Effect of collimators on the measurement of diagnostic x-ray spectra

One of the difficulties in the measurement of diagnostic x-ray spectra is the high fluence rate of the photons, which may exceed the capability of the spectrometer system. Pinhole collimators are often used which reduce the count rate to an acceptable level. The measured spectrum, however, may be distorted due to the penetration of photons through the collimator material. In this study, the effects of collimators on the measured spectra were evaluated. As an indicator of collimator performance, we determined the transmission equivalent aperture (TEA), through which a portion of the incident photons impinge on the detector. The dependence of the TEA on the geometry of collimation, pinhole area, collimator material and thickness, detector area and incident x-ray energy was investigated. The penetration correction factors (PCF) for the measured spectra were also derived. We obtained good correspondence between the theoretical spectra and the measured spectra which were corrected for collimator penetration by means of the calculated PCFS. Our results indicate that diagnostic x-ray spectra generated at high tube current can be measured if one uses a collimator with a small aperture together with its PCF.