Study Of The Chirality And Octupole Correlations In ⁷⁶Br By Lifetime Measurement

Spontaneous chiral symmetry breaking in nucleus was first proposed by Frauendorf and Meng.The restoration of spontaneous chiral symmetry breaking in the laboratory frame may give rise to pairs of nearly degenerate ?I=1 bands with the same parity,which are called chiral doublet bands.In addition to nearly degenerate levels,the intraband B(M1)values in the side and yrast bands should be almost equal as well as the B(E2)values,and the former should exhibit odd-even staggering.The properties of electromagnetic transitions carry more stringent information on the intrinsic structure and chiral geometry than excitation energies.Compared to the A ?100,130 and 190 mass region,the lifetime information in candidate chiral doublet bands is absent in the A ?80 mass region.Thus,making it essential to perform lifetime measurements in candidate chiral doublet bands in this mass region.The interaction between the nucleons of opposite-parity orbitals lying near the Fermi level whose angular momenta differ by 3h might lead to octupole correlations.The regions of nuclei with strong octupole correlations correspond to particle numbers near 34(g9/2(?)p3/2),56(h11/2(?)d5/2),88(i13/2(?)f7/2),and 134(j15/2(?)99/2).A common property of nuclei exhibiting the features of reflection asymmetry is the occurrence of relatively large E1 transition probabilities(the B(E1)? 10-5 W.u.)between the yrast positive-and negative-parity bands.In 78Br,the multiple chiral doublet bands with octupole correlations have been observed,which indicates that nuclear chirality can be robust against the octupole correlations.The experimental B(E1)/B(E2)branching ratios and the energy displacement ?E in 78Br were extracted and compared with those in 125Ba and 224Th,the results indicated that octupole correlations exist in 78Br.It is worth noting that the B(E1)/B(E2)is a good indicator to reflect the strength of B(E1)but also affected by B(E2)associated with the deformation.A large B(E1)/B(E2)branching ratio is probably due to the small B(E2)value.Thus,it is very important to perform lifetime measurements and obtain the absolute B(E1)values for the reflection asymmetry nuclei.In addition,the Br isotopes may systematically exhibit the coexistence between chirality and octupole correlations.It may provide a unique opportunity to research the interplay between chiral symmetry breaking and reflection symmetry breaking with the neutron number.High spin states in 76Br were populated by the 68Zn(12C,lp3n)reaction at a beam energy of 66 MeV.The present Letter report the lightest candidate chiral nucleus 76Br in the nuclide chart and present the reduced transition probabilities,obtained by lifetime measurements,in candidate chiral bands in A?80 mass region for the first time.A combination of the MDC-CDFT and TPRM reproduces the experimental data very well,substantiating the theoretical interpretation:The doublet bands in 76Br might correspond to a chiral vibration pattern.In addition,absolute B(E1)values,obtained by lifetime measurements,indicate that the the octupole correlations exists in 76Br.The MDC-CDFT calculations support the octupole correlations in 76Br as well.T he present letter provides the evidence for the octupole correlations having a chirality-independent characteristics in Br isotopes.