Characterization of Hydrogen in Concrete by Cold Neutron Prompt Gamma-Ray Activation Analysis and Neutron Incoherent Sca
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ABSTRACT A combination of cold neutron prompt gamma-ray activation analysis (PGAA) and neutron incoherent scattering (NIS) has been used for nondestructive characterization of hydrogen as a function of position in slabs of wet concrete of different compositions. Hydrogen was determined by PGAA by scanning each sample across a 5 mm diameter neutron beam in 10 mm increments, and measuring the 2223 keV prompt gamma ray. NIS measurements were performed by scanning the sample across a 5 mm diameter neutron beam at 5 mm increments and detecting scattered neutrons. The measurements demonstrate the feasibility of the techniques for 2D compositional mapping of hydrogen and other elements in materials, and indicate the potential of these methods for monitoring the uniformity of drying concrete. INTRODUCTION As building codes and manufacturing specifications are held to increasingly higher standards, the need for nondestructive evaluation of materials becomes of paramount importance. Of equal consequence is the capability for spatial resolution, i. e. the ability to map compositions and defects in materials as a function of position. As an example of the latter, scientists in the concrete industry are interested in determining whether the drying of concrete is uniform. One way to answer this question is to create a moisture profile of concrete samples during drying, that is to measure the H20 content as a function of position. Prompt gamma-ray activation analysis (PGAA) has proven useful for the nondestructive measurement of hydrogen and other elements in a wide variety of materials.' The sample is irradiated by a beam of neutrons; prompt gamma-rays, emitted upon neutron capture, are measured using a high purity germanium detector. The results are independent of the chemical form of the element in the sample, and because both neutrons and gamma-rays penetrate the target, the entire sample is analyzed. The use of cold neutrons results in higher capture rates, a lower background, and better detection limits. A 2223 keV gamma-ray in the spectrum indicates the presence of hydrogen. Two dimensional compositional mapping may be achieved by scanning the sample through the beam and making measurements at various points. Neutron incoherent scattering (NIS) analysis takes advantage of the fact that hydrogen has a large incoherent neutron scattering cross section (ob,.d.- 80 b). As with PGAA, the sample is irradiated by a beam of neutrons. But in this case the neutrons scattered from the sample are measured. The intensity of the scattered neutron signal is proportional to the amount of hydrogen in the sample. Because the neutron scattering cross section for hydrogen is much larger than the neutron absorption cross section (0.33 b), and the detection efficiency for neutrons is higher than that for gamma-rays, NIS has a higher sensitivity for hydrogen than PGAA, and good counting statistics may be achieved in a much smaller period of time. Unlike 175 Mat. Res. Soc. Symp. Proc. Vol. 591 02000 Materials Research Society
in PGAA, however,
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