A fast ionization chamber for the detection of fusion-evaporation residues produced by the exotic beams of SPES: design, tests and first experiments

This thesis work is divided in two parts: (i) designing and constructing a new ionization chamber with fast response (FastIC) for the study of near- and sub-barrier fusion of exotic systems using the very neutron-rich beams of the SPES facility in construction at the Laboratori Nazionali di Legnaro (LNL) of INFN, and (ii) carrying out, analyzing and interpreting the results of the sub-barrier fusion experiment for the two systems 36S + 50Ti,51V, performed using the FastIC. A new set-up for fusion cross-section measurements, especially designed for the low-intensity beams delivered by the SPES facility, has been developed at the LNL. This new set-up includes a fast IC designed to withstand a high counting rate particle identification for fusion studies involving exotic beams up to 105 pps. The readout of the FastIC was optimised and extensive tests using stable heavy-ion beams demonstrated its ability to operate up to rates of about140 kHz, compared to few kHz of the previous IC chamber. This feature and the much larger solid angle coverage will allow to detect fusion-evaporation residues with an efficiency close to 100%. The new FastIC has been employed for the experimental study of sub-barrier fusion in the 36S + 50Ti, 51V systems. The possible effect of the non-zero spin of 51V ground state on the sub-barrier excitation function and on the shape of the barrier distribution has been investigated. The two excitation functions have been measured down to cross sections of about 20 ub, and it appears that they are very similar to each other. A coupled channels analysis has been performed in order to highlight differences between the two systems, and a modified coupled-channels code was used in order to treat the odd nucleus 51V. The experimental excitation functions and the extracted barrier distributions have been compared to the theoretical predictions. This comparison suggests a small difference between the two systems above the barrier, that might be experimentally observed by measuring the backward-angle quasi-elastic scattering in that energy region.