Recent studies of heavy ion transfer reactions using large solid angle spectrometers

We present selected results recently obtained in the study of heavy ion transfer reactions at energies close to the Coulomb barrier by employing the large solid angle magnetic spectrometer PRISMA. We discuss the production of neutron-rich heavy nuclei via multinucleon transfer processes and the related effects of secondary processes, in particular nucleon evaporation, studied in a high resolution kinematic coincidence experiment. We also present the recent results in the studies of neutron-neutron correlations for closed shell and superfluid systems

Fusion Hindrance and Pauli Blocking in 58Ni + 64Ni

58Ni +64Ni is the first case where the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced, in a very well known experiment by Beckerman et al., by comparing with the two systems 58Ni + 58Ni and 64Ni+64Ni. Subsequent measurements on 64Ni + 64Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58Ni + 64Ni down to the measured level of 0.1 mb. In the present experiment the excitation function has been extended by two orders of magnitude downward. The cross sections for 58Ni + 64Ni continue decreasing very smoothly below the barrier, down to '1 µb. The logarithmic slope of the excitation function increases slowly, showing a tendency to saturate at the lowest energies. No maximum of the astrophysical S -factor is observed. Coupled-channels (CC) calculations using a Woods-Saxon potential and includinginelastic excitations only, underestimate the sub-barrier cross sections by a large amount. Good agreement is found by adding two-neutron transfer couplings to a schematical level. This behaviour is quite different from what already observed for 64Ni+ 64Ni (no positive Q-value transfer channels available), where a clear low-energy maximum of the S -factorappears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in 58Ni+ 64Ni in the measured energy range. This trend at deep sub-barrier energies reinforces the recent suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier

Study of the neutron-rich region in the vicinity of 208Pb via multinucleon transfer reactions

The multinucleon transfer reaction mechanism was employed to populate isotopes around the doubly- magic 208 Pb nucleus. We used an unstable 94 Rb beam on 208 Pb targets of different thickness. Transfer channels were studied via the fragment-γ and γ-γ coincidences, by using MINIBALL γ spectrometer coupled to a particle detector. Gamma transitions associated to the different Pb isotopes, populated by the neutron transfers, are discussed in terms of excitation energy and spin. Fragment angular distributions were extracted, andcompared with the reaction model

Multinucleon transfer reactions and proton transfer channels

Transfer reactions have always been of great importance for nuclear structure and reaction mechanism studies. So far, in multinucleon transfer studies, proton pickup channels have been completely identified in atomic and mass numbers at energies close to the Coulomb barrier only in few cases. We measured the multinucleon transfer reactions in the 40Ar+208Pb system near the Coulomb barrier, by employing the PRISMA magnetic spectrometer. By using the most neutron-rich stable 40Ar beam we could populate, besidesneutron pickup and proton stripping channels, also neutron stripping and proton pickup channels. Comparison ofcross sections between different systems with the 208Pb target and with projectiles going from neutron-poor to neutron-rich nuclei, as well as between the data and GRAZING calculations, was carried out.Finally, recent results concerning the measurement of the excitation function from the Coulomb barrier to far below for the 92Mo+54Fe system, where both proton stripping and pickup channels were populated with similar strength, will be discussed

Light and heavy fragments mass correlation in the 197Au+130Te transfer reaction

We studied multinucleon transfer (MNT) processes in the 197Au+130Te at Elab=1.07 GeV system coupling the PRISMA magnetic spectrometer to NOSE, an ancillary particle detector. We constructed a mass correlation matrix associating to each light fragment identified in PRISMA the corresponding mass distribution of the heavy partner detected in NOSE and, through the comparison with Monte Carlo simulations, we could infer about the role of neutron evaporation in multinucleon transfer reactions for the population of neutron-rich heavy nuclei

Multinucleon transfer in the 197Au+130Te reaction studied with a high-resolution kinematic coincidence

