X‐ray source and the output factor

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Abstract
When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x‐ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The source diameters in the two principal directions were clearly influenced by the design of the accelerators. The x‐ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal–film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.

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