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Abstract
Ionic copper-doped silica glasses are attractive materials for radiation detection and dosimetry based on radioluminescence (RL) and optically stimulated luminescence (OSL). We characterized the optical responses, under X-rays, of a Cu+-doped silica glass rod prepared via the sol-gel technique and spliced to transport pure silica optical fibers. Both the low-dose and the low-dose rate regimes were specially investigated at room temperature. RL and photoluminescence (PL) measurements confirmed that the Cu+ ions are the luminescent centers of interest for the targeted applications. RL dose rate response dependence shows a linear behavior from 260 μGy(SiO2)/s up to 23 mGy(SiO2)/s, allowing the monitoring of the dose rate evolutions during an irradiation run. The OSL response also linearly depends on the dose from 42 mGy(SiO2) to 200 Gy(SiO2), enabling a precise dose measurement shortly after the irradiation. The presented results confirm the potentialities of this material to monitor ionizing radiations in harsh environments.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The monovalent copper ions incorporated in various matrices have attracted attention of scientists from different fields due to their significant light emission in blue and green domains, especially when excited by ionizing radiation. Indeed, the optical properties of glasses and crystals doped with copper have been widely studied. By measuring the optically stimulated luminescence (OSL) and the thermo-luminescence (TL) responses of copper-doped alkali fluoro-silicates [1], lithium silicates [2] or lithium orthophosphate [3], it has been shown that their sensitivity is comparable to the one of reduced alumina (Al2O3:C). Other research teams have used co-doping with copper in their glass composition: for example, a zinc lithium borate glass system, doped with CuO and co-doped with Na2O, shows good dosimetry properties [4]. In addition, Mg-, Cu-, P-doped NaF phosphors were also studied to analyze the effect of the concentrations of these impurities on their TL and OSL properties. A linear and stable OSL response was obtained for one of these samples in the 10 – 1000 mGy dose range under a beta irradiation source 90Sr / 90Y [5].
Moreover, given the interest of pure silica glasses and in view of their radiation hardness, copper-doped quartz glasses were prepared in 1999 [6] by introducing the Cu+ ions into quartz rods using thermal diffusion. These samples were first investigated for their OSL response under γ-rays. The dose dependence of the OSL response, measured in the range from 0.1 mGy to 2 Gy, was found to be linear only between 0.5 mGy and 2 Gy. However, this material was subsequently implemented in an OSL optical fibered dosimetry system that exhibited linear OSL and RL responses over the 0.01-10 Gy dose range [7]. It has been shown that this system reacts identically to photons and electrons beams in the 4-20 MV acceleration voltage range. Another work was carried out with the same active material attached to the end of a transport optical fiber and using gated detection of the RL to eliminate the Čerenkov signal [8]. In this case, the irradiation source was a machine, used for radiotherapy, that delivered 6 MeV X-rays. The results show a linear scintillation response between 1 and 500 cGy, which was independent of the dose rate between 1 and 6 Gy.min-1.
Other studies, carried out on Cu-doped fused silica glasses obtained by immersion in a suspension of CuF2, showed a linear OSL response between 100 mGy and 5 Gy after γ-ray irradiation with a 90Sr/90Y source [9]. Finally, chemically synthesized Cu-doped polycrystalline SiO2, where the doping was achieved by sprinkling of the Cu solution on the dried SiO2 powder, has shown an OSL sensitivity 2.3 times higher than Al2O3:C [10].
In this study, the potential of a fibred dosimeter system, built with a Cu+-doped sol-gel-derived silica glass rod, to measure low doses and dose rates of X-rays has been assessed for the nuclear domain, where valuable information on low-dose areas is imposed. Such a system is suitable for environmental measurement requiring monitoring over a long distance, so as in nuclear power plants to control nuclear activities, for safety and radioprotection of workers, in recycling and dismantling operations, in long-term nuclear storage and industrial process controls.
