The Advanced Photon Source (APS) offers a unique opportunity to apply the nation’s most brilliant high-energy x‑ray source to fundamental problems in molecular to mesoscale environmental science and biogeochemistry, particularly elucidating the key plant, microbiological, and geochemical processes controlling mineral, nutrient, and contaminant transformations and mobility. The planned APS-U is projected to provide x-ray beams with increased brilliance of 2 to 3 orders of magnitude relative the present capabilities.
The APS is a third-generation synchrotron radiation source specifically designed to accommodate insertion devices (IDs) that cause electrons or positrons to wiggle or undulate through a series of alternating magnetic poles, resulting in orders-of-magnitude increases in brightness relative to previous approaches. Bending magnets are also used as x‑ray sources. Qualified users have access to the APS either as members of collaborative access teams (CATs) or as general users. Each CAT is responsible for building and operating the beamlines in one or more of the 35 APS sectors. The CATs are required to provide at least 25% of their sector beam time to general users. Allocation of beam time to a general user is determined by a review committee’s ranking of competitive proposals submitted by general user scientists. Over the past decade, the oversubscription rate of beam time requests (the ratio of the amount of beam time requested by general users to the amount of beam time supplied to them) for molecular to mesoscale environmental science has been ~2‑3. Recently, the oversubscription rate at MRCAT/EnviroCAT has climbed to 3-5. Typically, six months elapse between submission of an APS beam time proposal and the availability of beam time for approved experiments, and any follow-up work might need to pass the review process again. Our membership at the MRCAT/EnviroCAT beamline and the proximity of our laboratories to the APS give us and our collaborators the regular beam time access necessary for maintaining a synchrotron-based research program, as well as the ability to prepare additional samples during the beam run, depending on the outcome of a measurement. In addition, our close association with APS beamline staff gives us access to opportunistic beam time when a general user cannot make full use of allocated beam time.
At the MRCAT/EnviroCAT Insertion Device beamline, emphasis is on XAS-based studies. Currently, it is the only ID beamline in the United States dedicated to XAS measurements. A 13-element solid-state Ge detector and 4-element Si drift detector enable detection of XRF radiation during experiments on dilute environmental samples. We have redesigned the Kirkpatrick-Baez mirror setup to provide x‑ray microbeams as small as 1 µm for XRF elemental mapping and mXAFS and mXRD studies. This development directly benefits our investigation of typically spatially heterogeneous environmental samples.
To increase the availability of x‑ray beamlines with the characteristics required for molecular to mesoscale environmental science and biogeochemistry, we also have developed an experimental hutch at the back end of the first optical enclosure (FOE) of the MRCAT/EnviroCAT BM beamline, dedicated to bulk XAFS spectroscopy. The U.S. Environmental Protection Agency — a member of the MRCAT/EnviroCAT team — provided financial support for the development of XAS capabilities at the BM FOE. Additional support from other CAT members to purchase dedicated hardware (x‑ray slits, electronics, ion chambers, current amplifiers, a spectroscopy table, sample and detector positioning stages, etc.) and additional support provided by the Argonne Subsurface Biogeochemical Reseach Program for partial support of a beamline scientist enables the FOE BM beamline to provide monochromatic x‑ray beams for XAS and XRF experiments. These key pieces of infrastructure enable the XAFS capability of the MRCAT/EnviroCAT BM beamline to support Argonne biogeochemical research and molecular to mesoscale environmental science research as a whole.