Measurements of 6 MV X rays. II. Characteristics of secondary radiation

The measurements reported are for secondary radiation scattered from either a Presdwood phantom or a concrete barrier. The magnitude, angular distribution and attenuation characteristics of these secondary radiations are discussed. The relative dose from radiation scattered by a Presdwood phantom was evaluated at 1 m from the phantom. Expressed as a percentage of the dose at the centre of the phantom it varied from 0·04 per cent at 135 deg. to 0·8 per cent at 15 deg. Expressed as a tenth-value thickness the attenuation thickness for this scattered radiation in concrete varied from 14·7 cm at 135 deg. to 33·3 cm at 15 deg. The attenuation of 90 deg. scatter was also investigated for Presdwood, steel and lead. The 90 deg. scatter has a monochromatic equivalent energy of slightly less than 500 keV. The intensity and angular distribution of scatter from a concrete barrier was examined for both 90 and 60 deg. incidence of the primary beam. For these measurements the scattered dose at 1 m did not exceed 0·25 per cent of the dose maximum just below the barrier surface for a 48 × 48 cm field.