Search

Multineutron and multiproton transfer channels, populated in the inverse kinematics reaction 197Au+130Te at Elab=1.07 GeV, were measured at Laboratori Nazionali di Legnaro using the presently heaviest ion beam delivered by the PIAVE-ALPI accelerator complex and detecting both projectile-like and targetlike ions. To this end the large solid angle magnetic spectrometer PRISMA was coupled to a second arm for the detection of the heavy fragments in kinematic coincidence with the light ones selected and identified with the spectrometer. The data analysis is still in progress and will allow to compare the yields of both light and heavy partner with theoretical predictions performed with the GRAZING code to get quantitative information on transfer channels and the effect of evaporation and fission on the production rate of primary fragments. The mass integrated Z distribution, extracted from the experimental data, evidenced the population of proton pickup channels that, in conjunction with the neutron stripping ones from the 130Te, open the path for the production of neutron-rich heavy nuclei. In the following, we will present some preliminary results as well as details on the experimental configuration and perspectives for future investigations in the neutron-rich heavy region.

Excitation functions of one- and two-neutron transfer channels have been measured for the closed shell 40Ca+96Zr and superfluid 60Ni+116Sn systems from the Coulomb barrier energy to energies far below. By using the unique PRISMA performance in terms of resolution and efficiency, measurements at very low bombarding energies have been performed. In the case of the 60Ni+116Sn system, a complementary particle-γ coincident measurements provide the ground and excited state populations. The experimental transfer probabilities have been compared with semiclasical microscopic calculations that incorporate nucleon-nucleon correlations, together with known structure information of entrance and exit channels nuclei and reaction dynamics. These calculations well reproduce, in the whole energy range, one- and two-neutron transfer channels in shape and magnitude.

The fusion excitation function for the positive-Q-value system 12C+30Si (Qfus = +14.1 MeV) has been measured in inverse kinematics down to the μb level and compared with standard coupled-channel calculations. The appearance of the fusion hindrance phenomenon and the evidence of a S-factor maximum have been observed. This result can be significant to extrapolate the behavior of lighter astrophysical relevant systems at deep sub-barrier energies, where existing experimental data are still contradicting and affected by large errors.

The near- and sub-barrier fusion excitation function has been measured for the system 30Si+30Si at the Laboratori Nazionali di Legnaro of INFN, using the 30Si beam of the XTU Tandem accelerator in the energy range 47 - 90 MeV. A set-up based on a beam electrostatic deflector was used for detecting fusion evaporation residues. The measured cross sections have been compared to previous data on 28Si+28Si and Coupled Channels (CC) calculations have been performed using M3Y+repulsion and Woods-Saxon potentials, where the lowlying 2+ and 3− excitations have been included. A weak imaginary potential was found to be necessary to reproduce the low energy 28Si+28Si data. This probably simulates the effect of the oblate deformation of this nucleus. On the contrary, 30Si is a spherical nucleus, 30Si+30Si is nicely fit by CC calculations and no imaginary potential is needed. For this system, no maximum shows up for the astrophysical S-factor so that we have no evidence for hindrance, as confirmed by the comparison with CC calculations. The logarithmic derivative of the two symmetric systems highlights their different low energy trend. A difference can also be noted in the two barrier distributions, where the high-energy peak present in 28Si+28Si is not observed for 30Si+30Si, probably due to the weaker couplings in last case.

Fusion cross sections have been measured in the 40Ca + 58Ni and 40Ca + 64Ni systems at energies around and below the Coulomb barrier. The 40Ca beam was delivered by the XTU Tandem accelerator of the Laboratori Nazionali di Legnaro and evaporation residues were measured at very forward angles with the LNL electrostatic beam deflector. Coupled-channels calculations were performed which highlight possible strong effects of neutron transfers on the fusion below the barrier in the 40Ca + 64Ni system. Microscopic time-dependent Hartree-Fock calculations have also been performed for both systems. Preliminary results are shown.

In order to better understand the influence of transfer in sub-barrier nuclear reactions, cross sections for the system 58,64Ni+124Sn have been measured down to 0.5-1 µb and compared to detailed coupledchannel calculations. In agreement with a phenomenological Q-value systematics, calculations show the importance of including the coupling to the transfer channel for these heavy systems. No clear evidence of fusion hindrance is observed, probably due to the fact that the cross sections measured in this experiment are not low enough for the appearance of that phenomenon.

