Role of neutron transfer in sub-barrier fusion

Abstract

In heavy-ion induced reactions, the sub-barrier fusion cross-sections are enhanced as compared to the predictions of the one-dimensional barrier penetration model (1-D BPM). The enhancement in sub-barrier fusion has been interpreted by the inclusion of static deformations, the couplings of in-elastic excitations and non-fusion channels, especially the positive Q-value neutron transfer (PQNT) channels. However, the interplay of di erent factors responsible for sub-barrier fusion enhancement is yet fully understood. Aiming to investigate the factor which in uences sub-barrier fusion more and to understand the consequences of couplings, the fusion excitation functions for 35;37Cl+130Te systems have been measured from 10 % below to 15 % above the Bass barrier energies. Experiments have been carried out at the Inter-University Accelerator Centre, New Delhi, India using 15UD Pelletron Accelerator facilities. Di erent observables of nuclear reactions have been studied by employing the recoil mass spectrometer, Heavy-Ion Reaction Analyser (HIRA). In order to study the behavior of sub-barrier fusion, the excitation functions have been analyzed in the framework of coupled-channels code CCFULL. In the present work, the fusion cross-sections have been measured down to 1 b at the lowest measured energy, i.e., 10 % below the barrier for 37Cl+130Te, and down to 0.034 mb for 35Cl+130Te system. It has observed that the inclusion of couplings of low-lying excited states of interacting partners along with the modi ed radius parameter reproduces the fusion excitation function for the entire measured energy range satisfactorily for 37Cl+130Te system. However, in the case of 35Cl+130Te system, the couplings mentioned above are unable to reproduce the excitation function in the measured energy range. Thus, +2n transfer channel coupling has been implemented in the coupled-channels calculations. It has been found that the inclusion of +2n channel coupling well reproduces the fusion excitation function at sub-barrier energies. In order to understand the trend of fusion cross-section at sub-barrier energies in 37Cl + 130Te system, the fusion barrier distribution, the logarithmic derivative L(E)-factor, and the astrophysical S-factor have been extracted from the analysis of fusion excitation function. The analysis of astrophysical S-factor and the L(E)-factor suggest no signature of fusion hindrance in 37Cl + 130Te system even down to 1 b cross-section. However, for 35Cl + 130Te system no such analysis has been performed as deep sub-barrier cross-sections are not available for this system. For an insight into the e ect of positive Q-value neutron transfer (PQNT) channels, the excitation functions of both systems are compared. This particularly interesting because 35Cl + 130Te system has 6 PQNT channels as compared to none in 37Cl + 130Te system. The excitation function of 35Cl + 130Te system is found to be considerably enhanced as compared to 37Cl + 130Te system, indicating correlation between the sub-barrier fusion enhancement and the available PQNT channels. This thesis is organized as follows. Chapter 1 includes a brief introduction about the nuclear reactions around the barrier and some of the open questions related to the sub-barrier fusion. The experimental details and data reduction procedure are given in Chapter 2. The experimental outcome of this thesis is discussed in detail in Chapter 3 and 4. In these chapters, the excitation functions from 10 % below to 15 % above barrier en-ergies are analyzed in the framework of coupled-channels calculations. The fusion barrier distribution is extracted from the measured excitation functions to understand the structure of the potential landscape between them during the interaction. To understand the rate of reaction at astrophysical objects, the S(E)-factor and L(E)-factors are calculated. Information on the role of neutron transfer channels with positive Q-value in sub-barrier fusion enhancement is explored, especially for 35;37Cl + 130Te systems by comparing the measured excitation functions. The existing results of 130Te + 58;64Ni systems have been reanalyzed, in which PQNT channels have been found to be responsible for sub-barrier fusion enhancement unlike the interpretation of Z. Kohley et al., [PRL 107, 202701 (2011). In Chapter 5, summary and outlook of the work presented in this thesis are given.