Angular distributions and forward recoil range distributions of residues created in the interaction of 12C and 16O ions with 103Rh

We have measured the angular distributions and the forward recoil range distributions of residues produced in the interaction of, respectively, 151, 228 and 402 MeV ¹²C ions with ¹⁰³Rh and the forward recoil range distributions of residues produced in the interaction of 303 MeV ¹⁶O ions with ¹⁰³Rh. These data have been successfully reproduced by a theory which assumes that the dominant mechanisms are complete and incomplete fusion of the projectile with the target and single nucleon transfers from the projectile to the target and predicts that, starting from an incident energy of about 250 MeV, a large fraction of the residues has a mass and charge very close to those of the target nucleus. This is because, at incident energies of a few hundred MeV, a large fraction of the kinetic energy of ¹²C and ¹⁶O is carried away by fast ejectiles which then leave behind the intermediate equilibrated nuclei with a rather small excitation energy and small forward linear momentum.     

Heavy-ion inelastic scattering

The ²⁴Mg(¹⁶O, ¹⁶O'γ)²⁴Mg(2⁺) reaction has been investigated at 42 MeV incident energy. In-plane (¹⁶O, γ) angular correlations were measured for outgoing ¹⁶O ions between 6° and 40°. The results are compared with DWBA and coupled channels calculations.     

Heavy ion induced transfer reactions on 148Nd

Transfer reactions induced by ¹⁶O and ¹⁸O beams on ¹⁴⁸Nd were measured with a time-of-flight setup at 72 MeV incident energy. The angular distributions are bell shapes having their maxima at angles somewhat below the grazing angle. The excitation in the final nuclei takes place (if possible) near the optimum Q-value and is spread over 5 MeV for the one-particle transfer reactions and up to 10 MeV for the multiparticle transfers. The cross sections for the individual channels are explained mostly by Q-window considerations. In spite of the differences in the individual channels the total transfer cross section integrated over excitation energy, angle, and all channels turns out to be the same same for both ¹⁶O and ¹⁸O beams. This cross section amounts to 20 % of the total reaction cross section and nicely fills the gap between the measured fusion cross section and the total reaction cross section obtained from optical model calculations based on elastic scattering data.     

Transfer to the continuum of 14C via (18O, 16O) reaction

A study of the continuum of ¹⁴C populated by the ¹²C(¹⁸O, ¹⁶O)¹⁴C reaction at 84 MeV incident energy has been performed. The ejectiles have been momentum analyzed at forward angles by the MAGNEX magnetic spectrometer. The energy spectra were obtained up to about 20 MeV excitation energy. The scattering of two neutrons independently removed from the projectile as it passes the target nucleus has been described by means of an optical potential with a semi-classical approximation for the relative motion. The calculations describe a significant part of the continuum observed in the energy spectra. In particular, a resonance of the residual nucleus dominates the region near the two-neutron emission threshold.     

Energy dependence of the C(O, α) reaction

The energy dependence of the ¹²C(¹⁶O, α) reaction was measured at incident energies of E_lab=112−191 MeV. In the range ofE_x(²⁴Mg)=30−56 MeV, the excitation energies of the structures in the inclusive α spectrum were found to vary continuously as a function of incident energy in this region. This fact indicates that these structures do not represent excitations in ²⁴Mg, but rather that they originate from a different process such as a sequential ejectile decay.     

Gamma-ray spectroscopy with incomplete fusion reactions for the systems 16O +58,64Ni at 100 Me V

Energy spectra and angular distributions of ejectiles with 3 ⩽ Z ⩽ 7 have been measured for the reaction ¹⁶O on ⁶⁴Ni at 100MeV incident energy. Measured optimum Q-values and cross sections are accounted for by an incomplete fusion model. Coincidences between ejectiles detected at grazing angle and discrete γ-rays have been measured for the reactions 100 MeV ¹⁶O+^58,64Ni. The in-plane to out-of-plane anisotropies of discrete γ-rays show a high degree of spin polarization of incomplete fusion residues which can be exploited for spectroscopic studies.     

Resonant structures in the excitation functions of elastic and inelastic scattering of 16O on 24Mg

Excitation functions of ²⁴Mg(¹⁶O, ¹⁶O)²⁴Mg were studied between 25 and 64 MeV incident energy. A strongly resonant behaviour was observed for inelastic scattering at backward angles. The data were analysed in terms of coupled channel calculations. While agreement was good at forward angles, at backward angles the calculated cross sections are an order of magnitude too low suggesting that another reaction mechanism or neglected terms in the heavy ion interaction potential are important.     

