The anatomy of neck configuration in the fission decay of uranium and thorium isotopes is investigated in a microscopic study using relativistic mean field theory. The study includes ²³⁶U and ²³²Th in the valley of stability and exotic neutron rich isotopes ²⁵⁰U, ²⁵⁶U, ²⁶⁰U, ²⁴⁰Th, ²⁵⁰Th, ²⁵⁶Th likely to play an important role in the r-process nucleosynthesis in stellar evolution. Following the static fission path, the neck configurations are generated and their composition in terms of the number of neutrons and protons is obtained showing the progressive rise in the neutron component with the increase of mass number. Strong correlation between the neutron multiplicity in the fission decay and the number of neutrons in the neck is seen. The maximum neutron-proton ratio is about 5 for ²⁶⁰U and ²⁵⁶Th suggestive of the breakdown of the liquid-drop picture and inhibition of the fission decay in still heavier isotopes. Neck configuration as precursor of a new mode of fission decay like multi-fragmentation fission may also be inferred from this study.