Early data of sub-barrier fusion teached us that cross sections may strongly depend on the structure of colliding nuclei and on couplings to transfer channels. The influence of transfer is clearly indicated in the excitation functions of different nickel isotopes and various Ca+Zr systems. Fusion barrier distributions often yield the fingerprint of the relevant inelastic and transfer couplings. At lower energies, far below the barrier the slope of the excitation function keeps increasing in many cases, so that the cross sections are strongly over-predicted by standard coupled-channels (CC) calculations; this was named a hindrance effect. Furthermore, light heavy-ion systems show cross section oscillations above the Coulomb barrier. Recent experiments have been performed on the fusion of 28,30Si+28,30Si systems where all phenomena cited above show up. In particular regular oscillations that have been revealed above the barrier for 28Si+28Si and have been interpreted as the consequence of the strong channel couplings and/or the oblate deformation of 28Si.

In the last period, two classes of experiments have been carried out with the large acceptance magnetic spectrometer PRISMA. In particular, the one- and two-neutron transfer processes at energies ranging from the Coulomb barrier to deep below it and the population of exotic neutron rich nuclei in the A~130 and A~200 mass regions have been studied. Both kinds of experiments have been performed in inverse kinematics identifying in A, Z and velocity the light target-like recoils with PRISMA placed at very forward angles in order to have, at the same time, high efficiency and good energy and mass resolutions.

Strong coupling to a single channel with zero Q-value is predicted to produce a characteristic fusion barrier distribution with two peaks, one on each side of the original uncoupled Coulomb barrier. In practical cases, only coupling to an elastic transfer channel may produce such a distribution which, however, has never been observed sofar, probably because low-lying surface vibrations usually have a dominant role, and this may obscure the two-peak structure. The case of the two-neutron (2n) elastic transfer in 42Ca + 40Ca is particularly attractive, because of the relatively rigid nature of the two nuclei. We have measured the fusion excitation function of this system using the 42Ca beam of the XTU Tandem of LNL on a thin 40Ca target enriched to 99.96% in mass 40. Cross sections have been measured down to ≤1 mb. The extracted barrier distribution shows clearly two main peaks. We have performed preliminary CC calculations where the 2+ coupling strengths have been taken from the literature and the schematic 2n pair transfer form factor has been used, with a deformation length σt= 0.39 fm. The excitation function is well reproduced by the calculation including the 2n transfer channel. However, including the octupole excitations destroys the agreement.

The fusion excitation function of 48Ti + 58Fe has been measured in a wide energy range around the Coulomb barrier, covering 6 orders of magnitude of the cross sections. We present here the preliminary results of this experiment, and a full comparison with the near-by system 58Ni + 54Fe where evidence of fusion hindrance shows up at relatively high cross sections. The sub-barrier cross sections of 48Ti + 58Fe are much larger than those of 58Ni + 54Fe. Significant differences are also observed in the logarithmic derivatives, astro-physical S-factors and fusion barrier distributions. The influence of low-energy nuclear structure on all these trends is pointed out and commented. Coupled-channels calculations using a Woods-Saxon potential are able to reproduce the experimental results for 48Ti + 58Fe. The logarithmic derivative of the excitation function is very nicely fit, and no evidence of hindrance is observed down to around 1 μb. The fusion barrier distribution is rather wide, flat and structureless. It is only in qualitative agreement with the calculated distribution.

Fusion cross sections have been measured in the 40Ca + 58Ni and 40Ca + 64Ni systems at beam energies ranging from Elab = 104.75 MeV to 153.5 MeV using the Laboratori Nazionali di Legnaro electrostatic deflector. Distributions of barriers have been extracted from the experimental data. Preliminary coupled channel calculations were performed and hints of effects of neutron transfers on the fusion below the barrier in the 40Ca + 64Ni are discussed.

Spin distribution measurements were performed for the reaction 64Ni + 100Mo at three beam energies ranging from 230 to 260 MeV. Compound nucleus (CN) spin distributions were obtained channel selective for each evaporation residue populated by the de-excitation cascade. A comparison of the spin distribution at different beam energies indicates that its slope becomes steeper and steeper with increasing beam energy. This change in slope of the spin distribution is mainly due to the onset of fission competition with particle evaporation at higher beam energies.

