First Observation Of Two Pairs Of Triplet Bands In Atomic Nuclei:Study Of High-spin States In Kr-81

Chirality in atomic nuclei was first predicted by S.Frauendorf and J.Meng in 1997.They pointed out that chirality is expected to exist in triaxial nuclei whose configuration contains high-j particle and high-j hole orbits.A pair of nearly degenerate ?I=1 bands called chiral doublet bands with the same parity would be observed in experiment.In 2006,J.Meng predicted that there might be multiple chiral bands(MXD)in a single nucleus based on the relativistic mean field theory.Since then,multiple chiral bands have been observed in 133Ce,103Rh,78Br,136Nd,194T1 and 195T1 nuclei successively.Positive and negative parity rotation bands in 81 Kr based on the 3-qp config-urations ?and have been reported.These configurations satisfy one of the conditions for the construction of chiral dou-blet bands.Moreover,the ? band in 81Kr has the same core with the?bands in 80Br.The rotation bands involving the latter configuration have been reported as chiral doublet bands,which implies the core has a triaxial shape.Thus,a pair of chiral doublet bands based on ? configuration are likely to exist in 81Kr.In the same way,the chiral doublet bands based on?configuration may also exist in 81Kr.It should be not-ed that a valence proton of ? configuration occupies the pseudospin orbit,suggesting that ? rotation bands may also be a pair of pseudospin doublet bands.The experiment was performed at iThemba LABS in South Africa,High-spin states of 81Kr were populated following the 82Se(a,5n)81Kr reaction at beam energies of 65 and 68 MeV.Approximately 1.45×109)?-? coincidence events were accumulated.Based on the analysis of the data,a new level scheme of 81 Kr was built.A tot,al of seven rotation bands were est,ablished,three of them were completely new structures.Angular distribution of ?-rays deexciting the oriented states(ADO)ratios and linear polarization values were extracted to obtain the multipolarities of transitions and to determine the I? values of energy levels in the scheme.In this work,two pairs of nearly degenerate triplet bands have been observed among the nuclear system for the first time.According to angular momentums ix,level energies E(I),signature splittings S(I)and electromagnetic transition proba-bility ratios B(M1)/B(E2),configurations of rotation bands were determined and the existence of multiple chiral bands in the 81 Kr was reported.The RMF and MPRM calculations support the interpretation of multiple chiral bands.This means that 81Kr would be the first odd-A nucleus whose multiple chiral bands have been observed in the A-80 mass region,giving evidence for the coexistence of different triaxial deformations.In addition,three nearly degenerate bands with negative parity are designated as chirality-pseudospin triplet bands.The phase in the B(M1)/B(E2)staggering of the yrast band was the same as that of the chiral partner band,but opposite to that of the pseudospin partner band.The experimental results have been successfully described by the RMF theory and particle rotor model(PRM).Such triplet bands in 81Kr provide the first example in the A?80 mass region for the coexistence of chiral symmetry and pseudospin symmetry in one single nucleus.