Search Results (16 total)

The γ spectroscopy of ^25,27Ne and ^26,27Na was studied from the reaction of ²⁶Ne with a deuterium target in inverse kinematics at 9.7 MeV/nucleon. The selectivity of the (d,p),(d,t), and (d,n) transfer reactions provides new spectroscopic information on low-lying states. The validity of the sd shell-model space for these nuclei is discussed.

The spectroscopy of the unstable ⁸He and unbound ⁷He nuclei is investigated via the p(⁸He, d) transfer reaction with a 15.7A MeV ⁸He beam from the SPIRAL facility. The emitted deuterons were detected by the telescope array MUST. The results are analyzed within the coupled-channels Born approximation framework, and a spectroscopic factor C²S=4.4±1.3 for neutron pickup to the ⁷He_g.s. is deduced. This value is consistent with a full p3/2 subshell for ⁸He. Tentative evidence for the first excited state of ⁷He is found at E^*=0.9±0.5 MeV (width Γ=1.0±0.9 MeV). The second one is observed at a position compatible with previous measurements, E^*=2.9±0.1 MeV. Both are in agreement with previous separate measurements. The reproduction of the first excited state below 1 MeV would be a challenge for the most sophisticated nuclear theories.

To probe the ground state and transition densities, elastic and inelastic scattering on a proton target were measured in inverse kinematics for the unstable ¹⁰C and ¹¹C nuclei at 45.3 and 40.6 MeV/nucleon, respectively. The detection of the recoil proton was performed by the MUST telescope array, in coincidence with a wall of scintillators for the quasiprojectile. The differential cross sections for elastic and inelastic scattering to the first excited states are compared to the optical model calculations performed within the framework of the microscopic nucleon-nucleus Jeukenne-Lejeune-Mahaux potential. Elastic scattering is sensitive to the matter-root-mean square radius found to be 2.42±0.1 and 2.33±0.1 fm, for ^10,11C, respectively. The transition densities from cluster and mean-field models are tested, and the cluster model predicts the correct order of magnitude of cross sections for the transitions of both isotopes. Using the Bohr-Mottelson prescription, a profile for the ¹⁰C transition density from the 0⁺ ground to the 2₁⁺ state is deduced from the data. The corresponding neutron transition matrix element is extracted: M_n=5.51±1.09 fm².

The α+2n and t+t clustering of the ⁶He ground state were investigated by means of the transfer reaction ⁶He(p,t)⁴He at 25 MeV/nucleon. The experiment was performed in inverse kinematics at GANIL with the SPEG spectrometer coupled to the MUST array. Experimental data for the transfer reaction were analyzed by a distorted wave Born approximation (DWBA) calculation, including the two neutrons and the triton transfer. The couplings to the ⁶He→⁴He+2n breakup channels were taken into account with a polarization potential deduced from a coupled-discretized-continuum channels analysis of the ⁶He+¹H elastic scattering measured at the same time. The influence on the calculations of the α+t exit potential and of the triton sequential transfer is discussed. The final calculation gives a spectroscopic factor close to one for the α+2n configuration as expected. The spectroscopic factor obtained for the t+t configuration is much smaller than the theoretical predictions.

Central and mid-central collisions which lead to the formation of a heavy residue in the 37A MeV ³⁶Ar+⁹⁸Mo reaction have been studied with the MEDEA multidetector array coupled to a Parallel Plate Avalanche Counter. The dependence of the high energy gamma ray and light charged particle production as a function of the linear momentum transferred to the fused system has been studied. The unique potentialities of the MEDEA detector have made it possible to follow the evolution of the reaction dynamics from the pre-equilibrium stage to the formation of a heavy compound nucleus. The analysis of the correlation between the most energetic photons and protons shows how both of them are mainly produced in the most energetic primary nucleon-nucleon collisions. The multiplicity of high energy (E_γ>30 MeV) γ rays has been found to increase with the linear momentum transfer, showing the dominance of two-body dissipation in the transfer mechanism and giving a tool to correlate the momentum transfer with the centrality of the collision. Light charged particle kinetic energy spectra show how in these collisions, compound systems with excitation energies of more than 3A MeV and temperature up to 7 MeV are formed. The experimental findings are compared with Boltzmann-Nordheim-Vlasov calculations. A scenario is found where the nucleus-nucleus interaction starts with two-body nucleon-nucleon collisions in the overlap region of nuclear densities. These collisions give rise to the production of high energy nucleons and γ rays, and play a fundamental role in the energy transfer from the relative motion to internal excitation of the quasiprojectile and the target.

