Transmission in concrete of primary and leakage x rays from a 24-MV medical linear accelerator

The transmission of leakage radiation from a 24-MV/6-MV linear accelerator was measured through 52 in. of concrete forming the roof of a test cell, at angles of 30, 45, 90, 135 and 180 degrees relative to the direction of the electron beam at the target. Transmission of the primary beam was also included as a partial check on the suitability of the method. The latter results were in good agreement with recommended values. The leakage radiation tenth-value layers agreed well with theory at 90 degrees but, at greater and lesser angles, interpretation was less straightforward, owing it is believed to the multiple leakage sources and complex shielding geometries inherent in a bent-beam machine. The implications of the findings concerning shielding barrier design are discussed, including consideration of the new case presented by dual photon energy machines.