Determining the Uncertainty of X-Ray Absorption Measurements.
Determining the Uncertainty of X-Ray Absorption
Wojcik, G. S.
Journal of Research of the National Institute of
Standards and Technolgoy, Vol. 109, No. 5, 479-496,
x ray absorption; cements; cement paste; moisture;
mortar; uncertainty; wood; x ray attenuation; moisture
X-ray absorption (or more properly, x-ray attenuation)
techniques have been applied to study the moisture
movement in and moisture content of materials like
cement paste, mortar, and wood. An increase in the
number of x-ray counts with time at a location in a
specimen may indicate a decrease in moisture content.
The uncertainty of measurements from an x-ray absorption
system, which must be known to properly interpret the
data, is often assumed to be the square root of the
number of counts, as in a Poisson process. No detailed
studies have heretofore been conducted to determine the
uncertainty of x-ray absorption measurements or the
effect of averaging data on the uncertainty.
In this study, the Poisson estimate was found to
adequately approximate normalized root mean square
errors (a measure of uncertainty) of counts for point
measurements and profile measurements of water
specimens. The Poisson estimate, however, was not
reliable in approximating themagnitude of the
uncertainty when averaging data from paste and mortar
specimens. Changes in uncertainty from differing
averaging procedures were well-approximated by a Poisson
process. The normalized root mean square errors
decreased when the x-ray source intensity, integration
time, collimator size, and number of scanning
repetitions increased. Uncertainties in mean paste and
mortar count profiles were kept below 2% by averaging
vertical profiles at horizontal spacings of 1 mm or
larger with counts per point above 4000. Maximum
normalized root mean square errors did not exceed 10% in
any of the tests conducted.