A COMPARISON OF MEASURED AND CALCULATED GAMMA RAY ATTENUATION FOR A COMMON COUNTING GEOMETRY

Technical Report · Thu Feb 26 00:00:00 EST 2004

DOI:https://doi.org/10.2172/15009802· OSTI ID:15009802

Gaylord, R F

In order to perform quantitative gamma spectroscopy, it is necessary to know the sample-specific detection efficiency for photons as a function of energy. The detection efficiency, along with the branching ratio for the isotope and gamma ray of interest, is used to convert observed counts/second to actual disintegrations/second, and, hence, has a large effect on the accuracy of the measurement. In cases where the geometry of the source is simple and reproducible, such as a point source, small vial of solid, or jar of liquid, geometry-specific standards may be counted to determine the detection efficiency. In cases where the samples are large, irregular, or unique, this method generally cannot be used. For example, it is impossible to obtain a NIST-traceable standard glovebox or 55-gallon drum. In these cases, a combination of measured absolute detector efficiency and calculated sample-specific correction factors is commonly used. The correction factors may be calculated via Monte Carlo simulation of the item (the method used by Canberra's ISOCS system), or via semi-empirical calculation of matrix and container attenuations based on the thickness and composition of the container and radioactive matrix (ISOTOPIC by EG&G Ortec uses this method). The accuracy of these correction factors for specific geometries is often of vital interest when assessing the quality of gamma spectroscopy data. During the Building 251 Risk-Reduction Project, over 100 samples of high activity actinides will be characterized via gamma spectroscopy, typically without removing the material from the current storage containers. Most of the radioactive materials in B-251 are stored in cylindrical stainless steel canisters (called USV containers, after the Underground Storage Vaults they are commonly stored in), 13 cm in diameter, by 28 cm high, with walls that are 1.8 mm thick. While the actual samples have a variety of configurations inside the USV container, a very common configuration is the material (usually as an oxide powder pellet of approximately 2 cm diameter by {approx}2 mm thick) in a squat glass jar, with the jar placed in a thin steel food-pack can, which is then placed in the bottom of the USV canister. During data acquisition, the USV containers are typically rotated at approximately 4 rpm on a turntable to eliminate errors due to the material not being centered in the can, or attenuation not being isotropic. An aluminum plate is placed over the container, secured by three vertical rods, to securely hold the container. Pictures of both the containers, and this typical counting configuration are shown below.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: US Department of Energy (US)
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 15009802
• Report Number(s): UCRL-TR-202608; TRN: US0406625
• Resource Relation: Other Information: PBD: 26 Feb 2004
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
36 MATERIALS SCIENCE
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
ALUMINIUM
ATTENUATION
BRANCHING RATIO
CONTAINERS
DATA ACQUISITION
GAMMA SPECTROSCOPY
GEOMETRY
OXIDES
PHOTONS
POINT SOURCES
RADIOACTIVE MATERIALS
STAINLESS STEELS
UNDERGROUND STORAGE