N. J. Sigurd Hansen

Isomers in odd-mass Lu isotopes, ascribed to the [541] 12− Nilsson configuration

Abstract Irradiation of Yb isotopes with pulsed beams of 8–13 MeV protons and deuterons has led to the discovery of the following new isometric transitions in the Lu isotopes: a 29 keV, 160 sec, E3 transition in 169Lu; a 70 keV, 76 sec, E3 transition in 171Lu; a 345 keV, 1.3 μsec, E1 transition in 175Lu; a 0.67 sec level in 170Lu decaying by a 48 keV, M2 transition and a 44 keV, E2 transition; and a 64 keV, 430 μsec, E1 transition in 172Lu. The isomerism in the odd-mass isotopes is ascribed to the [541] 1 2 − Nilsson state, in analogy to the well-studied case in 173Lu. The results are discussed in relation to the parameters of the Nilsson model.

A spin 112 isomer in 157Gd

Abstract Through the reaction 154Sm(α, n), a 17 μm half-life isomer at 425 keV in 157Gd is investigated by means of a pulsed-beam technique. For several reasons, the isomeric level is believed to be the 11 2 − [505] Nilsson state. The decay is studied by using a Ge detector and by prompt as well as delayed coincidence techniques. The decay proceeds through two rotational bands, viz., the ground state band and a band built on the 5 2 + [642] Nilsson state, itself forming an isometric level at 64 keV and with half-life of t 1 2 = 0.46 μ s . Comparison of stripping and pick-up reaction data with theory shows that the 11 2 − [505] Nilsson state is a hole state. By bombardment of 152Sm and 150Sm with α-particles, isomeric decays were observed; the half-lives being 31 ms and 84 μs, respectively.

The Ω-forbidden decay of the 4.1 μs isomeric state in 170Tm

Abstract An isomeric state with a half-life of 4.12±0.13 μs has been found in the doubly odd nucleus 170 Tm. It decays by strongly retarded E1 transitions. The high hindrance factors of ≈ 10 8 seem to be caused by the fact that the projections Ω of the angular momenta of the two odd nucleons are directed antiparallel in the initial state and parallel in the final states. The projection Ω n for the neutron which undergoes the transition changes sign with respect to the projection Ω p of the non-participating proton. This leads to the selection rule |Ω ni |+|Ω nf | = 4 ≦ L and thus threefold Ω-forbiddenness for the E1 transitions.

LIFETIMES OF E3 TRANSITIONS IN THE ODD-MASS Ho ISOTOPES

Abstract The half-lives of the 206 keV E3 transition in 159 Ho and the 211 keV E3 transition in 161 Ho have been measured to be 6.9±0.3 s and 6.1±0.3 s, respectively, and the existence of a 305 keV E3 transition in 163 Ho with a half-life of 1.1±0.1 s has been confirmed.

Observation of the [505]112− Nilsson state as an isomer in 159Dy

Abstract The 115±10 μsec isomer in 159Dy has been produced by bombardment of 159Tb with pulsed beams of protons and deutrons. The delayed γ-rays were recorded with a NaI scintillation spectrometer and the delayed conversion lines with a magnetic spectrometer. The results are interpreted in terms of an isomeric level at 356±3 keV with (K, I− equal to ( 11 2 , 11 2 − ) , decaying by K-forbidden M1 and E2 transitions to the 9 2 − and 7 2 − members of the ground-state rotational band in 159Dy. These properties serve to identify the isomeric level as the [N, n z , Λ]Ωπ = [505] 11 2 − neutron state predicted by Nilsson.

Evidence for j-forbidden isomeric transition in 208Bi

The decay scheme of the 2.5 ms isometric state in 208Bi has been established with the aid of high-resolution γ-ray and electron spectrometry. The multipolarity of the 921.0 keV isomeric transition has been determined as E3 and the 1571.1 isomeric state therefore probably has spin and parity 10−. The isomeric transition is hindered by a factor of 3.200 with respect to the Weisskopf estimate. This can be explained as being due to j-forbiddenness.