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Characterization of Spectral Changes with Measurement Geometry and Magnetic Field Strength in Light Guides Used for Scintillation Dosimetry

E Simiele1*, R Kapsch2 , U Ankerhold2 , W Culberson1 , L DeWerd1 , (1) Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI, (2) Physikalisch-Technische Bundesanstalt, Braunschweig, Germany


(Monday, 7/15/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 3

Purpose: To characterize the stem-effect signal and Cerenkov light ratio (CLR) as functions of measurement geometry and magnetic field strength in various light guides used for scintillation dosimetry.

Methods: Two PMMA-, two silica-, and one polystyrene-based light guides were considered in this work. Spectra measurements were performed as functions of depth, fiber-beam angle, and magnetic field strength using an optical spectrometer. All measurements were performed using a clinical linac at a photon beam energy of 6MV. Depths ranging from 1cm to 10cm, fiber-beam angles ranging from 90degrees to 30degrees, and magnetic field strengths ranging from 0T to ±1.40T were investigated. The CLR was calculated from each spectrum by taking the ratio of the integral signal between 400nm and 500nm to the signal between 500nm and 600nm.

Results: The spectral responses of the fibers increased as a function of magnetic field strength where the positive field spectra exhibited greater responses than their negative field analogs. Variations in spectral shape and consequently, the CLR, were observed for all the fibers as functions of magnetic field strength and measurement geometry. The plastic fibers exhibited decreases in the CLR as a function of magnetic field strength, whereas the silica fibers exhibited increases in the CLR with decreasing magnetic field strength. A maximum variation of 11.1% in the CLR was observed for the polystyrene fiber due to the magnetic field. The sensitivity of the CLR to the magnetic field decreased as the fiber-beam angle decreased.

Conclusion: The measured spectral response, shape, and CLR were found to be sensitive to the magnetic field strength and polarity. These variations in spectral response, shape, and the CLR were unique to each fiber. If care is not taken, these variations in the stem-effect can significantly impact the noise-subtracted scintillator response in the presence of a magnetic field.


Scintillators, Magnetic Fields, Dosimetry


Not Applicable / None Entered.

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