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An Ionization Chamber Mounting Jig for In-Air Output Calibration and Quality Assurance of Commercial Cs-137 Irradiators

S Jayarathna*, R Tailor , M F Ahmed , S H Cho , The University of Texas MD Anderson Cancer Center, Houston, TX


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To develop an ionization chamber mounting jig for in-air output calibration and quality assurance of commercial Cs-137 irradiators.

Methods: The irradiator “J.L Shepherd Mark I-25� has a Cs-137 line source (vertical), a shielded cubical enclosure, two turn-tables, and three sample rotation positions. A jig for mounting an ionization chamber has been designed to perform in-air dose output calibration in an efficient and reproducible manner. Since in-air dose measurement requires absence of any scattering material, the jig design employs minimal amount of material (Lexan) to prevent significant scatter reaching the ionization chamber’s thimble. The jig consists of a bottom-plate (14 cm × 13 cm) supporting a thin-wall vertical tube (inner diameter 8 mm, wall thickness 1.2 mm, height 18 cm). The tube holds an ionization chamber vertically, and is provided additional support by means of four triangular shaped ramps glued to the bottom plate and outer surface of the tube. The jig’s bottom plate is taped symmetrically onto a turn-table in order to precisely align the vertical tube with the sample rotation axis. The ionization chamber is gently inserted through the tube’s top. The center of an ionization chamber thimble is adjusted to line up with the ink mark on the tube (11.6 cm from the turn-table surface). The turn-table is gently placed at a desired sample rotation position, and ionization measurements are then performed to provide in-air output calibration.

Results: The AAPM TG-61 was employed to calculate in-air dose rates (to striated muscle) based on the measured ionization data. Dose output results using the currently described jig agreed well (within 2%) with the reference dose rates decayed to the measurement dates.

Conclusion: This study presents a simple yet highly effective ionization chamber mounting jig that can be used for periodic quality assurance of commercial Cs-137 irradiators.


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