High-spin Level Structures Of Semimagic Nuclei ⁸⁹Y And ¹⁴¹Pr

The structures of nuclei in the A?80 and 140 mass regions have attracted considerable theoretical and experimental attention in recent years because of anumber of interesting nuclear phenomena predicted and also observed,such as chiral doublet bands,shape coexistence,octupole correlations and superdeformed bands.In this two mass regions,the shell structure effects are quite strong and the level structures of the N = 50 and 82 isotones and the nuclei around themmostly come from various proton excitations.This is because the magic number 50 and 82 locate in the A?80 and 140 mass regions,respectively.The properties of nuclei are essentially influenced by the proton subshells and shell closures.In addition,high-spin states corresponding to the neutron excitations across the N=50 and 82 gaps have been identified in a lot of nuclei in the two mass regions.In this thesis,we will introduce the high-spin level structures of ⁸⁹Y and ¹⁴¹Pr in the N = 50 and 82 isotones,respectively.We will focus on the phenomenon of neutron cross-shell excitation and the systematic features of the N = 50 and 82 isotones.Using the in-beam ?-spectroscopic techniques,we have populated the high-spin states of ⁸⁹Y by the heavy-ion fusion evaporation reaction ⁸²Se(¹¹B,4n)at beam energies of 48 and 52 MeV,and also populated the high-spin states of ¹⁴¹Pr by the ¹³⁸Ba?⁷Li,4n?reaction at a beam energy of 38 MeV.The ¹¹B and ⁷Li beams were provided by the HI-13 tandem accelerator at the China Insti-tute of Atomic Energy?CIAE?in Beijing.In the former experiment,the target consisted of 1.35 mg/cm² of enriched ⁸²Se rolled onto a 9.1 mg/cm² Pb backing,and the ?-rays were detected by an array of 12 Compton-suppressed hyperpure-germanium?HPGe?detectors and two Low-Energy Photon Spectrometer?LEPS?detectors.In the experiment of ¹⁴¹Pr,the target consisted of 3.1 mg/cm² of en-riched ¹³⁸BaCO₃ rolled onto a 1.2 mg/cm² carbon backing,and the ?-rays were detected by eleven Compton-suppressed HPGe detectors.Besides,two LEPS detectors were also used to measure the x-rays and low-energy ?-rays in the sec-ond experiment.In total,about 1.3×10⁸ and 9×10⁷ ?-? coincidence events were recorded in the two experiments,respectively.Through the detailed off-line analyses of the data,we have extended the level schemes of ⁸⁹Y and ¹⁴¹Pr for both positive and negative states.We have added 13 new levels and 24 new transitions to ⁸⁹Y,and added 14 new levels and 30 new transitios to the level scheme of ¹⁴¹Pr.Based on the previous level schemes,ADO ratios,internal conversion coefficients and systematic features,we have assigned spin and parity to most of the levels in the both nuclei.It should be noted that,we have constructed two new negative parity sequences in the level scheme of ⁸⁹Y.Though the analyses of the level structures of ⁸⁹Y and ¹⁴¹Pr,we have found that the low-lying level structures of the two nuclei are mainly due to proton excitations.As the spin and excitation energy increase,the proton pairs will be broken to form higher spin.In the high-lying states,the neutron cross-shell excitation may occur in ⁸⁹Y and ¹⁴¹Pr.We have performed the shell model calculations using the large-scale shell model code OXBASH to semimagic nuclei ⁸⁹Y and ¹⁴¹Pr.Using the GWB model space and GWBXG interactions,we have performed two sets of shell model calculations to ⁸⁹Y,one allowing neutron excitations across the N = 50 shell and the other not.The two sets of shell model calculations could both well reproduce the low-lying levels.However,to the high-spin states,only the calculated results allowing neutron excitations can well reproduce the experimental observations.Through the detailed analyses of the calculated results,we have assigned the configurations to the known levels.We have not only observed the positive parity level structures from neutron cross-shell excitations,but also indicated that there is a strong competition between the pure proton excitations and neutron-core excitations.In addition,we have also observed the negative states from neutron core excitations for the first time.As to ¹⁴¹Pr,we have performed the calculations using the N82 model space and N82K interaction.The remaining nine valance protons are distributed over the orbits between Z = 50 and 82.The calculated excitation energies are in reasonable agreement with the experimental levels for both positive and negative parity states.Based on the calculated results,we have assigned pure proton excitations to most of the levels of ¹⁴¹Pr.However,the 23/2₂⁺ state can not be explained by the present calculations.Hence,we have assumed this level come from neutron core excitation.To verify the present cross-shell excitation assignments to the levels in ⁸⁹Y and ¹⁴¹Pr,and get a better understanding of the systematic features of cross-shell excitation in the N = 50 and 82 isotones,we have compared the excitation ener-gies as a function of angular momentum of the states from the neutron excitations in the N = 50 and 82 isotones for different parity and mass odevity.Possible evidence for the 23/2₂⁺ state come from the neutron cross-shell excitations and competition between the pure proton excitations and neutron-core excitations have been found in ¹⁴¹Pr based on the comparisons.Besides,we have found that the required energy for exciting one neutron across the N = 50 shell gap is at least 4 MeV and the energies of negative parity states from the neutron-core excitations are higher than these of the positive parity states.The systematics of the states from the neutron-core excitations in odd-A isotones are very similar with the those in even-even isotones,which indicate that the additional valance proton do not evidently influence the neutron-core excitations.In the meantime,there are also some differences between the two isotones.The negative parity states from neutron cross-shell excitations in the N = 50 isotones need to break another pair of protons relative to the positive parity states,but the negative parity states in the N = 82 isotones come from a different neutron cross the N =82 gap with respect to the positive states.In addition,due to the proton-neutron interaction,the nuclei between ⁸⁶Kr and ⁸²Ge need less than 4 MeV to form the the states come from cross-shell excitations.