EXPERIMENTAL-DETERMINATION OF THE CONVOLUTION KERNEL FOR THE STUDY OFTHE SPATIAL RESPONSE OF A DETECTOR

One of the most important parameters in the characterization of a dete ctor is its spatial convolution kernel. This kernel contains all of th e information about the influence that the detector size has on the me asured beam profile. In this paper we present an experimental method f or the determination of the spatial convolution kernel for commonly us ed detectors that are employed in the x-ray profile measurement: filmdensitometer, diode, and ionization minichamber. Our work is based on first assuming a step function pattern on a photographic film is known and is a perfect step function, The kernel of the densitometer system was then derived from the deconvolution of the scanned profile to the step function. Next a film was exposed to a penumbra area of an x-ray beam from a linac. The film was scanned using the same densitometer. The ''real profile'' that emerges from a linear accelerator was derive d by the deconvolution of the scanned profile using the now known kern el of the film densitometer, Under the same irradiation condition the x-ray profile was measured with other detectors and with this informat ion we obtained the convolution kernels for these detectors by solving numerically their basic convolution integrals. The results show that the Gaussian convolution kernel is the most consistent with the measur ements. The best numerical values for the FWHM of the kernels are 1.1 mm, 2.2 mm, and 5.4 mm for densitometer, diode, and minichamber, respe ctively. (C) 1998 American Association of Physicists in Medicine.