Student Abstracts: Physics at PNNL

Ion Beam Implantation Induced Au Nano-Cluster Formation in MgO (100). ANDREW CAVANAGH (Fort Lewis College, Durango, CO 81301) THEVA THEVUTHASAN (Pacific Northwest National Laboratory, Richland, WA 99352) .
The formation of nano-clusters within an oxide via ion beam implantation is of significant interest because of the ability to parametrically alter the physical characteristics of the cluster formation. This allows for the creation of a variety of optical properties by optimizing the size and density distribution of the nano-clusters within the crystal. These parameters include altering the energy of the implantation ion, the fluence, the temperature of the crystal and post implantation annealing. For this experiment the implantations were carried out on MgO (100) with a 2 MeV Au2+ ion beam at a range of temperatures from 300K to 975K with fluences ranging from 1 to 20 x 1016 ions/cm2. Characterization of the crystals was completed using Rutherford backscattering analysis (RBS) with multi-axial channeling, high-resolution transmission electron microscopy (TEM) and optical absorption. Measurements of the samples were conducted directly after implantation and following ex-situ annealing at 1475K. RBS and channeling measurements were used to characterize the individual samples for both virgin and implanted regions in three of the major crystalline axis, (100), (110) and the (111). These measurements will be discussed further.

Evaluation of the Performance of PGT RG-11A/C and Amptek A250 Preamplifiers in Configuration with a Germanium Detector. SHELECE EASTERDAY (University of Notre Dame, Notre Dame, IN 46556) HARRY MILEY (Pacific Northwest National Laboratory, Richland, WA 99352) .
Radiation detection is an increasingly important branch of science. Gamma-ray detection has applications in several fields, including high-energy physics. Germanium detectors are employed in many experiments of this sort. Gamma ray events picked up by a germanium detector are sensed by a charge-integrating preamplifier. A charge-integrating preamplifier is comprised of a field effect transistor (FET), an operational amplifier, and an RC circuit that integrates the charge of the radiation event and dissipates the charge over a time constant. In determining the performance of a particular preamplifier, one must test the energy resolution of the detector-preamplifier configuration and analyze the fall time of the preamplifier signal. The resolution of a preamplifier can be tested by exposing it to a radioactive source and analyzing the resulting energy peaks with data acquisition equipment. Using a pulser to simulate a radioactive source of a particular energy, one can look at the fall time of the preamplifier signal with an oscilloscope and determine its quality. In this project, the performance of a Princeton Gamma Tech RG-11A/C preamplifier and an Amptek A250 preamplifier were evaluated through the testing of energy resolution, the observation of leakage current on the Ge detector, and the analysis of preamplifier signal characteristics and baseline noise measurements.

Development of an Ultra High Vacuum End Station with Surface Science Capabilities. CARA GAINCOLA (Columbia Basin College, Pasco, WA 99301) DR SHUTTA SHUTTANANDAN (Pacific Northwest National Laboratory, Richland, WA 99352) .
There is a growing interest in physics labs about the epitaxial growth of model oxides on various oxide and metal substrates to obtain high-quality surfaces and films. A number of single crystal oxide films on various substrates have recently been synthesized in our laboratory using oxygen-plasma-assisted molecular beam epitaxial growth. In the present work, we have developed an Ultra High Vacuum (UHV) end station with surface science capabilities including Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) for oxide studies. This end station will be connected to the channeling beam line at the accelerator facility to incorporate ion beam capabilities in addition to the surface science capabilities. A TiO2 (110) single crystal was used to test the processing and analytical capabilities in the chamber. Ceria single crystal thin films on yitria stabilized zirconia were used for channeling investigations in a similar chamber. A brief description of the UHV chamber, preliminary results from the spectrometers and Rutherford Backscattering Spectroscopy (RBS)/channeling results from the ceria films will be presented.

Development of novel infrared photonic devices in bulk chalcogenide glasses. ANDREW LAFORGE (University of Puget Sound, Tacoma, WA 98416) RICHARD M. WILLIAMS (Pacific Northwest National Laboratory, Richland, WA 99352) .
We explore the permanent photomodification of bulk chalcogenide glasses, with the prospect of incorporating the processes into the development of infrared photonic devices. Effects include photoexpansion, photodarkening, and change of refractive index. Illumination of AsS and GeAsSe with HeNe laser light produced surface expansions of up to 5 microns and darkened regions 175 microns into the material, values considerably larger than those typically reported for thin films. Optical microscopy shows evidence of the creation of subsurface lenses. Although intensity variations affect the speed at which the process occurs, the type and degree of modification are largely dependent upon wavelength and exposure of writing light. The results suggest bulk samples can be used in the fabrication of discrete waveguide-based photonic devices for infrared laser applications.

Laser Fluorimetric Characterization of Sorption of Gd3+ by ä-Alumina and Mesoporous Silica. JENNIFER WASSMUTH (Lewis Clark State College, Lewiston, ID 83501) ZHEMING WANG (Pacific Northwest National Laboratory, Richland, WA 99352) .
With the growing concern for environmental cleanup, it is important to understand how radioactive materials interact with soils. Radioactive waste from underground tanks has leaked to the soil at the Hanford Site. The radioactive elements in this waste such as americium and curium migrate through the soil. Determining how these materials migrate is essential to selecting the appropriate soil cleanup strategy. Americium and curium are very difficult to study in a lab. Because of this, gadolinium is used due to its analogous properties. Soil is made up of many different components including ä-alumina and mesoporous silica, which were used to study gadolinium sorption. Nine samples with different concentrations of gadolinium were analyzed in four trials. During the first two trials, the adsorption of gadolinium to silica was studied by adding 0.4 grams of silica to each sample. During the next two trials, the adsorption of gadolinium to ä-alumina was studied by using an ä-alumina suspension of 4 g/L concentration in each sample. The samples were placed in a shaker and then centrifuged to separate the supernatant from the paste. Samples were analyzed using a laser fluorimeter. The resulting graphs show that as concentrations of gadolinium increased, so did fluorescence intensity of the gadolinium peaks. The ä-alumina paste graphs showed that an impurity might be interfering with the fluorescence measurements. Further experiments will determine where the impurity comes from and how gadolinium is adsorbed onto soil. Preliminary indications are that laser fluorimetric analysis will be useful in characterization of sorption of gadolinium.