C. Bourgeois

Collapse of the N=28 shell closure in (42)Si.

The energies of the excited states in very neutron-rich (42)Si and (41,43)P have been measured using in-beam gamma-ray spectroscopy from the fragmentation of secondary beams of (42,44)S at 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that (42)Si is best described as a well-deformed oblate rotor.

Decays of 185m + gHg: Low-spin levels of 185Au as a test of nuclear models

The decay of 185m + gHg has been studied on-line with mass-separated sources from the ISOCELE facility. Precise conversion-electron measurements were performed with a 180° spectrograph. The 132+ isomeric state of 185Hg (T12 = 28 ± 5 s) was located with respect to the 12− ground state (T12 = 55 ± 10 s). A level scheme of 185Au has been established. Two abnormally converted M1 transitions de-excite a state local at 330.2 keV. Excited states of 185Au have been discussed in the framework of a “quasi-particle + axial rotor” approach, quasi-particle states being issued from Hartree-Fock plus BCS calculations using the SIII Skyrme force. Most of the low-spin negative-parity levels have been identified as h92 + f 52 or p32 + f72 mixed states. The h112 system has also been discussed using a model of asingle-j quasi-particle coupled to a triaxial rotor.

Decays of 183Hg and 183Au

Abstract The β + /EC decays of 183 Hg and 183 Au have been studied on-line with mass-separated sources from the ISOCELE facility. A conversion-electron measurement was performed by means of a 180° spectrograph and abnormally converted transitions were found. Level schemes of 183 Au and 183 Pt have been established. The isomeric state of 183 Pt has been located at 34.6 keV with respect to the ground state. A prolate shape for 183 Pt can be deduced from these results and for the first time there is good evidence to believe that the positive-parity states in odd- A gold isotopes correspond to oblate-shaped nuclei. The existence of a high-spin isomeric state in 183 Hg is discussed.

High-spin excitations of 187, 188Hg

Abstract High-spin states in 187, 188 Hg have been investigated via the 163, 164 Dy ( 28 Si , x n ) reactions by in-beam γ-ray spectroscopic techniques using the “Chateau de Cristal” 4π-multidetector array. Level schemes have been established. Spins and parities of the states have been deduced from transition multipolarities obtained from measurements of the angular distribution and linear polarization of the γ-rays. The shape coexistence of both prolate and oblate band structures has been observed for the first time in 187 Hg. In 188 Hg negative-parity bands are well developed. The band structure is interpreted within the frame of the Strutinsky method with the deformed Woods-Saxon average field. Pairing behaviour with increasing spin and general features of shape coexistence in mercury nuclei are illustrated.

Decay of 185Au: Sign of shape coexistence in 185Pt

Abstract The decay of 185 Au has been studied on-line with mass-separated sources from the ISOCELE facility. Precise conversion-electron measurements have been performed with a 180° magnetic spectrograph. The level scheme of 185 Pt has been established, and the T 1 2 = 33 min isomeric state has been located at 103.2 keV with respect to the T 1 2 = 71 min ground state. Two highly converted transitions have been observed. The level scheme is discussed in the framework of an “axial-rotor + quasiparticle” approach: numerous states are interpreted assuming a prolate shape of the nucleus. Ten levels with low spin and negative parity ( I ⩽ 7 2 − ) decay mainly to the 1 2 − [521] band via strong M1 transitions and are not expected from the calculations performed with the prolate cores. The possibility of shape coexistence is discussed.

Decay of 183Ir: Shape of 183Os and 185Pt in their low-spin states

Abstract The decay of 183 Ir has been studied with mass-separated sources from the ISOCELE facility. A level scheme of 183 Os has been established. It is discussed in the framework of an “axial-rotor + quasi-particle” model: all the states located below 700 keV are interpreted assuming a prolate shape of the nucleus. This study has shed a new light on the interpretation of the 185 Pt low-spin states. Highly converted transitions in 185 Pt are discussed.

Beta decay studies of neutron-rich 21Sc27Co nuclei at GANIL

The neutron-rich nuclei ~:‘5sS~, ;i-60Ti,!

Shape coexistence in the odd-odd 186Au nucleus

-62V, ;z-66Cr, ;:-68Mn, ige7’Fe and ;F-71Co have been produced at Ganil via interactions of a 61.8 A.MeV 76Ge beam with a 58Ni target. Beta-decay studies have been achieved, bringing new half-lives and a better nuclear structure knowledge in this region. These results could help for understanding the overabundances of the neutron-rich stable 58Fe, 64Ni in certain inclusions of meteorites.

Decay of 183Au: Low-spin states of 183Pt

Abstract Low-spin states of 186 Au have been studied from the β + /EC decay of 186 Hg using the ISOCELE facility. Precise conversion-electron measurements were performed with a semicircular magnetic spectrograph. The 186 Au level scheme has been established with spin and parity assignments deduced from transition multipolarities and log ƒt values. It shows two separate groups of levels which could be interpreted as due to shape coexistence: prolate deformation for the first low-lying states, and oblate deformation for the 251.5 keV (2 − ) and 363.6 keV (1 + ) states.

Lifetimes of the superdeformed band in 192Hg

Abstract The decay of 183 Au (T 1 2 = 42 s ) has been studied with mass-separated sources from the ISOCELE facility. A level scheme of 183 Pt has been established. Most of the states located below 800 keV are interpreted assuming a prolate shape of the nucleus from comparison with isotones or isotopes.