FUSION08: New Aspects of Heavy Ion Collisions Near the Coulomb Barrier
1098(2009); http://dx.doi.org/10.1063/1.3108858View Description Hide Description
Fusion hindrance well below the Coulomb barrier has been observed in several systems in recent years showing systematic but also unsystematic behaviours. Our understanding of this phenomenon has greatly advanced in the last 2–3 years. Coupled‐channel effects leading to barrier distributions, influence the threshold energy for hindrance. Very recent measurements on performed at LNL, are presented. The two excitation functions are very different below the barrier, possibly due to the Q‐value for CN formation. A few other systems, for which previous experiments were done, show interesting features, suggesting measurements at still lower energies.
1098(2009); http://dx.doi.org/10.1063/1.3108759View Description Hide Description
The coupled channels model has generally been very successful in describing the different reaction outcomes following the collision of heavy nuclei. However, several outstanding problems remain—amongst them the inability to simultaneously describe elastic scattering and fusion data, and more recently the failure to consistently describe deep‐sub barrier and above‐barrier fusion. Possible reasons include deviation of the nuclear potential from the assumed Woods‐Saxon form, and the emergence of processes which cannot be described within the coherent coupled channels framework. The question is whether there are inadequacies in the current models which need to be addressed before we can hope to get a consistent description of the main reaction processes. We put forward the possibility that dissipation near the fusion barrier and the consequent loss of coherence need to be considered for a realistic description of nuclear collisions.
1098(2009); http://dx.doi.org/10.1063/1.3108791View Description Hide Description
For a single potential barrier, the barrier penetrability can be inverted based on the WKB approximation to yield the barrier thickness. We apply this method to heavy‐ion fusion reactions at energies well below the Coulomb barrier and directly determine the inter‐nucleus potential between the colliding nuclei. To this end, we assume that fusion cross sections at deep subbarrier energies are governed by the lowest barrier in the barrier distribution. The inverted inter‐nucleus potentials for the and reactions show that they are much thicker than phenomenological potentials. We discuss a consequence of such thick potential by fitting the inverted potentials with the Bass function.
1098(2009); http://dx.doi.org/10.1063/1.3108820View Description Hide Description
We present our investigations of double‐folding, heavy‐ion potentials and of the role played by nucleon‐nucleon interactions that incorporate medium effects in accordance with the saturation properties of nuclear matter. The dependence on the medium is included directly via a repulsive term in the heavy‐ion potential, and/or via a density dependent term as in the case of zero‐or finite‐range forces in mean‐field nuclear structure calculations. We illustrate this point with the coupled‐channels calculations we performed for systems that display a hindrance in fusion at deep sub‐barrier energies.
1098(2009); http://dx.doi.org/10.1063/1.3108851View Description Hide Description
We propose a two‐step model for heavy‐ion fusion reactions based on the adiabatic approach in order to account for steep fall‐off of fusion cross sections at deep subbarrier energies. The two‐step model consists of the capture process in the two‐body potential pocket, which is followed by the penetration of the adiabatic one‐body potential to reach a compound state after the touching configuration. We argue that although the sudden and adiabatic approaches provide a similar result to each other for the fusion cross section, the two approaches can be discriminated by detecting average angular momenta of a compound nuclei.
1098(2009); http://dx.doi.org/10.1063/1.3108856View Description Hide Description
The fusion excitation function for with positive Q‐value for the compound nucleus formation, has been measured from near the barrier down to very low energies, where “fusion hindrance” may be expected. No evidence of hindrance shows up in the measured energy range. Coupled‐channels calculations have been performed with a large diffuseness parameter of the Wood‐Saxon potential, reproducing the data above and below the barrier.
1098(2009); http://dx.doi.org/10.1063/1.3108859View Description Hide Description
A novel quantum dynamical model based on the dissipative quantum dynamics of open quantum systems is presented. It allows the treatment of both deep‐inelastic processes and quantum tunneling (fusion) within a fully quantum mechanical coupled‐channels approach. Model calculations show the transition from pure state (coherent) to mixed state (decoherent and dissipative) dynamics during a near‐barrier nuclear collision. Energy dissipation, due to irreversible decay of giant‐dipole excitations of the interacting nuclei, results in hindrance of quantum tunneling.
