Development of photon counting system with FPGA for precise measurement of radiophotoluminescence
Introduction
Silver-activated phosphate glass, of which radiophotoluminescence (RPL) has been investigated (Kirilova and Kozhukharov, 1995, Schneckenburger et al., 1981), is used as material of glass dosimeters because of its excellent characteristics of high sensitivity, low fading, and reliable repeatability in dose measurement. In dose reading, the photoluminescence (PL) component with a decay time shorter than ∼0.2 μs immediately after the pulse excitation is excluded, and the sequent tail component is integrated for about 10 μs soon after the PL component decays sufficiently. It is clear from previous papers (Piesch et al., 1986, Schneckenburger et al., 1981) that this component with a decay time of 2–5 μs originates mainly from RPL photons related with radiations.
With the aim of more precise dose measurement, we examined the response of optical components to a UV light pulse. The PL of optical components used for a dose reader becomes background noise. Therefore, the selection of low-noise optical components is important for the construction of a sensitive dose reader. In addition, the stabilization of the temperature of the UV light source and the RPL photon detector is important for keeping the sensitivity of a dose reader constant. This paper describes a compact and stable photon counting system for the RPL measurement of glass dosimeters. The photon counting system was made by using a UV LED, a photomultiplier tube (PMT), appropriate optical filters and electronic circuits based on a field programmable gate array (FPGA). At first this paper describes the construction of the new system and then shows results of some experiments with the system.
Section snippets
Apparatus
Fig. 1 shows a schematic drawing of the RPL photon counting system for glass dosimeters. With the aim of the stable RPL measurement, the temperature of the PMT (R7400U-02) and the UV LED (UVTOP280) was stabilized by Peltier devices, which were controlled by the feedback system with the FPGA. In this system, the single photon counting technique was used for low RPL intensity, while for high RPL intensity the current averaging technique was done for the avoidance of counting loss. The individual
Results and discussion
Fig. 3 shows measured results of the temperature effect on the UV LED and the PMT. The temperature coefficient for the light power of the LED was about −0.3%/°C and that for the gain of the PMT was about −0.4%/°C. It was confirmed from these that the LED and the PMT had negative temperature effects for the generation and the detection of RPL photons, respectively and eventually that the sensitivity of the RPL measurement system decreased with temperature. However, the temperature of the LED and
Conclusion
With the aim of the use in a room without temperature adjustment, we developed the compact and stable RPL photon counting system for glass dosimeters. The temperature of the UV LED and the PMT was stabilized by the Peltier devices which were controlled by the feedback system with the FPGA. Also, the low-noise optical components were selectively used for the construction of the stable dose reading system. It was confirmed that the present system with the homemade glass dosimeter had good
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