Sensitivity of amorphous selenium to x rays from 40 kVp to 18 MV: measurements and implications for portal imaging

Recently, the clinical application of electronic portal imaging devices has enabled more frequent verification of patient setup for radiation treatment. However, the image quality has sometimes proven to be inadequate, motivating the investigation of alternative sensors with better image quality. Amorphous selenium (a-Se) is potentially one such sensor since the electrostatic image formation process has high resolution. To fully evaluate the potential of a-Se for portal imaging, it is necessary to investigate all the imaging properties at high x-ray energies. Here, measurements of the sensitivity of a-Se to incident x-ray spectra ranging in energy from 40 kVp to 18 MV and for a-Se thicknesses ranging from approximately 10 to 300 microns under full buildup conditions are described. When x rays or energetic electrons deposit energy in a photoconductor with an applied electric field, F, electrons and holes are released. The x-ray conversion sensitivity may be defined as 1/W +/-, where W +/- is the energy required to release an electron-hole pair. Consistent with the results of previous investigators, W +/- is found to vary approximately with F-2/3. Unexpectedly, over the energy range of 40 kVp to 18 MV, W +/- was found to decrease by a factor of nearly 3. These dependencies are compared to the predictions of two competing charge recombination models, geminate and columnar. The results are explained by a microdosimetric model in which the sensitivity at megavoltage energies is governed by geminate recombination, but at lower energies, both mechanisms are involved. Thus, the sensitivity of a-Se to x rays spanning the diagnostic and radiotherapy range has been measured and the physical basis for this behavior established.