2. Experimental
Copper-doped glassy rod was synthetized by the sol-gel technique as described in our previous paper [11]. The doped glass was then drawn down to a millimeter-sized cane and a small portion (of around 1 cm) was fusion-spliced, at its two ends, to a 5 m-long multimode pure silica fiber having a core diameter of 0.5 mm and coated with a low refractive index clad. The RL or OSL signal at room temperature was guided toward a photomultiplier (PMT, H9305-03 Hamamatsu) using one of these fibers. For the detection, the PMT module was connected to an oscilloscope (500 MHz band pass, Tektronix). A bandpass spectral filter at 550 ± 40 nm was used to select the OSL signal and remove the stimulating light of a 660 nm laser diode. The stimulation light was guided through the second fiber to the doped glassy rod while evaluating the OSL signal. This light was delivered, through a microscope objective, by a fibered laser diode at 660 nm, using its highest power of 51 mW. The external X-ray beam was delivered by the MOPERIX facility of Laboratoire Hubert Curien. The machine was operated at 100 kV with a current of 45 mA, generating photons of ∼40 keV average energy. An ionization chamber was used to measure the dose rates at various locations. Figure 1 shows the experimental setup used to characterize the OSL response of the glassy rod under X-ray beam. RL measurements were performed using the same setup, except that the spectral filter was removed and the laser diode was switched-off.
RL spectrum was recorded by connecting the fiber to a spectrometer (QE Pro Ocean optics). The dark signal has been systematically subtracted from the RL spectra. Photoluminescence spectra were also recorded, in the same conditions, under a He-Cd UV laser probe excitation at 325 nm.
3. Results and discussion
3.1 RL vs. PL
Figure 2 shows the X-ray emission spectrum (RL) and the photoluminescence spectrum under laser excitation at 325 nm. Both spectra show a broad emission band peaking around 2.21 eV (560 nm) and 2.24 eV (553 nm) with a full width at half-maximum of 0.51 and 0.35 eV, for PL and RL, respectively. The spectra maximal positions between 550 and 560 nm can be attributed to the transition from 3d94s1 excited state to the 3d10 ground state of Cu+ ions [12,13]. However the different line shapes and the slight red-shift of the maximum in the case of the PL emission lead us to believe in different excitation-emission mechanisms, probably involving defects states when the excitation energy is lower than the matrix bandgap.
Figure 3 shows the decomposition into three Gaussian bands of both radioluminescence (Fig. 3(a)) and photoluminescence (Fig. 3(b)) spectra. The first band peaking at about 1.98 eV is attributed to the emission of non-bridging oxygen hole centers (NBOHC). In such silica material, NBOHC could be generated during the rod drawing process [14,15] as well as during the irradiation run. The other two bands peaking at 2.21 eV and 2.28 eV are attributed to monovalent copper ions (Cu+) emissions [12]. The ratio of the area of NBOHC band over the total area is 10% and 23% in the case of the RL and the PL respectively. This result suggests that under UV excitation, the NBOHC defects are more directly excited than under X-rays.
3.2 RL measurements
Figure 4 shows the typical RL signal evolution versus time. As the X irradiation starts, the RL signal increases for a few seconds to finally tend toward a plateau.
Fig. 4. Typical RL response of the Cu+-doped rod spliced to the transport fiber during an X-ray irradiation run.
Figure 5 shows the evolution of the RL maximum signal of the Cu-doped glass versus dose rate. The presented data are taken on the plateau after a background subtraction. Only the low-dose rate measurements have been reported here. A linearity of the response has thus been highlighted at least up to 23 mGy/s, the dosimeter being sensitive to dose rates as low as 256 μGy/s. To check the repeatability of the response, the measurements were performed several times under the same conditions, leading to at most 7% variations of the output average voltage.
Figure 6(a) shows the different radioluminescence spectra obtained for several dose rates ranging from 17 to 783 mGy/s. All the spectra show an identical broad emission band centered at 544 nm (2.28 eV) with a full width at half maximum of 0.37 eV. The evolution of the maximum of the bands is linear with the dose rate up to at least 783 mGy/s (Fig. 6(b)).
Fig. 6. (a) X-ray RL spectra under different dose rates. (b) Maximum RL intensity evolution plotted against the dose rate for the Cu-doped rod.
3.3 OSL measurements
Figure 7 presents the OSL decay curves for the Cu+-doped sample under stimulation at 660 nm, recorded almost immediately (2 s) after X-ray exposures at a dose rate of 171 mGy/s (SiO2) for different irradiation durations (10 s, 30 s and 60 s), yielding different accumulated doses (1.7 Gy, 5.1 Gy and 10.2 Gy). It is noticeable that, about 10 s after the beginning of the laser stimulation, almost all the trapped carriers have been released, independently of the accumulated dose.
