Radiation Interactions and Dosimetry Group

Wide Angle Free Air Chamber

The Wide-Angle Free Air Chamber (WAFAC) serves as the NIST primary standard for low-energy photon-emitting brachytherapy sources such as iodine-125. The WAFAC is a variable volume, circular free-air chamber, symmetrical about the beam axis, with lines of force parallel to the beam axis. There is an 80 mm diameter tungsten aperture (not visible on the right of the picture) at the front of the chamber used to define the beam.

The chamber has three electrodes: the front one (closest to the source) is the polarizing electrode, a middle electrode, and a collecting electrode (farthest from the source and left in the picture) about 150 mm in diameter. The chamber volume is varied from approximately 75 cm³ to 804 cm³ by changing the length of middle electrode from 11 mm to 152 mm. The polarizing and collecting electrodes are constructed of aluminized Mylar, about 1 mg/cm² thick, while the middle electrode is an aluminum tube 250 mm in diameter. The difference in ionization currents measured with the 152 mm and the 11 mm middle electrode, respectively, is used, along with other parameters, to calculate the air kerma strength of the source.

For information on calibrations contact:

Dr. Michael Mitch [michael.mitch@nist.gov]

Inquiries or comments: s.seltzer@nist.gov
Online: September 1997   -   Last update: February 2004   -   Reviewed: May 2005