SU-E-J-50: Measurement of the HVL Value for the 120keV Photon Beam of the X-Ray Volume Imager (XVI) Using Ionization Chamber, Gafchromic Film XRQA2 and Diode.

Abstract

The value of Half Value Layer (HVL) is essential in quantifying the ability of an x-ray beam to penetrate the material being studied. This paper examines the use of three different types of detectors for measuring HVL of the 120keV photon beam of the X-ray Volume Imager (XVI) cone beam CT system mounted on the Elekta Synergy (Crawley, UK) Linac: ionization chamber, Gafchromic film and diode. The effects of collimator diameters are presented and the chamber tip and chamber horizontalmethods are compared to investigate the source of variations in the measurement of HVL. The measurements are done in free airboth using an ionization chamber calibrated for absolute dosimetry [University of Wisconsin Accredited Dosimetry Laboratory] and the Gafchromic film dosimeters [XR-QA2 (International Specialty Products, Wayne, NJ)]. The beam is collimated to the size of the detector to measure the exposed dose. The different thicknesses of the aluminum attenuators are added to beam to measure individual doses every time and repeated until the dose fell to below 50% of the initial unattenuated value. Dose measurements with a semiconductor diode detector [Barracuda MPD (RTI Electronics AB, NJ)] is also carried out. We plotted all results on a semi-log graph to find HVL value. When the measurements are made using ionization chamber with different collimator diameters we obtained 1.6% difference in HVL values. Measurements with chamber tip and horizontal methods agree with each other to within approximately 1.6% percentage difference. The percentage difference of the HVL values measured using the diode and ionization chamber is about 6.4% and that of using Gafchromic film and ionization chamber is about 8% percent. Gafchromic films and diodes can be used to measure HVL value of 120 keV photon beams. Recommendations for consistency in HVL measurements are provided.