Experimental Research For The Lifetimes Of The Low-excited States In Nuclei ³⁷S And ¹⁴³Eu

The lifetimes of the nuclear excited states are directly related to the transition probabilities between the states,which provide sensitive probe for the structure of the energy levels.The excited-state lifetime measurement can deduce the reduced electromagnetic transition probabilities especially the reduced electric quadrupole transition probability B(E2)and reduced magnetic dipole transition probability B(M 1).Among many research topics in the field of nuclear structure,the interplay between single-particle and collective degrees of freedom have been being the most hottest research focus.For the neutron-rich37 S nucleus,the transition energy from the 3/2-first excited state to the 7/2-ground state is relatively low,which may involve the mixing between single-particle and collective intruder-state transitions.The B(E2;3/2-? 7/2-)value calculated with the lifetime of 3/2-state from experiment can provide the percentages of the single-particle and collective intruder-state transitions.For the spherical143 Eu nucleus,the M1 transition from the 7/2+excited state to the 5/2+ground state can be classified as either the L-forbidden M 1 transition from the point of view of single-particle transition or the magnetic-rotation transition from the point of view of collective transition,which can be determined from the B(M1;7/2+? 5/2+)value extracted from the lifetime of 7/2+state from the experiments.The experiment for the lifetime measurement of the 3/2-state in37 S was carried out at the Radioactive Ion Beam Line at the Heavy Ion Research Facility in Lanzhou(HIRFL-RIBLL1).The projectile fragmentation reaction of40 Ar primary beam with Be target produced the secondary beam of37 P,which populated the 3/2-state in37 S via ?decay.For the first time,the ?-? coincidence technique deduced the lifetime of the 3/2-state as 193 ± 4 ps,which was used to calculate the B(E2;3/2-? 7/2-)value as5.12 ± 0.11 W.u..The configuration mixing under the framework of shell model was introduced to analyze the 3/2-? 7/2-transition.It was found that this transition can be described as the mixture of a major single-particle transition and a certain amount of collective intruder-state transition.The experiment for the lifetime measurement of the 7/2+state in143 Eu was performed at the gas-filled recoil separator,Spectrometer for Heavy Atom and Nuclear Structure,ofthe Heavy Ion Research Facility in Lanzhou(HIRFL-SHANS).The excited states of143 Eu were populated with the123Sb(24Mg,4n)fusion-evaporation reaction.The143 Eu nuclei at isomeric state were separated and purified by SHANS.The lifetime of the 7/2+state in143 Eu was determined as 109 ± 17 ps by the implantation-?-? delayed coincidence technique for the first time.The B(M1;7/2+? 5/2+)was calculated as(1.3 ± 0.2)×10-2W.u.,which indicates the 7/2+? 5/2+indeed belongs to the L-forbidden M1 transition.The systemics of the B(M1)values of the ?(1g7/2? 2d5/2)L-forbidden M 1transition in N=78,80 and 82 odd-mass isotones were studied.Likewise,we performed some simple calculation using the shell-model configuration mixing and the numbers of the valence nucleons,and the calculated results successfully explain the evolution trend of B(M1)values with the varying proton number.