Characterization of the linear scintillator array signal response as a function of x-ray impact parameter

Many imaging systems are based in linear scintillator scanning arrays. These indirect x-ray detection elements are commonly used both in medical and industrial imaging applications. The aim of this work has been the evaluation of the kernel response fuction for these arrays. The kernel response fuction k(x) is the average number of photoelectrons detected in each array channel per x-ray photon impinging on the array in perpendicular direction at a distance x from the center of the element. The kernel function has an intrinsic dependence both on the radiation transport in the detector and on the scintillating light yield and its optical path and absorption. The objective of the present work is to achieve a more detailed parametrization of these devices for their simulation and optimization inside imaging systems. We have performed measurements with a commercial 1.6 mm photodiode array optically coupled to CsI(Tl) and a 0.8 mm pitch photodiode array optically coupled to two CsI(Tl) and Gadox scintillators. These devices were irradiated with a highly collimated (FWHM 0.25 mm) 50 kV tungsten target x-ray beam. Signal measurements were compared with simulation data from MANTIS (an optical and radiation transport Monte Carlo code based on PENELOPE and DETECT-II). We have been able to achieve good agreement (relative difference normalized to maximum less than 2%) between measurements and Monte Carlo. These results show that in the case of CsI(Tl), where a discrete cristal array is used, there is a significative light cross-talk between adjacent channels that reaches a 5% of maximum signal corresponding to a distance of 2.1 mm (pitch + 0.5 times crystal width). This is a consequence of a very thin backing wall between crystal elements, that tends to diminish dead areas between the detector elements. Nevertheless, although intrinsic Csl detection efficiency is almost 100% at these energies, due to the photodiode fill factor and optical transport there is a 40% signal drop when the beam impact parameter coincides with the cell array intermediate point. Thus the average efficiency of the array is actually below 84% from the kernel expresion. For the Gadox scintillation we observe a higher light dispersion that accounts for a wider kernel behaviour. As for CsI(Tl), the Gadox array exhibits an important signal loss in the intermediate array positions. The spectral sensivity of the commercial arrays were also measured.