Rotational band structure and deformation properties of states in strontium-81

The level structure and lifetimes of excited states in 81Sr were investigated using the techniques of in beam γ ray spectroscopy. The reactions 56Fe(28Si,2pn)81Sn and 55Mn(29Si,p2n)81Sr were used at beam energies of 103.6 and 95.2 MeV, respectively. The targets consisted of 0.1 mm thick natural Fe (92% 56Fe abundance) and ≅ 2 mg/cm255Mn evaporated on a thick lead backing. The previously known level scheme was confirmed and a total of 15 new levels were placed in 81Sr. Tentative spin assignments were based on level systematics, directional correlations between γ rays and effective level lifetimes. The mean lifetimes of 12 states were measured and lifetime limits were assigned for 10 others using the Doppler shift attenuation method. The deformation properties of states in 81Sr were determined from the lifetimes and compared to the deformation inferred from level energies. The four rotational band structures in 81Sr were found to have differing deformations which persist to high spins. The magnitude of this deformation ranges from β ≅ 0.2 to ≅ 0.4. Band crossings in the moderately deformed g9/2 yrast and K = 5/2- bands were observed and a very deformed K = 1/2(+) band was seen up to a tentative spin of 31/2h. The experimental results were compared to theoretical projected and deformed shell model calculations. The crossing observed in the yrast band has been attributed to the alignment of two g9/2 protons. The properties of the K = 1/2(+) band are shown to be consistent with a configuration based on a d5/2 intruder orbital. This is the first nucleus in the mass 80 region to exhibit such a strongly deformed band based on this configuration