Spectroscopy of Rf257

The isotope Rf257 was produced in the fusion-evaporation reaction Pb208(Ti50,n)Rf257. Reaction products were separated and identified by mass. Delayed spectroscopy of Rf257 and its decay products was performed. A partial decay scheme with configuration assignments is proposed based on α hindrance factors. The excitation energy of the 1/2⁺[620] configuration in No253 is proposed. The energy of this 1/2⁺ state in a series of N=151 isotones increases with nuclear charge, reflecting an increase in the N=152 gap. This gap is deduced to grow substantially from 850 to 1400 keV between Z=94 and 102. An isomeric state in Rf257, with a half-life of 160_-31⁺⁴²μs, was discovered by detecting internal conversion electrons followed by α decay. It is interpreted as a three-quasiparticle high-K isomer. A second group of internal conversion electrons, with a half-life of 4.1_-1.3^+2.4 s, followed by α decay, was also observed. These events might originate from the decay of excited states in Lr257, populated by electron-capture decay of Rf257. Fission of Rf257 was unambiguously detected, with a branching ratio of b_Rf^SF=0.02±0.01.