Exact finite-range DWBA description of the60Ni(160,12C)64Zn reaction at 60 MeV incident energy

Cross section angular distributions of⁶⁰Ni(¹⁶O,¹²C)⁶⁴Zn reactions leading to three strongly excited states at 60 MeV incident energy and the¹⁶O+⁶⁰Ni and¹²C+⁶⁴Zn elastic scattering at 60 MeV respectively 45 MeV and 54 MeV have been measured using aQ3D magnetic spectrograph. EFR-DWBA calculations assuming the transfer of anα-cluster in its 0s ground state are able to describe the general features of the strongly oscillating experimental angular distributions using a surface transparent optical model potential. The optical model parameters used in the DWBA calculations are obtained from fits of the elastic scattering data of incident and exit channels. The importance of “correct” optical model parameters in the exit channels for relative spectroscopic factors will be discussed and the extracted relative spectroscopic factors will be compared to previous (⁶Li,d) results.     

Energy dependence of peripheral reactions induced by heavy ions

Collisions of ¹⁶O ions on targets of ⁹⁴Zr, ¹⁹⁷Au, ²⁰⁸Pb, and ²³²Th are investigated at incident energies of 140 and 315 MeV and compared to published data on peripheral collisions at 33.6 GeV. At 140 MeV, the isotope-production cross sections resemble the ground-state Q-value systematics characteristic of a partially-equilibrated, di-nuclear system formed in deeply-inelastic scattering. The yields are independent of shell and pairing effects in the target and residual nuclie. These Q-value systematics fail at 315 MeV, where the relative element yields are similar to those at 33.6 GeV. A theory of projectile fragmentation accounts for the shapes of the inclusive energy spectra as well as for the isotope yields at both energies. The relative cross sections are target independent (i.e., factorize) for the data at 315 MeV incident energy, but limiting fragmentation (i.e. isotope yields independent of energy) applies only at higher energies. The differential cross sections peal at the grazing angle or decrease exponentially, depending on the incident energy and the mass of the ejectile; the distributions are described by classical trajectories and diffraction models for which the reaction time is estimated to be typical of direct reactions. The kinetic energies of the reaction products depend primarily on their charge and only slightly on their mass number. For the 315 MeV results, the velocities of the reaction products at the maximum of the spectrum and the grazing angle are slightly less than the beam velocity and decrease rapidly for larger scattering angles. The results are interpreted with simple fraction and fragmentation models, and are compared to observations at 33.6 GeV.     

Trends of light particle spectra observed in nucleus-nucleus collisions

The emission of energetic light particles (p,d,t) has been studied for ¹⁶O induced reactions on Al, Zr and Au targets at the incident energies of 140, 215 and 310 MeV. The light-particle energy spectra have been analyzed in terms of a moving thermal source. The apparent temperatures exhibit a systematic variation as a function of the incident energy per nucleon above the Coulomb barrier. The observed trend can be extrapolated in a smooth fashion to temperatures obtained in relativistic heavy-ion collisions.     

On the energy dependence of the real central optical potential for proton-nucleus scattering

The energy dependence of the real central optical potential in the energy range 25–1000 MeV has been determined from optical model analyses of p+¹²C, p+¹⁶O, p+²⁷Al, p+⁴⁰Ca and p+²⁰⁸Pb elastic scattering data. The volume integral and the strength can be represented by a relation linear in the incident energy only if a limited energy range is chosen. When the energy range 25–1000 MeV is considered a logarithmic energy dependence gives a better representation of the phenomenological results, especially for the light nuclei.     

Emission of intermediate-mass fragments in the interaction of 16O with 59Co, 93Nb and 197Au

In this paper we study the emission of ⁸Be_gs, B and N fragments in the interaction of ¹⁶O ions with ⁵⁹Co, ⁹³Nb and ¹⁹⁷Au at incident energies varying from 6 to 25 MeV/nucleon. The spectra of these fragments, as well as those of C fragments studied in a previous paper, are dominated at forward angles by a component originating from break-up of ¹⁶O. At the higher incident energies break-up occurs after quite a sizeable projectile energy loss. Another mechanism which dominates at large emission angles, favours the emission of low-energy fragments and is attributed to the coalescence of nucleons during the cascade of nucleon-nucleon interactions by means of which the excited nuclei produced in the primary two-ion interaction thermalize. Received: 21 January 2003 / Accepted: 27 March 2003 / Published online: 5 June 2003     

Elastic and inelastic scattering of nucleons from 16O

Measurements of differential elastic and inelastic cross sections for neutron scattering from ¹⁶O at incident energies 18 to 26 MeV are presented. In addition to cross sections for neutron scattering differential cross sections for proton scattering up to 66 MeV are described in terms of phenomenological optical model potentials. At 24.5 MeV incident energy inelastic scattering up to 11.5 MeV excitation was measured. The elastic and inelastic compound nucleus contributions were examined. Direct inelastic scattering from the normal parity states was calculated using the DWBA and coupled-channel formalisms. The inelastic scattering cross section from non-normal parity state 2⁻ was calculated using the coupled-channel formalism via multi-step processes. Cross sections due to inelastic scattering from some of the states, which are thought to be members of an excited state rotational band were calculated using both vibrational and rotational approaches and were compared.     

