Photofission at sub-barrier excitations

The review is devoted to investigations of the probability of photofission in the threshold region and in the sub-barrier region of energies. The following questions of the physics of low-energy fission are the main subjects of interest: the spectrum of transition states (fission channels) of cold nuclei, the shape of the barrier, the symmetry of the nuclear configuration in the process of deformation, the features of the passage through the barrier associated with the existence in the second well of the quasistationary fission and nonfission modes, and the interaction between them and between them and the analogous states in the first well. The nature of this interaction depends strongly on the energy, which explains the increasing interest in a new field of investigation: deep sub-barrier fission near the bottom of the second well, in particular, the isomer shelf effect observed at such energies. The present state of work in this direction is analyzed critically