[en] This talk will cover some of the activities at ITU that involve physics and materials science of the actinides. Basic research aims to be intellectually challenging and involves many collaborators. Applied materials science aims to address some issues of importance for the future of nuclear energy, in particular with our capability to examine irradiated materials and perform leaching experiments on strongly radioactive materials. In condensed-matter physics of the actinides the central question is the state of localisation of the 5f electrons and their interaction (hybridization) with other electron states. The most recent discovery of superconductivity in PuCoGa₅ at a surprisingly high temperature of 18 K is simply one highlight of the extraordinary complexity of the physics of 5f systems. Some other examples will be given, as well as a brief description of our Actinide User Laboratory, which allows outside teams of researchers to profit from our infrastructure. In materials science we have a major effort on irradiated fuel. The crucial aspect for the mechanical properties is the change of the form of the material, especially due to the release of helium. The self-damage is particularly severe over a long time, and can lead finally to a non-crystalline fine-grain amorphous material. Changes of thermal properties are studied as a function of burn up in irradiated fuel. Laser melting of surfaces has established the phase diagrams as a function of oxygen stoichiometry. New laser capabilities are planned. Leaching experiments on UO₂, as well as alpha-doped material, show the importance of oxygen concentrations in the environment and especially of radiolysis. (author)

[en] We performed inelastic neutron scattering (INS) in order to re-investigate the crystal-field ground state and the level splitting in UO₂. Previous INS studies on UO₂ by Amoretti et al. [Physical Review B 15, 1989, 1856] uncovered four excitations at low temperatures in the 150-180 meV range. Considering the dipole-allowed transitions, only three of these transitions could be explained by the published crystal-field model. Our INS results on a different UO₂ sample revealed that the unaccounted peak at about 180 meV is a spurious one, and thus not intrinsic to UO₂. In good agreement with Amoretti's results, we corroborated that the ground-state of UO₂ is the Γ₅ triplet, and we computed that the fourth-and six-order crystal field parameters are V₄ = -116 meV and V₆ = 26 meV, respectively. We also studied the INS response of the non-magnetic U₀₄Th₀₆O₂. The splitting for this thorium-doped compound is similar to the one of UO₂, which orders antiferromagnetically at low temperatures. Therefore, we can conclude that magnetic interactions only weakly perturb the energy level splitting, which is dominated by strong crystal fields.

[en] In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO₂ below its antiferromagnetic ordering temperature of 30 K. They showed the strong magnon–phonon coupling in this material and that the excitations appeared to contain an additional mode that was not anticipated. Cowley never returned to UO₂, but showed a keen interest in the developments. Forty years after this pioneering work, unambiguous evidence was found (using resonance x-ray techniques) for the ordering below T _N of the electric quadrupoles involving the anisotropy of the 5f charge distribution around the uranium nuclei. A further 10 years later, now armed with a full theory for the excitation spectrum expected for phonons, magnons, and quadrupoles, we can identify the latter as the source of the ‘extra’ mode reported first in 1966. The story is a long winding one, with the expected serendipity and dead ends, but is now (almost) completed. (paper)

[en] We report on a study of the magnetic structure and phase transitions of the UAs_1-xSe_x solid solutions employing x-ray and neutron scattering. We have focused this study on compositions 0.18< x<0.22, in which magnetic structures having double-k and triple-k configurations are found with increasing x. In general, our findings are in excellent agreement with those reported by Kuznietz using solely neutron diffraction. However, our x-ray studies have added several important details about (1) the incommensurate transitions near T_N, (2) the nature of the 2k-3k phase boundary, and (3) the discovery of a new magnetic phase

[en] The most remarkable advance that one can report as regards transuranium systems is certainly the discovery of superconductivity above 18 K in PuCoGa₅. Motivated by this discovery, we have investigated bulk samples of both PuCoGa₅ and its Rh analogue. Characterizations of polycrystalline Co and Rh compounds are reported. The new PuRhGa₅ compound is also found to become superconducting above 8 K

[en] X-ray Magnetic Circular Dichroism (XMCD) is a powerful spectroscopic technique that offers a possibility to determine quantitatively element and shell specific spin and orbital magnetic moments in ferro-, ferri- or even paramagnetic samples. At the third generation synchrotron radiation facilities, small x-ray beam with flexible polarization sized down to few microns can be routinely achieved. This technique is thus perfectly suited for studying minute samples (a few micrograms) of transuranium materials

[en] Neutron diffraction results, in the vicinity of the magnetic phase transition of USb and MnF₂, are reported. The thermal evolution of the magnetic diffuse signal and nuclear Bragg reflections demonstrate that the centre of gravity of the magnetic signals does not lie at the predicted position as calculated from nuclear reflections. This phenomenon, called the q-shift, was first found using resonance x-ray scattering (RXS). The present results show that, (i) the effect is not an artefact of RXS and is also found with neutrons (ii) that the effect arises from the bulk of the sample and is not restricted to the near surface layer (∼2000 A) associated with the RXS probe in actinide systems, (iii) the effect is not restricted to actinide compounds.

[en] The anisotropy of magnetic fluctuations propagating along the [1 1 0] direction in the ordered phase of uranium antimonide has been studied using polarized inelastic neutron scattering. The observed polarization behavior of the spin waves is a natural consequence of the longitudinal 3-k magnetic structure; together with recent results on the 3-k-transverse uranium dioxide, these findings establish this technique as an important tool to study complex magnetic arrangements. Selected details of the magnon excitation spectra of USb have also been reinvestigated, indicating the need to revise the currently accepted theoretical picture for this material.

[en] The experimental data on incommensurate phase versus temperature, as they have been obtained by high resolution inelastic neutron scattering, are compared to theoretical models. In the experiments reported we have set out to examine the phonon dispersion surface to verify that the charge-density wave (CDW) state is present continuously at temperature even above T_CDW. The position of the phonon minimum (q_min = q_CDW), as well as the detailed shape of the dispersion surface of phonon modes over an extended areas in the (E, q) space, is important in view of the debate on the origin of the CDW at the Peierls transition (2nd_order phase) and gives rise to a further modulation of the structure. This work confirms that the CDW state has precursor effects before the phase transition, there is a strong coupling between phonon and electronic states and that a 'soft-mode transition' occurs at T_CDW. This result is compared to theories discussing the origin of the CDW state

[en] Epitaxial layers of uranium have been grown on a variety of buffer/seed layers on sapphire substrates by UHV magnetron sputtering and their structure determined using x-ray diffraction. The buffer layers were epitaxial layers of niobium, tungsten and niobium covered by a seed layer of hcp gadolinium, on which uranium layers were grown to a thickness of 600 A. The x-ray diffraction results establish that the α-orthorhombic phase of uranium grows epitaxially in the (110) orientation on the niobium (110) buffer, while on the tungsten (110) buffer the growth planes of the α-uranium were (002) and for the growth on the gadolinium buffer the α-uranium was predominantly (021) oriented. These results show that epitaxial uranium films in selected orientations can be grown by using an appropriate buffer. To our knowledge this is the first report of epitaxial α-uranium films, and it is significant because of the difficulty of growing single crystals of α-uranium due to the occurrence of high temperature structural transformations