Features of the (t,p) reaction below the Coulomb barrier of the entrance channel

The differential cross sections for the reactions 63,65Cu(t,p)65,67Cu were measured at E/subt/=3.2, 3.0, and 2.8 MeV. For L=0 ground-state transitions, the energy dependence of the values of the differential cross sections is strong and exponential, while the angular distribution shape does not change with small energy variation. The reactions proceed well below the Coulomb barrier of the entrance channel, yet through a direct process. A definite difference is observed between the angular distributions for the two ground-state transitions. Zero-range distorted wave Born approximations can reproduce the energy dependence of the differential cross sections and the trend of isotope dependence of the angular distribution. In DWBA calculations, a definite nuclear optical potential is needed for the triton-nucleus channel which is below the Coulomb barrier. The triton-nucleus optical potential below the Coulomb barrier is nearly equal to the optical potential that works at energies of a few tens of MeV. The angular distribution for an L=2 transition is forward-rising, though not diffraction-like, and can be reproduced on the whole by the zero-range DWBA. It is confirmed for the first time that the (t,p) reaction below the Coulomb barrier of the entrance channel takes place in an extended region outside the nucleus. It it concluded that the (t,p) angular distribution shape below the Coulomb barrier of the entrance channel is sensitive to the radial two-neutron form factor outside the nucleus. Physical implications of the revealed features are discussed, especially on the coherence of the two-particle form factor and the proton and neutron distributions in the nuclear surface region