Obliquity factors for 60 Co and 4, 10, and 18 MV x rays for concrete, steel, and lead and angles of incidence between 0 and 70 degrees.

Abstract

The attenuation of 60 Co gamma rays and photons of 4, 10, and 18 MV x-ray beams by concrete, steel, and lead has been studied using the Monte Carlo technique for angles of incidence 0, 30, 45, 60, and 70 degrees. Transmission factors have been determined down to <2x10(-5) in all cases. The results show that deviation from the obliquity factor increases with angle but is not significant for angles <45 degrees. At a 70 degree angle of incidence and a transmission factor of 10(-5), the obliquity factor varies between 1.2 and 1.9 for concrete, between 1.4 and 1.7 for steel, and between 1.4 and 1.5 for lead for the range of energies investigated. This amounts to an additional 86 and 50 cm of concrete, 25 and 23 cm of steel, and 8 and 14 cm of lead for 60 Co and 18 MV x rays respectively. The results for 60 Co in concrete and lead are in good agreement with previously published experimental work. Fits to the data using mathematical models allow reconstruction of all data curves to better than 1% on average and 7% in the worst single case.