A Concrete Low-Background Counting Enclosure

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Abstract
A massive concrete low-background gamma-ray counting enclosure has recently been constructed at the Health Physics Department, Lawrence Radiation Laboratory (University of California), Berkeley. Walls and roof are 4–5 ft thick; inside dimensions are 23.5 × 11.5 × 11.5 ft. A substantial effort was made to secure materials adequately low in radioactivity for the concrete. Of primary concern were the concentrations of potassium, uranium and thorium in possible low-radioactivity sources of aggregate and cement materials. Geologic considerations indicated that ultramafic igneous rocks are the lowest in these radioisotopes. An examination was conducted of possible sources of low-radioactivity aggregate material; samples were analyzed for radioactivity on a 100-channel gamma-ray spectrometer. Evaluation of the spectrometer analyses and economic considerations resulted in the choice ofserpentinized ultramafic rock from a deposit in the foothills of the Sierra Nevada as the source of low-background aggregate. Raw materials from twelve portland cement plants in California, and finished products from 114 plants in the United States and Canada, were also examined for radioactivity by gamma-ray spectrographic analysis. Cement from a producer on Vancouver Island, British Columbia, was chosen as the lowest-radioactivity cement product commercially available. The completed enclosure furnishes a nearly twofold reduction in background in comparison with the previous counting arrangement. Equally important, the thick concrete shield with its included water of hydration keeps the present level of radioactivity non-fluctuating, whereas the background in the previous counting arrangement varied in proportion to the external fast-neutron flux produced by nearby accelerators. Future installation of a 1-ft-thick ultra-low-background liner and (or) an approximately 1/8-in.-thick lead membrane on the inside surface of the enclosure should significantly reduce the present background. A comparison of costs indicates that to shield a similar-sized counting space with steel of equivalent attenuation and low-background characteristics would cost (in place) about four times as much as the existing concrete structure.

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