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This paper describes a technique for nondestructive, in situ analysis of building condition that involves the use of neutrons from a portable source and the detection of the gamma rays resulting from the interactions of the neutrons with the building materials. Applications of this technique include the mapping of salt and moisture profiles within the walls and the location of voids or inclusions.
The technique uses a californium-252 neutron source (106 N/s) for composition measurements and a cesium-137 gamma-ray source for density measurements and energy calibration. Emitted gamma rays are measured with a high-purity germanium detector. The intensity of discrete gamma-ray lines is a function of the concentrations of the various elements encountered within the wall by the neutron flux. Using characteristic gammaray energy signatures for materials of interest, a measured gamma-ray spectra can be processed to yield information on the relative elemental distribution of a variety of substances, including water, salt, and building materials such as limestone, brick, and iron reinforcing bars.
Applications of this technique to several common problems of building diagnosis are discussed, including the case of an 18th-century building at Colonial Williamsburg suffering from salt damage.
neutron-gamma technique, salt damage, moisture, building diagnosis, nondestructive evaluation, brick, Colonial Williamsburg
Research associate, University of Maryland, College Park, MD
Principal scientist, Computer Sciences Corp., Silver Spring, MD
Chief architectural conservator, The Colonial Williamsburg Foundation, Williamsburg, VA
Astrophysicist, Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD