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Abstract

A neutron/gamma ray technique for mapping the distribution of contaminants in an extended medium such as the wall (10) of a building. A neutron excitation source (14) is located on one side of the wall (10) and a gamma ray spectrometer, including a gamma ray detector (16), is located on the opposite side of the wall (10) facing the excitation source (12). The source and detector are moved in unison in discrete steps over opposing wall surfaces (12, 18) so as to determine the chemical composition of the elements in a hemispheric region (20) of the wall adjacent the detector with the radius of the region being substantially that of the mean free path distance of gamma rays emitted from elements interacting with neutrons on the detector side of the wall. The source (14) and detector (16) are reversed for relatively thick walls for mapping the distribution of elements on the other side of the wall thickness. The output of the detector (16) is fed to a multi-channel pulse height analyzer (24) where the intensity of the various gamma ray spectral lines are indicated relative to a dominant constituent element such as silicon. Concentrations of water in the wall (10) are determined by detecting the presence of hydrogen while salt concentrations are detected by the presence of chlorine. Resolution of anomalies such as the presence of voids and/or determining the bulk density of the medium is achieved by substituting a gamma ray source (38) for the neutron source (14). The neutron/gamma ray technique is also applied to metal alloys, such as iron alloys, in either the solid or molten state.

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First Claim