Measured X-ray Spectra and Absorbed Dose Conversion Factors in a Water Equivalent Phantom Irradiated with Diagnostic X-rays

The diagnostic X-ray spectra in a water equivalent phantom have been measured. From these measured spectra, the absorbed dose conversion factors of water were derived. The primary X-ray spectra were also measured and the scattered X-ray spectra were calculated by subtraction. The measurements were made at the depths of 0, 5, 10, 15, and 20 cm in a 20 cm-thick phantom and at the X-ray tube voltages 60, 90, and 120 kV by using a small silicon diode detector. The radiation field size was 30×30 cm2 at the phantom surface. In the obtained spectra, the fraction of the scattered photon number is increased with the depth. The X-ray qualities of the spectra in the phantom were near the qualities of primary X-rays when the depth is 0 cm, and became near the qualities of scattered X-rays as the depth increases. The changes of the X-ray qualities due to the depth change were small; photon mean-energy changes were within 4.6 keV. The changes in the absorbed dose conversion factors were also small (within 0.68%). These conversion factors were 0.4–2.3% larger than those obtained from the effective energy of incident X-rays and only −0.3 to 0.5% larger than those obtained from the X-ray spectra calculated from the aluminum half value layer and the tube voltage of incident X-rays. This study shows experimentally that the absorbed dose in a water-like phantom can be calculated with good accuracy by using the absorbed dose conversion factor obtained from the incident X-rays.