One- and three-quasiparticle states in 249Cf populated by the electron capture decay of 1.70-h 249Es

The level structure of 249Cf has been investigated by measuring the γ rays and conversion electrons associated with the electron capture decay of 1.70-h 249Es. The γ-ray spectra were measured with a 2-cm2 x 5-mm planar and a 25-cm3 coaxial Ge(Li) detector and the conversion-electron spectra were measured with a cooled Si(Li) detector. Forty-six γ rays were identified and multipolarities of most of the transitions were deduced. The half-life of 249Es was determined from the decay rate of the 379.5-keV photopeak and was found to be 102.2 +- 0.6 min. The logft values for the observed electron capture transitions were derived from the measured electron capture intensities. On the basis of the present work the following intrinsic states were identified: 9/2-[734], 0; 5/2+[622], 145.0; 7/2+[624], 379.5; 7/2+[613], 443.0; Kπ=5/2-, 813.2; 9/2+[615], 1007.9; and Kπ=7/2-, 1218.5. The 5/2- band at 813.2 keV has been interpreted as the γ-vibrational band coupled to the 9/2-[734] ground state. The 7/2- state at 1218.5 keV has been interpreted as a three-quasiparticle state with configuration ]p[633]7/2+; p[521]3/2-; n[622]3/2+closing-brace7/2-. This is the first time a three-quasiparticle state in an actinide nucleus has been identified. An enhancement in the B (E3) value of the 145.0→0 transition has been observed which suggests collective admixture in the 145.0-keV state. From the experimental B (E3) value the admixture (amplitude squared) of the ]9/2-[734]direct-product2-closing-brace5/2+ configuration in the 145.0-keV state has been derived to be approx.29%, in excellent agreement with the results of (d,d') reaction studies