Effects of Surface Paint Coatings on 232Th Surface Contamination Detection

J.S. Bland; A. Reyes; and J.K. Doan (Chesapeake Nuclear Services, Inc.)

In conducting surface contamination measurements, the efficiency for measurement is highly dependent on surface conditions – surface depth of contamination and surface coatings. A common coating encountered for building surfaces is paint. Paint coatings can cause significant attenuation of the electron fluence from the surface, thereby reducing the overall detection efficiency. A study was conducted using paint samples from a 232Th decommissioning project to evaluate electron attenuation for paint coating of varying thicknesses and to determine the overall effect on detector efficiency for thin window gas flow detectors. For 53 mg/cm2 paint density thickness, the source efficiency for the 232Th decay decreased by a factor of 4.0. For 103 mg/cm2, the reduction was a factor of 6.5; and for 174 mg/cm2, the reduction was a factor of 11. The relatively minor decrease between 50 and 100 mg/cm2 density thickness appears to reflect the attenuation of essentially all low energy electrons at 50 mg/cm2 and the higher penetrating factor for the remaining higher energy electrons. In summary, for 232Th surface contamination and paint density thickness exceeding a few 10's of mg/cm2, the electron attenuation can be significant; the reduction the source efficiency could jeopardize the ability for detecting low levels of 232Th (decay product) radioactivity under painted surfaces. The results of this study were compared with those performed by ORISE as presented in NUREG-1507. From these data, a correlation was developed between electron energy, paint thickness and detector efficiency. This correlation provides a basis for establishing acceptance criteria for conducting characterization and final status surveys for painted surfaces in keeping with established DCGL's, considering detector efficiency as a function of the contamination electron energy and paint thickness.

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