SU-GG-T-218: Measurements of X-Ray Spectra Emergent From Low-Energy Photon- Emitting Brachytherapy Sources

Abstract

Purpose: To investigate changes in the emergent spectra of low‐energy photon‐emitting brachytherapy sources as a function of angle in the plane of the source long axis. Method and Materials: X‐ray spectrometry was performed utilizing a high‐purity germanium detector, having a 1 cm diameter aperture, located at a distance of 177 cm from the center of the source. The source, with the long axis parallel to the floor of the laboratory, was mounted on a rotating vertical post. Emergent spectra were measured at rotation intervals that varied from 5 degrees to 15 degrees for a total of forty spectra per source. The intensity ratio of Kβ to Kαx rays from the decay of the radionuclide, the intensity ratio of Ag Kαfluorescencex rays to the 35 keV emission line from I‐125 sources that contained silver, and the air‐kerma strength were all calculated from the measuredspectra as a function of source rotation angle. Results: The degree of anisotropy about the axis perpendicular to the mid‐point of the long axis of the source was characterized by calculating the ratio of the air‐kerma strength of the source positioned perpendicular to and parallel to the detector face. Values of this “air‐anisotropy ratio”, αS, varied from 0.05 for a Pd‐103 source with thick end caps to about 1 for an I‐125 source with a uniform encapsulation thickness. Sources with low values of αS showed a significant increase in the Kβ‐to‐Kα intensity ratio for emissions from the seed ends. Conclusion: Variations in the designs of brachytherapy sources significantly influence their emergent spectra.Measuring the spectra around these sources has lead to an understanding of the response of well‐ionization chambers, used in therapy clinics to verify source air‐kerma strength prior to treatment, relative to the NIST Wide‐Angle Free‐Air Chamber (WAFAC) primary standard.