Evidence of an isoscalar E3 giant resonance in 16O

Differential cross sections and analyzing powers for the inelastic scattering of polarized protons from ¹⁶O leading to the 2^? state at 8.88 MeV excitation have been measured at incident energies of 31.7, 33.8, 35.8, 36.8 and 39.9 MeV. These data have been analyzed using a distorted wave theory in which the effects of virtual excitation of E1, E2 and E3 giant resonances as doorway states are included explicitly. This analysis shows that the strong energy variation in the data between 30 and 40 MeV may be predominantly due to a new isoscalar E3 resonance with the contributions from the E1 and E2 resonances corroborating earlier findings.     

Unnatural parity levels populated in the 16O(6Li,d)20Ne reaction

The ¹⁶O(⁶Li, d)²⁰Ne reaction to the 2^?, 4.97 MeV, 3^?, 5.63 MeV, and 4^?, 7.01 MeV members of the K^π = 2^? band has been studied. Angular distributions were measured at 32 MeV from 7.5° to 145° (lab). Excitation functions were measured at 15° (lab) and 145° (lab) from 31 to 33 MeV and 31.75 to 32.5 MeV, respectively. Results of multi-step and compound nuclear calculations are compared to the data. At this incident energy, both mechanisms appear to contribute to the population of the unnatural parity levels.     

The orthogonality condition model applied to the inelastic two-step reaction process

An approximate method to take the Pauli principle into account is applied to the analysis of inelastic scattering of α-particles from the 8.88 MeV 2^? state of ¹⁶O, and is found to reproduce the magnitudes and the angular distributions of the cross sections at the incident energies of 40.5 and 65 MeV.     

Interpretation of airy minima in 16O+16O and 16O+12C elastic scattering in terms of a barrier-wave/internal-wave decomposition

The observation of refractive effects in ¹⁶O+¹⁶O and ¹⁶O+¹²C elastic scattering data has definitively established the fact that the optical potential for some light heavy-ion systems is relatively transparent and that its real part is deep. Most of the interpretations of the rainbow features of these data rely on the so-called nearside-farside decomposition of the scattering amplitude. Starting from recent optical model analyses of ¹⁶O+¹⁶O and ¹⁶O+¹²C elastic scattering around 100 MeV incident energy as an example, we present an alternative interpretation based on the barrier-wave/internal-wave decomposition first proposed by Brink and Takigawa. This method, which complements the nearside-farside approach, demonstrates clearly the exceptional transparency of the ¹⁶O+¹⁶O, and to a lesser extent ¹⁶O+¹²C, interactions at the investigated energies and makes possible the extraction of the two contributions whose interference explains the Airy oscillations seen in the farside amplitude.     

Study of new resonances at high excitation energy by the 120Sn(p,t)118Sn reaction at 35 MeV

The ¹²⁰Sn(p,t)¹¹⁸Sn reaction was investigated at 35 MeV incident energy. The ¹¹⁸Sn excitation energy spectrum was reconstructed up to about 16 MeV. Preliminary results show the presence of a broad resonance at high excitation energy, compatible with the predicted population of the Giant Pairing Vibration (GPV).     

New excited states of the nucleus129Xe

Excited states of¹²⁹Xe have been investigated through the¹²⁶Te(α,n)¹²⁹Xe reaction at the incident energy E=16MeV by means of in-beam gamma ray spectroscopy. Numerous levels below the excitation energy of 2.5MeV were found. Spin assignments were made from side feeding excitation functions. A new level at 274.2keV, which is suggested as spin 9/2 negtive parity bandhead, was observed for the first time.     

Selectivity of the 12C(18O, 16O)14C reaction

A study about the ¹²C(¹⁸O, ¹⁶O)¹⁴C two-neutron transfer reaction was performed at the Catania INFN-LNS laboratory at 84 MeV incident energy. The ¹⁶O ejectiles produced in the reactions were momentum analyzed and identified by the MAGNEX spectrometer. The Q-value spectrum of ¹⁴C shows several known bound and resonant states, in particular states with 2p-4h configuration respect to the ¹⁶O core. The integrated cross sections show an enhanced yield for the two-neutron transfer compared to the one-neutron transfer. These results are some experimental evidences that the (¹⁸O, ¹⁶O) reaction proceeds mainly by the direct transfer of the neutron pair, instead of a second order process.