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1.
《Nuclear Physics A》1998,636(4):427-451
A systematic study of the damping of the giant dipole resonance (GDR) in 90Zr, 120Sn and 208Pb as a function of temperature T is performed. The double-time Green function technique is employed to determine the single-particle and GDR dampings. The single-particle energies, obtained in the Woods-Saxon potential for these nuclei, are used in the calculations. The results show that the coupling of collective vibration to the pp and hh excitations, which causes the thermal damping width, is responsible for the enlargement of the total width with increasing temperature up to T ≈ 3MeV and its saturation at higher temperatures. The quantal width, which arises from the coupling of the collective mode to the ph excitations decreases slowly with increasing temperature. The effect of single-particle damping on the GDR width is small. The results are found in an overall agreement with the experimental data for the GDR width, obtained in the inelastic α scattering and heavy-ion fusion reactions at excitation energies E* ⩽ 450 MeV. At high excitation energies (E* > 400 MeV) a behavior similar to the transition from zero to ordinary sounds is observed.  相似文献   

2.
《Nuclear Physics A》1997,614(2):217-237
The properties of the giant-dipole resonance (GDR) are calculated as a function of excitation energy, angular momentum, and the compound nucleus particle decay width in the nuclei 120Sn and 208Pb, and are compared with recent experimental data. Differences observed in the behavior of the full-width-at-half-maximum of the GDR for 120Sn and 208Pb are attributed to the fact that shell corrections in 208Pb are stronger than in 120Sn, and favor the spherical shape at low temperatures. The effects shell corrections have on both the free energy and the moments of inertia are discussed in detail. At high temperature, the FWHM in 120Sn exhibits effects due to the evaporation width of the compound nucleus, while these effects are predicted for 208Pb.  相似文献   

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A study of the spectral and angular distributions of the high energyγ rays emitted by the giant dipole resonance (GDR) in the compound nuclei165,167Er and110Sn at a variety of excitation energiesE *=50–90 MeV) is presented. A comparison of the data with model predictions at the equilibrium deformation and with thermal fluctuations in the adiabatic limit is discussed. In general, the adiabatic calculations give a fair account of the data exception made for Sn at very high spins.  相似文献   

6.
Inelastic scattering of α-particles can excite the isovector giant dipole resonance (GDR) via the Coulomb interaction. In spite of their isoscalar nature α-particles can also excite the GDR via the nuclear interaction due to the difference in the radii of the neutron and proton density distributions. The absolute cross section to excite the GDR in inelastic α-scattering is therefore a measure of this radial difference, the so-called neutron-skin thickness. Furthermore, since the GDR strength distribution has a centroid energy which depends on the nuclear radius, these studies, when performed in deformed nuclei, can measure the neutron-skin thickness along both the short and the long axes independently. Results of an experiment performed at KVI atE α=120 MeV and small scattering angles, including 0°, to determine the neutron-skin thickness in208Pb,116Sn,124Sn, and the deformed150Nd are discussed and compared to earlier measurements and theoretical predictions. Future improvements in the experimental set-up are also discussed.  相似文献   

7.
Continuumγ- ray spectra from the decay of59Cu formed at an excitation energy of 100 MeV and angular momenta up to 43? by means of the reaction 190 MeV32S +27Al have been measured and analyzed. The parameters of the Giant Dipole Resonance (GDR) have been extracted using the statistical model. The derived GDR width confirms the sizeable broadening of this resonance in59Cu already reported in our earlier investigation at 77 MeV excitation energy (Jcrit=38?). Estimates of the GDR width have been performed in the adiabatic approximation. Predicted values account qualitatively for the experimental data of59Cu as well as of the heavier isotope63Cu, in which the broadening was not seen up to 77MeV excitation (Jcrit=35?). The present analysis demonstrates the strong sensitivity of the GDR to spin effects in this mass region.  相似文献   

8.
Differential cross-section angular distributions for the elastic scattering of 270 MeV3He particles from58Ni,90Zr,116Sn and208Pb have been measured. Optical model analysis of the cross-sections has yielded the optical model parameters for3He particles at 270 MeV. Angular distributions have also been measured for the inelastic excitation of the low-lying levels in the above mentioned nuclei. A collective model analysis using the distorted wave Born approximation (DWBA) of these cross-sections with the distorted waves generated by the optical model parameters determined from the elastic scattering analysis, has yielded the reduced transition probability (B(EL)) values consistent with those reported in the literature.  相似文献   

9.
In a systematic study of nucleon transfer reactions accompanied by Coulomb excitation we have bombarded152Sm,160Gd and232Th with206, 208pb beams at incident energies close to the Coulomb barrier. Particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and in nucleon transfer reactions. One-neutron stripping and pick-up reactions on152Sm were observed leading to known states of the rotation alignedi 13/2-bands in153Sm and151Sm. In the160Gd+206, 208Pb systems no significant population of low lying states of product nuclei was found in the nucleon-transfer channels. Large cross sections were observed for one- and two-neutron pick-up from232Th at an incident energy of 6.4 MeV/u. Around the grazing angle they are of the same order of magnitude as the cross section measured for inelastic scattering. The results are analyzed in the framework of semiclassical models.  相似文献   

10.
《Physics letters. [Part B]》1987,191(3):222-226
We study the damping width of the giant dipole resonance (GDR) built on the ground state and on excited states of 90Zr, which is spherical at zero spin and temperature, and 164Er which is deformed. Neither of these nuclei changes appreciably their equilibrium shape when excited. Nonetheless, fluctuations due to finite temperature produce a marked increase in the width of the GDR of 90Zr, leaving essentially unchanged that of 164Er. This is because in the case of deformed nuclei thermal fluctuations imply the sampling of both larger and smaller deformations around the equilibrium shape, while spherical nuclei can only feel larger distortions. In spite of the stability of the GDR of 164Er, the angular distribution of the associated γ-rays is strongly affected by the temperature.  相似文献   

