Affiliations: Division of Engineering Materials, Department of
Mechanical Engineering, Linköpings Universitet, Sweden. | Division of Radiation Physics, Department of Medicine
and Care, Linköpings Universitet, Sweden. | Division of Engineering Materials, Department of
Mechanical Engineering, Linköpings Universitet, Sweden.
Note: [] Corresponding author.
Abstract: 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.
