NEUTRONS IN ACTION: EXPLORING THEIR IMPACT IN THE GEOSCIENCES (Invited Presentation)

Neutron scattering and imaging are powerful probes for the geosciences, despite that neutron sources are much less intense as compared to synchrotron X-ray sources. Neutrons lack charge so they are highly penetrating in most materials. As a result, complex sample environments can be used to provide broad ranges of temperature, pressure, magnetic field, electrochemical cell, electric field, light excitation, as well as fluid/gas flows. Signature geoscience applications include the use of Small Angle Neutron Scattering (SANS) to characterize pore structure of gas reservoir rocks, neutron diffraction (ND) and inelastic neutron scattering (INS) to determine the structure and dynamics of hydrogenous minerals and water-mineral interfaces, total scattering to probe the structural dynamics of solution and amorphous phases, neutron radiography to map porosity and fluid flow in rocks, and neutron diffraction to determine magnetic structures and phase transitions in minerals. These applications are realized from neutrons interacting with atomic nuclei and with the magnetic fields from unpaired electrons. Light elements (e.g., hydrogen, lithium) generally scatter neutrons as well as heavy elements. Neutron scattering and absorption cross-sections vary in a non-periodic way for each element and their isotopes.

Neutron sources are few world-wide but continual improvements and expansions of the major facilities, and their sample environments, are creating opportunities to grow geoscience research applications. Neutron sources are generally of two types, accelerator-based or reactor-based. In the U.S., currently two neutron sources are operating, both at ORNL, the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR), which are DOE Office of Science User Facilities. SNS and HFIR host 31 neutron scattering instruments for various types of analyses including diffraction (powder and single-crystal), spectroscopy, reflectometry, small-angle neutron scattering, and imaging. Beam-time proposals at these facilities are selected through a competitive peer review process, and those scientists who have their proposals approved use the facilities and instruments free of charge in return for making their data and findings public. Proprietary use of instruments is also available on cost recovery basis.