To study the effect of the weak binding energy on the interaction potential between a light exotic nucleus and a target, elastic scattering of ⁶He at 38.3 MeV/nucleon on a ¹²C target was measured at Grand Accélérateur National d’Ions Lourds (GANIL). The ⁶He beam was produced by fragmentation. The detection of the scattered particles was performed by the GANIL spectrometer. The energy resolution was good enough to separate elastic from inelastic scattering contributions. The measured elastic data have been analyzed within the optical model, with the real part of the optical potential calculated in the double-folding model using a realistic density-dependent nucleon-nucleon interaction and the imaginary part taken in the conventional Woods-Saxon (WS) form. A failure of the “bare” real folded potential to reproduce the measured angular distribution over the whole angular range suggests quite a strong coupling of the higher-order breakup channels to the elastic channel. To estimate the strength of the breakup effects, a complex surface potential with a repulsive real part (designed to simulate the polarization effects caused by the projectile breakup) was added to the real folded and imaginary WS potentials. A realistic estimate of the polarization potential caused by the breakup of the weakly bound ⁶He was made based on a parallel study of ⁶He+¹²C and ⁶Li+¹²C optical potentials at about the same energies.

The fusion-fission cross sections of the ⁴He+²³⁸U and ⁶He+²³⁸U systems have been measured, at Louvain-la-Neuve, for energies around and below the Coulomb barrier, using an array of Si detectors surrounding a UF₄ target. The data taken with ⁴He are in good agreement with previous data and with the coupled channel fusion calculation performed with ECIS. The ⁶He data show a regular trend with a large enhancement below the barrier which is attributed to the halo structure of the ⁶He nucleus.

We present the results of an experimental comparative study of the ^40,48Ca resonance spectra between 6 and 12 MeV. The power of the heavy ion reaction ^40,48Ca(⁸⁶Kr, ⁸⁶Kr^′)^40,48Ca^* at 60 MeV per nucleon was exploited to enhance greatly the low energy part of inclusive spectra in order to look for the possible presence of a low energy dipole mode in ⁴⁸Ca due to a neutron skin. We did not observe any difference in the l=1 channel and therefore found no evidence of this mode. In the l=2 channel, an important excess of strength was observed in ⁴⁰Ca compared to ⁴⁸Ca.

Gamma rays, light charged particles, and evaporation residues emitted from hot nuclei formed in the ³⁶Ar+⁹⁰Zr reaction at 27 MeV/nucleon have been measured at the GANIL facility with the 4π barium fluoride multidetector MEDEA. The combination of the residue and particle measurements shows that nuclei with masses around 115 and excitation energies between 350 and 550 MeV are produced. The γ spectra measured in coincidence with the evaporation residues exhibit three components: a low-energy statistical component, a high-energy contribution due to nucleon-nucleon bremsstrahlung during the initial stages of the collision, and a contribution from the decay of the giant dipole resonance built on highly excited states. The characteristics of the bremsstrahlung component are in agreement with previously published systematics. The γ yield from the decay of the giant dipole resonance remains constant over the excitation energy range studied. A comparison with other experiments shows that the N/Z asymmetry in the entrance channel does not affect the γ yield. Statistical calculations performed using the code CASCADE and supposing a fixed width and full sum rule strength for the dipole resonance strongly overpredict the data. The hypothesis of a continuously increasing width of the resonance with temperature gives a better agreement with experiment near the centroid of the resonance but overpredicts the γ spectra at higher energies. The best account of the data is given by assuming a cutoff of γ emission from the resonance above an excitation energy of approximately 250 MeV. This cutoff is discussed in terms of the time necessary to equilibrate the dipole oscillations with the hot compound nucleus. Finally, some evidence is given for a possible new low-energy component of the dipole strength at very high temperatures. © 1996 The American Physical Society.

