Charge transfer prior to fusion

Abstract

In reactions between very asymmetric projectile-target systems charge is preferably transferred from the lighter to the heavier nucleus. This leads to a decrease of the potential barrier between the nuclei and increase of the fusion cross-section. We perform trajectory calculations with energy dissipation and proton transfer to estimate the magnitude of the effect.Our theoretical understanding of heavy-ion induced fusion reactions has been substantially extended during the past few years. The simple model that two nuclei fuse when they approach closer than their separation at the Coulomb barrier has been modified in the following important respects:(i) a dissipative force has been introduced which, at sufficiently high incident energies diverts flux from the fusion into the deep inelastic channels [1, 2]; (ii) this strong dissipative force has been shown to lead to oblate dynamical deformations of the incident nuclei, thereby increasing the barrier to be surmounted and decreasing the fusion cross section (iii) neck-formation, after the nuclei have penetrated their outer barrier, allows the system to evolve on a multi-dimensional potential energy surface, with eventual fusion being determined by the location of saddle points on this surface [4,5]. This note investigates the effect on the fusion crosssection of proton transfer during the approach phase of projectile and target, before these have reached the Coulomb barrier. The calculations below include the gradual energy loss along a trajectory due to the dissipative force ((i) above) but ignores deformation.Its inclusion is not expected to change the conclusions. Nucleon exchange is believed to be the first stage of neck formation. We however do not include the later effects of the neck degree of freedom discussion in (iii) above.For intermediate to heavy mass systems (At, to j + Atarget >150) the fusion cross-section is composed to a large extent of fission fragments. Hence such fusion cross-sections are measured most unambiguously when the projectile and target masses are well separated. It is for just such projectile-target combinations that one observes a net charge transfer from the light to the heavy nucleus i.e. away from symmetry. This is most easily understood by considering the dynamical distortion of the potential energy surface of the combined projectile-plus-target system. The mean Coulomb field of the heavier partner raises the proton Fermi level in the lighter partner by an amount (Z2-Z1)eZ/R more than the weaker Coulomb field of the lighter partner the proton Fermi level in the heavier partner. Here Z2(Z1) is the charge number of the heavy (light) nucleus and R is their separation. Thus, provided Z 2 >>Z1, a potential head and therefore a driving force on the protons develops in the direction from the lighter to the heavier nucleus. The magnitude of the net charge transfer will depend on the proton single particle potential barrier which forms between projectile and target as these pass each other on their classical trajectories [6] (see below...

Odd-Z Transactinide Compound Nucleus Reactions Including the Discovery of 260Bh

“…Hartmann and Gross [84] have shown that in asymmetric nuclear reactions, there is a charge transfer from the lighter to the heavier nucleus immediately 2.9 ± 1.6 Table 8.2: Capture cross sections for all experiments used in the systematic study of paired reactions, calculated by the method described in [29] with updated parameters [32]. Peak 1n cross sections and ratios for these values are included for comparison.…”

Transfer reactions inS32+92,Liang1, Lee2, Mahon3 et al. 1994Phys. Rev. C20030View full textAdd to dashboardCite

“…Although the neutron transfer for ' Mo and 3Nb has positive Q values, particularly for two-neutron pickup, the yields for proton transfer are not much less than the neutron transfer. It has been pointed out [28,29] that charge transfer with a negative Q value could also enhance fusion if the Q-matching conditions for transfer are valid. Furthermore, coupled-channel calculations show that multinucleon transfer plays an important role in the fusion enhancement [30].…”