Evaluation of energy distribution of quasi-monochromatic x-ray beams for sources with extremely high instantaneous flux using a k-edge subtraction technique

The characterization of novel x-ray sources includes the measurement of the photon flux and the energy distribution of the beam produced. The aim of the BEATS2 experiment at INFN-LNF is the study of medical applications of an x-ray source based on Thomson relativistic backscattering. This source is expected to produce pulsed quasi-monochromatic x-ray beam with an instantaneous flux of 1020 ph/s in pulses 10 ps long and with an average energy of 20 keV. A direct measurement of energy distribution of this beam with traditional detectors, such as HPGe, CZT or CdTe, is very difficult because of the extremely high photon flux. For this reason we have planned to use a technique based on beam filtration using k-edge absorbing foils in the energy range of interest (16–22 keV). By measuring the photon flux of the photon beam filtered with appropriate thicknesses of k-edge absorbers, it is possible to retrieve the energy distribution of the incident x-ray beam. The energy resolution obtainable depends on the number and energy separation of k-edges of filter used.A preliminary test of the technique was made using an x-ray tube with a tungsten anode at 22 kVp, filtered with 3.1 mm of Al providing an x-ray spectrum with an energy distribution similar to that expected from a Thomson source. The comparison between the results obtained directly measuring this spectrum with an HPGe detector and using the k-edge subtraction technique showed good agreement.