Session Y10: Invited Session: Advances in Actinide Measurement Techniques

Anomalous thermodynamic behavior in actinides

The thermal expansion of some of the actinides metals are strongly dependent upon doping. Extreme examples involve a change of sign of the thermal expansion coefficient upon few percent Ga doping. In contrast, resonant ultrasound spectroscopy of these doping series reveals very weak dependence of the elastic moduli on Ga content. We suggest that the anomalous thermodynamic behavior in these systems has dynamic rather than static origin.   

Multiconfigurational nature of 5f orbitals in uranium and plutonium and their intermetallic compounds

The structural, electronic, and magnetic properties of U and Pu elements and intermetallics remain poorly understood despite decades of effort, and currently represent an important scientific frontier toward understanding matter. The last decade has seen great progress both due to the discovery of superconductivity in PuCoGa$_5$ and advances in theory that finally can explain fundamental ground state properties in elemental plutonium, such as the phonon dispersion curve, the non-magnetic ground state, and the volume difference between the $\alpha$ and $\delta$ phases. A new feature of the recent calculations is the presence not only of intermediate valence of the Pu 5f electrons, but of multiconfigurational ground states, where the different properties of the $\alpha$ and $\delta$ phases are primarily governed by the different relative weights of the 5f$^4$, 5f$^5$, and 5f$^6$ electronic configurations. The usual method for measuring multiconfigurational states in the lanthanides is to measure the lanthanide $L_{III}$-edge x-ray absorption near-edge structure (XANES), a method that is severely limited for the actinides because the spectroscopic features are not well enough separated. Advances in resonant x-ray emission spectroscopy (RXES) have now allowed for spectra with sufficient resolution to resolve individual resonances associated with the various actinide valence states. Utilizing a new spectrometer at the Stanford Synchrotron Radiation Lightsource (SSRL), RXES data have been collected that show, for the first time, spectroscopic signatures of each of these configurations and their relative changes in various uranium and plutonium intermetallic compounds. In combination with conventional XANES spectra on related compounds, these data indicate such states may be ubiquitous in uranium and plutonium intermetallics, providing a new framework toward understanding properties ranging from heavy fermion behavior, superconductivity, and intermediate valence to mechanical and fundamental bonding behavior in these materials.   

Observation of $^{239}$Pu NMR in PuO$_{2-}$A new frontier for the physics and chemistry of plutonium compounds

In actinide science, in general, NMR studies have been forced to limit their scope to nuclei associated with ligand atoms. The only exception of direct observation of NMR in actinide nuclei is that of $^{235}$U NMR in UO$_{2}$. There have been extensive efforts to realize NMR in actinide compounds since the electronic properties of these materials are predominantly governed by the actinide atom itself. We report the first observation of Nuclear Magnetic Resonance (NMR) on the $^{239}$Pu nucleus in any material. Our $^{239}$Pu NMR measurements were performed on plutonium dioxide, PuO$_{2}$, for a wide range of external magnetic field values (Ho$=$3$\sim $8T) at a temperature of T$=$4K. By mapping the external field dependence of the measured resonance frequency, we determined the nuclear gyromagnetic ratio to be $^{239}\gamma_{n}$(PuO$_{2})=$2.856 $\pm$ .001 MHz/T. Assuming a free ion value for the Pu$^{4+}$ hyperfine coupling constant, we estimated a bare value of $^{239}\gamma_{n}=$2.29MHz/T for the $^{239}$Pu nucleus, hence a nuclear magnetic moment of $\mu _{n}=$.15$\mu_{N}$ (where $\mu_{N}$ is the nuclear magneton). Our findings put an end to a fifty-year long search for Pu NMR and open potentially a new horizon for the solid state physics, nuclear materials science and complex chemistry in Pu compounds.\\[4pt] Work done in collaboration with G. Koutroulakis, S. Richmond, K. Veirs, E. D. Bauer, J. D. Thompson, G. Jarvinen, and D. L. Clark, Los Alamos National Laboratory, Los Alamos, NM.   

An instrument for the investigation of actinides with spin resolved photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy

A new system [1] for spin resolved photoelectron spectroscopy [2,3] and bremsstrahlung isochromat spectroscopy [4] has been built and commissioned at Lawrence Livermore National Laboratory for the investigation of the electronic structure of the actinides. Actinide materials are very toxic and radioactive and therefore cannot be brought to most general user facilities for spectroscopic studies. The technical details of the new system and preliminary data obtained therein will be presented and discussed. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy (DOE), National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering.\\[4pt] [1] S.-W. Yu, J. G. Tobin, and B. W. Chung, Rev. Sci. Instrum. \textbf{82}, 093903 (2011).\\[0pt] [2] S.W. Yu and J. G. Tobin, Phys. Rev. B \textbf{77}, 193409 (2008).\\[0pt] [3] J.G. Tobin, S.W. Yu, T. Komesu, B.W. Chung, S.A. Morton, and G.D. Waddill, EuroPhysics Letters \textbf{77}, 17004 (2007).\\[0pt] [4] J.G. Tobin and S.-W. Yu, Phys. Rev. Lett, \textbf{107}, 167406 (2011).   

Transuranic Photoemission Using a Unique Light Source

There has been a remarkable advance in the understanding of electronic structure for complex materials in recent years. Much of this advance in understanding has been realized through advanced spectroscopy capabilities available at public synchrotron facilities. While the vast majority of materials can take advantage of facilities at public synchrotrons, transuranic materials are excluded from these facilities when multiple containment barriers are incompatible with the chosen spectroscopy. We have developed an advanced spectroscopy capability at Los Alamos for photoemission on transuranic materials including Pu. Using several different variants of photoemission we have explored a wide range of Pu materials which has lead to a significant improvement in our understanding of transuranic electronic structure. Examples of these successes will be given along with details of the unique facility. Using the unique capabilities of our transuranic photoemission system we exploit opportunities in angle-resolved photoemission (ARPES) providing insight into the details of both the energy and crystal momentum for a material. Additional information is obtained using tunable photons which may be used to isolate the 5f electron contribution to the valence electronic structure. Between ARPES and tunable photoemission, one may construct a fairly detailed picture of the bonding and hybridization for transuranic materials. By adding temperature-dependent (10 - 350K) photoemission to the suite of tools, we may cross over phase transition boundaries as well as quantify electron-phonon coupling. We also have the capability for 1.5 and 3 KeV core-level spectroscopy using a monochromatized x-ray source. By combining the above photoemission tools with a variety of surface preparation capabilities including cleaving, laser ablation, and thermal desorption, we have a flexible and capable spectroscopy facility that provides unique insight into the electronic structure of transuranic materials.