Fig. 7. OSL decay curves for different X-ray exposure times at a dose rate of 171 mGy/s (silica) during the stimulation with a laser diode at λ = 660 nm.
Figure 8(a) shows the OSL response against the accumulated dose. This response is defined as the integral over the whole decay time interval until the stabilization of the signal, namely well after 20 s. The noise background and the offset due to the residual stimulating laser signal have been subtracted before integration. Figure 8(b) more particularly shows the low-dose range of the same data. The copper-doped rod exhibits a linear OSL response, independently of the dose rate, from 42 mGy to 200 Gy in silica. A sublinear behavior can be observed beyond this upper limit.
Fig. 8. (a) Dose dependence of the OSL response of the Cu+-doped rod exposed to irradiation at different X-ray dose rates. Each point represents the normalized integrated OSL response. (b) Zoom on the low dose range of the same curve.
3.4 OSL kinetics
Figure 9 shows the OSL decay profiles of the Cu-doped rod after two exposure times (120 s and 1000 s) with a dose rate of 384 mGy/s, yielding respectively accumulated doses in and out of the linearity domain in Fig. 7(b). For all the deposited doses, a good fit of the OSL measured intensity versus time could be obtain using three exponential decay functions:
Fig. 9. OSL decay curves after X-ray irradiation at 46 Gy and 384 Gy accumulated doses with a 384 mGy/s dose rate.
Table 1. Different decay times of the OSL signal for three different doses
A non mono-exponential decay of the OSL signal is a signature of multiple trap levels involved in the OSL processes. The obtained values are comparable to those published by Barve et al. [9], who also fitted the OSL responses of their Cu-doped glasses stimulated by infrared light with three exponential functions. However, at this stage, we cannot assign each decay time to a specific and defined type of trap. This will require complementary experimental investigations (such as those from the TL) and a thorough theoretical study. As a second interpretation, re-trapping phenomena could also lead to OSL decay which could be fitted with a sum of exponential functions [1].
3.5 Fading of the OSL signal
The stability of the measured OSL signal with time, in both short and intermediate time scales, has also been studied. It is important to notice that such a comprehensive study is rarely reported in the literature. Figure 10 presents an example of this fading phenomenon of the OSL signal for an accumulated dose of 17 Gy. The data are normalized to the response obtained directly after the end of irradiation. For each measurement plot, the sample was kept in the dark and at ambient temperature for a given delay before stimulation. Due to thermal fading, a loss of about 50% of the OSL integrated intensity occurred in 1 hour approximately. After that time, the signal remains stable, which means that the spontaneous depopulation of the trapped levels seems to be completed. At first sight, this fading may appear very important, but in the nuclear applications intended for this system, the dose evaluation can be done within a few seconds, which implies a moderate decrease of the signal (10% after 10 s).
Fig. 10. Fading behavior of the integrated OSL signal for an accumulated dose of 17 Gy at a dose rate of 170 mGy/s. The red plot is reported as a guideline.
4. Conclusion
A fibered system has been designed to measure the RL and OSL responses of a sol-gel copper-doped silica rod under X-ray irradiations. We have shown that the RL signal of such device presents a linear behavior versus dose rate in a domain ranging from 0.26 to 800 mGy/s. Moreover, the OSL signal was found to be linear in the 42 mGy - 200 Gy dose range with a high reproducibility and independently of the dose rate. A comprehensive study of the fading of the OSL signal showed that a loss of 50% of the OSL signal has been recorded after about 1 hour without light stimulation. However, after that time, the OSL signal remains stable, which indicates that Cu-doped silica could also be used for longer time periods. All the results presented in this paper confirm the potential of this material for ionizing radiation dosimetry.
Funding
Agence Nationale pour la Gestion des Déchets Radioactifs (ANDRA); Agence Nationale de la Recherche (ANR) (ANR-11-EQPX-0017, ANR-11-LABX-0007); Ministère de l'Education Nationale, de l'Enseignement Superieur et de la Recherche (MESR) (CPER Photonics for Society P4S); Région Hauts-de-France (CPER Photonics for Society P4S); European Regional Development Fund (ERDF) (CPER Photonics for Society P4S).
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