A method to remove residual signals in fibre optic luminescence dosimeters

Abstract

Whenever a fibre optic is used to convey a light signal through a radiation field, it is likely that an unwanted background signal will arise from Cerenkov or fluorescent light which will contaminate the signal. In luminescence dosimetry of high energy beams, when a fibre optic is used to convey the signal from the radiation field to the detector, Cerenkov light is the dominant contributor to the background signal and must be corrected for. In this work, a novel method is demonstrated to separate the signal from the unwanted background. A remotely operated shutter is used to block the signal, allowing the residual background in the fibre optic to be quantified. This background is subtracted from the total measurement acquired in a subsequent irradiation, enabling the luminescence signal to be extracted. Two types of shutter mechanism are considered: an electro-mechanical device to intercept the light path and an LCD device to block the light by cross-polarization. Both shutters were characterized and incorporated into a fibre optic dosimetry system used to measure the radiation dose produced by external beam radiation linear accelerators. The dosimeter using each of the shutters in turn was exposed to a 6 MV photon beam to determine their performance, including the measurement of field size dependent output factors. The mechanical shutter determined the output factors to within 0.29% of those measured with an ionization chamber, whereas the LCD shutter gave results that deviated by up to 2.4%. The switching precision of both shutters was good with standard deviations of less than 0.25% and both were able to completely block the light signal when closed. The use of shutters could therefore be applied to any fibre optic based system to quantify and remove a reproducible background arising from any source including ambient, fluorescent and Cerenkov light.

Dosimetría por fibra óptica basada en YVO4 : Eu3+

“…La rigidez mecánica que presenta este diseño genera dos inconvenientes: cierta pérdida de la robustez y facilidad en el uso del equipo, algo importante para su empleo clínico rutinario, y la imposibilidad de utilizarlo de manera intracavitaria. Además, la señal es fuertemente atenuada en la región de aire, por lo que la sensibilidad del detector disminuye [Lambert et al, 2008, Lee et al, 2013.…”

Twisted pair of optic fibers for background removal in radiation fields

“…A disadvantage of the spectral method is the vulnerability to changes in the light spectrum that may be associated with spectral changes arising from varying attenuation of light in the optic fiber or a varying relative contribution of fluorescent and Cerenkov light [16]. A disadvantage of the air core method is the presence of a residual Cerenkov background generated in the readout fiber [17].…”

“…Lee et al [17] report the residual Cerenkov background signal to be 0.5% of the Cerenkov background signal that would have been generated in the absence of an air core. However, depending on factors such as the size of the scintillator and the efficiency of light transport in the air core, the residual Cerenkov signal can still be a significant fraction of the total measured signal.…”

“…The residual Cerenkov signal has previously been subtracted by one of two methods that enable measurement of the Cerenkov signal in the absence of a scintillation signal. The first method is the removal of the scintillator [13], and the second is the use of a shutter to interrupt the light path [17]. While both of those methods provide accurate dose measurements, they require a second irradiation.…”