Heavy ion transfer reactions play an important role for the definition of the reaction mechanism that describes the evolution from the quasi-elastic to the more complex deep-inelastic regime. At energies close to the Coulomb barrier these processes are mainly governed by optimum Q-value considerations and nuclear form factors. As a consequence, for nuclei close to the stability line the dominant channels are the neutron pick-up and the proton stripping. The use of neutron-rich projectiles is expected to open the possibility to populate neutron stripping and proton pick-up paths. This corresponds, for the heavy partner, to the population in the “south-east” direction of the nuclear chart, leading to the neutron-rich heavy region, like that below 208Pb or in the actinides and trans-actinides, which can be hardly accessed by fragmentation or fission reactions. Multinucleon transfer may represent a suitable mechanism to approach these neutron-rich areas. In this work we studied the multinucleon transfer reaction 197Au+130Te at Elab = 1.07 GeV in inverse kinematics to extract absolute yields for neutron and proton transfer channels in the vicinity of 132Sn and, at the same time, to obtain information on the associated heavy partner. We adopted for the first time a method which makes use of a particle-particle coincidence in which one of the two devices is a high-resolution mass spectrometer, PRISMA, which can completely identify the light partner of the reaction in A, Z and Q value; the coincident detector works as a second arm of PRISMA and is placed at the kinematically correlated angle for the detection of the heavy partner with lower resolution but comparable efficiency. The obtained yields can then be compared with theoretical calculations which predict, for this system, the population of neutron stripping and proton pick-up channels, in turn leading to the production of neutron-rich nuclei approaching the region close to 208Pb. This thesis presents the results of this experiment. Through an event-by-event trajectory reconstruction in PRISMA we could achieve enough mass resolution to distinguish final isotopes and extract the corresponding yields for neutron transfer channels. For proton ones, with about ten times less cross sections, we could extract the mass-integrated yields corresponding to each nuclear charge. As theoretically predicted, the neutron stripping and proton pick-up channels have been identified and their yields compared with the calculations performed with the GRAZING model for transfer reactions. For neutron transfer the comparison showed a quite good agreement; for proton transfer the model is able to reproduce quite well the stripping channels but largely underestimates the pick-up ones. After verifying the correct functioning of the kinematic coincidence, we reconstructed the mass of the heavy partner in the second arm by assuming a binary character of the reaction. From the correlation between the mass of the Te isotopes identified in PRISMA and the mass distributions of the Au ions detected in the second arm, we could assess that the reaction maintains its (quasi) binary character up to the transfer of 3-4 neutrons. This result is consistent with the behaviour of the total kinetic energy loss distributions, analyzed and compared, on the same footing, with and without using the information of the coincident second arm. The yields of the Au mass distributions correlated to each Te isotope in PRISMA were compared with GRAZING calculations. The trend of the yields corresponding to the population of neutron-rich isotopes of the heavy partners well agrees with the GRAZING predictions. The limited effect of the evaporation on the final yields when a low bombarding energy is employed has been argued from the comparison between experimental widths of the mass distributions and Monte Carlo calculations.Le reazioni di trasferimento tra ioni pesanti giocano un ruolo importante nel definire i meccanismi di reazione che descrivono l'evoluzione dal regime quasi-elastic al più complesso deep-inelastic. A energie vicine alla barriera Coulombiana questi processi sono governati principalmente da considerazioni di ottimo Q-valore e fattori di forma nucleari. Di conseguenza per nuclei vicini alla stabilità i canali dominanti sono il pick-up di neutroni e lo stripping di protoni. Ci si aspetta che l'uso di proiettili ricchi di neutroni apra la possibilità di popolare i canali di stripping di neutroni e pick-up di protoni. Questo corrisponde, per il partner pesante, alla popolazione in direzione "sud-est" nella carta dei nuclidi, verso la regione pesante ricca di neutroni, come quella sotto