SU-E-T-318: A Simulation Study for Active Scanning Nozzle Design Using Beam Optic Parameters

Abstract

Purpose: The beam conditions at the end of HEBT (High Energy Beam Transport) line affect on active scanning nozzle design and scanning parameters. In this study, the different initial conditions at HEBT were applied to determine an appropriate active scanning nozzle dimension. Using determined beam conditions, the dose distribution at the Io-center was calculated to provide a spot spacing at active scanning scanning. Methods: In this study, the initial geometry dimension was set to about 9 m from the end of HEBT to Iso-center. The beam line was considered an vacuum space until 7.3 m from HEBT. Two different initial beam conditions were applied. The first was a simple gaussian distribution for simple divergence beam case. And, the second was a focused beam based on Twiss parameters. The beam line was produced by Geant4 simulation tool kit. For the drifted beam, the position and momentum distribution were considered as 3 mm and 0.6 mradian as sigma. For the focused beam, the given emittance was 10 pi mm mradian. Twiss parameter were derived by drift length to create 3 mm of sigma, as waist point, at the iso-center. The required inner diamater for HEBT and scanning magnet were predicted by the size of the beam, which was obtained by above calculations. Also, the dose distribution near the iso-center was obtained to get optimized scan spacing. Results: Based on the size of the transported beam, the minimum inner diameter of HEBT and Scanning Magnet should be larger then 21mm of sigma. Also, the sigma difference of dose distribution at water phantom shows about 20 % at given beam conditions. Conclusion: Depending on the initial beam condition at the end of HEBT, the beam nozzle dimension and scan spacing should be determined. Through this simulation study, more precise data could be given.