Giant Resonance Region Research Articles

Asymmetry of the angular distributions of products of photoneutron direct-semidirect reactions in the region of an isovector giant quadrupole resonance

A semimicroscopic approach based on the random-phase approximation exactly taking into account a single-particle continuum and on a phenomenological description of the fragmentation effect is used to describe the basic properties of an isovector giant quadrupole resonance. The same approach is employed to perform a quantitative analysis of the asymmetry in the angular distributions of products of photoneutron “direct + semidirect” reactions in the region of such a resonance. The results of the calculations performed for the target nucleus 208Pb are compared with available experimental data.

Photoabsorption onHe4with a Realistic Nuclear Force

The 4He total photoabsorption cross section is calculated with the realistic nucleon-nucleon potential Argonne V18 and the three-nucleon force (3NF) Urbana IX. Final state interaction is included rigorously via the Lorentz integral transform method. A rather pronounced giant resonance with peak cross sections of 3.0 (3.2) mb is obtained with (without) the 3NF. Above 50 MeV strong 3NF effects, up to 35%, are present. Good agreement with experiment is found close to threshold. A comparison in the giant resonance region is inconclusive, since data do not show a unique picture.

Isovector Quadrupole Resonance in13C(γ,n) Reaction

The photoneutron cross section for 13 C was measured in the tagged photon energy range E γ =17–70 MeV. Substantial forward to backward asymmetry was observed above the giant dipole resonance region...

Isoscalar giant resonances for nuclei with mass between 56 and 60

The giant resonance region from 10 MeV $<{E}_{x}<62$ MeV in $^{56}\mathrm{Fe}$, $^{58}\mathrm{Ni}$, and $^{60}\mathrm{Ni}$ has been studied with inelastic scattering of 240 MeV $\ensuremath{\alpha}$ particles at small angles, including 0\ifmmode^\circ\else\textdegree\fi{}. Most of the expected isoscalar $E0$ and $E2$ strength has been identified below ${E}_{x}=40$ MeV. Between 56 and 72% of the isoscalar $E1$ strength has been located in these nuclei. The mass dependence of the giant monopole energy between $A=40$ and 90 is compared to relativistic and nonrelativistic calculations for interactions with compressibility of nuclear matter ${K}_{\mathrm{NM}}~211--225$ MeV.

Weak response of nuclei

We discuss some differences and similarities between electron and neutrino scattering off atomic nuclei. We find that, in the giant resonance region, the two processes excite different nuclear modes, therefore the weak and the electromagnetic nuclear responses are rather different. In any case, the scattering of electrons and photons is the best guide we have to test the validity of our nuclear models and their prediction power. The experience in describing electromagnetic excitations of the nucleus, suggests that, when the nucleus is excited in the continuum, the re-interaction between the emitted nucleon and the remaining nucleus should not be neglected. A simple model taking into account this final state interaction is proposed, and applied to the neutrino scattering off 16O nucleus.

A microscopic investigation of the transition form factor in the region of collective multipole excitations of stable and unstable nuclei

We have used a self-consistent Skyrme–Hartree–Fock plus continuum-RPA model to study the low-multipole response of stable and neutron/proton-rich Ni and Sn isotopes. We focus on the momentum-transfer dependence of the strength distribution, as it provides information on the structure of excited nuclear states and in particular on the variations of the transition form factor (TFF) with the energy. Our results show, among other things, that the TFF may show significant energy dependence in the region of the isoscalar giant monopole resonance and that the TFF corresponding to the threshold strength in the case of neutron-rich nuclei is different compared to that corresponding to the respective giant resonance. Perspectives are given for more detailed future investigations.

New facet in the study of Gamow–Teller transitions

We introduce high-resolution (3He, t) reaction at 0° and at an intermediate beam energy as a new spectroscopic tool for studying Gamow–Teller (GT) excitations. Owing to the high energy resolution of the reaction (≈ 30 keV), individual transitions can be observed up to the region of GT giant resonance. In addition, the strengths of GT transitions can now be compared with the strengths of analogous GT transitions from β decays, M1 transitions from γ decays or (e, e′) reactions. The study on GT transitions with astrophysical interest, isospin symmetry of nuclei and other related topics are discussed.

Electromagnetic dissociation of ultrarelativistic heavy ions and cross sections for photonuclear reactions in the region of a giant resonance

In calculating cross sections for the electromagnetic dissociation of heavy ions by the Weizsacker-Williams method, use is made of approximations and extrapolations of experimental data on photonuclear reactions. On the basis of the predicted cross sections for the mutual dissociation of nuclei in beams of ultrarelativistic colliders, it is proposed to measure, among other things, the yields of neutrons from such a dissociation in order to monitor the luminosity of accelerators. Considerable discrepancies between the results of different photonuclear experiments impose limitations on the accuracy of the method. The reasons behind these discrepancies are determined on the basis of a systematic analysis of available data on the cross sections for photoneutron reactions, and a method for removing them is proposed. By considering the example where new data on the dissociation of 208Pb nuclei at an energy of 30 GeV per nucleon are compared with the results of calculations, it is shown that the use of evaluated cross sections for partial photoneutron reactions of the (γ, n) and (γ, 2n) types makes it possible to improve the accuracy in calculating cross sections for electromagnetic dissociation.

