?-decay Studies Of The Neutron-deficient Nuclides ²¹⁸Pa And ²¹⁹U

For isotopes in the light actinide region near the N=126 shell closure,? decay is one of the dominant radioactive decay mode.Fine structure in the ? spectrum can always be observed because ? decay can occur in the different levels between mother and daughter nuclei.This phenomenon reveals the discrete energy levels inside the nuclei and thus ?-decay spectroscopy plays crucial role to obtain information on nuclear structureThe ?-decay studies on the very neutron-deficient nuclides 218Pa and 219U were carried out at the gas-filled recoil separator SHANS(Spectrometer for Heavy Atoms and Nuclear Structure)at HIRFL(Heavy Ion Research Facility in Lanzhou).Fusion evaporation reaction 182W(40Ar,1p3n)218Pa and 183W(40Ar,4n)219U were used to produce 218Pa and 219U,respectively.The evaporation residues(ERs)were separated in flight from the background of primary beam and other unwanted products by SHANS and then transported into a focal plane detector system,where their implantation and subsequent decays were measured.A fast electronics system based on waveform digitizers was used in the data acquition and the sampled pulses were processed by digital algorithms.The identication of the products was made using the ?-? and ?-? coincidence measurementsFor 218Pa,the previously known ?-decay transitions were measured with improved precision based on a better statistics and unambiguously assigned to the decay of the ground state in 218Pa,which clarified the ambiguities in the previous assignment.A new isomeric state in 218Pa decaying with ?-particle energies of 9691(15)keV and 9596(21)keV and a half-life of 135-32+63 was identified.The excitation energy of the isomeric state was determined to be 83(6)keV.In order to reproduce the experimental data on low-lying levels in the odd-proton N=127 isotones,shell model calculations using the modified Kuo-Herling interaction were performed.According to the N=127 level systematics and calculation results,the spin and parities of the ground state and new isomeric state in 218Pa were tentatively assigned to be 8-and 1-,respectively.This indicates that the spins and parities of 218Pa do not follow the trend of the 1-ground state and the 9-or 8-isomeric state as observed in lighter odd-proton N=127 isotones.For 219U,the ?-decay properties were measured with improved precision.The ground to ground ?-decay properties of 219U were determined to be Ea=9763(15)keV and T1/2=60(7)?s.Two new ?v decays from the(9/2+)ground state to the(5/2-)and(3/2-)states in 215Th were observed.The exciation energy of the(3/2-)state in 215Th was determined to be 803 keV for the first time.The systematics of ?-decay reduced widths was established and the results indicated that the 9/2+?3/2-transitions are favored by the nuclear structure.