[en] This thesis is devoted to the study of the nuclear structure of neutron-rich nuclei, via the search of isomeric nuclear states. Neutron-rich nuclei were produced in the spontaneous fission of ²⁵²Cf. The experimental study of isomeric states in these nuclei was performed with the γ-array EUROGAM II, coupled to an additional and original fission fragment detector composed by photovoltaic cells, SAPhIR. The photovoltaic cells are well adapted to detect low energy heavy ions and have good energy and time resolutions to obtain a good fission fragment detection. This experiment led to the discovery of new isomeric states in ¹³⁵Xe, ¹⁰⁴Mo, ^146,147,148Ce and ^152,154,156Nd, with lifetimes between 60 ns and 2 μs. Level schemes of these nuclei have been completed. An interpretation of the isomeric states in the nuclei ^154,156Nd and ^156,158Sm was performed by Hartree-Fock-Bogolyubov calculations using the DIS Gogny force with two quasi-particles excitations. The confrontation with the experimental results led to an interpretation of these isomeric states as K-isomers. (author)

[en] The nucleus ⁸⁸Sr has been produced at high spin as fission fragment following the fusion reaction ²⁸Si + ¹⁷⁶Yb at 145 MeV bombarding energy. The γ-rays were detected with the Eurogam2 array. The level scheme of ⁸⁸Sr has been extended up to 8.5 MeV excitation energy and spin values around 14ℎ. The structure of the high-energy states is discussed by analogy with those of the even-even neighbouring N=50 isotones and possible configurations are proposed. They are compared with results obtained within the Quasiparticle-Phonon Model

[en] The spectroscopy of nuclei produced as fragments in the fission process has been undertaken using the EUROGAM II γ-ray multidetector array. The first experiment involved a spontaneously fissioning ²⁴⁸Cm source and produced neutron-rich nuclei. The data analysis concentrated on the odd-A Ce isotopes and the present contribution details the structure of ¹⁵¹Ce which results from the strong coupling of the odd neutron to the core. The results of a preliminary analysis of the yrast structure of ¹³⁸Te will also be given. In a second experiment performed at the VIVITRON accelerator in Strasbourg, nuclei on the neutron-rich side of the valley of stability were produced via the ²⁸Si + ¹⁷⁶Yb reaction at 145 MeV bombarding energy. The level schemes of ⁹⁹Mo, ¹⁰¹Tc and ¹⁰³Ru have been extended to high spins (∼ 20h). Two new high lying structures in ¹⁰¹Tc are explained with the help of cranked shell model calculations

[en] High-spin states have been studied in ¹⁰¹Tc produced as a fission fragment in the reaction ¹⁷⁶Yb+²⁸Si at 145 MeV. Gamma rays were detected with the EUROGAM2 array. The level scheme of ¹⁰¹Tc has been extended up to 4.2 MeV excitation energy and several band structures are observed. Configurations are assigned to two new bands on the basis of their behaviour and of cranked Hartree-Fock-Bogolyubov calculations. (orig.)

[en] The prompt γ emission in the spontaneous fission of ²⁵²Cf is characterized by an energy spectrum which extends up to 20 MeV. It was established that the spectrum presents in the neighbourhood of symmetric fission an intensity bump in the 3-8 MeV γ energy interval. The origin of this phenomenon is still not well understood, so that it was found interesting to carry out new measurements. The spectrum of the γ rays emitted in spontaneous fission of ²⁵²Cf has been measurement in the EUROGAM II multidetector using photovoltaic cells to detect fragments. The aim of the experiment was to investigate the γ yield enhancement which appears for mass fragment ratio near 132/120. This enhancement was found to be composed of two peaks located at 4 MeV and 5.5 MeV respectively. The results obtained confirm the intensity bound in the 3-8 MeV region but this augmentation reaches the maximum when the heavy fragment is near the mass 132. Beyond mass 140 the phenomenon diminish and the γ spectrum regains the behaviour expected for a statistic emission. The additional structure at 5.5 MeV does not vary with excitation energy while the excitation function of the 4 MeV structure is more structured and presents a maximum when the excitation energy is near 8 MeV. It is likely that all or part of this observed phenomenon is due to a particular excitation mode of this isotope associated for instance with a low energy dipole resonance. A theoretical study of this collective effect is under way

