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Tim Elam

Principal Physicist

Email

wtelam@apl.washington.edu

Phone

206-685-3092

Research Interests

X-ray Spectroscopy

Biosketch

Dr. 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 an X-ray fluorescence spectrometer (XRFS) to measure heavy metal contaminants in soils and sediments that is directly pushed via cone penetrometery without drilling. His latest project was a borehole XRFS for use in the Mars Subsurface Access Program and it is now being adapted to make in-situ measurements of diffusion of stable isotopes of nuclear waste elements through native rock without radioactivity. Dr. Elam is also the hardware lead for the APL Ice Diver.

He is Chair of the Denver X-ray Conference, serves on the Advisory Board for X-ray Spectrometry, and is a member of the International Center for Diffraction Data. He 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.

Education

B.S. Physics, Mississippi State University, 1973

M.S. Physics, University of Maryland, 1977

Ph.D. Physics, University of Maryland, 1979

Publications

2000-present and while at APL-UW

Low-energy shelf response in thin energy-dispersive X-ray detectors from Compton scattering of hard X-rays

Michel-Hart, N., and W.T. Elam, "Low-energy shelf response in thin energy-dispersive X-ray detectors from Compton scattering of hard X-rays," Nucl. Instrum. Meth. A, 863, doi:10.1016/j.nima.2017.04.039, 2017.

More Info

1 Aug 2017

Silicon drift detectors have been successfully employed in both soft and hard X-ray spectroscopy. The response function to incident radiation at soft X-ray levels has been well studied and modeled, but less research has been published on response functions for these detectors to hard X-ray input spectra above 20 keV. When used with hard X-ray sources a significant low energy, non-peak response exists which can adversely affect detection limits for lighter elements in, for example, X-ray fluorescence spectroscopy. We present a numerical model that explains the non-peak response function of silicon drift detectors to hard X-rays based on incoherent Compton scattering within the detector volume. Experimental results are presented and numerically compared to model results.

Origins of extreme broadening mechanisms in near-edge x-ray spectra of nitrogen compounds

Vinson, J., T. Jach, W.T. Elam, and J.D. Denlinger, "Origins of extreme broadening mechanisms in near-edge x-ray spectra of nitrogen compounds," Phys. Rev. B, 90, 205207, doi:10.1103/PhysRevB.90.205207, 2014.

More Info

10 Nov 2014

We demonstrate the observation of many-body lifetime effects in valence-band x-ray emission. A comparison of the N K α emission of crystalline ammonium nitrate to molecular-orbital calculations revealed an unexpected, extreme broadening of the NO σ recombination — so extensively as to virtually disappear. GW calculations establish that this disappearance is due to a large imaginary component of the self-energy associated with the NO σ orbitals. Building upon density-functional theory, we have calculated radiative transitions from the nitrogen 1s level of ammonium nitrate and ammonium chloride using a Bethe–Salpeter method to include electron-hole interactions. The absorption and emission spectra of both crystals evince large, orbital-dependent sensitivity to molecular dynamics. We demonstrate that many-body effects as well as thermal and zero-point motion are vital for understanding observed spectra. A computational approach using average atomic positions and uniform broadening to account for lifetime and phonon effects is unsatisfactory.

Comparison of transport theory predictions with measurements of the decrease in shallow water reverberation level as the sea state increases

Thorsos, E., J. Yang, W.T. Elam, F.S. Henyey, F. Li, and J. Liu, "Comparison of transport theory predictions with measurements of the decrease in shallow water reverberation level as the sea state increases," Proc., Meetings on Acoustics, 19, 070024, doi:10.1121/1.4800711, 2013.

More Info

2 Jun 2013

Transport theory has been developed for modeling shallow water propagation and reverberation at mid frequencies (1-10 kHz) where forward scattering from a rough sea surface is taken into account in a computationally efficient manner. The method is based on a decomposition of the field in terms of unperturbed modes, and forward scattering at the sea surface leads to mode coupling that is treated with perturbation theory. Reverberation measurements made during ASIAEX in 2001 provide a useful test of transport theory predictions. Modeling indicates that the measured reverberation was dominated by bottom reverberation, and the reverberation level at 1 and 2 kHz was observed to decrease as the sea surface conditions increased from a low sea state to a higher sea state. This suggests that surface forward scattering was responsible for the change in reverberation level. By modeling the difference in reverberation as the sea state changes, the sensitivity to environmental conditions other than the sea surface roughness is much reduced. Transport theory predictions for the reverberation difference are found to be in good agreement with measurements.

More Publications

Inventions

In-Situ Elemental Analyzer Using Wavelength Dispersive X-ray Fluorescence

Record of Invention Number: 48011

Tim Elam, Gerald Seidler

Disclosure

28 Mar 2017

Reactive landing for modification of graphene and other nanocarbon materials

Record of Invention Number: 46967

Tim Elam, Joe Rolfs, Frantisek Turecek, Michael Volny

Disclosure

27 May 2014

Reactive Landing Resistance Biosensors

Record of Invention Number: 45652

Tim Elam, Frantisek Turecek, Michael Volny

Disclosure

8 Jun 2011

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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