1098(2009); http://dx.doi.org/10.1063/1.3108860View Description Hide Description
SOLITAIRE is a powerful tool to study nuclear fusion, having a very large angular acceptance and good rejection of beam particles. This report shows how this device, combined with a high quality ray‐tracing code, can give information about both the evaporation residue angular distribution and cross section from a single measurement. Results are shown for and fusion reactions.
1098(2009); http://dx.doi.org/10.1063/1.3108861View Description Hide Description
Over the last decade, we have performed in‐beam experiments using to measure excited states in proton‐rich Au, Hg, Tl and Pb isotopes. In these studies, the use of the FMA is essential in order to differentiate evaporation residues from the large fission background which dominates the reaction cross‐section. In addition, we have found that using near‐symmetric reactions at bombarding energies near the Coloumb barrier is beneficial in performing these studies. By keeping the bombarding energy low, fission is minimized and the reaction products are concentrated in only a few channels. New results have recently been obtained using the reaction to study shape co‐existence in via the lp evaporation channel. In addition, we have measured the total γ‐ray energy and multiplicity associated with the surviving compund system, following the fusion reaction,
1098(2009); http://dx.doi.org/10.1063/1.3108862View Description Hide Description
Investigations into the role of the N/Z ratio on the decay modes of compound nuclei are presented. Characteristics of fragments with atomic number and light charged particles emitted in at 5.5 MeV/A reactions were measured at the GANIL facility using the 4π‐INDRA array. Data are compatible with an emission process from a compound nucleus. Persistence of structure effects and emission before full separation of fission fragments are evidenced from elemental cross‐sections and coincidence data between light charged particles and fragments. Data are discussed in the framework of the transition state model.
1098(2009); http://dx.doi.org/10.1063/1.3108863View Description Hide Description
Nucleus‐nucleus interaction potentials in heavy‐ion fusion reactions are extracted from the microscopic time‐dependent Hartree‐Fock theory. When the center‐of‐mass energy is much higher than the Coulomb barrier energy, extracted potentials identify with the frozen density approximation. As the center‐of‐mass energy decreases to the Coulomb barrier energy, potentials become energy dependent. This dependence indicates dynamical reorganization of internal degrees of freedom and leads to a reduction of the “apparent” barrier. Including this effect leads to the Coulomb barrier energy very close to experimental one. Aspects of one‐body energy dissipation extracted from the mean‐field theory are discussed.
1098(2009); http://dx.doi.org/10.1063/1.3108864View Description Hide Description
As an alternative to the well known Hauser‐Feshbach analysis and statistical fission model, a dynamical collective clusterization model, called the dynamical cluster‐decay model (DCM), is developed for the decay of hot and rotating compound nuclei (CN) formed in the low‐energy heavy ion reactions. The model is a non‐statistical description for the decay of a CN to light particles (LPs), intermediate mass fragments (IMFs), fusion‐fission (FF) and quasi‐fission (QF) (equivalently, capture) processes. The model considers all decay products as dynamical mass motions of preformed fragments or clusters through the interaction barrier, thereby including structure effects of the CN, and is applicable to CN from different mass regions.
1098(2009); http://dx.doi.org/10.1063/1.3108865View Description Hide Description
Classical novae are a site of explosive nucleosynthesis where hydrogen rich material from a companion giant star accretes onto the surface of a white warf. Critical to our understanding of nova explosions are proton‐capture reaction rates involved in the nucleosynthesis. While, ideally, all of the relevant reactions would be measured directly, in practice, such measurements are very challenging and are only possible in a few cases. This provides considerable scope for indirect measurements including transfer reactions, mass measurements, beta‐decay and gamma‐ray spectroscopy. The latter technique, until recently largely neglected as an input in nuclear astrophysics analyses, has clear advantages in locating resonances with high energy precision and assisting in determining the spin and parity of resonances. Such information is very valuable in a complementary approach to indirect determinations of key reaction rates.
