Fiber-coupled organic plastic scintillator for on-line dose rate monitoring in 6 MV X-ray beam for external radiotherapy

Fiber-coupled organic plastic scintillators enable on-line dose rate monitoring in conjunction with pulsed radiation sources like linear medical accelerators (linacs). The accelerator, however, generates a significant amount of stray ionizing radiation. This radiation excites the long optical fiber (15-20 m), connecting the scintillator, typically with a diameter of 1 mm and 5 mm in length, with the optical detector circuit, causing parasitic luminescence in the optical fiber. In this paper we propose a method for circumventing this problem. The method is based on the use of an organic scintillator, 2-Naphthoic acid, doped in an optical polymer. The organic scintillator possesses a long luminescent lifetime (room temperature phosphorescence). The scintillator is molded onto the distal end of a polymer optical fiber. The luminescent signal from the scintillator is detected by a PMT in photon-counting mode. The long lifetime of the scintillator signal facilitates a temporal gating of the dose rate signal with respect to the parasitic luminescence from the optical fiber. We will present data obtained using a solid water phantom irradiated with 6 MV X-rays from a medical linac at the Copenhagen University Hospital. Also issues pertaining to the selection of proper matrix as well as phosphorescent dye will be presented in this paper.