Reflected flux in reactions of 40Ca+208Pb at below-barrier energies

The measurements of the reflected flux were performed at Legnaro National Laboratory XTU Tandem-ALPI accelerator complex, using the PRISMA magnetic spectrometer. PRISMA features a large solid angle (80 msr, ∆θlab = ±6◦, ∆φ = ±11◦), momentum acceptance ∆p = ±10%, mass-resolution ∆A/A ∼ 1/200, and energy resolution up to 1/1000 (via time-of-flight measurement). In this experiment, PRISMA was located at θlab = 115◦. The magnetic fields were set for each energy to maximise the transmission for the dominant charge state of the elastically scattered beam. Thus, the measurements focus on the evolution of quasi-elastic scattering to multinucleon transfer (or deep-inelastic scattering). Beams of 40Ca were produced in 12 energy steps between Ecm=189.0 and 230.5 MeV. For the energies above 213 MeV, where the ALPI booster accelerator was used, carbon degrader foils of 135 μg/cm^2 or 205 μg/cm^2 were employed to provide three beam energies for each accelerator tune. The 40Ca beams were delivered onto ~150 μg/cm^2 208PbS targets oriented with their normals at 60 degrees to the beam axis. The targets had 20 μg/cm^2 carbon backings which were placed upstream of the target such that the particles accepted into PRISMA did not pass through the carbon backing. The atomic (proton) number Z, mass number A and energies of the scattered beam-like particles passing through PRISMA were determined using the ToF − Bρ − ∆E technique. Ions pass through a position-sensitive microchannel plate timing detector (MCP) before entering the quadrupole and dipole magnets. At the focal plane, ions first pass through a multi-wire parallel plate avalanche counter (MWPPAC) then into a segmented ionisation chamber. The measured positions of the ions in the MCP and MWPPAC define the trajectory of the ions through the magnetic elements, determining the magnetic rigidity Bρ. The energy loss of ions in the ionisation chamber enables the determination of Z, and with Bρ, the charge-state q. Together with time-of-flight (TOF), this allows determination of A and the kinetic energy of the projectile-like nuclei. Following Z,N determination, the ground-state to ground-state energy difference (Q-value, Qgg) could be obtained for each event. With the kinetic energy information, the total excitation energy Ex could be derived (Ex=Qgg-Q), making use of two-body kinematics. In the data files, the center of mass energy is indicated, along with the measurement angle of 115 degrees.