il 208Pb o negli attinidi e trans-attinidi, difficilmente raggiungibile tramite reazioni di frammentazione o di fissione. Il trasferimento di molti nucleoni (MNT) può rappresentare un metodo adatto per raggiungere queste regioni. In questo lavoro abbiamo studiato la reazione di MNT 197Au+130Te a Elab = 1.07 GeV in cinematica inversa per estrarre le yield assolute dei canali di trasferimento di neutroni e protoni nelle vicinanze del 132Sn e, allo stesso tempo, ottenere informazioni sul partner pesante associato. Si è adottato per la prima volta un metodo di coincidenza particella-particella in cui uno dei due rivelatori è uno spettrometro magnetico ad alta risoluzione, PRISMA, che identifica completamente il partner leggero della reazione in A, Z e Q valore; il rivelatore coincidente è usato come secondo braccio di PRISMA posizionato all'angolo cinematicamente correlato per la rivelazione del partner pesante con minore risoluzione ma comparabile efficienza. Si possono confrontare le yield ottenute con calcoli teorici che predicono, per questo sistema, la popolazione di canali di stripping di neutroni e pick-up di protoni, che conducono a loro volta alla produzione di nuclei ricchi di neutroni in prossimità della regione vicina al 208Pb. Questa tesi presenta i risultati di questo esperimento. In PRISMA si è ottenuta una risoluzione in massa sufficiente per estrarre le correspondenti yield per i canali di trasferimento di neutroni. Per quelli di protoni, con sezioni d'urto circa 10 volte inferiori, abbiamo estratto le yield massa-integrate corrispondenti ad ogni Z. Come predetto teoricamente, si sono identificati i canali di stripping di neutroni e pick-up di protoni e le loro yield confrontate con le predizioni del modello GRAZING per reazioni di trasferimento. Per i neutroni il confronto ha mostrato un buon accordo tra dati sperimentali e calcoli. Per i protoni il modello riesce a riprodurre abbastanza bene i canali di stripping ma sottostima di molto quelli di pick-up. Dopo aver verificato il corretto funzionamento della coincidenza cinematica, abbiamo ricostruito la massa del partner pesante nel secondo braccio assumendo un carattere binario della reazione. Dalla correlazione tra la massa degli isotopi del Te identificati in PRISMA e le distribuzioni di massa degli ioni Au rivelati nel secondo braccio, si è potuto dimostrare che la reazione mantiene il suo carattere (quasi) binario fino al trasferimento di 3-4 neutroni. Questo risultato è consistente con il comportamento delle distribuzioni di perdita di energia cinetica totale, analizzate e confrontate, allo stesso livello, con e senza l'utilizzo del secondo braccio coincidente. Le yield delle distribuzioni di massa del Au correlate ad ogni isotopo del Te in PRISMA sono state comparate con calcoli di GRAZING. Il trend delle yield corrispondenti alla popolazione di isotopi del partner pesante ricchi di neutroni è in buon accordo con le predizioni di GRAZING. L'effetto limitato dell'evaporazione sulle yield finali quando viene impiegata una bassa energia di bombardamento è stato derivato dal confronto tra le larghezze sperimentali delle distribuzioni di massa e i calcoli Monte Carlo

Recent studies of heavy ion transfer reactions using large solid angle spectrometers

We present selected results recently obtained in the study of heavy ion transfer reactions at energies close to the Coulomb barrier by employing the large solid angle magnetic spectrometer PRISMA. We discuss the production of neutron-rich heavy nuclei via multinucleon transfer processes and the related effects of secondary processes, in particular nucleon evaporation, studied in a high resolution kinematic coincidence experiment. We also present the recent results in the studies of neutron-neutron correlations for closed shell and superfluid systems

Study of sub-barrier fusion of 36S+50Ti,51V systems

A detailed comparative study of the sub-barrier fusion of the two near-by systems 36S+50Ti,51V was performed at the National Laboratories of Legnaro (INFN). Aim of the experiment was the investigation of possible effects of the non-zero spin of the ground state of the 51V nucleus on the sub-barrier excitation function, and in particular on the shape of the barrier distribution. The results sh w that the two measured excitation functions are very similar down to the level of 20 - 30 μb. The same is observed for the two barrier distributions. Coupled-channels calculations have been performed and are in good agreement with the experimental data. This result indicates that the low-lying levels in 51V can be interpreted in the weak-coupling scheme, that is, 51V(I) = 50Ti(2+)⊗ p(1 f7/2)