A well-type ionization chamber geometric correction factor

To correct for the influence of source configuration on the measured activity of spherical and cylindrical brachytherapy sources, a geometric correction factor was calculated for the Standard Imaging HDR-1000 well-type ionization chamber. A Fonran program modelled each source as a lattice of point sources. Because of the cylindrical symmetry of the well chamber, it could be uniquely modelled by point detectors along the perimeter of the radial plane of the detection volume. Path lengths were calculated and attenuation factors were applied to each source-detector point combination individually. The total dose rate at each detection point was found through a Sievert summation of the point source contributions. For Cs-137 sources with identical activities, a correction factor of 0.965 +/- 0.005 was calculated, equal to the ratio of the dose rate of the cylindrical source to that of the sphere. Experimental verification using a Nuclear Associates 67-809 series cylindrical source and an Amersham spherical Cs-137 source yielded a correction factor of 0.958 +/- 0.016.