The Dosimetry Group has developed a new facility for the calibration of the Xoft miniature x-ray source, which provides low-energy x-rays (< 50 keV) for electronic brachytherapy applications.
A reference air-kerma rate of the sources has been directly realized through use of the Lamperti free-air chamber. The Lamperti chamber has a history of direct traceability at the BIPM and is designed for the energy range required in electronic brachytherapy. The energy spectrum from individual sources is measured using a high-purity germanium (HPGe) spectrometer, which is mounted opposite the free-air chamber to allow simultaneous measurement of the air-kerma rate and tube output spectrum. The spectral information has been used to determine correction factors for the Lamperti chamber. Comparison measurements between the Ritz and Lamperti primary standard chambers were conducted, and the Lamperti chamber was also compared to the Attix chamber using the rhodium x-ray tube in an effort to further investigate the direct determination of air attenuation corrections. A measurement comparison with the University of Wisconsin began in January 2013, where four electronic brachytherapy sources were evaluated using the Lamperti primary free-air ionization chamber and two well chambers. The NIST determined the air kerma rate for each source at a distance of 50 cm using the Lamperti chamber. The well chambers are used to measure the stability of the source output and serve as transfer instruments.