Fine structure in the energy region of the isoscalar giant quadrupole resonance: characteristic scales from a wavelet analysis.

Fine structure in the energy region of the isoscalar giant quadrupole resonance in nuclei is observed in high-resolution proton scattering experiments at iThemba LABS over a wide mass range. A novel method based on wavelet transforms is introduced for the extraction of scales characterizing the fine structure. A comparison with microscopic model calculations including two-particle two-hole (2p2h) degrees of freedom identifies the coupling to surface vibrations as the main source of the observed scales. A generic pattern is also found for the stochastic coupling to the background of the more complex states.

Compression mode resonances inZr90

The giant resonance region from $10\phantom{\rule{0.3em}{0ex}}\text{MeV}l{E}_{x}l55\phantom{\rule{0.3em}{0ex}}\text{MeV}$ in $^{90}\mathrm{Zr}$ has been studied with inelastic scattering of $240\phantom{\rule{0.3em}{0ex}}\text{MeV}$ $\ensuremath{\alpha}$ particles at small angles including $0\ifmmode^\circ\else\textdegree\fi{}$. The isoscalar monopole resonance was found to contain $100\ifmmode\pm\else\textpm\fi{}12%$ of the $E0$ energy weighted sum rule with a centroid of $(17.81+0.32\ensuremath{-}0.20)\phantom{\rule{0.3em}{0ex}}\text{MeV}$. Eighty one percent of the isoscalar $E1$ energy weighted sum rule was located in two peaks having ${E}_{x}=(17.1\ifmmode\pm\else\textpm\fi{}0.4)$ and $(26.7\ifmmode\pm\else\textpm\fi{}0.5)\phantom{\rule{0.3em}{0ex}}\text{MeV}$, $\ensuremath{\Gamma}=(5.4\ifmmode\pm\else\textpm\fi{}0.3)$ and $(8.8\ifmmode\pm\else\textpm\fi{}1.0)\phantom{\rule{0.3em}{0ex}}\text{MeV}$, and containing $13\ifmmode\pm\else\textpm\fi{}3%$ and $70\ifmmode\pm\else\textpm\fi{}10%$, respectively, of the $E1$ energy weighted sum rule.

Cross Sections and Angular Distributions for the10B(e,p) and10B(e,d) Reactions in the Giant Resonance Region

The cross sections and angular distributions of (γ,p) and (γ,d) reactions on 10 B have been measured in the giant resonance region by the (e,p) and (e,d) reactions applying the virtual photon theory. The E2 cross section of the (γ,p 0 ) reaction derived by a model dependent analysis of the angular distributions shows contributions of 26.5 ±4.5% to the E2 energy weighted sum rule. The (γ,p 0,2 ) integrated cross section up to 27 MeV is 6.1 ±0.2% of the E1 sum rule value which is smaller than 30% for 10 B(γ,n). This suggests that the (γ,p) reaction leaves high-lying residual states as the prediction of the shell model calculation. The E2 strength deduced from the 10 B(γ,d 0 ) reaction is 0.14 ±0.05% of the E2 energy weighted sum rule, which is one order of magnitude smaller than the case of neighboring odd–odd nuclei; 6 Li and 14 N. This may be suppressed by a competition with (γ,d 1 ).

Isoscalar giant dipole resonance for several nuclei with A ≥ 90

The giant resonance region in several nuclei in A ≥ 90 have been re-measured using 240 MeV α particle scattering with a detector provides that both horizontal and vertical angles. Slice analyses with multipole fits were performed and strength due to both low and high energy components of the isoscalar giant dipole resonance (ISGDR) was identified. E1 strengths for the high energy component corresponding to 70±15%,68±15% and 67±14% of the isoscalar E1 energy-weighted sum rule were identified, with centroid energies of 26.3±0.6 MeV, 24.7±0.6 MeV and 21.7±0.6 MeV for90Zr,144Sm and208Pb, respectively. The centroid of the high energy component of the strength distribution is in closer agreement with the microscopic prediction generated using compressibilities obtained from the isoscalar giant monopole resonance data.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range fromA = 11 toA = 23 with mass-over-charge ratios up toA/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lyingE1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range fromA = 11 toA = 23 with mass-over-charge ratios up toA/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lyingE1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Isoscalar giant dipole resonance for several nuclei with A ≥ 90

The giant resonance region in several nuclei in A ≥ 90 have been re-measured using 240 MeV α particle scattering with a detector provides that both horizontal and vertical angles. Slice analyses with multipole fits were performed and strength due to both low and high energy components of the isoscalar giant dipole resonance (ISGDR) was identified. E1 strengths for the high energy component corresponding to 70±15%,68±15% and 67±14% of the isoscalar E1 energy-weighted sum rule were identified, with centroid energies of 26.3±0.6 MeV, 24.7±0.6 MeV and 21.7±0.6 MeV for 90Zr, 144Sm and 208Pb, respectively. The centroid of the high energy component of the strength distribution is in closer agreement with the microscopic prediction generated using compressibilities obtained from the isoscalar giant monopole resonance data.

