Impact of SEM acquisition parameters on the porosity analysis of irradiated U-Mo fuel

In this study, the effect of various scanning electron microscope (SEM) imaging parameters, including voltage, beam current, and magnification, on the porosity analysis of U-Mo fuel was investigated by varying one parameter at a time during back-scattered electron (BSE) micrograph acquisition. Among the parameters assessed, porosity analysis is most sensitive to the SEM voltage. In the range from 5 kV to 30 kV, we reported relative differences as high as 30%, 10%, and 20% in the total porosity, average diameter, and pore density, respectively. Monte Carlo simulations were also performed to determine the influence of SEM voltage on the probing depths of back-scattered electrons. Increasing the voltage from 5 kV to 30 kV resulted in a change in probing depth from few tens of nanometers to several hundreds of nanometers. Portions of pores or entire pores residing below the polished plane that could not be seen at low voltages, and should not be counted for in the estimation of the open porosity, became visible at high voltages. Interestingly, in addition to allowing for a better estimation of the open porosity, the higher sensitivity to surface features at low voltages seems to reduce the number of pixels with intermediate gray levels in the final BSE image. This, in turn, could limit the ground for different interpretation by different analysts, and, as such, has the potential to facilitate a more consistent and uniform porosity analysis across different research laboratories.