Improving HEXITEC gain calibration through charge-shared and fluorescent multi-pixel events

We describe two techniques to verify and improve the energy calibration of pixelated solid-state X-ray detectors by specifically analyzing multiple-pixel events. There are two primary types of multiple-pixel events. In fluorescent events, some of the incident photon energy is re-emitted as a Cd or Te characteristic X-ray, with mean free path (∼80-200 μm) comparable to the pixel pitch. Deposition of the fluorescence photon commonly occurs in an adjacent pixel. Isolating fluorescent double-pixel events provides low-energy (<15 keV) escape lines not intrinsic to the source. These lines reveal that the non-linearity of the gain at low energies is larger than previously thought, and can now be corrected. In charge-shared events, the cloud of deposited charge from one incident photon spreads across pixel boundaries. This causes charge loss due to variation in applied field strength within inter-pixel regions. The charge loss is a smooth function of the ratio between component energies, with more loss seen when a photon hits closer to the boundary and its energy is more evenly split. Characterization of this energy-split-dependent loss lets us extrapolate †split event behavior below the signal threshold. We can then verify consistency of the non-linear gain calibration at the lowest energies.