Elsevier

Nuclear Physics A

Volume 135, Issue 1, 22 September 1969, Pages 36-48
Nuclear Physics A

A low-lying 2 state in 236U

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Abstract

A low-lying KIπ = 22 state of 236U, first characterized in measurements of 240Pu alpha decay, has been more thoroughly studied through the electron-capture decay of 236Np. The energy of the state is 687.70 ± 0.10 keV, its half-life is 4.4 ± 0.6 nsec, and it decays by M2 and E1 transitions to members of the ground state band. It is populated with an intensity of 1.5 ± 0.3% in the decay of 236Np, and (2.1 ± 0.4) × 10−5% in the decay of 240Pu, in the latter case presumably via an alpha transition to the unobserved spin-3 member of the band. Besides having the lowest energy of any known 2 state in an even nucleus, the state is characterized by fast M2 transitions and highly hindered El transitions.

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      The branching ratio for the competing decay branches from the 236mNp state are listed as 50(3)% for each decay mode in the most recent nuclear data sheets evaluation (Browne and Tuli, 2006). This majority of the decay strength from the 236mNp state decays directly to the 0+ ground states and first Iπ = 2+ states in the 236Pu and 236U daughter nuclei, with a weak direct decay branch also identified the Kπ = 2- excited state at 687.7 keV in 236U (Lederer et al., 1969). The electromagnetic (M5) direct decay branch from the 236mNp (1−) metastable state to the proposed Kπ = 6− 236gNp ground state has not been identified experimentally to date.

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    This work was performed under the auspices of the U.S. Atomic Energy Commission.

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