11.
Quasielastic and deep inelastic processes in the reaction 16O + 208Pb have been studied at bombarding energies of 10–80 MeV per nucleon. The excitation probability of the giant modes becomes larger with increasing energies. At a given bombarding energy the multiple excitation of low-lying modes becomes predominant as the angle of observation moves away from the grazing angle. Near grazing, no appreciable background of this character is found for bombarding energies above 30 MeV per nucleon. At these energies the electromagnetic interaction becomes the most effective way to excite the giant modes in the excitation energy interval of 10–20 MeV. The importance of Coulomb excitation for the highest bombarding energies makes the excitation of isovector modes no longer negligible.  相似文献   

12.
The angular distribution of γ-rays from fast proton capture in 208Pb has been examined in detail by the direct-semidirect model. The interference of the dipole, quadrupole and octupole amplitudes was considered. The excitation of the isovector giant quadrupole resonance was assumed to occur at the excitation energy of 22.5 MeV, having the width of 5 MeV. The excitation of this resonance strongly influences the asymmetry of the angular distribution but the major part of the asymmetry is a consequence of the direct capture of the incident proton. Several groups of optical-model parameters were used in the calculation in order to see how they affect the results. The complex interaction coupling between the incident proton and the quadrupole giant resonance is essential to obtain good fits to the experimental asymmetry of the angular distribution of γ-rays.  相似文献   

13.
S MUKHOPADHYAY 《Pramana》2014,82(5):779-787
High energy photons from the decay of giant dipole resonances (GDR) built on excited states provide an excellent probe in the study of nuclear structure properties, damping mechanisms etc., at finite temperatures. The dependence of GDR width on temperature (T) and angular momentum (J) has been the prime focus of many experimental and theoretical studies for the last few decades. The measured GDR widths for a wide range of nuclei at temperatures (1.5 < T < 2.5 MeV) and spins (upto fission limit) were well described by the thermal shape fluctuation model (TSFM). But, at low temperatures (T < 1.5 MeV) there are large discrepancies between the existing theoretical models. The problem is compounded as there are very few experimental data in this region. At Variable Energy Cyclotron Centre, Kolkata, a programme for the systematic measurement of GDR width at very low temperatures has been initiated with precise experimental techniques. Several experiments have been performed by bombarding 7–12 MeV/nucleon alpha beam on various targets (63Cu, 115In and 197Au) and new datasets have been obtained at low temperatures (T < 1.5 MeV) and at very low spins ( $J < 20 \hbar $ ). The TSFM completely fails to represent the experimental data at these low temperatures in the entire mass range. In fact, the GDR width appears to be constant at its ground state value until a critical temperature is reached and subsequently increases thereafter, whereas the TSFM predicts a gradual increase of GDR width from its ground state value for T > 0 MeV. In order to explain this discrepancy at low T, a new formalism has been put forward by including GDR induced quadrupole moment in the TSFM.  相似文献   

14.
The region of the giant resonances in 208Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3?, 5?, 2+ and 4+ collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV (Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV (Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV (Γ = 2.6 MeV).A resonance located at 21.5 MeV (Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).  相似文献   

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16.
《Physics letters. [Part B]》1986,167(4):379-382
The inelastic scattering of 600 MeV 20Ne from 208Pb has been reinvestigated. Cross sections for giant resonance excitation have been extracted and analysed in terms of DWBA. No evidence has been found for higher excitation energy structures.  相似文献   

17.
Elastic scattering cross sections of 86 MeV/N 12C ions on 12C, NATCa, 89Y and 208Pb targets has been measured together with inelastic scattering to the 4.4 MeV state of 12C. There is some indication for giant (quadrupole) resonance excitation in 40Ca. Optical model and DWBA analyses are reported. Nuclear transparency effect is discussed.  相似文献   

18.
《Nuclear Physics A》1997,617(3):316-330
Double-differential cross sections of the 208Pb(n,p) reaction have been measured at 97 MeV in the angular range 0°–30° for excitation energies up to 40 MeV. The experimental proton spectra have been compared with calculated spectra obtained with a statistical multistep direct reaction theory, in which charge exchange and inelastic response functions are described microscopically in the quasiparticle random phase approximation. The direct parts of the spectra have also been distributed on different multipole components by using a decomposition technique, based on sample angular distributions calculated within the distorted-wave Born approximation.  相似文献   

19.
《Nuclear Physics A》1986,452(3):513-530
A study is made of the electromagnetic decay of the 208Pb giant quadrupole resonance excited by inelastic scattering of 17O at 380 MeV. Due to the instability of the giant quadrupole resonance with respect to compound-nucleus formation and to neutron emission, the time over which the radiation is produced is not much greater than the time in which the direct reaction occurs. Angular distributions of the γ-rays produced in transitions to several low-lying states of the residual nucleus are calculated and their dependence on effects due to the proximity of the interacting nuclei is discussed.  相似文献   

20.
The matrix element for inelastic scattering of protons leading to the excitation of collective states in even-even nuclei is calculated in the framework of Dirac phenomenology using a DWBA approach. The deformed parts of the Lorentz scalar and four-vector optical potentials serve as the transition operators. The results are compared with the recently measured polarization transfer coefficients on 40Ca and 208Pb at 500 MeV. The agreement with experiment is good.  相似文献   

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