Modeling of x-ray fluorescence full field imaging using planar square pore micro-channel plate optics

Abstract

X-ray fluorescence imaging using perfect planar square pore micro-channel plate x-ray optics (MPO) is investigated through the modeling of the MPO point spread function (PSF). A semi-continuous model based on the use of a simplified two parameters reflectivity curve is developed including, in particular, three kinds of contributions. A validation of this model is carried out by calculating variations of several PSF characteristics with the MPO and fluorescence imaging parameters and comparing the results with ray-tracing simulations. A good agreement is found in a large range of x-ray energies; however, it is shown that for the lower values of the working distance a discrete model should be used to take into account the periodic nature of the PSF. Ray-tracing simulated images of extended monochromatic sources are interpreted in light of both the semi-continuous and discrete models. Finally, solutions are proposed to improve the imaging properties of the MPOs.

Characterization of micro pore optics for full-field X-ray fluorescence imaging

“…This has been simulated for large distances (10 cm) and short distances (0.5 cm). The image obtained using the randomly oriented squares is not very different at a short distance to the image obtained using a regular MPO [18]. This can be explained by the limited number of channels involved for each point of the extended source, which is also the origin of the periodicity of the PSF discussed above in the case of a regular MPO.…”

“…Lens thickness, channel size, and internal coating are the main parameters that can be adjusted in the MPO design. In previous ray tracing studies, two different methods have been proposed to reduce the impact on the MPO [18]. The first approach involves an array of square pores with a random orientation of the square cross section.…”

“…The main features of a square-channel MPO are a central spot (which is desirable for image formation), two perpendicular lines forming a cross, and weaker intensity in the quadrants delimited by the cross. The central spot intensity, or focusing reflection in figure 1, is the result of an odd number of reflections in one plane [18]. A second effect is from the diffuse signal from direct transmission.…”

“…Both the object material and the system parameters influence these PSFs. The intensity of the cross-arms in the region around the PSF central spot depends on geometric factors, and reflectivity, which changes with the material and X-ray energy [18]. For a fixed geometry, the material and the X-ray energy will mainly affect the length of the cross-arms.…”

“…The understanding of the square MPO properties relies on the modelling of ideal structures, as shown by one study carried out already in 1991 [14]. XRF imaging using a perfect planar square-channel MPO has been further investigated through the modelling of the PSF, whereby Monte Carlo simulations were developed to evaluate the effects of instrument geometry and MPO characteristics, including various types of defects [18]. However, the MPOs have imperfections that might add complexity to applications and create difficulties for understanding the performance of MPOs in practice.…”