First investigation of prompt neutrons from spontaneously fissioning isomers produced in multinucleon transfer reactions

This work presents the first experimental results on prompt neutron multiplicities from short-lived spontaneously fissioning isomers produced in the multinucleon transfer (MNT) reaction of 26 Mg + 238 U. The observed prompt neutron multiplicity distribution yields an average neutron number of ν ¯ = 2.65 ± 0.20 and a variance of σ ν 2 = 1.4. The half-life of the spontaneously fissioning products was determined to be T 1 / 2 = 5 . 3 − 0.4 + 0.5 μ s . The shape of the multiplicity distribution of prompt fission neutrons was restored using the Tikhonov regularization technique and is characterized by a symmetrical distribution. Among the known spontaneous fission isomers, 239 m Pu and 243 m Am exhibit half-lives and prompt neutron multiplicities that agree with the measured values within uncertainties. GEF model estimates yield ν ¯ = 2.54 ± 0.20 for 239 m Pu and ν ¯ = 2.91 ± 0.20 for 243 m Am, with the former showing slightly better agreement. Whether the activity arises from a single isomer or a mixture of both remains unresolved, as the current data do not allow for a clear separation of their contributions. Further high-statistics measurements, including fission fragment total kinetic energy and gamma-ray spectroscopy, are required for unambiguous identification of the fissioning nucleus.