Scintillator wavelength influence in an optical dosimeter
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Green and red scintillator crystals with plastic fiber optics were used to investigate their application as optical fiber dosimeters. In Part 1, the radiation beam is perpendicular to the system with 20x20cm2 half field. We have a linear response for both systems with doubling the MU starting from 1MU up to 1024MU. Here we define the SOBR (Signal to Optical Background Ratio) to be the response of total signal (crystal and fiber) divided by the background (fiber) signal. The SOBR of red and green with no filter were 16.58 and 17.74 respectively. When we added the filter, the SOBR for red and green became 11.03 and 66.72 respectively. In Part 2, we changed the field to X=1cm, Y1=5cm and Y2=1cm. The SOBR for red and green with no filter are 44.43 and 45.47. After we added the filter, the SOBR for red and green became 15.62 and 1684. This change in field shape gave us a higher SOBR, especially when the filter was added. In Part 3, we tested the angular response of our detector. Both systems increased their response when gantry angle reach 45° and -45° (315°). When the filter was added, a change of a factor of 2 in response remained. When the crystal was then rotated and pointed in the direction of the gantry, good response was obtained from range 90° to -90° (270°). The response of green system was within 2.5%. For the red system, a large step about 10% was observed. Conversion of the fiber fluorescence and Cerenkov radiation in the scintillator crystal and transmission to the detector is a problem in all optical fiber systems, including the dual fiber system, that remains to be addressed.