Gamma rays emitted from hot nuclei with mass around 115 and excitation energies between 350 and 500 MeV, formed in the ³⁶Ar + ⁹⁰Zr reaction at 27 MeV/nucleon have been measured. The γ-ray yield from the decay of the giant dipole resonance in these nuclei remains constant over the excitation energy range studied. This quenching of the γ multiplicity cannot be explained by a continuous increase with temperature of the width of the resonance. Better agreement with the data is obtained by assuming a cutoff of γ emission from the resonance above an excitation energy of 250 MeV. The existing data do not show entrance channel effects.

The high-energy continuum from inelastic scattering of 84 MeV/nucleon ¹⁷O by ²⁰⁸Pb was studied by measuring ¹⁷ray coincidences. The particle spectra in coincidence with γ rays above ∼1 MeV show structures up to 45-MeV excitation energy that can be explained in terms of the statistical decay of the highly excited target nucleus. The observation in the γ-ray spectrum of the giant dipole resonance built on excited states in ²⁰⁸Pb confirms that the continuum up to ∼60-MeV excitation energy is dominated by the excitation of ²⁰⁸Pb and offers the unique possibility of detailed studies of the giant dipole resonance as a function of excitation energy decoupled from the influence of angular momentum.

We have investigated the excitation and ground-state photon decay of the giant resonance region in ²⁰⁸Pb and ²⁰⁹Bi due to inelastic scattering of 84 MeV/nucleon ¹⁷O. The inelastic spectra are dominated by the isovector giant dipole resonance. The spectra of ²⁰⁸Pb and ²⁰⁹Bi are almost identical in the giant resonance region. We conclude that the resonance strength functions are very nearly the same for these nuclei. Significant differences are, however, found in the distribution of ground-state decay photons from the two nuclei. These differences appear to be due to a large contribution from decay of fully damped compound states in ²⁰⁸Pb.

The excitation and electromagnetic decay of the giant resonance region (8–15 MeV) in ²⁰⁸Pb have been measured with the ²⁰⁸Pb(¹⁷O,¹⁷O’) reaction at 84 MeV/nucleon. The study of coincidences between scattered ¹⁷O ions and photon decays to the ²⁰⁸Pb ground state, including angular correlations, serves to isolate the isovector giant dipole resonance and enable us to investigate the mechanisms of its excitation and decay in detail. The angular correlations and yields are accounted for quantitatively by a pure Coulomb excitation model of the reaction process. The distribution in energy of the ground-state photon decay cross section is well described by an approximate application of the multistep theory of nuclear reactions. The photon coincidence measurements enable us to extract the differential cross sections for the 10.6-MeV giant quadrupole and 13.9-MeV monopole resonances with low uncertainty. An analysis of the Coulomb-nuclear interference for the 10.6-MeV excitation indicates that the ratio of neutron to proton matrix elements, M_n/M_p, is 1.7±0.4, consistent with a predominantly isoscalar character for the giant quadrupole resonance.

Masses, atomic numbers, and energies of heavy (M≥15 nucleons) particles emitted in the reactions induced by ⁸⁴Kr on ¹²C and ²⁷Al at 35 MeV/nucleon have been measured by means of two time-of-flight silicon counter telescopes. Experimental results from single and coincidence measurements are presented and discussed in the framework of the statistical sequential decay model following complete and incomplete fusion in the entrance channel.

The elastic-scattering angular distribution for the reaction of ¹⁶O on ¹²C has been measured at E_lab=1503 MeV. Optical-model analysis shows that the data determine the nuclear potential between 4 and 6 fm, i.e., well inside the strong-absorption radius. Above θ_c.m.=3°, the main contribution to the cross section comes from negative-angle scattering; however, the measured angular distribution is not sensitive to the position of the nuclear rainbow.

The inclusive energy spectra of the ¹⁶O(¹⁶O,¹²C)²⁰Ne alpha transfer reaction and of the ²⁸Si(¹⁶O,¹²C)³²S alpha transfer reactions have been studied at four different ¹⁶O incident energies: 50, 60, 68, and 72 MeV. The shape of the various energy spectra and the corresponding angular distributions are reproduced by a diffractional model calculation based on mutual excitation of the two residual partners.

NUCLEAR REACTIONS Alpha transfer induced by ¹⁶O on ¹⁶O and ²⁸Si. Energy spectra and angular distributions. Diffractional model fits.