Microscopic dose measurement with thin radiophotoluminescence glass plate
Introduction
Silver-activated phosphate glass is well known as a radiophotoluminescence (RPL) material (Yokota, 1969). The RPL glass is widely used as an accumulation-type solid state dosimeter for individual and environmental dosimetry, which has excellent radiation sensitivity and reliable repeatability for radiation dose measurement. When ionizing radiations produce electron-hole pairs in the RPL glass, stable-state Ag0 centers are formed by electron trapping in the Ag+ site. In addition, Ag+ centers are transformed into Ag++ centers through hole trapping in the PO4 site. Both the Ag0 and Ag++ centers are photoluminescent centers and they emit orange lights following exposure to UV light. However, the photoluminescence has two different components (Ihara et al., 2008). The intrinsic photoluminescence (IPL) of ∼0.3 μs short-lifetime around 450 nm in wavelength is related to the pre-dose. The photoluminescence of ∼4 μs lifetime around 650 nm is linked to radiation dose (RPL). It is possible to determine the dose by discriminating the RPL from the photoluminescence in the time and the wavelength. Therefore, the lowest detectable dose and the spatial resolving power obviously depend on the discrimination technique of RPL readout system.
Moreover, it is not easy to accurately evaluate doses of low-energy X-rays or energetic charged particles because these are absorbed in micrometer-order surface layer of RPL materials. A conventional RPL readout system is not applicable to the microscopic dose detection due to IPL from whole bulk degrades the S/N ratio of RPL intensity. Therefore, a microscopic RPL readout system and an appropriate glass detector are required for microscopic dose measurement for low-energy X-rays or energetic charged particles.
The aim of this study is to develop thin RPL glass plates for the detection of low-energy X-rays or energetic charged particles. Handing of RPL glass materials is easier than other dosimeter materials owing to the low melting point and the appropriate mechanical hardness. Thin RPL glass plates below 200 μm in thickness were successfully fabricated by melting and polishing processing. The property of thin RPL glass plates was investigated by microscopic dose measurement system based on a laser scanning microscope and a photon counting detector. Then, RPL dose measurements for low-energy X-rays and alpha-rays were performed with the thin RPL glass plates.
Section snippets
Microscopic RPL readout system
Fig. 1 shows a layout of microscopic RPL measurement by the use of laser scanning microscope. The detail of the RPL measurement system has been reported in the paper (Maki et al., 2011). The RPL measurement system was composed mainly of an inverted-type microscope (I81, Olympus), a continuous waveform ultraviolet laser diode (BCL-005-375, Crystal Laser), an XY movable stage, photon-counting circuits with a photomultiplier tube (PMT, R1894, Hamamatsu), a photon spectrometer (PMA-10, Hamamatsu
Spatial resolving power
Fig. 3 shows an example of RPL image drawn by the electron beam lithography system. For low-energy electron, it has been estimated from an electron transport simulation (Sempau et al., 1997) and the condition of electron beam and that the absorbed dose on the surface of the glass sample was about 10 mGy. The electron beam size was much smaller than the widths of lines and gaps on the test pattern. There were the bright lines scanned by the electron beam and some bleary areas of the narrow gaps
Conclusions
Thin RPL glass plates were fabricated by melting and polishing processes. The property of thin RPL glass plates was investigated by an RPL readout system based on a laser scanning microscope and a photon counting. The spatial resolving power of RPL image was estimated to be about 3 μm by electron beam lithography. The absorbed doses for low-energy X-rays were successfully measured with a set of thin RPL glass plates. The RPL intensity was probably in agreement with the absorbed dose calculated
Acknowledgments
The authors sincerely thank N. Zushi of Osaka University for their valuable suggestions in electron beam lithography. This work was partially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology.
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