Characterization of Plastic Scintillation Detector Response as a Function of Measurement Geometry and Magnetic Field Strength

Graduate Investigator: Eric Simiele

Summary of Research: In recent years, there has been a growing interest in performing magnetic resonance (MR) imaging during radiotherapy treatments to allow for real-time motion and target tracking. Integrating MR imaging with radiotherapy treatment presents challenges for dosimetry due to the skewing of the secondary electron trajectories, and consequently the dose distribution, from the Lorentz force. Solid-state dosimeters have been shown to be a favorable alternative to ionization chambers for measurements in magnetic field environments since the density of the sensitive detector volume is similar to that of the surrounding medium. Organic plastic scintillation detectors (PSD) are becoming a popular solid-state detector for dosimetry measurements in megavoltage (MV) photon beams due to their near-water equivalence in the MV energy range, small detector volume, small energy dependence, and real-time readout. Although PSDs possess many attractive dosimetric properties, they suffer from an unwanted noise signal that degrades the overall detector signal-to-noise ratio (SNR). This noise or stem-effect signal is caused by Cerenkov radiation and fluorescence generated in the light guide of these detectors. This work explores the properties of the stem-effect signal in various types of light guides used in PSDs in linac-based measurement geometries. In collaboration with the German primary standards laboratory (PTB), the stem-effect signal was characterized as a function of measurement geometry and magnetic field strength in these various light guides. In addition, Monte Carlo models of the optical properties of these light guides were constructed to understand the observed changes in response.