We report the first direct measurement of differential transfer cross sections using a Recoil Mass Spectrometer. Absolute differential 1p and 2p-stripping cross sections at θ c.m. = 180 • have been determined for the system 16 O+ 142 Ce by detecting the heavier target-like ions at the focal plane of the Heavy Ion Reaction Analyzer. Focal plane spectra have been compared with the results of a semi-microscopic Monte-Carlo simulation to unambiguously identify the transfer channels. Transmission efficiency of the target-like ions through the spectrometer has also been estimated using the simulation which has been crucial to extract the cross sections from the yields of ions measured during the experiment. The methodology adopted in this work can be applied to measure multi-nucleon transfer cross sections using other similar recoil separators. The experimental excitation functions for the reactions 142 Ce( 16O, 15 N) 143 Pr and 142 Ce( 16 O, 14 C) 144 Nd have been compared with coupled reaction channel calculations. An excellent matching between measurement and theory has been obtained. For 1p-stripping, major contribution to the cross section has been found to be the transfer of a proton from 16 O to the 2d 5 2 excited state of 143 Pr, leaving behind 15 N in the 1p 1 2 ground state. Transfer of a cluster of two protons from 16 O to the 2 + excited state of 144 Nd, resulting in 14 C in the 0 + ground state, appears to be the most probable cause for 2p-stripping. Measured transfer probabilities for 1p and 2p channels have been compared with Time-Dependent Hartree-Fock calculations. Proton stripping channels are found to be more favourable compared to neutron pick-up channels. However, the theory overpredicts the measurement hinting at the need for extended approaches with explicit treatment of pairing correlations in the calculations.
A steeper fall of fusion excitation function, compared to the predictions of coupled-channels models, at energies below the lowest barrier between the reaction partners, is termed as deep subbarrier fusion hindrance. This phenomenon has been observed in many symmetric and nearlysymmetric systems. Different physical origins of the hindrance have been proposed. This work aims to study the probable effects of direct reactions on deep sub-barrier fusion cross sections. Fusion (evaporation residue) cross sections have been measured for the system 19 F+ 181 Ta, from above the barrier down to the energies where fusion hindrance is expected to come into play. Coupled-channels calculation with standard Woods-Saxon potential gives a fair description of the fusion excitation function down to energies ≃ 14% below the barrier for the present system. This is in contrast with the observation of increasing fusion hindrance in asymmetric reactions induced by increasingly heavier projectiles, viz. 6,7 Li, 11 B, 12 C and 16 O. The asymmetric reactions, which have not shown any signature of fusion hindrance within the measured energy range, are found to be induced by projectiles with lower α break-up threshold, compared to the reactions which have shown signatures of fusion hindrance. In addition, most of the Q-values for light particles pick-up channels are negative for the reactions which have exhibited strong signatures of fusion hindrance, viz. 12 C+ 198 Pt and 16 O+ 204,208 Pb. Thus, break-up of projectile and particle transfer channels with positive Q-values seem to compensate for the hindrance in fusion deep below the barrier. Inclusion of break-up and transfer channels within the framework of coupled-channels calculation would be of interest.
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