Student Abstracts: Nuclear Science at PNNL

Preparation of specimens for nondestructive testing. ADAM MORASCH (University of Idaho, Moscow, ID 83843) GEORGE SCHUSTER (Pacific Northwest National Laboratory, Richland, WA 99352) .
The researchers, to whom I was assigned, were conducting studies using nondestructive testing to estimate the occurrence of fabrication flaws in welding material for light-water reactor pressure vessels. Researchers at the Pacific Northwest National Laboratory do nondestructive testing on material from cancelled nuclear power plants. The researchers are developing fabrication flaw density and distribution functions for the materials used in fabrication of nuclear reactor pressure vessels. This includes all product forms for vessels shell cores, welding processes, and the stainless steel cladding applied to the inside of the vessel. The information gathered will be used in future machine analysis by the U. S. Nuclear Regulatory Commission to support/improve the technical basis of assessing potential vessel failure due to postulated failure devices, such as pressurized thermal shock. Preparing the metal specimen is one of the most important steps of the nondestructive testing process. The surface of the metal will be sanded, polished, and etched to reveal the specific area of interest. Sanding is perhaps the most important part of specimen preparation because great care must be used to assure that there is no surface damage. During my internship, I was responsible for preparing the metal specimens so that the researchers could then perform ultrasonic testing on them. The material used by the researchers is carbon steel. The pieces that I prepared for testing were small sections that were ultrasonically tested. This paper details my internship and the appendix includes exact steps to perform the preparation of metal specimens.

The Development of Cd1-xZnxTe for X-ray and Gamma Ray Radiation Detection. CHARLES SHAWLEY (Whitworth College, Spokane, WA 99251) GLEN DUNHAM (Pacific Northwest National Laboratory, Richland, WA 99352) .
Cadmium Zinc Telluride crystals have properties conducive for room temperature radiation detection. Due to the high atomic mass, wide band gap, and good charge carrier mobility, it is a very attractive material. However, lack of understanding of the behavior of trapping levels in the band gap has restricted its advancement as a commercial detector. Such levels are caused by intrinsic defects and impurities, which control carrier mobility and electrical compensation of the material. The focus of this project was to discover electron mobility by grid-probing the crystal with varying length sources. The electron mobility was then plotted against volume to determine surface impurities from polishing, the effective volume of the crystal, and how changes in the average lifetime of the electron (Üe) are caused. The driving force of the experiment is the need for large crystal detectors with high resolution. Large crystal detectors have better radiation stopping power, but have considerably worse resolution than smaller crystals. Being a large, single crystal capable of excellent resolution makes CZT a good prospect for commercial radiation detectors.