1098(2009); http://dx.doi.org/10.1063/1.3108866View Description Hide Description
Binary reactions at energies close to the Coulomb barrier received recently a significant boost thanks to the advent of the large solid angle magnetic spectrometer PRISMA coupled to the γ array CLARA. In the present paper different aspects of recent results of nuclear structure and reaction dynamics will be presented, focusing more closely on the reaction mechanism, in particular on the properties of quasi‐elastic and deep inelastic processes and on studies aiming at getting information on nucleon‐nucleon correlations.
1098(2009); http://dx.doi.org/10.1063/1.3108867View Description Hide Description
The distributions of products of and collisions have been determined in thick target gamma‐ray coincidence experiments. Analysis of gamma cross‐coincidence data allowed to account for neutron evaporation to reconstruct primary product distributions. For the three studied systems the neutron‐to‐proton ratio equilibration as a function of the mass transfer have been obtained from the experimental data and compared to a simple Liquid Drop energy minimization.
1098(2009); http://dx.doi.org/10.1063/1.3108753View Description Hide Description
The large solid angle magnetic spectrometer for heavy ions PRISMA was operated at LNL in conjunction with the highly efficient CLARA set‐up up to the end of March 2008. The AGATA Demonstrator is being mounted around the target area of PRISMA replacing the CLARA set‐up. It will allow to carry out nuclear structure and reaction mechanism studies with better resolution and higher statistics. New detectors for light ions and slow moving heavy ions have been designed for the focal plane of the spectrometer.
1098(2009); http://dx.doi.org/10.1063/1.3108754View Description Hide Description
Quasi‐elastic barrier distributions are analysed in a semiclassical model that incorporates both the excitation of surface modes and the particle transfer degrees of freedom. It is emphasised that the contributions of transfer channels are essential for the understanding of these barrier distributions over all the energy range.
Systematic study of the nuclear potential through high precision back‐angle quasi‐elastic scattering measurements of and on various targets1098(2009); http://dx.doi.org/10.1063/1.3108756View Description Hide Description
Elastic and inelastic scattering have proven an excellent method to probe the nuclear potential in the surface region . A surface diffuseness parameter of around 0.63 fm is accepted as describing elastic and inelastic scattering data. A new method to determine the diffuseness was proposed  using high precision quasi‐elastic excitation functions at sub‐barrier energies, and at backward angles. In response to this proposal, we have made measurements for , and more recently for the reactions .
Both single channel and coupled‐channels calculations have been performed to extract the diffuseness parameter of the nuclear potential, assuming a Woods‐Saxon shape. For the reactions involving near‐spherical targets, both theoretical analyses give the same diffuseness parameter, as had been hoped, showing that the effect of couplings to intrinsic excited states in the target and projectile is negligible. On the other hand, for deformed systems, coupling effects are surprisingly important even at deep sub‐barrier energies, reducing the flux at backward angles.
Energy spectra recorded with the back‐angle Si detector show a large number of distinct peaks as well as a broad structure at even higher excitation energies. A comparison of the back‐scattered energy spectra with coupled‐channels calculations demonstrates that including couplings only to low lying collective states in coupled‐channels calculations is inadequate in describing the reaction mechanism even at energies close to the barrier.
1098(2009); http://dx.doi.org/10.1063/1.3108762View Description Hide Description
In order to study the nucleus‐nucleus interaction in Pb‐based cold fusion, we have measured excitation functions for quasi‐elastic scattering of and projectiles on target at backward angles. The barrier distributions were derived from the first derivative of measured quasi‐elastic scattering cross sections relative to the Rutherford scattering cross section. The centroids of the barrier distributions show a deviation from several predicted barrier heights toward the low energy side. The shape of the barrier distributions is well reproduced by the results of a coupled‐channel calculation taking account of the coupling effects of two phonon excitations of the quadrupole vibration for the projectiles and of the octupole vibration for the target.