Investigation of random gain variations in columnar CsI:Tl using single x-ray photon imaging

The x-ray imaging performance of an indirect flat panel detector (I-FPD) is degraded by random variations in its scintillator's conversion gain. At energies below the K-edge, these variations are caused by depth-dependence in light collection from within the scintillator, and intrinsic fluctuations in the number of optical photons (N_ph) emitted per absorbed x-ray. At fixed energy, the former effect can be quantified by the average depth-dependent gain N_ph (z). The latter effect can be evaluated using a Fano factor F_N, defined as the variance in N_ph divided by its mean at fixed interaction depth. Neither phenomenon has been directly measured in non-transparent scintillators used in medical I-FPDs, namely columnar CsI:Tl. This work presents experimental measurements of N_ph(z) and F_N in a columnar CsI:Tl scintillator with 1000 μm thickness. X-ray interactions were localized to fixed depths (±10 μm, 100 μm intervals) in the scintillator using a microslit beam of parallel synchrotron radiation (32 keV). Light bursts from single interactions at each depth were imaged using an II-EMCCD optical camera, and their magnitude was characterized by 2D summation of their image pixel values. The II-EMCCD camera was calibrated to convert summed pixel values to numbers of optical photons detected per event. The number distributions of photons collected per event were represented in histograms as "depth-localized pulse height spectra" (DLPHS), from which N_ph(z) and F_N were derived. The II-EMCCD's noise contribution to these measurements was estimated and removed from F_N. Depth-dependent and intrinsic variations in the gain of columnar CsI:Tl are compared.