Session DD05: X-ray Diagnostics

Absolute x-ray spectra measurements at the Dynamic Compression Sector

The x-ray properties at the Dynamic Compression Sector can be tailored in peak energy, bandwidth, and spot size to accommodate a broad range of real-time, in-situ, x-ray measurements during dynamic compression experiments for addressing the scientific needs of each research campaign.  Accurate characterization of the x-ray beam properties used for each campaign is critical for performing quantitative analysis of the x-ray data obtained. We provide each DCS experimentalist with an accurate measurement of the beam profile on target as well as the full x-ray energy spectral profile – using a small, Si (220) double crystal monochromator and photodiode – to aid in the analysis of their data. However, we have not been able to provide our users with quantitative flux measurements, and the total number of x-ray photons per pulse have been estimated using simulations. Using a newapproach  that accounts for the monochromator characteristics, electronic equipment response, and photodiode calibration, we can now quantitively determine the absolute flux for each spectral measurement. The measurements are in good agreement with calorimetry data and with x-ray ray trace simulations that account for the beamline optics setup. These measurements constitute a significant enhancement to our x-ray diagnostics capabilities and are critical for real-time evaluation of the performance of the beamline, accurate forward simulations of experimental results, and predictive calculations for future experiments.   

X-ray absorption spectra of explosives for shocked chemical dynamics

Accurate prediction of the reaction of explosives to shocks requires knowledge of the chemical reaction rates that occur as a function of pressure and temperature. Reactive molecular dynamics, both quantum and classical, provide the best current insight into these reaction kinetics. Unfortunately, there are very few experiments that directly measure shock induced chemistry in explosives on time and length scales that allow direct validation of molecular dynamics predictions. This paper reports ultrafast laser driven shock experiments performed at the Linac Coherent Light Source that were designed to look for new observations of shock induced chemistry through changes in the soft X-ray absorption spectra (XAS) at the oxygen K shell edges. Static XAS measurements of explosive films at the carbon, nitrogen, and oxygen K shell edges measured at the Advanced Light Source will also be reported. The motivation for use of XAS is that the near edge structure is sensitive to the local chemical environment, but the broadening due to pressure and temperature is expected to be less problematic than competing chemically specific measurements using vibrational spectroscopies.   

X-ray diagnostics and dynamic compression experiments at the Linac Coherent Light Source

The behavior and physical properties of matter under extreme conditions are of fundamental scientific interest since extremes induce new physical phenomena that don’t exist in ordinary conditions. Furthermore, it provides a clue to the understanding of dense matter phase in Earth’s inside and planetary science. Extreme conditions created by intense light source generate dense state with densities of up to several times of solid density, temperatures of 0.1 eV to 100s eV, and pressures of 10s kbar to 10s Mbar. Model calculations in this regime predict electronic and structural phase transitions with new atomic and electronic band structure, anomalous transport, and changes of scattering properties and opacity.

The unique characteristics of free electron lasers, with combined high peak brightness, quasi-monochromaticity, and tunable x-ray photon energy over femtosecond timescales, has given rise to many new opportunities expanding our understanding of the kinetics and dynamics of material deformation during shock compression. Several x-ray diagnostics using the x-ray free electron laser source are tailored and improved at Matter in Extreme Conditions (MEC) instrument to study wide range of extreme conditions in phase space. The MEC instrument features high power short pulse and long pulse optical laser systems as well as x-ray diagnostics including diffraction, spectroscopy, and imaging. The research at MEC have addressed dynamic behaviors under high-pressure and phenomena of shock-compressed condition. In this talk, the details of x-ray diagnostics and highlight of several experiments will be presented.

    

Development of SAXS models to elucidate particulate formation during detonations

Recent progress in the application of time resolved small angle x-ray scattering (TR-SAXS) has enabled the monitoring of nanosecond particulate growth in detonations. Particularly useful for monitoring formation of various carbon allotropes, TR-SAXS can differentiate between the phases of carbon (sp² vs. sp³) and characterize the average particulate size, morphology, and surface structure. Despite the use of SAXS to probe carbon particulate growth in detonations, usually as diamond or graphite, the possibility of other products is not generally considered in the analysis. Taking into account other species in the SAXS models may impact the morphological description of carbon particles. Here, various model dependent and model independent analysis methods are considered for the description of carbon particulate growth during detonations. Models which accept the possibility of additional products result in a different radius of gyration compared to a standard model considering only the formation of carbon agglomerates. Guinier-Porod equations, as well as the SAXS invariant, are used to identify the primary contribution to the scattering based off of the scattering length density contrast. Accurately defining the contrast allows models to be developed with fewer parameters and are expected to provide more accurate estimates of particle size. The same analysis model proposed here can be used for all detonation synthesis systems and has the potential to change the existing understanding of carbon particulate morphology.   

Synchrotron spectroscopies to study electronic and magnetic materials under high pressure at HPCAT

The structural, electronic and magnetic properties of materials under high pressure are of fundamental interest in physics, chemistry, materials science, and earth sciences. The 16 ID-D beamline of the High Pressure Collaborative Access Team (HPCAT) at the Advanced Photon Source (APS) is dedicated to high pressure research using X-ray spectroscopy techniques typically integrated with diamond anvil cells. The beamline provides X-rays of 4.5-37 keV, and current available techniques include X-ray emission spectroscopy (XES), inelastic X-ray scattering (IXS) and nuclear resonant scattering (NRS).[1] In this presentation, we will cover mainly two techniques: XES and IXS. [2-3]

In this presentation, we discuss on the particular requirements and instrumentation for XES and IXS. We then present several examples to illustrate the recent progress in high pressure XES and IXS studies at HPCAT. [4-5] An outlook toward future developments in high pressure X-ray spectroscopy with new diffraction-limited storage rings (APS-U etc.) is also discussed.

[1] Xiao, Y.M., Chow, P., Boman, G., Bai, L.G., Rod, E., Bommannavar, A., Kenney-Benson, C., Sinogeikin, S. & Shen. G.Y. (2015).  Rev. Sci. Instrum. 86, 072206.

[2] Xiao, Y.M., Chow, P. & Shen G.Y. (2016). High Pressure Res.,36, 315-331.

[3] Chow P., Xiao Y.M., Rod E., Bai L.G., Shen G.Y., Sinogeikin S., Gao N., Ding Y., Mao H.K.. (2015) Rev. Sci. Instrum. 86,072203.

[4] Wang, P., Zhu,S.C., Zou,Y.T., Chen,H.Y., Liu,Y., Li,W.W., Chen,J., Zhu,J.L., Wu,L.S., Wang,S.M.,  Yang,W.G., Xiao,Y.M., Chow,P., Wang,L.P.,  Zhao, Y.S.  (2022) Chem. Mater. 34, 3931

[5] Jiang, Z.M., Wang,Y.M., Jiang,D.Q., Li,C., Liu,K.,  Wen,T., Xiao,Y.M., Chow,P., Li,S., Wang,Y.G.  (2021)  J. Phys. Chem. Lett.  12,  6348