Α-clustering In Nuclei Research Articles

Pre-equilibrium Emission and α-clustering in Nuclei

The study of nuclear states built on clusters bound by valence neutrons in their molecular configurations is a field of large interest, which is being renewed by the availability of exotic beams: clustering is, in fact, predicted to become very important at the drip-line, where weakly bound systems will prevail. Although for light nuclei at an excitation energy close to the particle separation value there are experimental evidences of such structure effects, this is still not the case for heavier nuclear systems. Many attempts have been done using preformation alpha clustering models, but there is still a lack of experimental data capable to give a direct feedback. In particular, searching for alpha clustering effects in medium mass systems is still a challenge, which can give new hints in this subject. In the past we have studied the reactions 250, 192 and 130 MeV 16O + 116Sn, observing a significant increase in the fast emitted α-particle yield. This effect was ascribed to the presence of preformed a-clusters in the 16O projectile nucleus. In order to investigate these aspects, in a model independent way, a new experimental campaign has been performed with the GARFIELD + RCo set up, to compare results from two different reactions: a double magic a-cluster (16O) and a non-magic α-cluster projectile (19F) at the same beam velocity (16AMeV) have been chosen, impinging respectively on 65Cu and 62Ni targets, thus leading to the same 81Rb* compound nucleus. The angular distributions and the light charged particles emission spectra in coincidence with evaporation residues have been measured and analyzed. The preliminary results of the data analysis and the main features of the theoretical model used for their interpretation are presented.

Pre-equilibrium α-particle emission as a probe to study α-clustering in nuclei

A theoretical approach was developed to describe secondary particle emission in heavy ion collisions, with special regards to pre-equilibrium {\alpha}-particle production. Griffin's model of non-equilibrium processes is used to account for the first stage of the compound system formation, while a Monte Carlo statistical approach was used to describe the further decay from a hot source at thermal equilibrium. The probabilities of neutron, proton and {\alpha}-particle emission have been evaluated for both the equilibrium and pre-equilibrium stages of the process. Fission and {\gamma}-ray emission competition were also considered after equilibration. Effects due the possible cluster structure of the projectile which has been excited during the collisions have been experimentally evidenced studying the double differential cross sections of p and {\alpha}-particles emitted in the E=250MeV 16O +116Sn reaction. Calculations within the present model with different clusterization probabilities have been compared to the experimental data.

Surface alpha-clustering in the lead region

The effect of shell model configuration mixing on the reduced α-amplitude in heavy nuclei is investigated and discussed with respect to the question of α-clustering in nuclei. It is shown that extensive configuration mixing may enhance the α-amplitude under the Coulomb barrier by huge, largely arbitrary factors. In contrast to this the actual αclustering in the nuclear surface may well be described within the shell model including only relatively few configurations in the open shell.

Inelastic α-scattering to the continuum: A probe of α-clustering in nuclei

The statistical multistep direct theory is applied to the 90Zr(α, α′) reaction at 140 MeV. Using the averaging procedure already tested in (p, n) and ( p , p′0) reactions, DWBA matrix elements are employed to construct both the single-step and multistep cross sections in the continuum. It is shown that good fits for angular distributions in the 10–60 MeV excitation range can be achieved only by taking into account the interaction of the incoming α-particles with preformed α-particles in the target nucleus. By comparing both the shape and absolute value of cross sections with data, an α-clustering probability is extracted which is shown to be in agreement with independent analysis in the same mass region. Due to its surface localization, the possible role of inelastic α-scattering in the 100 MeV region to investigate α-clustering in nuclei is suggested.

The probability of pion absorption by α-clusters in nuclei

A multistep direct reaction model with pickup is extended to described the formation of tritons and 3He-particles in nucleon- and pion-induced precompound reactions. In the latter ones, those tritons not accounted for by pickup can be explained as primary ones from absorption by α-clusters, for which a 10% probability is deduced.

Studies in the α-particle model of nuclei: IV. The polarisation forces between α-clusters in nuclei

The polarisation forces between α-particles are calculated for central forces between nucleons. There are two distinct possibilities to consider. The first is that the nuclear potential does not contain a direct part, when the polarisation energy is the only source of an attraction between α-particles. The Rosenfeld mixture is treated as an example of such a potential, and it is shown that the polarisation energy due to this potential is too small to account for the attraction required by the α-particle model. The second possibility to consider is that the nuclear potential does contain a direct part, which is assumed to be attractive, when there is a direct attraction between α-particles (analogous to the attraction between oppositely charged ions) which the polarisation forces supplement. The Serber potential is treated as an example of a potential with a direct part, and it is shown that the polarisation forces due to this potential cannot play more than a minor role in the attraction between α-particles; however, the magnitude of the polarisation energy is not negligible, and is of the order of the α-α binding energy per pair of nearest neighbours in the α-nuclei. The distortion of α-particles to which polarisation give rise may be interpreted as a tendency for the α-particles to dissolve. The probability of solution is estimated, and is shown to be small for A ≦ 40 even if polarisation does provide most of the attraction between α-particles, perhaps through the action of tensor forces.