[en] About eighty nuclei in the A≅100 mass region have been observed as fission fragments following the fusion evaporation ^28,30Si+¹⁷⁶Yb at 145 MeV bombarding energy. These nuclei have been identified from their γ-ray cascades detected with the EUROGAM2 array. The level schemes of several stable or neutron-rich nuclei have been extended to higher spins. From coincidences between transitions in complementary fragments, γ-rays de-exciting new high-spin states of hitherto unexplored isotopes can be assigned and some aspects of the fission mechanism can be studied

[en] Isomeric states have been studied in fission fragments produced by spontaneous fission of ²⁵²Cf. 34 isomeric nuclei have been identified by using coincidences between γ-rays detected in EUROGAM II and fission fragments detected in photovoltaic cells. Lifetimes from 20 ns up to 2 μs have been measured. Microscopic interpretation of the isomeric levels discovered has been tried by means of the Hartree-Fock-Bogolyubov procedure using Gogny force. It was found that the ^152,154,156Nd nuclei have prolate deformation in their ground state; the rotational band built on this ground state is well reproduced by the calculation. For these nuclei the 2 quasi-particle excited states energies are above 1 MeV in agreement with the experiment. The computation confirms the similitude of the ¹⁵⁶Nd and ¹⁵⁸Sm isomeric states associated to neutron 2 quasi-particles of J^π K^π = 5^- (ν 5/2 (642) x ν 5/2 (523)) while such similitude does not occur for the isotone nuclei ¹⁵⁴Nd and ¹⁵⁶Sm. The computation predicts a proton 2 quasi-particle excited states of J^π = 5^-, near the isomeric level measured in ¹⁵⁶Sm but not for that of ¹⁵⁴Nd. Concerning the ¹⁵²Nd the calculated level density is very near that of the states measured in the 1.6 to 2.3 MeV interval. However, the lack of precise information on spins and parities of the measured levels does make not possible a confrontation with the calculations. An up-graded equipment implying 32 photovoltaic cells instead of 2 cells is to be developed and installed by the EUROGAM

[en] The ^107,109Rh nuclei have been produced as fission fragments following the fusion reaction ²⁸Si+¹⁷⁶Yb at 145 MeV bombarding energy and studied with the Eurogam2 array. In both nuclei three new rotational bands with the odd proton occupying the πg_9/2, πp_1/2 and π(g_7/2/d_5/2) sub-shells have been observed. In ¹⁰⁷Rh, two other bands involving strong M1 transitions have been identified at excitation energy larger than 2 MeV. They can be interpreted in terms of three quasiparticle excitations. In addition new structures consisting of four transitions, built on states located at low excitation energy (680 keV in ¹⁰⁷Rh and 642 keV in ¹⁰⁹Rh), point out the importance of triaxial deformation in these two isotopes. (orig.)

[en] Isomeric states have been observed in fission-fragments produced by spontaneous fission of ²⁵²Cf. These states are found in neutron rich nuclei of different structure and deformations. About 50 isomeric nuclei have been observed using coincidences between γ-rays identified in EUROGAM II and fission fragments detected in photovoltaic cells (SAPhIR). Lifetimes in the range from 20 ns to 2μs have been measured. Presented calculations based on HFB+D1S force on new measured isomeric states in the ^152,154,156Nd show evidence for K-isomers. (orig.)

[en] SAPhIR is the acronym for S_aclay A_quitaine P_hotovoltaic cells for I_somer R_esearch. It consists of solar cells, used for fission-fragment detection. It is a collaboration between 3 laboratories: CEA Saclay, CENBG Bordeaux and CEA Bruyeres le Chatel. The coupling of a highly efficient fission-fragment detector like SAPhIR with EUROBALL will provide new insights in the study of very deformed nuclear matter and in the spectroscopy of neutron-rich nuclei