D. Barnéoud

Rotational states and coriolis interaction in 171Yb

Abstract The 170 Er(α, 3n) 171 Yb reaction is used to populate rotational states built on the 7 2 + [633], 1 2 − [521], 5 2 − [512] and 11 2 − [505] intrinsic states. Particular attention is paid to the positiveparity band. The rotational energies within this band are successfully explained within the unified model with pairing and Coriolis interactions included, provided the theoretical Coriolis matrix elements are reduced. With the wave functions obtained from this energy fit the experimental E2/M1 amplitude mixing ratios are well reproduced. These wave functions also reproduce the cross-section pattern obtained in transfer-reaction experiments very well.

High spin rotational levels in 177Ta

Abstract Gamma rays following the (α, 2n) reaction leading to the nucleus 177Ta have been studied by means of γγ coincidences. Unambiguous assignments have been made for the bands built on the Nilsson states 7 2 + [404], 9 2 − [514], 5 2 + [402] and 1 2 − [541].

New insight in the level structure of 153Er

Abstract: The high-spin level structure of the 153Er nucleus has been reinvestigated by in-beam γ-ray and electron spectroscopy with (14N,pxn) reaction. Excitation energies, spin and parity assignments are unambiguously established for the two lowest isomers T1/2≃ 380 ns and T1/2\simeq 10 ns. The characteristics of the third one (T1/2≃ 250 ns) at 5.2 MeV are still questionable but its main decay modes are better known. An extension of the level scheme is proposed up to 8.4 MeV.Experimental results have been discussed in terms of shell model multiplets and compared with the other structures observed in N=85 neighbouring nuclei.

Level structures of 131,129Ce observed in beta decay

Abstract The low-lying levels of the odd-mass nuclei 131 Ce and 129 Ce have been investigated by means of the β + EC decays of 131g+m Pr and 129 Pr, respectively, The Pr nuclei were obtained by bombarding 94,96 Mo targets with a 255 MeV 40 Ca beam. The radioactivities produced in the reactions were transported with a He-jet device and γγt , X γt , e − γt coincidence measurements were performed. Conversion electrons were measured with a magnetic spectrometer and transition multipolarities were deduced. On-line mass separation was used to select the 129 Pr β-decay. The resulting level schemes of 131,129,127 Ce are discussed in connection with level systematics and calculations performed by using the Interacting Boson-Fermion Model (IBFM).

Low-lying states in the doubly odd nucleus 132Pr from beta decay of 132Nd

Abstract The low-lying level scheme of the odd-odd nucleus 132Pr has been investigated for the first time. The radioactive nuclei were produced by bombarding 96Mo targets with a 225 MeV 40Ca beam. Levels in 132Pr have been populated from the 132Nd β-decay. A new value of its half-life has been measured: T 1 2 = 88 ± 7 s . The activities produced in the reaction have been transported with a He-jet device and γγt, Xγt, e−γt coincidence measurements performed. Internal conversion coefficients have been measured with a magnetic lens spectrometer. The experimental level scheme of 132Pr is compared to theoretical calculations performed with the interacting boson-fermion-fermion model (IBFFM).

127Ce levels populated in the 4.2 s127Pr beta-decay

From new measurements, a half-live of 4.2±0.3 s has been clearly established in127Pr. This value confirms our first result [4] and rules out other data [5]. The127Pr activity was produced by bombarding92Mo or94Mo targets with 210 MeV40Ca beams. The mass identification has been made using an ion-guide coupled with an on-line isotope separator. The partial level scheme proposed for127Ce has been deduced from β-gated γ spectra and Xγ- and γγ-coincidences. A new band structure based on a 1/2+ state is established.

High spin states in the transitional odd-odd nuclei150Eu and152Tb

The (7Li, 5n) and (11B, 5n) reactions have been used to study the high-spin states in the two odd-odd nuclei150Eu and152Tb. Three decoupled bands have been evidenced in each nucleus belonging to the same configurations [f7/2]n [h11/2]p, [h9/2]n [h11/2]p and [i13/2]n [h11/2]p. The latter one is well developped and improves our knowledge of this system between the spherical and deformed region. The analysis of the collective moment of inertia and transition ratios strongly suggests an increase of the deformation when the rotational frequency increases in these two transitional nuclei150Eu and152Tb.

First identification of rotational bands in 121Ba

Abstract Collective excitations have for the first time been identified in 121 Ba using the particle-γγ coincidence technique in the 19 F + 106 Cd reaction at 95 MeV. Two main rotational bands, both of ΔI = 1 character, have been observed in this nucleus: A positive-parity band without signature splitting based on the 5 2 + ground state and a level structure with signature splitting assigned as the h 11 2 neutron-hole band. It is proposed that this negative-parity band is based on the 5 2 − state generated from the 5 2 − [532] orbital. The upbends observed in these band structures originate from the alignment of two h 11 2 quasiprotons.

Non-yrast level structures in the isotopes 131pr and 131La

Abstract Low-lying levels of 131 Pr and 131 La isotopes have been investigated by means of β/EC decays of 131 Nd and 131 Ce, respectively. The radioactive nuclei were produced in the 94 Mo+ 40 Ca reaction at a bombarding energy of 255 MeV and transported with a He-jet system. The identification and the characteristics of the 131m Pr isomer (5.7 s) were obtained by applying both He-jet techniques and on-line gamma-ray spectroscopy with mechanically pulsed beams. γγt , X γt , e − γt coincidence measurements as well as time multi-analysis were performed. Internal conversion electrons were recorded with a magnetic selector and multipolarities deduced. Level schemes of 131 Pr and 131 La are discussed and compared to those of neighboring odd-Z Pr and La isotopes using calculations made in the frame of the Interacting Boson-Fermion Model.

First observation of low-energy long-lived isomeric transitions in 166Tm

Abstract A study of 166 Tm by the 160 Gd( 11 B,5nγ) reaction was performed with the aim for searching for low-energy isomeric transitions using the Recoil Shadow Method (RSM). γ-rays from the production target and from the recoil ions stopped on an annular carbon catcher placed 16 cm downstream were observed with Ge detectors. Singles spectra following bombardment at three different energies, and coincidence spectra, were measured. Six isomeric γ-rays were observed. New levels are established and their configuration assignments are proposed. One of these levels, at ( x + 109.3) keV, is isomeric with a half-life of 340(25) ms. An upper limit for x was set to 25 keV. The half-life of the yrast band head was determined to be 2(1) μs. Two alternatives for the decay path of the 340(25) ms level to the ground state are proposed. The structure of the low-lying { p 7 2 + [404] + n 5 2 − [523]}K π = 6 − band is established for the first time.