Evolution of the prompt dipoleγ-ray emission with incident energy in fusion reactions

Abstract

MeV/nucleon, with the aim to probe its evolution with incident energy. These reactions populate, through entrance channels having different charge asymmetries, the 132 Ce compound nucleus at an excitation energy of 117 MeV with identical spin distribution. Fusion events were selected by detecting high-energy γ-rays in coincidence with evaporation residues. The present results can be compared directly with those related to the same reactions at Elab � 9 MeV/nucleon where the compound nucleus excitation energy was equal to 173.5 MeV [1]. This study could be of aid in the formation of superheavy elements. In fact, the emission of pre-equilibrium dipole photons could represent an efficient new cooling mechanism of the composite system in charge asymmetric ”hot” fusion reactions, increasing thus its survival probability against fission [2]. EXPERIMENTAL SET-UP The experiment was performed by using the pulsed beams of 32 S and 36 S provided by the heavy-ion 15 MV Tandem-XTU accelerator of the Laboratori Nazionali di Legnaro (Italy) at Elab = 196 MeV and 214.2 MeV, respectively, impinging on 550 � g/cm 2 thick 100 Mo ( 97.42%) and 96 Mo ( 95.9%) self-supporting targets. The beam consisted of � 2 ns wide bunches with a 400 ns separation and it was measured in a Faraday cup shielded with lead and paraffin to reduce the background due to γ-rays and neutrons. Beam current was about 10 nA. The γ-rays were detected by using six seven-pack clusters of BaF2 scintillators situated at 28 cm from the target at different θ angles with respect to the beam direction. The total solid angle covered by all the clusters was 1.6 sr. Each cluster was surrounded by a 3 mm thick lead shield which reduced the counting rate due to the low energy γ-rays to 50% and stopped the charged particles. The evaporation residues were detected by means of four position sensitive Parallel Plate Avalanche Counters (PPAC’s) located at θ = 7 ◦ symmetrically around the beam direction at 70 cm from the target. The total solid angle covered by the PPAC’s was 0.089 sr. They provided the time of flight (TOF) with respect to the radiofrequency signal of the accelerator, the energy loss (�E) and the position of the reaction products. The energy calibration of the γ-ray detectors was obtained by using the sources 60 Co, 88 Y, the composite source of 241 Am+ 9 Be and the 15.1 MeV γ-rays from the p+ 12 C reaction. Down-scaled single events together with coincidence events between a PPAC and at least one fired BaF2 scintillator were collected during the experiment. A coincidence event was accepted if the deposited energy in a BaF2 cluster was greater than � 5 MeV. The threshold of each BaF2 scintillator was set at � 100 keV. The coincidence request eliminated any cosmic ray contamination of the γ-ray spectra.