- Pressure and chemical dependent electron-capture radioactivity. The half-lives of radioactive isotopes are often considered constant and are determined to high precision. For this reason, many radioactive isotopes are used to date geological and astronomical processes at all time and length scales. Among important radioisotopes are those that decay by electron capture, namely 26Al (-> 26Mg, half-life ~ 720 kyr) and 40K (-> 40Ar, half-life ~ 1.28 Gyr). Heat production due to the decay of 26Al and 40K were also important during the Earth's accretion process and the current heat budget respectively. Because of their widespread use, any change in the decay rate is fundamental to understanding and implementing these relations to the Earth.
For electron-capture decay schemes, external forces such as chemical state, pressure, temperature and ionization can affect the half-life. This is a new effort combining experimental measurements of the changes in decay constant with pressure and chemistry of important electron-capture isotopes with theoretical predictions. This effort brings together state-of-the-art ab-initio computations of electron density with that of high-pressure diamond-anvil cell experiments. Experiments and computations will be combined jointly to investigate how chemical composition and pressure affect the electron-capture portion of their half-lives. Computations will be used to study isotopes under variable conditions in both chemistry and compression that are not easily accessible by experiments either due to very long half-lives or lack of isotopically-enriched samples. See CV for reprints.
- High-pressure/temperature alloying of potassium and iron. Radioactive decay through the incorporation of 40K into the core could be an important source of energy deep inside the Earth, helping to power the geodynamo and mantle dynamics. We have found that under high pressures and temperature, potassium and iron form a solid solution using both experiments and ab-initio calculations. See CV for reprints.
- Pressure-induced siderophile behavior of normally lithophile, chalcophile and atmophile elements. Pressure can make normally lithophile (rock-loving), chalcophile (sulfur-loving) and atmophile (atmosphere-loving) elements into iron-loving elements. Using both ab-initio quantum-mechanical calculations and experiments, I am investigating how pressure affects the electronic character of alkali (K, Rb), alkaline-earth (Ca, Sr) metals and noble gases (e.g., Xe). The possible alloying behavior between iron and these elements is important for understanding the accretion and evolution of the Earth. Partitioning of these elements between iron and rocky-portion of the Earth can also provide insight into the light-element composition of the Earth's core. See CV for reprints.
- Constraining the Earth's Lower Mantle composition.
To better constrain the chemical composition of the Earth's Mantle, I characterized an undepleted natural peridotite at high pressures and temperatures to determine the plausibility of a homogeneous Mantle. By taking a natural peridotite (an Upper-Mantle rock and direct analog to the pyrolite model) and compressing it to Lower-Mantle conditions, I compare density measurements obtained by high-resolution synchrotron x-ray diffraction with seismological observations of the Preliminary Reference Earth Model (PREM). The surprising finding is that the density of Upper-Mantle peridotite is 1-4% less dense than the Lower Mantle, implying that the Upper and Lower Mantle may be compositionally distinct. See CV for reprints.
I continue to explore alternative Lower Mantle compositions -- pyroxenite, enstatite, komatiite, chondritic, etc. -- through experiments, semi-empirical thermodynamic modeling and normal mode comparison.
- Combining static and dynamic techniques: laser-driven shockwave experiments on pre-compressed (in a diamond cell) samples. In collaboration with our colleagues at Lawrence Livermore National Laboratory, the Commissariat a l'Energie Atomique and Rutherford Appleton Laboratory's VULCAN Laser Facility, we have developed a novel technique which allow us to shock materials at a higher initial density thereby tracing a different Hugoniot (higher pressures with lower temperatures). This technique works well for very compressible fluids (we've tried this on water, nitrogen, hydrogen, and helium) as precompression to even nominal pressures (~1 GPa) produces large increases in density. See CV for reprints.