Experimental Study On Coexistences Of Multiple Modes Of Motion In ⁷⁴As And ⁷⁵As

Nuclei in the A～80 mass region exhibit rich patterns of motion and deformation,especially when they were excited to high spin states.The high spin states in 74As and 75As have been studied by heavy ion fusion evaporation reactions.The experiment has been completed at iThemba LABS in South Africa.The reaction channel generated 74As was the main and that of 75As was the side.Based the off-line γ-γ analysis and the analysis for the multipolarities of transitions,the level schemes of 74As and 75As have been established.Two positive-and two negative-parity rotational bands have been identified in 74As.We have corrected the excitation energies of the positive-parity bands reported in the previous work.The excitation energies of the positive-parity bands was increased about 11.8 keV,and the spin-parity of the states in the positive-parity bands have been reassigned,which were supported by the systematic comparison between 74As and the neighbouring odd-odd A isotopes.The positive-parity doublet bands have been assigned the chiral doublet bands.The negative-parity bands were established through rearranging and extending the negative-parity level structures,and have been assigned the pseudospin doublet bands.In our previous work,the positive-parity doublet bands were assigned the chiral doublet bands.The assignments to the positive-and negative-parity bands were supported by the calculations based on the triaxial particle rotor model.Between the yrast positive-and negative-parity bands,three E1 transitions were newly observed in this work,which imply that the octupole correlations exist in 74As.To investigate the possible octupole correlations in 74As,the experimental B(E1)/B(E2)values and energy displacement δ between the positiveand negative-parity bands been extracted and compared with those in the nuclei reported have octupole correlations and stable octupole deformation.Based the comparison,we suggest that octupole correlations exist in 74As.In the study of 75As,five bands were observed.We assigned the new observed positive-parity band as the γ vibrational band of the yrast positive-parity band.Three E1 transitions were observed between the yrast positive band and the negative-parity established on the 13/2-state.Based on the same method as the study on the octupole correlations in 74As,we also suggest that octupole correlations exist in 74As.In summary,we have observed the coexistence of the chirality,octupole correlations,and pseudospin symmetry in 74As,it is the first observation of the coexistence for these three phenomena in A～80 mass region.The γ vibration and octupole correlations were observed in 75As,it is the first observation of the coexistence for these two patterns in As isotopes.