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The new Hope College Ion Beam Analysis Laboratory provides the capability to study a wide range of research projects ranging from materials analysis, environmental chemistry, electrochemistry, biochemistry, paleontology and forensic science. The specific applications of Particle-Induced X-ray Emission (PIXE) spectrometry, Rutherford Back Scattering (RBS) analysis and Proton Elastic Scattering Analysis (PESA) are used to explore new areas of interdisciplinary research.
The services provided by the Hope Ion Beam Accelerator Laboratory can be divided into three broad categories. The first group of services includes those that are routine, the second group is made up of those that are special, and the third group are those that are experimental or developmental. In general, all of these techniques are non-destructive. The samples are placed in high vacuum, however, so if the surface chemistry of the sample is volatile or otherwise unstable then some sample degradation may occur.
There are several standard ion beam analysis techniques we perform to measure the “bulk” properties of a sample or a confirmation of the layer structure of a particular film. These services deal with samples where the analysis is relatively straightforward and where the samples are relatively simple. Typically in these measurements, a circular area with a diameter a few millimeters of the sample is probed with the ion beam for a few minutes per sample.
The majority of these measurements are Particle Induced X-ray Emission (PIXE) studies of all sorts of samples. The PIXE technique excites or removes the inner-shell electrons in the sample via Coulomb interactions with a 2.3 MeV proton beam. As the atoms decay back to their ground state, x rays are emitted. The energies of the emitted x rays are characteristic of the elements present in the sample. The x rays are detected by a Si(Li) detector with a thin entrance window. This type of detector gives a very precise measure of the x-ray energies and this allows one to identify and quantify all heavy elements present in a sample with part-per-million sensitivity typically. Routine analysis of samples would require samples that are vacuum compatible and that are conducting, or could be made conductive by sputtering. The measurements can be analyzed by laboratory staff (with a commercial program GUPIX).
Another routine service available is surface film characterization with the ion beam analysis technique of Rutherford Back Scattering (RBS). For routine measurements, the alpha particle beam spot samples a few millimeters of each sample. The alpha particle energy can be as high as 5 MeV for thicker samples but typically energies below the oxygen alpha-alpha resonance of ~3 MeV is recommended. This technique can be used to characterize thin films that are either self-supporting or evaporated on a substrate (mylar, glass, sapphire, etc.). The fundamental physics behind this technique involves measuring the energy loss of charged particles in bulk material and elastic (Rutherford) scattering. The scattering angle is chosen to be large to maximize sensitivity. The alpha particles in the beam lose energy as they move into the thin film. Then they scatter off the elements present as they move into the film with energies corresponding to the mass of the scattering center. These scattered particles also lose energy as the move back out of the sample. The measured peak shapes and intensities can be used with RUMP modeling software to determine the thickness and composition of the layers. In routine RBS analysis, one assumes that there is a certain level of knowledge about the approximate structure of the film. As with PIXE the data analysis can be done at Hope.
A third routine analysis available is Proton Elastic Scattering Analysis (PESA). This energy-loss and Rutherford scattering technique is performed on thin films to quantitatively determine the hydrogen content of the films. By using a transmission proton beam around 3.4 MeV, and detecting scattered particles out to forward angles of around 30 degrees, scattering from hydrogen nuclei in the sample can be readily distinguished from scattering off all heavier elements. For films where hydrogen density is important, this technique can provide quantitative, non-destructive results.
In the category of special services, we find various extensions to the basic techniques. Generally, these services are special because they require special beam properties or extended time for measurement.
The main special service available is micro-focusing. It is possible to focus the beam to a 10 micron diameter to study very localized portions of a sample. For example, studies of individual sand grains or contaminant grains are possible. Micro-PIXE and micro-RBS measurements and analyses are available, but there is extra time required to maneuver the tightly focused beam around each sample.
We have processed samples that had very complicated film structure with a non-uniform concentration (versus depth) of one or more of the constituents. Since the analysis of such complicated structures is time intensive, such samples are considered special.
Another special service is simultaneous PIXE and RBS measurements. This allows measurements of very low level trace elements at the same time that the thin film structure and stoichiometry is determined.
Double blind handling of samples is available. In this situation, only one staff member knows the origin of the sample and the results of the measurement are only known to a different scientist. This too would qualify as a special service.
Other special services would include proton RBS or alpha PIXE.. These measurements are desirable in certain situations, but the analyses are harder and the sensitivity is lower than with with routine RBS and PIXE measurements.
The last category of special services might include sample preparation and/or disposal. In most circumstances target materials are prepared by the “user” and removed after analysis. Preparation of one or more targets from raw materials for routine analysis can be performed at Hope, but would be handled as a special service.
The scientists at the Hope Ion Beam Accelerator Laboratory are very willing to partner with clients to develop new or improved measurements. Some examples of possible services in this category would include Scanning Tunneling Ion Microscopy, oxygen profiling using the oxygen alpha-alpha resonance, or some combination of the above techniques.
We have worked with Bryce Bergethon from Huron Technologies (Lansing) on various aspects of quality control and identification of unknown contaminants in mold-release agents.
We have worked with Tom Guarr (VP research) from Gentex Corporation (Zeeland) on various aspects of quality control in evaporated metal layers on a glass substrate.
We have worked with Donnelly Corporation (Holland), Pfizer Corporation (Holland) and Hansen Industries (Holland) in the past on one or more occasion to identify unknown contaminants.
We have worked with Prof. Rick Rediske of the Annis Water Resources Institute of Grand Valley State University to identify metal contaminants in lake sediment for an EPA-funded study on several occasions.
We have worked with Dr. David Borok from the Geology Department at the University of Notre Dame to develop a method to characterize metal content in bacteria and humic sludges.
We work with Prof. Mark Little in the Physics Department at Hope College to characterize thin films of GaN and ScN.
We work with Prof. Ken Brown in the chemistry department at Hope College to characterize thin electropolymerized organic films on solid substrates.
We work with Prof Ed Hansen in the Geological and Environmental Sciences Department at Hope College to analyze trace element content in quartz minerals and sand grains.
We work with Prof. Brian Bodenbender in the Geological and Environmental Sciences Department at Hope College to characterize the mineral content of Wyoming soils surrounding dinosaur bones.