Figure 2.

Block diagram representing the various stages in the serial cascaded systems analysis performed in this paper for prototype PVD#4. The x-ray spectrum (Boone and Seibert 1997) used in the calculations corresponds to that used in our measurements and is characterized by a fluence of ${\stackrel{‒}{q}}_0$ = 262,410 × rays/mm²/mR. The HgI₂ photoconductor is described by a quantum efficiency ${\stackrel{‒}{g}}_1$ = 0.643 with a variance of 0.229. It is also described by a photoconductive gain represented by ${\stackrel{‒}{g}}_2$ = 11,246 (mean number of electron-hole pairs generated in the photoconductor per interacting x ray) with a variance of 9.86×10⁶ - as determined via EGS4 Monte Carlo simulations (El-Mohri et al 2007) assuming a thickness of 210 μm for HgI₂, a density of 5.72 g/cm³ and an internal ionization energy of 4.2 eV.(Schieber et al, 1997; Alexiev et al, 2004) The stochastic spatial spreading is characterized by T₃ obtained from the empirically determined MTF divided by the sine function for a square pixel aperture of 127 μm. The gain ${\stackrel{‒}{g}}_4$ = 0.392 with a variance of 0.238 corresponds to the collection efficiency of charges created in the photoconductor. The value of ${\stackrel{‒}{g}}_4$ was obtained by fitting the calculated x-ray sensitivity to the empirically determined value, with ${\stackrel{‒}{g}}_4$ taken as a free parameter. The array pixels are characterized by a spatial spreading represented by the aforementioned sine function, T₅, and an additional stage (labeled a_pix) representing the sampling of the signal. The additive noise of the electronic acquisition system is characterized by σ_ADD, which was empirically determined from dark NPS measurements to be ~2000 e (rms). Note that, for simplicity, k-fluorescent interactions have not been separated as a parallel branch in the model since their effect on the MTF, as determined from Monte Carlo simulations that include such interactions, was found to be negligible.