The Dosimetry Group calibrates well-type ionization chambers using Xoft miniature x-ray sources, which provide low-energy x-rays (< 50 keV) for electronic brachytherapy applications. Calibrations are performed by comparing the well-type ionization chamber response to air kerma realized by a NIST primary x-ray standard.
The increased use of limited-lifetime electronic brachytherapy sources which are miniature, low-energy x-ray tubes for clinical applications resulted in the need of a measurement of air-kerma dose from NIST. The type of electronic brachytherapy sources, Axxent S700, used at NIST operates at tube potentials of 50 kV and anode currents of 300 μA.
The electronic brachytherapy calibration range incorporates the Lamperti free-air chamber (FAC), a well-ionization chamber, and a high-purity germanium (HPGe) spectrometer to determine the air-kerma of miniature x-ray sources. The calibration service determines the air-kerma rate in terms of Gy/s for each source at 50 cm using the Lamperti free-air ionization chamber. A customer-provided well chamber is used to measure the stability of the source output and serves as a transfer instrument. A NIST-maintained well chamber is also calibrated to provide quality assurance to the process. The measurement process results in a NIST calibration coefficient for the well chambers for each source in units of the NIST air-kerma rate (Gy/s) at 50 cm per ampere, Gy/(A s) normalized to 295.15 K (22 oC) and 101.325 kPa (1 atm).
A report describing the air-kerma determination of the electronic brachytherapy sources using the Lamperti chamber is available in the NIST Journal of Research. A description of the service and procedures of the measurement of air-kerma and the calibration of well-ionization chambers using the electronic brachytherapy sources is available as Procedure 18.