Experimental verification of reverse order of diagnostic X-ray beam quality in voltage-ripple dependence

It has been generally expected that, as X-ray tube-voltage ripple increases, the X-ray spectrum shifts to the low photon-energy side, and therefore the mean energy decreases, i.e. the beam quality is softened. This is the normal order. Previous calculation (Birch et al., 1979), however, showed that the beam quality generated by 100% ripple was harder than that by 50% ripple. This is the reverse order, against general expectation. To verify the reverse order, X-ray spectra are measured using a germanium detector system. The measurements are performed with various thicknesses of aluminium object. To obtain large ripples at small tube currents, small-capacity high-voltage cables are made. To perform efficient measurements, the homogeneous sensitive region of the germanium crystal is determined, and detector-collimators with various diameters are prepared. As a result, experimental verification of the reverse order is performed for various thicknesses of object. The reverse order also appears in photon spectra and mean energies calculated using three models from previous work (Kramer, 1923; Birch and Marshall, 1979; Tucker et al., 1991) by inserting the tube-voltage waveforms measured with the authors' system. The experimental results agree well with the results using the Birch-Marshall model. Mean energies reach the minimum levels for the three models at 20-60% ripple, as do the experimental results. The reason for the phenomenon is clarified by qualitative illustration.