Dealing with Cerenkov radiation generated in organic scintillator dosimeters by bremsstrahlung beams

Abstract

An organic scintillator detector system has been developed for radiotherapy bremsstrahlung dosimetry. The scintillators are connected to photodiodes by light pipes as the photodiodes must be removed and shielded from the incident radiation. The photodiodes see visible and near-visible light emissions from the scintillator as well as Cerenkov and fluorescence radiation that has been generated and trapped in the scintillator and light pipe. The Cerenkov and fluorescence radiation limits the accuracy of the dosimeter. This work examines a range of methods for diminishing the signal contribution of Cerenkov and fluorescence radiation while optimizing the scintillator signal. Three methods of achieving these goals have been used. They are: reflective coatings on the scintillator, long-wavelength-emitting scintillators used in conjunction with the photodiode, and absorptive filters placed between the light guide and photodiode. The contribution of the Cerenkov radiation to the light seen by the photodiode has been modelled and the model predictions have been tested using bremsstrahlung beams of peak energy between 13 and 20 MV, showing agreement with measurement.

Plastic scintillation dosimetry and its application to radiotherapy

“…Issues concerning S/N ratio can be handled by subtraction of the background signal due to Cerenkov radiation. Other methods for removing the background signal include optical filtering (de Boer et al, 1993;Clift et al, 2000a;Fontbonne et al, 2002) and temporal avoidance (Clift et al, 2002;Justus et al, 2004). It is hoped that improvement in signal collection efficiency and reduction in background will make these detectors more widely utilized.…”

“…Using optical filters, de Boer et al (1993) were able to reduce Cerenkov emission by 50%, while Clift et al (2000a) obtained an 82% reduction.…”

“…This allows subtraction of the signals and is especially important for electron beam dosimetry. Other systems use a light pipe instead of fiber light guides and/or photodiodes instead of PMTs (Beddar, 1994;Létourneau et al, 1999;Clift et al, 2000a). Techniques other than direct subtraction of the background signal include optical filtration (de Boer et al, 1993;Clift et al, 2000a;Fontbonne et al, 2002) and temporal avoidance of the Cerenkov radiation signal (Clift et al, 2002;Justus et al, 2004).…”

Feasibility study for the use of Ce3+-doped optical fibres in radiotherapy

“…The irradiation of optical fibres with high-energy beams, like those employed in radiotherapy, produces a luminescence signal due to the Cherenkov effect [9,10]. Cherenkov radiation, generated when a charged particle passes through a medium of refractive index n with a velocity greater than that of light in the medium, may become a significant source of stem effect, especially in the case of electron irradiation [11,12].…”

Embedded structure fiber-optic radiation dosimeter for radiotherapy applications

“…As the scintillator is exposed to ionizing radiation, an optical signal is generated and this is guided by the optical fiber toward a detecting device placed remotely from the irradiation zone. Plastic scintillator dosimeters have many desirable dosimetric characteristics when compared with traditional detector systems, as extensively described in the literature [8][9][10][11][12][13][14][15][16][17]. However, plastic scintillator dosimeters have at least one shortcoming; a low signal-to-noise ratio (SNR) resulting from the Cerenkov radiation emission.…”