Katharina Luening

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Determination of copper nanoparticle size distributions with total reflection X-ray fluorescence spectroscopy

Total reflection X-ray fluorescence (TXRF) analysis is extensively used by the semiconductor industry for measuring trace metal contamination on silicon surfaces. In addition to determining the quantity of impurities on a surface, TXRF can reveal information about the vertical distribution of contaminants by measuring the fluorescence signal as a function of the angle of incidence. In this study, two samples were intentionally contaminated with copper in non-deoxygenated and deoxygenated ultrapure water (UPW) resulting in impurity profiles that were either atomically dispersed in a thin film or particle-like, respectively. The concentration profile of the samples immersed into deoxygenated UPW was calculated using a theoretical concentration profile representative of particles, yielding a mean particle height of 16.1 nm. However, the resulting theoretical profile suggested that a distribution of particle heights exists on the surface. The fit of the angular distribution data was further refined by minimizing the residual error of a least-squares fit employing a model with a Gaussian distribution of particle heights about the mean height. The presence of a height distribution was also confirmed with atomic force microscopy measurements.

The Nucleation and Growth of Cu Nanoclusters on Silicon Surfaces

Due to the recent adoption of copper interconnect technology by the semiconductor industry, there has been great interest in understanding the kinetics and mechanisms of copper metal deposition on silicon wafer surfaces in ultra pure water (UPW) solutions. To study the kinetics of the copper deposition mechanism on silicon surfaces, silicon [100] samples were immersed in non‐deoxygenated and deoxygenated UPW solutions contaminated with a copper concentration of 100 ppb with dipping times ranging from 5 to 300 seconds and then measured using total reflection x‐ray fluorescence (TXRF) at the Stanford Synchrotron Radiation Laboratory (SSRL). By measuring the Cu fluorescence signal as function of angle of incidence of the incoming x‐rays, it was possible to ascertain whether the deposited copper was atomically dispersed or particle‐like in nature. It was established that in non‐deoxygenated UPW, the copper is incorporated atomically into the silicon surface oxide as a copper oxide, while in deoxygenated UPW, ...

XRF microCT study of space objects at SSRL

Study of the composition and 3D chemical distribution of the particles that come from space are of great interest since they can provide information about the early stages and evolution of the solar system. The size of these samples varies with the smallest ones in the micron and even sub-micron range. X-ray fluorescence microCT (computed tomography) with focused X-ray beam can be successfully used to study these kinds of samples. This is especially important when sectioning is not feasible, or it is undesirable either due to the risk of contamination, as is the case with comet particles recently collected by the NASA Stardust mission, or the requirement for further analysis by different characterization techniques. X-ray fluorescence microCT measurements on several space samples were performed at the beamline 6-2 using the existing microprobe setup. Two mirror optical system is used for beam focusing with an additional set of KB mirrors located in the hutch near the sample to focus the beam further down to 2x4 microns. Incident X-ray energy is selected with a monochromator in the range of 5 to 20 keV. Fluorescence data was collected with Si(Li) fluorescence detector and PIN diode was used to collect attenuation data that provides additional information for fluorescence tomography reconstruction. The results of the measurements of two micrometeorites with sizes of approximately 100 microns, are presented.

C-10 INVITED—ANALYTICAL METHODS FOR DISCRIMINATING STARDUST IN AEROGEL CAPTURE MEDIA

Methods using X-ray fluorescence have been developed to identify cometary material captured in aerogel during the NASA Stardust mission to Comet 81P/Wild 2. These analytical methods are necessitated by the levels of trace contaminants present in the aerogel. The cometary material disaggregates during deceleration in the aerogel, so fluorescence mapping of the entire track (which can be several millimeters long) is necessary. Distinguishing those pixels which have cometary material and aerogel from those which have only cometary material can be very challenging. We have chosen a “dual threshold” method, with some pixels clearly having only aerogel (plus contaminants) and other pixels clearly having cometary and aerogel material. Between these two threshold levels is a set of pixels which cannot be easily ascribed to one or the other. By leaving these pixels out of the analysis, the estimate of cometary material is improved.

Surface trace element characterization of synthetic single crystal Al2O3 at the SSRL

Detailed surface trace impurity element analyses (Ti, Cr, Fe, Co, Ni, Cu, and Zn) have been performed on synthetic sapphire grown by the HEM™ technique. Both bulk and surface measurements, yielding information on the spatial distribution of the elements are needed as part of our effort to obtain correlations between optical absorption at 1064 nm and trace element concentrations. Transition metal elements (e.g. Ti, Cr, and Fe) are known to produce absorption in sapphire and therefore were the focus of our study. We report results from our use of total reflection x‐ray fluorescence (TXRF) to determine trace transition elements on the surface of synthetic sapphire. The measurements show concentrations for Cr at 109 atoms/cm2 to Fe at 1012 atoms/cm2 and are consistent with bulk trace element studies done using instrumental neutron activation analysis (INAA).