway-state parameter (the energy, entrance and partial nucleon, escape widths); (iii) the phenomenological consideration for the doorway-state coupling to many-particle configurations with the use of reasonable statistical assumptions and derivation on this base the expressions for the energy-averaged reaction amplitudes. The nuclear mean field and quasiparticle interaction are the input quantities for calculations. Examples of description for the direct nucleon decay of a number of giant resonances in the $^{208}Pb$ parent nucleus are given together with the comparison with corresponding experimental data.

Internal pair decay of giant resonances in hot 200Pb

Electron-positron pairs emitted during the de-excitation of hot 200Pb from the reaction 95 MeV 19 F + 181Ta were detected with the Stony Brook pair detector. The internal pair decay from the giant dipole resonance (GDR) built on excited states in a heavy nucleus is cleanly observed for the first time. The results show that the GDR dominates the pair decay from a hot, heavy nucleus. Superior statistics is needed in order to extract weaker modes like the monopole or quadrupole giant resonances. Attempts to extract E0 pair transitions by use of kinematic cuts and by dividing out the GDR pair strength are presented.

Dilepton decay of giant resonances built on excited states of 28Si.

Nuclear states in ${}^{28}\mathrm{Si}$ were populated at an initial excitation energy ${E}^{*}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50$ MeV via the isospin $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ reaction ${}^{16}\mathrm{O}{+}^{12}\mathrm{C}$. The dilepton- and photon-decay yields were measured simultaneously for the first time up to transition energies of 30 MeV. Statistical decay calculations describe well both measurements on an absolute scale. The measured absolute dilepton cross section, integrated over the transition energy region 15--30 MeV, amounts to $\ensuremath{\sigma}({e}^{+}{e}^{\ensuremath{-}})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}84\ifmmode\pm\else\textpm\fi{}6$ nb. Implications for measuring a possible monopole strength are discussed.

Internal pair decay of giant resonances in hot 200Pb

Electron-positron pairs with transition energies of <or=25 MeV emitted during the deexcitation of highly excited 200Pb produced in the 19F+181Ta reaction were detected in the Stony Brook pair detector in order to study internal pair conversion from giant resonances in a hot nucleus. The observed kinematic e+-e-variables show that the pair decay is overwhelmingly of E1 character. The extracted transition spectrum confirms the statistical model prediction that the giant dipole resonance in the hot (Ex=57 MeV) 200Pb nucleus dominates internal pair conversion from 4 to 20 MeV transition energy. E1 pair conversion coefficients extracted from a comparison with available gamma spectra obtained in the same reaction show that the point nucleus and Born approximation can be used for energies above 6.5 MeV even in this high-Z nucleus. Results from a search for e+-e-pairs from giant monopole resonances are presented.

Excitation and fission decay of 232Th by inelastic scattering of 17O at 84 MeV/u

Inelastic scattering of 84 MeV/u 17O projectiles has been used to excite the giant resonances (GR) in 232Th. The deduced parameters (width, energy, EWSR) of the isoscalar giant quadrupole resonance are in agreement with the known systematics. The observed giant monopole resonance strength, 100% of the EWSR, is consistent with other recent experimental results using heavy-ion projectiles. Strength is also observed at high excitation energy. The fission probability of GR in 232Th has been measured by detecting the fission fragments in coincidence with the inelastically scattered particles. A structure is observed in the fission coincidence spectrum around 10 MeV, which can be attributed to the fission decay of giant resonances. The measured fission probability is consistent with statistical decay of the GQR.

Excitation and photon decay of giant resonances from high-energy collisions of heavy ions

We develop closed-form expressions for the cross section for the electromagnetic excitation induced by heavy-ion collisions at intermediate and high energies. These expressions directly relate the excitation cross section to the corresponding photonuclear cross section. The effects of strong absorption, relativity and retardation are incorporated from the outset and are shown to be important. We apply our results to several situations of recent interest, including the excitation and photon decay of giant resonances.

On the direct nucleon decay of giant resonances

A method is proposed for calculating the partial nucleon widths of giant resonances basing on the random phase approximation in the continuum. Within the framework of this method the partial neutron widths of the isoscalar monopole giant resonance in the nucleus 208Pb were calculated. These results are compared with experimental data and with the results obtained within other theoretical approaches.

