Jing Qian

To improve the understanding of the evolution of nuclear structure near the shell closures at N = 126, recoil-decay tagging, which provides a very high channel selectivity, is a very important tool. At Yale, a focal plane detection system of SASSYER, a gas-filled recoil separator at A. W. Wright Nuclear Structure Laboratory, is built to utilize the recoil-decay tagging technique. Here the commissioning of the detection system is presented. Results from a commissioning experiment are compared to literature data.

In this work, delayed spectroscopy of 257Rf and its decay products was also performed at Argonne National Laboratory. The experimental results are interpreted in the framework of the Microscopic-Macroscopic model. The isotope 257Rf was produced in the fusion-evaporation reaction 205Pb(50Ti, n)257Rf. Reaction products were separated and identified by mass. A partial decay scheme with configuration assignments is proposed based on alpha hindrance factors. The excitation energy of the 1/2+[620] configuration in 253No 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 257Rf, with a half life of [special characters omitted] μs, was discovered by detecting internal conversion electrons followed by alpha decay. It is interpreted as a three-quasiparticle high-K isomer. A second group of internal conversion electrons, with a half-life of [special characters omitted] s, followed by alpha decay, was also observed. These events might originate from the decay of excited states in 257Lr, populated by electron-capture decay of 257Rf. Spontaneous fission (SF) of 257Rf was unambiguously detected, with a branching ratio of [special characters omitted] = 0.02 ± 0.01.