The sub-barrier fusion excitation function of 40Ca + 96Zr has been measured down to cross sections ≃2.4µb, i.e. two orders of magnitude smaller than obtained in the previous experiment, where the sub-barrier fusion of this system was found to be greatly enhanced with respect to 40Ca + 90Zr, and the need of coupling to transfer channels was suggested. The purpose of this work was to investigate the behavior of 40Ca + 96Zr fusion far below the barrier. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with 48Ca + 96Zr is very useful in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon Q >0 transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few µb. Locating the hindrance threshold, if any, in 40Ca + 96Zr would require challenging measurements of cross sections in the sub-µb range.

This paper is devoted mainly to the fusion hindrance phenomenon, in its various aspects. Recent experimental results on medium-light systems where the fusion Q-value is positive are discussed. The application of the coupled-channels model using a shallow ion-ion potential is illustrated for 32,36S + 48Ca. The detailed influence of nuclear structure on the low-energy fusion cross sections is shown for the pair of systems 48Ti + 58Fe and 58Ni + 54Fe, where the low-lying quadrupole modes have a different degree of collectivity. The sub-barrier excitation function of 48Ti + 58Fe is much larger than for 58Ni + 54Fe. The lighter symmetric system 28Si +28Si has been the object of recent experimental investigations. Its fusion cross sections have been measured in a wide energy range down to ≤1µb. Above the barrier, we have a clear indication of oscillations in the excitation function, probably due to the penetration of successive centrifugal barriers, that are in rather good agreement with previous calculations. The CC model using the shallow M3Y+ repulsion potential is able to reproduce also the sub-barrier part of the excitation function of 28Si +28Si.

We measured excitation functions for the main transfer channels in the 116Sn+60Ni reaction from above to well below the Coulomb barrier. The experiment has been performed in inverse kinematics, detecting the lighter (target-like) ions with the magnetic spectrometer PRISMA at very forward angles. The comparison between the data and microscopic calculations for the present case and for the previously measured 96Zr+40Ca system, namely superfluid and near closed shells nuclei, should significantly improve our understanding of nucleon-nucleon correlation properties in multinucleon transfer processes.

The detection of the 1809 keV emission line associated with the decay of 26Al in the interstellar medium provides a direct evidence of recent nucleosynthesis events in our galaxy. 26Al is thought to be mainly produced in massive stars, but in order to have a quantita- tive understanding of the 26Al distribution, the cross section of all the nuclear reactions involved in its production should be accurately known. A recent sensitivity study demonstrated that the 25Mg(α,n)28Si is the reaction with the strongest impact on the synthesis of 26Al during explosive Neon and Carbon burning [4]. In order to improve the experimental knowledge of the 25Mg(α,n)28Si cross section, a new direct measurement has been performed at Legnaro National Laboratories. The experimental setup, the data analysis and preliminary results are discussed.

The neutron-rich isotopes of tungsten, osmium and platinum have different shapes in their ground states and present also shape transitions phenomena. Spectroscopic information for these nuclei is scarce and often limited to the gamma rays from the decay of isomeric states. For the neutron-rich even-even osmium isotopes 194Os and 198Os, a shape transition between a slightly prolate deformed to an oblate deformed ground state was deduced from the observed level schemes. For the even-even nucleus lying in between, 196Os, no gamma ray transition is known. In order to elucidate the shape transition and to test the nuclear models describing it, this region was investigated through gamma-ray spectroscopy using the AGATA demonstrator and the large acceptance heavy-ion spectrometer PRISMA at LNL, Italy. A two-nucleon transfer from a 198Pt target to a stable 82Se beam was utilized to populate medium-high spin states of 196Os. The analysis method and preliminary results, including the first life-time measurement of isomeric states with AGATA, are presented.

Fusion excitation functions of light heavy-ion systems show oscillatory structures above the Coulomb barrier, caused by resonances or due to the penetration of successive centrifugal barriers well separated in energy. In heavier systems, the amplitude of oscillations decreases and the peaks get nearer to each other. This makes the measurements very challenging. We have performed a first experiment for 28Si + 28Si, by measuring fusion cross sections (σ) in an energy range of ≃15 MeV above the barrier, with a small ΔElab = 0.5 MeV step. Three regular oscillations are clearly observed, which are best revealed by plotting the energy-weighted derivative of the excitation function. The excitation function has been recently measured down to cross sections ≤1μb with larger energy steps. Coupled-channel (CC) calculations based on a shallow potential in the entrance channel are able to reproduce the oscillations. A further analysis will provide a stringent test for the calculations, in particular for the choice of the ion-ion potential, because the subbarrier excitation function has to be reproduced as well. Coupled-channel (CC) calculations based on a shallow potential in the entrance channel are able to reproduce the oscillations. A further analysis will provide a stringent test for the calculations, in particular for the choice of the ion-ion potential, because the subbarrier excitation function has to be reproduced as well.

