Measurement of backscattered x-ray spectra at the water surface in the energy range 60 kV to 120 kV.

Backscattered x-ray spectra at the water surface have been measured by using a small silicon diode detector. The measurements have been made at tube voltages 60 kV to 120 kV (HVL 2.4-6.1 mm Al) and field sizes 5 x 5 cm2 to 30 x 30 cm2. The measured spectra are corrected for detector distortion and for the angular dependence of detector efficiency. The obtained backscattered spectrum has a lower mean energy and a narrower shape than the primary spectrum. The ratio of the mean energy of the backscattered spectrum to that of the primary spectrum is between 0.83 and 0.94. The ratio of the spectrum width at 10% of the continuous spectrum maximum is between 0.65 and 0.78. The change of spectral shape due to the field size is slight. In the high-voltage spectra, the peak due to the Compton scattering of tungsten Kalpha x-rays is observed. The backscatter factors (BSFs) calculated from the obtained spectra show a satisfactory agreement with other studies. The difference between the BSF defined as the ratio of air kerma and the BSF defined as the ratio of water kerma is also calculated; the maximum difference is 0.43%. The empirical equation showing the relation between the two BSFs is presented.