P. Paris

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.

États excités des isotopes de plomb de masse 202, 200 et 198

Abstract The low-lying excited levels of 198, 200, 202Pb have been studied by observing γ- and β-rays and γ−γ coincidences in bismuth decay. Decay schemes have been proposed for these nuclei. Half-lives were determined for the 9−, 7−, 5− and 4+ levels in 200Pb, and for the 5− level in 198Pb. Microscopic calculations have been made with Gaussian and surface delta interactions to account for level schemes. The results are discussed.

Conversion electron measurements in the124Laβ-decay. Spin and parity assignments in124Ba

Low-lying energy levels in124Ba have been investigated from124Laβ-decay by using He-jet technique. Spin and parity assignments have been deduced from internal conversion electron measurements. Aβ-vibrational band based on a second 0+ state has been identified. Experimental level energies have been compared with IBFA-2 calculations.

New isomers and their decay in odd-odd neutron-deficient cesium isotopes

AbstractBy a systematic on-line cesium mass separation fromA=122 toA=132 and subsequent gamma and electron decay spectroscopy at very low energy, new isomers have been precisely identified:122mCs (T1/2=0.36±0.02 s),124mCs (T1/2=6.3±0.2 s) and130mCs (T1/2=3.46±0.06 min). Detailed level schemes are given for both124mCs and130mCs. Comparison of excited levels known in the odd-odd nuclei of the same region shows that more experimental informations are needed to propose a clear and realistic picture of the nuclear states structure. Radioactivity:122m, 124m, 130mCs [from La or Ce,3Hexn]-measuredT1/2,Eγ,Iγ,ECE,ICE,γ-γ-t, γ-ce coinc.; deduced ICC,124, 130Cs deduced levels,J, π. Online mass separated sources, Ge(Li), intrinsic Ge, Si(Li), magnetic electron selector.

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).

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.

On-line separation of163Ta and162Ta

By on-line selective separation on molecular fluoride compounds at the ISOCELE 2 facility, two new isotopes:163Ta (T1/2=10.5+-1.8s) and162Ta (T1/2=3.5+-0.2s) have been identified.

Odd-proton alpha-decay systematics and nuclear structure just beyond208Pb

The level structures populated in alpha decay of all odd-Z-even-N nuclei withZ=(83–93) andN=(126–142) up to 500 keV are presented. More data on the 4n+3 nuclear sequences are given from215Bi to235Np. Particular emphasis is placed on the hindrance factors to alpha decay in interpreting the level structures in term of the shell model, the octupole-quadrupole model and the Nilsson model. The level structures and the hindrance factors go through transition region in which mixtures of the properties of two different nuclear models are appropriate. These transition regions represent a challenge to nuclear theorists to develop more all-encompassing nuclear models.

New investigation of83Zr and85Zr beta decay

The decay of on-line mass separated83Zr and85Zr sources has been studied by singles and coincident X-ray andγ-ray and conversion electron measurements. The previous level scheme of83Y is confirmed and 5 new levels have been added up to 2.74 MeV excitation. A decay scheme of85gZr with 40 levels up to 2.66 MeV in85Y is deduced. The experimental observation below 2 MeV excitation in85Y agrees qualitatively with the predictions of the interacting boson-fermion model.

THE ODD-A Ac NUCLEI — TESTS OF NUCLEAR MODELS

The remarkable features of the spectra of the odd-A Ac isotopes from 215Ac (with 126 neutrons) through 231Ac (with 142 neutrons) serve as a sequence of examples for the application of a variety of nuclear models. As the shapes vary from spherical to spherical with strong octupole correlations to octupole-deformed, the corresponding symmetry breaking leads to increasingly complex spectra. The spectrum of 215Ac can be described in terms of the shell model configurations π(h9/2)4f7/2 and π(h9/2)4i13/2. However, even in this 126-neutron nucleus there is already a strongly enhanced E3 transition. The spectra of 217Ac and 219Ac can be interpreted in terms of the weak coupling of the h9/2 proton to the 216Ra and 218Ra cores, respectively. However, the existence of parity doublet bands (same spins but opposite parities) in 219At and the interleaving of positive and negative states in 218Ra clearly demonstrate the increasing importance of octupole deformation. Parity doublet bands are also present in the spectra of 221Ac. However, in both 219Ac and 221Ac the positive parity members of the parity doublets appear to cut off before reaching the low spin values expected from the negative parity bands. In 223Ac, 225Ac, and 227Ac the experimental spectra clearly show the existence of parity doublet bands built on the parity mixed configurations 5/2±(0.0; −0.2) and 3/2±(0.0; −0.3). The fact that the ordering of these two configurations changes for 223Ac results from decreasing quadrupole deformation. Other spectroscopic properties also confirm octupole deformation in these three isotopes. In 229Ac and 231Ac the only detailed but limited spectroscopic information comes from the (t, α) reaction on 230Th and 232Th. While it is consistent with the presence of parity doublets, much more detailed spectroscopic studies could give information about the possible coexistence of reflection asymmetry and reflection symmetry in these nuclei.