We measured excitation functions for the main transfer channels in the 116Sn+60Ni reaction at different bombarding energies from above to well below the Coulomb barrier. The experiment has been performed in inverse kinematics, detecting the lighter (target-like) ions with the magnetic spectrometer PRISMA at very forward angles. Good mass, nuclear charge and kinetic energy resolutions have been achieved. The comparison between the data and microscopic calculations for the present case and for the previously measured 96Zr+40Ca system, namely superfluid and near closed shells nuclei, should significantly improve our understanding of nucleon-nucleon correlation properties in multinucleon transfer processes.

The fusion cross section of 60Ni + 100Mo has been measured down to microbarn level, looking for hindrance at low energy, in a system with positive Q-values for neutron transfer. The measured cross sections look similar to those of the nearby 64Ni + 100Mo, but no conclusive statement can be made at this stage, as to the onset of hindrance in this system.

Multinucleon transfer cross sections in the system 40Ca+96Zr have been measured at bombarding energies ranging from the Coulomb barrier to ~ 25% below. Target-like (lighter) recoils in inverse kinematics have been completely identif ed in A,Z and Q-value with the large solid angle magnetic spectrometer PRISMA. The experimental slopes of the neutron transfer probabilities at large internuclear separation are consistent with the values derived from the binding energies. A phenomenological interpretation of the transfer probabilities indicates the presence of enhanced values for the even number of neutron transfers.

The recent experiment on fusion of 40Ca + 40Ca is described in some detail and the results are reported. A full excitation function has been measured from well above the Coulomb barrier, down to low energies where the cross section reduces to ≃20 µb. A comparison is done with the recently published data on fusion of 40Ca + 48Ca and of 48Ca + 48Ca. The trends are different, in particular as far as the logarithmic derivatives (slopes) are concerned, in the interesting energy region below the barrier. The slope for systems where 40Ca is involved, show a characteristic behavior with a tendency to saturate in a limited energy range just below the main barrier. The slopes resume increasing at lower energies, possibly indicating the influence of nuclear structure (the strong octupole vibration of 40Ca) at such low energies, together with the clear presence of the fusion hindrance phenomenon. The results of coupled-channels calculations are presented.

Multinucleon transfer reactions have been investigated in 40Ar+208Pb with the Prisma+Clara set-up. The experimental differential cross sections of different neutron transfer channels have been obtained at three different angular settings taking into account the transmission through the spectrometer. The experimental yields of the excited states have been determined via particle-γ coincidences. In odd Ar isotopes, we reported a signif cant population of 11/2− states, reached via neutron transfer. Their structure matches a stretched conf guration of the valence neutron coupled to vibration quanta.

A recent experiment on 12C + 24Mg sub-barrier fusion was performed. Standard coupled-channels calculations clearly over-estimated the low energy cross sections and there was indication of a maximum of the astrophysical S factor vs energy. In the present work further measurements have been performed on the same system, and we extended the excitation function to lower energies down to σ ≃ 4 ÎĽ b, confirming the existence of the hindrance phenomenon. Additional energy points near the barrier were added as well. In agreement with the result of previous experiment, the hindrance energy threshold is observed to correspond to a remarkably large cross section. The present new data allow to better define the S -factor maximum, that is well fitted using two parametrizations of either the hindrance or the adiabatic model. We also performed coupled-channels calculations using a Woodsâ€"Saxon potential. Far below the barrier we observe that the coupling strengths decrease and tend to vanish, indeed one-dimensional tunnelling calculations well reproduce the excitation function. Further precise cross section measurements at slightly lower energies are needed to discriminate between the adiabatic and hindrance models. This would give a reliable guidance for the extrapolation to the systems of astrophysical interest, like 12C + 12C and 16O + 16O. [ABSTRACT FROM AUTHOR]