Gamma — decay of giant resonances

The partial widths of the γ-decay of the giant E2 and E4 resonances in 90 Zr and 208 Pub to low-lying states are calculated within the quasiparticle-phonon model. The complex configurations of the wave function can be concluded to be rather important for correct discription of the decay properties of highly excited nuclear states. It is shown that due to the core polarization the decay of the isovector resonances is more intensive compared to the isoscalar ones.

Neutron decay of the giant resonance region in 124Sn and especially of the giant monopole resonance

The decay of the isoscalar giant monopole and quadrupole resonances and of the underlying continuum in 124Sn has been studied with the 124Sn(α, α'n) 123Sn reaction. The data are compared with calculations based on the statistical model to extract the non-statistical decay component. The giant monopole resonance (GMR) decay mode is predominantly statistical; non-statistical processes account for 15% of the decay. Possible evidence for a pre-equilibrium decay branch of the GMR between 5% and 10% was found. The non-statistical fraction of the decay of the GMR decreases with increasing excitation energy. The contribution of neutron knock-out was found to be in qualitative agreement with PWIA calculations.

Statistical theory of decay of giant resonances

A general formulation of the decay of giant resonances is presented. Giant resonances are assumed to be excited by a γ-ray through the semidirect process, and decay by particle emission in compound or pre-equilibrium processes. The S-matrix for the semidirect process is evaluated and inserted into the general expression of the transmission matrices, which are used as inputs to the compound or pre-equilibrium fluctuation cross section. The results are compared with those of Dias et al. [Phys. Rev. Lett. 57 (1986) 1998], which are based on a different approach and have been used in analysis of the data.

Particle decay of the giant resonance region in 90Zr and especially of the giant monopole resonance

The decay of the isoscalar giant monopole and quadrupole resonances in 90Zr has been studied with the 90Zr(α, α'n) 89Zr and the 90Zr(α, α'p) 89 reactions. For the giant monopole resonance (GMR), a branching ratio for (neutron : proton : alpha) decay of (85(20) : 10(2) : (<2)) was found, in agreement with statistical model calculations. The neutron decay of the GMR is predominantly statistical, except for a 5% direct decay branch to the case9 2 + ground state of 90Zr. The proton decay spectrum of the GMR is also in overall agreement with statistical model calculations. The proton as well as neutron decay from the region of the giant quadrupole resonance (GQR) indicate that in the singles (α, α') cross section the L>2 excitation amounts to 25–50% of the L = 2 cross section.

Heavy-ion excitation and photon decay of giant resonances in 208Pb.

We have determined the ground-state photon decay probability and the branching ratios to a number of low-lying states as a function of excitation energy in $^{208}\mathrm{Pb}$ up to \ensuremath{\sim}15 MeV. Results for the giant quadrupole resonance at 10.6 MeV include determination of a ground-state electromagnetic transition strength B(E2)\ensuremath{\uparrow}=5800\ifmmode\pm\else\textpm\fi{}1600 ${e}^{2}$${\mathrm{fm}}^{4}$ and observation of strong suppression of the E1 transition from the resonance to the ${3}^{\mathrm{\ensuremath{-}}}$ state at 2.6 MeV. These results confirm the common assumption of the predominately isoscalar character of the 10.6-MeV resonance. Ground-state gamma coincidences enable us to isolate the isovector giant dipole resonance, which is too weak to be seen in singles, and to test models for its excitation in heavy-ion scattering.

Excitation and photon decay of giant multipole resonances — the role and future of medium-energy heavy ions

Inelastic scattering of medium energy heavy ions provides very large cross sections and peak-to-continuum ratios for excitation of giant resonances. For energies above about 50 MeV/nucleon, giant resonances are excited primarily through Coulomb excitation, which is indifferent to isospin, thus providing a good probe for the study of isovector giant resonances. The extremely large cross sections available from heavy ion excitation permit the study of rare decay modes of the giant resonances. In particular, recent measurements have been made of the photon decay of giant resonances following excitation by 22 and 84 MeV/nucleon 17O projectiles. The singles results at 84 MeV/nucleon yield peak cross sections for the isoscalar giant quadrupole resonance and the isovector giant dipole resonance of approximately 0.8 and 3 barns/sr, respectively. Data on the ground state decay of the isoscalar giant quadrupole and isovector giant dipole resonances are presented and compared with calculations. Decays to low-lying excited states are also discussed. Preliminary results from an experiment to isolate the 208Pb isovector quadrupole resonance using its gamma decay are presented.