Purpose: Edge-on silicon (Si) photon counting detectors (PCDs) promise to provide lower material cost and faster count-rates compared to other semiconductor PCDs. However, the lower atomic number of Si introduces more x-ray scatterings within the sensors. The purpose of this work was to experimentally quantify intra-detector scattering in edge-on Si PCD and study the corresponding impact on quantum efficiency and spatial resolution.
Methods: To measure inter-Si detector row scattering, a sub-mm pencil beam was created by collimating polychromatic x-rays from a diagnostic tube using two lead pinholes. A 500 um-thick, 4 cm-wide Si module was placed edge-on to this pencil beam. A zero-electronic noise CdTe-based PCD operating under the ultra-high sensitivity mode was placed behind or to the side of the Si module to measure the number of penetrated primary x-rays and side-scattered x-rays, respectively. Measurements were performed at 7 tube potentials ranging from 60 to 120 kV with different filtration. Finally, the experimental data was used to validate a Monte-Carlo based simulation to help understand the detector performance at arbitrary energy levels and detector row widths.
Results: The percentage of input x-rays scattered to the sides of each detector row ranges from 14% (60 kV) to 62% (120 kV with 2mm Cu), while 1–14% penetrated through the 4 cm-wide Si PCD. Results from the simulation agree well with the experimental data (Δ<3%). With inter-detector row septa, the overall quantum efficiency is reduced to 26% at 120 kV with 2mm Cu. Without septa, the efficiency is increased to 53%, but 33% of scattered x-rays are absorbed more than 20 detector rows away (1 cm).
Conclusion: It is experimentally demonstrated that a large fraction of input x-rays are side-scattered by each row of edge-on Si PCDs. Removing the inter-row septa may improve the quantum efficiency at the cost of z-resolution.