Upgrading DRACULA setup to be used for light products - fission fragments coincidence measurements

At low bombarding energy (E/A < 10 MeV) nuclear reactions induced by light ions on high fissility parameter targets such as 238 U give rise to a number of fission processes, all leading to very similar fission products. Therefore, in order to understand the fission processes in this energy domain it is of interest to determine the amount of fission occurring after a peripheral interaction relative to that originating from compound nucleus formation. Although the detection of a projectile residue (PLF) in coincidence with the fission fragments is a very promising probe for the macroscopic features of the mechanism of induced fission, at incident energies in the vicinity of the Coulomb barrier (E/A < 10 MeV) only few detailed experimental investigations exist. In order to elucidate the possible presence of dissipative collisions in competition with the direct transfer or incomplete fusion processes in the reaction mechanism, a complete isotopic identification of the PLF fragments detected in coincidence with a target-like nucleus or/and a light particle is required. This type of experiments can be successfully done using an up-graded version of DRACULA setup, successfully used previously in studies of deep inelastic processes in light heavy ion collisions. This gives the possibility to rotate the reaction chamber in the horizontal plane by coupling it to the beam transport pipe in steps of 15 angle and to attach to it large area detection systems via three windows with an angular opening of 70 angle. Thus, for the PLF detection, the DRACULA setup whose main components are two large area position sensitive ionization chambers placed up and down relative to the horizontal reaction plane covering a polar angular range of 24 angle, will stay at negative detection angles, by convention. Moving the whole setup at three successive positions in steps of 15 angle, with a superposition for relative normalization, a laboratory angular domain of 54 angle (between - 45 angle and -99 angle will be covered. The big ionization chambers are coupled to one of the 70 angle angular opening windows of the reaction chamber by a special extension which gives access to a flight path of 600 mm. Thus, a small start PPAD and two stop PPAD's in front of the ionization chambers will allow to measure the mass of the PLF's. For light particle detection a hybrid detector will be mounted in this extension too. The fission fragments will be detected on the opposite side of the beam using two arrays of phoswich detectors. For the fission fragment emitted at more forward angles the array contains twenty detection elements packed in three vertical layers. The other fragment is detected with an array made by thirteen elements packed in two vertical layers. The single element of 25 cm2 cross section area uses the phoswich technique by coupling a thin fast NE102A plastic scintillator to a 10 cm long BaF2 crystal of hexagonal section. The BaF2 crystal detectors have been successfully used in modular multielement detector ARGOS in the context of GANCT and HOTCT researches at LNS. The light response of the phoswich configuration as a function of the plastic thickness and of the energy and charge of the incident ion has been studied at Tandem energies. Both arrays will be placed in separate vacuum chambers attached to the remaining large angular opening windows of the reaction chamber. By rotating the whole device the fission fragment detection arrays will cover a range of 96 angle in the horizontal plane. The main advantage of this setup is that it allows to perform continuous measurements in energy and angle of the reaction products. The geometry of the whole device has been tested by Monte Carlo calculations using the code ELPHIC. The coincidence condition is completely fulfilled for the first two positions of the setup and partially for the third one. Measurements are intended to be performed at the SMP Tandem from LNS-Catania using light beams (16 O, 19 F, 20 Ne, 32 S) at ∼ 6 MeV/A on high fissility parameter targets. (authors)