Experimental investigation of the range of existence of asymmetrical fission type in light nuclei

Mass-energy distributions of fragments from fission of nuclei 201Tl-209Bi have been measured in the reaction induced by protons and α particles. The results evidence that the contribution from the asymmetric component detected previously in nuclei 208Po-213At falls rapidly with a decrease in the nuclear mass and, probably, vanishes at 201Tl. Thus a lower boundary for the range of existence of the asymmetrical fission type A approximately 200, Z approximately 80 is established. A qualitative interpretation is given for the main properties of the asymmetrical fission for pre-actinide nuclei