PERFORMANCE LIMITS OF PLASTIC SCINTILLATION DETECTOR CCD-BASED DOSIMETERS USING CURRENT TECHNOLOGY

Abstract

Purpose: A numerical model was developed that allows the calculation of the image signal-to-noise ratio (SNR) as a function of the system parameters for a plastic scintillation detector (PSD). The reason that SNR is interesting is that it is inversely related to the standard deviation in the spot Analog Digital Unit (ADU) counts (i.e. image pixel values) and therefore characterizes the measurement precision of the dosimeter as a function of the system parameters (scintillator volume, deposited dose, coupling efficiency to the CCD, etc.). The SNR has therefore the potential to become an important parameter in the design of PSDs. The SNR calculation model is used to explore the fundamental precision and spatial resolution limits in scintillation dosimetry. Delimiting the performance limits of PSDs will help position this technology amongst the other dosimeter technologies available. Delimiting the optimal application areas of PSDs might help drive the development of novel commercial PSD systems and speed up the adoption of this technology in the clinic. Materials: The SNR in based optoelectronic systems is given by 1, appropriately called the CCD equation [8]. Where is the photon fluence (photons pixel s), is the quantum efficiency of the photodetector, T is the integration time for one image in seconds, D is the variance of the dark noise (electrons) and is the variance of the readout noise (electrons). In general, the imaged object (the fiber) subtends a finite number of pixels (n). To obtain the SNR for a collection of pixels, a summation of the signal and noise for each pixel over n pixels must be performed. The number of pixels included in one optical spot will depend on the optical magnification of the objective lens used. The scintillation photon production rate is known for each Gy of dose deposited for most plastic scintillators [9]. The photon fluence rate on the can be calculated by taking into account all the losses suffered by the scintillation photons throughout the optical train (scintillator to fiber coupling loss, fiber attenuation loss, objective lens coupling loss). These losses are either provided by the component manufacturer, can be extracted from the scientific literature or can be calculated from geometrical optics considerations. Calculating the SNR for a specific PSD system is therefore fairly straightforward. SNR=ΦpηqT √ ΦpηpT+DT+N2 r