Dipole excitations of neutron-proton asymmetric nuclei

Dipole excitations of unstable short-lived nuclei has been investigated experimentally by utilizing the electromagnetic-excitation process with high-energy secondary beams. From an exclusive measurement of the neutron-decay channels, differential cross sections with respect to excitation energy, which are directly related to the photo-absorption cross section and accordingly to the dipole-strength function, have been derived. Light neutron-rich nuclei in the mass range from A = 11 to A = 23 with mass-over-charge ratios up to A/Z≈ 2.8 have been investigated systematically. Much in contrast to stable nuclei, low-lying dipole excitations well below the giant dipole resonance region have been observed as a general phenomenon for these neutron-proton asymmetric nuclei. For the neutron-rich oxygen isotopes, for instance, low-lying E1 strength has been observed exhausting about 10% of the classical dipole sum rule below 15 MeV excitation energy. A quantitative analysis of low-lying threshold strength for loosely bound nuclei indicates that the characteristics of the dipole strength is directly related to the ground-state single-particle structure of the valence nucleon in the projectile.

Energy-weighted and non-energy-weighted Gamow-Teller sum rules in the relativistic random phase approximation

The non energy-weighted Gamow-Teller(GT) sum rule is satisfied in relativistic models, when all nuclear density-dependent terms, including Pauli blocking terms from nucleon-antinucleon excitations, are taken into account in the RPA correlation function. The no-sea approximation is equivalent to this approximation for the giant GT resonance state and satisfies the sum rule, but each of the total $\beta_-$ and $\beta_+$ strengths is different in the two approximations. It is also shown that the energy-weighted sum of the GT strengths for the $\beta_-$ and $\beta_+$ transitions in RPA is equal to the expectation value of the double commutator of the nuclear Hamiltonian with the GT operator, when the expectation value is calculated with the ground state in the mean field approximation. Since the present RPA neglects renormalization of the divergence, however, the energy-weighted strengths outside of the giant GT resonance region become negative. These facts are shown by calculating in an analytic way the GT strengths of nuclear matter.

Statistical and direct aspects of64Zn(γ,n)and(γ,np)decay channels in the giant dipole resonance and quasideuteron energy regions

The investigation of statistical and direct aspects related to the $(\ensuremath{\gamma},n)$ and $(\ensuremath{\gamma},np)$ decay channels of ${}^{64}\mathrm{Zn}$ in the giant dipole resonance (GDR) and quasideuteron (QD) energy regions was performed by a trial function fitting to the respective $(e,n)$ and $(e,np)$ electrodisintegration yields measured by residual activity. The trial function incorporated the GDR and QD models to describe the initial photoabsorption mechanism and the geometry dependent hybrid exciton model used in the ALICE/LIVERMORE-82 code to calculate the relevant branching ratios, with the $E1$ virtual photon spectra being calculated in the distorted wave Born approximation. We compared our results for the $(\ensuremath{\gamma},n)$ cross section with other existing experimental measurements, and the long-standing normalization issue among different laboratories was revisited and addressed. We obtained for the first time the absolute $(\ensuremath{\gamma},np)$ cross section from threshold to 60 MeV. We succeeded in separating statistical and direct contributions of the $(\ensuremath{\gamma},np)$ process, the latter being remarkably well described by the QD model in the interval 40--60 MeV. A possible direct contribution for the $(\ensuremath{\gamma},n)$ decay in the GDR is also addressed. Finally, the total photoabsorption cross section of ${}^{64}\mathrm{Zn}$ was reevaluated up to 21 MeV, and the results were compared with previous estimates performed by other groups.

Nuclear reactions with 14 MeV neutrons and bremsstrahlungs in giant dipole resonance (GDR) region using small accelerators

In 1974 an accelerator of deterium, namely neutron generator NA-3-C was put into operation and in 1982 another accelerator of electron Microtron MT-17 started its work in the Institute of Physics. Though very modest these accelerators are useful for developing countries as Vietnam in both Nuclear Physics Research and Training. In this report we present some results obtained in studies on Nuclear Data, Nuclear Reactions as well as nuclear activation analysis methods. We also would like to discuss about the possibilities of collaboration in the future.

Continuum quasiparticle random phase approximation and the time-dependent Hartree-Fock-Bogoliubov approach

Quadrupole excitations of neutron-rich nuclei are analyzed by using the linear response method in the Quasiparticle Random Phase Approximation (QRPA). The QRPA response is derived starting from the time-dependent Hartree-Fock-Bogoliubov (HFB) equations. The residual interaction between the quasiparticles is determined consistently from the two-body force used in the HFB equations, and the continuum coupling is treated exactly. Calculations are done for the neutron-rich oxygen isotopes. It is found that pairing correlations affect the low-lying states, and that a full treatment of the continuum can change the structure of the states in the giant resonance region.