Proton states in the Z=99 nucleus Es251 excited by Fm251 electron capture decay and Cf250(α,t) reaction

Level structure of the Z=99 nucleus, Es251, has been investigated by measuring the γ rays and conversion electrons associated with the electron capture decay of 5.3-h Fm251, and from triton spectra produced in the Cf250(α,t)Es251 reaction. The γ-ray spectra were measured with a 2-cm2 × 5-mm planar and a 25-cm3 coaxial Ge(Li) detector and the electron spectra were measured with a cooled Si(Li) detector. Fortyfive γ rays were identified in Es251 and the multipolarities of most of the transitions were deduced. Logft values for the observed electron capture transitions were derived from the measured electron capture transition intensities. A two-parameter γ-γ coincidence experiment was performed to determine the relationships between various γ transitions. Triton spectra from the Cf250(α,t)Es251 reaction were analyzed with an Enge split-pole magnetic spectrograph. On the basis of the present investigation the following intrinsic states were identified in Es251: 3/2 - [521], 0; 7/2 + [633], 8.3; 1/2 - [521], 411; 7/2 - [514], 461.4; 9/2 + [624], 777.9; {7/2+[633]-2+}11/2+ vibrational state, 889.1; {n[734]92-; n[620]12+; p[521]32-}112+ three-quasiparticle state, 1239.0 keV. A Kπ=112+ state at 1264.9 keV was interpreted as a three-quasiparticle state but could not be given a definite assignment. This is the first time a definite identification of the proton orbitals 1/2 - [521] and 7/2 - [514] has been made in any nucleus. RADIOACTIVITY Fm251 [from Cf249(α,2n)]; measured Eγ, Iγ, Ece, Ice, γγ coin, α branching. NUCLEAR REACTIONS Cf250(α,t), E=28.0 MeV; measured E (excitation), σ. Es251 deduced levels, logft(EC), γ multipolarity, I, π, five single-particle states, one vibrational state, and two three-quasiparticle states. Mass-separated Fm251 and Cf250.