Proučavanje teškoionskih reakcija magnetskim spektrometrom velikoga prostornoga kuta

Nucleon transfer reactions at energies close to the Coulomb barrier always played an important role in nuclear structure and reaction dynamics studies. It has been shown that heavy-ion reactions are an ideal tool for the study of the residual interaction in nuclei, in particular the components responsible for the couplings and correlations. The main subject of the present thesis is the study of the properties of the nuclear force which are not accounted for by the mean field description, especially the role played by neutron-proton correlations. This has been addressed by measurements of multinucleon transfer reactions performed with the new generation magnetic spectrometer PRISMA and its ancillary detectors. The experimental differential and total cross sections and total kinetic energy loss distributions for the final reaction products have been extracted and compared with the predictions of the GRAZING model for heavy-ion transfer reactions, based on semi-classical theory. The main objective was to search for an experimental signature of transfer of correlated neutron-proton pairs in the region near the Z =20 and N =20 shell closures. Through the simultaneous study of the absolute cross sections for all channels populated via transfer of nucleon pairs (±nn), (±pp) and (±np), and their comparison with the theoretical predictions, we explored the possible effect of (np) correlation. The findings have been also corroborated with results obtained at energies below the Coulomb barrier, where the excitation energies of the final reaction products are expected to be small. The comparison of the experimental results with the semi-classical calculations that include only the transfer of independent particles shows that the experimental cross section of (+1p+1n) channel is enhanced, indicating the possibility of a presence of a (np) pair degree of freedom. The analysis of these inclusive data was corroborated by a complementary analysis of γ-particle coincidences, where PRISMA has been used in coupled operation with the CLARA γ array. From the analysis of these coincidences, the population strength of excited states has been extracted for different transfer channels. In particular, for 42K, reached via the (+1p+1n) channel, a strong population of the 7+ state has been evidenced. Since this can happen when a neutron and a proton occupy the same f7/2 orbital, the finding may give a further support to the presence of a neutron-proton correlation.Reakcije prijenosa nukleona na energijama blizu kulonske barijere važne su i u proučvanju strukture jezgara i mehanizma reakcije. Pokazalo se da su teškoionske reakcije odličan način proučavanja rezidualne interakcije u jezgrama, naročito dijelova odgovornih za vezanje i korelacije. Glavna tema ovog doktorskog rada je proučavanje svojstava nuklearne sile koja nisu uključena u opis pomoću srednjeg polja, posebno uloga korelacija između neutrona i protona. To se proučavalo mjerenjem reakcije prijenosa mnogo nukleona s magnetskim spektrometrom velikoga prostornoga kuta PRISMA i njegovim pomoćnim detektorima. Dobiveni su diferencijalni i ukupni udarni presjek, raspodjela gubitka kinetičke energije za sve produkte reakcije, te su uspoređeni s teorijskim predviđanjima modela GRAZING. Glavna ideja je potraga za eksperimentalnim potpisom prijenosa koreliranog para neutron-proton u području blizu Z = 20 i N = 20 zatvorenih ljusaka. Istodobnom usporedbom apsolutnog udarnog presjeka za sve kanale koje su pobuđeni prijenosom para nukleona, (±nn), (±pp) i (±np), te njihovom usporedbom s teorijskim predviđanjima, traženi su mogući efekti (np) korelacija. Rezultati su potvrđeni s rezultatima dobivenim na energijama ispod kulonske barijere, gdje se očekuju male energije pobuđenja produkata reakcije. Usporedba mjerenih rezultata s polu-klasičnim računima koji uključuju samo prijenos neovisnih čestica pokazuje da je (+1p+1n) kanal pojačan, što može ukazivati na moguć doprinos (np) korelacija. Analiza inkluzivnih podataka je nadopunjena dodatnom analizom γ-fragment koincidencija, kada je PRISMA korištena s CLARA γ detektorom. Ove koincidencije omogućuju proučavanje snage pobuđenja različitih stanja. 42K kanal, pobuđen putem prijenosa jednog protona i neutrona, pokazuje jako pobuđenje 7+ stanja. Taj rezultat se može shvatiti kao dodatna potvrda prisutnosti korelacije neutron-proton, jer se 7+ stanje može opisati kao stanje u kojem su neutron i proton u istoj f7/2 orbitali

Fusion Hindrance and Quadrupole Collectivity in Collisions of A≃50 Nuclei: The Case of 48Ti + 58Fe

International audience; The fusion excitation function of Ti-48 + Fe-58 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 Ni-58 + Fe-54 where evidence of fusion hindrance shows up at relatively high cross sections. The sub-barrier cross sections of Ti-48 + Fe-58 are much larger than those of Ni-58 + Fe-54. Significant differences are also observed in the logarithmic derivatives, astrophysical 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 Ti-48 + Fe-58. The logarithmic derivative of the excitation function is very nicely fit, and no evidence of hindrance is observed down to around 1 mu b. The fusion barrier distribution is rather wide, flat and structureless. It is only in qualitative agreement with the calculated distribution

Evidence of proton-proton correlations in the 116Sn+60Ni transfer reactions

One and two proton transfer channels have been measured in 116Sn+60Ni with the magnetic spectrometer PRISMA by making an excitation function at several bombarding energies, from above to well below the Coulomb barrier. The total kinetic energy loss distributions show the predominance of quasi-elastic processes in the sub-barrier regime. The data have been compared with calculations performed with the GRAZING program, based on semiclassical formalism, and in the Distorted Wave Born Approximation (DWBA), which provided a good theoretical description of the extracted transfer probabilities for the one proton transfers. The much larger values of the experimental two proton transfers compared with those evaluated within an independent particle transfer mechanism, indicate the presence of strong proton-proton correlations. The results complement the ones of the previously analyzed one- and two-neutron transfers, providing significant new information on the subject compared to past works

Study of interstrip gap effects and efficiency for full energy detection of Double Sided Silicon Strip Detectors

In this work is reported a study on the response of double sided silicon strip detectors. In order to investigate the effect of the electrode segmentation on the detector response, two experiments were performed aimed to measure the efficiency for full energy detection. Results show that the efficiency for full energy detection, that is directly related to effective width of the inter-strip region, varies with both detected ion energy and bias voltage. The experimental results are qualitatively reproduced by a simplified model based on the Shockley-Ramo-Gunn framework