the ray-sum is product of linear attenuation coefficient (l) with
thickness (x), (ii) density measurement and (iii) thickness measurement
(Limkitjaroenporn et al., 2011). Finally, the total standard error
has been determined by combining errors for the ray-sum
measurement, density measurement and thickness measurement
in quadrature.
5. Results and discussions
The density of Inconel 738 superalloy is 8.1771 ± 0.0006 g/cm3.
The thickness of the superalloy is 1.108 ± 0.003 cm. The chemical
compositions of Inconel 738 superalloy are shown in Table 1. It
was seen that the major composition of the superalloy is Ni.
The values of mass attenuation coefficients and partial interactions
of Inconel 738 superalloy are shown in Table 2. The experimental
values of mass attenuation coefficients were evaluated
from incident (I0) and transmitted (I) intensities of each gammaray
energy. The partial interactions and theoretical values of mass
attenuation coefficients were calculated from WinXCom program.
It has been found that the total mass attenuation coefficients were
increased with decreasing of gamma ray energy, which indicates
the total interaction is increased. The experimental values of mass
attenuation coefficient are in good agreement with the theoretical
values as shown in Fig. 2.
Differences between experimental and theoretical values may be
attributed to the effects of mixture rule on mass attenuation coefficient
values. The mass attenuation coefficient values are believed to
be affected by the chemical, molecular and thermal environments.
These phenomena lead to the deviation of the experimental mass
attenuation coefficient value from that of theoretical value, since
the calculation of the theoretical value has been done by considering
the cross-section for an isolated atom. Difference between theoretical
and experimental values can be resulted from the description of
mixture rule. This rule is a standard practice to assume that the contribution
of each element to attenuation is additive. This assumption
yields the well known mixture rule, which gives the
attenuation coefficient of any substance as the sum of the appropriately
weighted contributions from the individual atoms. The mixture
rule is valid when the effects on the atomic wave function of
molecular bonding and chemical or crystalline environment are
negligible. As a result, differences between experimental and theoretical
values can be attributed to breakdown from mixture rule.
For the partial interactions of Inconel 738 superalloy, the results
show that photoelectric absorption, coherent and incoherent