Measurements of absolute energy spectra for an industrial micro focal X-ray source under working conditions using a Compton scattering spectrometer

Knowledge of absolute photon energy spectrum is essential for image quality analysis and optimisation for any X-ray imaging method, for example, radiography and computerised tomography (CT). Conventional quantities such as half-value layer (HVL) and effective energy are easily calculated from energy spectra. These quantities are, however, of limited value and use for image quality analysis. For example, two energy spectra with the same effective energy but different distributions will not yield the same signal in energy-dependent (read 'most') detectors. Accurate absolute energy spectra are, unfortunately, hard to generalise, since they depend on the specific X-ray source characteristic, that is, target material, internal filtration, high-tension generator, working load etc. They are also laborious to measure, which makes them hard to obtain. In this work absolute energy spectra [1 /(keV mAs sr)] for an industrial micro focal X-ray source have been measured under working conditions, using a Compton scattering spectrometer. The energy spectra were measured as a function of tube potential (30-190 kV for every 10 kV) at maximum tube charge (8 W, i.e., tube potential × tube current) for the smallest focus diameter (~5µm). This is because the micro focal X-ray source in the application in mind is used mainly for high resolution CT, where its maximum fluence is required to shorten scanning times. Target material was tungsten. The spectra were measured for a highly focused fresh focal spot. Neither focal spot wear (age) nor defocusing of the focal spot was considered. The measured spectra were compared to simulated spectra for the same source supplied by the X-ray source manufacturer. It was found that the measured spectra have slightly different energy distributions with a lower mean energy even though their emitted numbers of photons were similar. The energy calibration, δhv = 0.5 keV, was shown to be accurate compared to the energy resolution used. This work is a part of a larger project, where image quality dependence on X-ray equipment parameters has been studied. Even though the main interest has been in high resolution CT, much of the results and general discussions have wider applications. The full spectra data files are available on the Internet.