Tim Elam

Dr. Tim Elam’s main research interest is X-ray spectroscopy. He has worked in the areas of X-ray absorption, emission, fluorescence, and non-resonant inelastic scattering. His present efforts focus on using X-ray fluorescence in difficult environments. He has built several downhole X-ray fluorescence spectrometers to measure heavy metal contaminants in soils and sediments and to make in-situ measurements of diffusion of stable isotopes of nuclear waste elements through native rock without radioactivity. He is now the Chief Spectroscopist for the Planetary Instrument for X-ray Lithochemistry (PIXL) on the Perseverance rover and the hardware lead for the APL-UW Ice Diver.

He is past Chair of the Denver X-ray Conference and was the American Institute of Physics Congressional Science Fellow for 1991. He has more than 100 publications in refereed scientific journals and holds 5 patents.

• Optimized Compton fitting and modeling for light element determination in micro-X-ray fluorescence map datasets

  O’Neil, L.P., D.C. Cating, and W.T. Elam, “Optimized Compton fitting and modeling for light element determination in micro-X-ray fluorescence map datasets,” Nucl. Instrum. Methods Phys. Res., Sect. B, 436, 173-178, doi:10.1016/j.nimb.2018.09.023, 2018.

  The Planetary Instrument for X-ray Lithochemistry (PIXL) is an X-ray fluorescence instrument scheduled to fly to Mars on NASA’s 2020 rover (Allwood et al., 2015). It will be capable of quantifying elements with an atomic number of at least 11 using X-ray fluorescence (XRF), but the detector window blocks fluorescence from lighter elements. Important elements otherwise invisible include carbon, oxygen, and nitrogen, which can make up anions in minerals of scientific interest. X-rays scattered by all elements can be detected, so the ratio of Compton to Rayleigh scatter may be measured and used to infer the presence of elements for which there is no detectable fluorescence. We have refined a fundamental parameters model to predict the Compton/Rayleigh ratio for any given composition that can be compared to an experimentally measured ratio. We compare with a published Monte Carlo model (Schoonjans et al., 2012) and to experimental values for a set of seven materials. Compton/Rayleigh ratios predicted by the model are in good, though imperfect, agreement with experimental measurements. A procedure for consistently computing the Compton/Rayleigh ratio from a noisy spectrum has also been developed using a variation on a common background removal method and peak fitting.

   

• In-situ X-ray fluorescence to investigate iodide diffusion in opalinus clay: Demonstration of a novel experimental approach

  Jaquenoud, M., and 9 others including W.T. Elam, “In-situ X-ray fluorescence to investigate iodide diffusion in opalinus clay: Demonstration of a novel experimental approach,” Chemosphere, 269, doi:10.1016/j.chemosphere.2020.128674, 2021.

  During the last two decades, the Mont Terri rock laboratory has hosted an extensive experimental research campaign focusing on improving our understanding of radionuclide transport within Opalinus Clay. The latest diffusion experiment, the Diffusion and Retention experiment B (DR-B) has been designed based on an entirely different concept compared to all predecessor experiments. With its novel experimental methodology, which uses in-situ X-ray fluorescence (XRF) to monitor the progress of an iodide plume within the Opalinus Clay, this experiment enables large-scale and long-term data acquisition and provides an alternative method for the validation of previously acquired radionuclide transport parameters.

  After briefly presenting conventional experimental methodologies used for field diffusion experiments and highlighting their limitations, this paper will focus on the pioneer experimental methodology developed for the DR-B experiment and give a preview of the results it has delivered thus far.

   

• Avoiding slush for hot-point drilling of glacier boreholes

  Hill’s, B.H., D.P. Winebrenner, W.T. Elam, and P.M.S. Kintner, “Avoiding slush for hot-point drilling of glacier boreholes,” Ann. Glaciol., 62, 166-170, doi:10.1017/a0g.2020.70, 2021.

  Water-filled boreholes in cold ice refreeze in hours to days, and prior attempts to keep them open with antifreeze resulted in a plug of slush effectively freezing the hole even faster. Thus, antifreeze as a method to stabilize hot-water boreholes has largely been abandoned. In the hot-point drilling case, no external water is added to the hole during drilling, so earlier antifreeze injection is possible while the drill continues melting downward. Here, we use a cylindrical Stefan model to explore slush formation within the parameter space representative of hot-point drilling. We find that earlier injection timing creates an opportunity to avoid slush entirely by injecting sufficient antifreeze to dissolve the hole past the drilled radius. As in the case of hot-water drilling, the alternative is to force mixing in the hole after antifreeze injection to ensure that ice refreezes onto the borehole wall instead of within the solution as slush.