McLean D
University of Sydney, Faculty of Health Sciences, School of Medical Radiation Technology, Libcombe, NSW, Australia.
Med Phys. 1993 Sep-Oct;20(5):1549-54. doi: 10.1118/1.597118.
Significant differences between broad- and narrow-beam transmission measurements for lead protective barriers have been noted for x-ray beams in the diagnostic energy range. Work reported in this paper indicates that these differences in transmitted exposure between broad- and narrow-beam geometry measurements are primarily the result of two independent interaction processes; the transmission of K-characteristic photons produced in the barrier, and coherently scattered photons. The magnitude of these effects was studied for x-ray spectra between 70 and 150 kVp. The K-characteristic component was found to be maximal at 150 kVp, while the coherently scattered contribution was found to be maximal for thick barriers and at lower energies. The effect of beam field size on relative transmitted intensities was also studied and found to increase with increasing field size up to 30 cm. A detector-to-barrier distance of 10 cm was found to maximize the K-characteristic component. For a typical diagnostic x-ray potential of 100 kVp, the K-characteristic component was found to be less than 7% of the total transmitted beam. Voltage wave form considerations were found to be significant as has been previously reported. The contribution of K-characteristic photons has been mathematically modeled and a computer program written to allow the rapid computation of broad beam transmission exposures for lead barriers, once the appropriate beam specifications have been entered.
