Calibration of X-ray imaging devices for accurate intensity measurement

Abstract

National Security Technologies (NSTec) has developed calibration procedures for X-ray imaging systems. The X-ray sources that are used for calibration are both diode type and diode/fluorescer combinations. Calibrating the X-ray detectors is a key to accurate calibration of the X-ray sources. Both energy dispersive detectors and photodiodes measuring total flux were used. We have developed calibration techniques for the detectors using radioactive sources that are traceable to the National Institute of Standards and Technology (NIST). The German synchrotron at Physikalische Technische Bundestalt (PTB) was used to calibrate the silicon photodiodes over the energy range from 50 to 60 keV. The measurements on X-ray cameras made using the NSTec X-ray sources included quantum efficiency averaged over all pixels, camera counts per photon per pixel, and response variation across the sensor. The instrumentation required to accomplish the calibrations is described. The X-ray energies ranged from 720 to 22.7 keV. The X-ray sources produce narrow energy bands, allowing us to determine the properties as a function of X-ray energy. The calibrations were done for several types of imaging devices. There were back and front illuminated CCD (charge-coupled device) sensors, and a CID (charge injection device) type camera. The CCD and CID camera types differ significantly in some of their properties that affect the accuracy of the X-ray intensity measurements. All the cameras discussed here are silicon based. The measurements of the quantum efficiency variation with the X-ray energy are compared to the models for the sensor structure. The cameras that are not back-thinned are compared to those that are.