Spectra of scattered radiation at points off the beam axis
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In radiological physics, a knowledge of the spectral distribution or radiation reaching a point is very important. Although the dose gives the amount of radiation at the point, it is the spectral distribution which determines the effects, for instance, the energy absorbed by the irradiated medium. In an irradiated medium radiation reaching a point consists of primary and scattered radiation. The scattered radiation becomes more important as the depth is increased. In this investigation, the spectral distributions of scattered radiation at 1.47 cm inside, at 1.88 cm outside the boundary of the field and at the central axis were measured with a scintillation spectrometer for two qualities of X-rays, 280 KVP, HVL of 2.5 mm Cu and 140 KVP, HVL of 1.45 mm Al. A circular beam of 100 cm2 at FSD of 60 cm was used. Measurements were carried out at 5 cm depth for 280 KVP and 2 cm depth for 140 KVP. It was found that for 280 KVP radiation, the shapes of the integrated spectra inside the field, outside the field and at the centre are nearly the same. However, the dose received at 1.47 cm inside is 73% of that at the centre and 1.88 cm outside is 37% of that at the centre. It should be pointed out that at the centre, the scattered radiation contributes about half the total dose. For 140 KVP radiation, the peak of the scattered spectrum becomes broader as the distance from the centre is increased. The dose received at 1.47 cm inside is 83% or that at the centre and 1.88 cm outside is 25% of that at the centre. For the central point the scattered radiation accounts for approximately 50% of the total dose received. Comparisons were made with doses calculated by a method suggested by Clarkson together with the depth dose data. Reasonable agreement was found.