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Aluminum Nitride Ceramic as an Optically Stimulable Luminescence Dosimeter Plate

Aluminum Nitride Ceramic as an Optically Stimulable Luminescence Dosimeter Plate hv photonics Article Aluminum Nitride Ceramic as an Optically Stimulable Luminescence Dosimeter Plate 1 , 2 3 1 Go Okada *, Kentaro Fukuda , Safa Kasap and Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan; t-yanagida@ms.naist.jp Tokuyama Corporation, 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648, Japan; ken-fukuda@tokuyama.co.jp Department of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Dr., Saskatoon, SK S7N 5A9, Canada; safa.kasap@usask.ca * Correspondence: go-okada@ms.naist.jp; Tel.: +81-743-72-6144 Received: 4 April 2016; Accepted: 27 April 2016; Published: 30 April 2016 Abstract: Photostimulable storage phosphors have been used in a wide range of applications including radiation measurements in one- and two-dimensional spaces, called point dosimetry and radiography. In this work, we report that an aluminum nitride (AlN) ceramic plate, which is practically used as a heat sink (SHAPAL , Tokuyama Corp., Yamaguchi, Japan), shows good optically-stimulated luminescence (OSL) properties with sufficiently large signal and capability for imaging applications, and we have characterized the AlN plate for OSL applications. Upon interaction with X-rays, the sample color turns yellowish, due to a radiation-induced photoabsorption band in the UV-blue range below ~500 nm. After irradiating the sample with X-rays, an intense OSL emission can be observed in the UV (360 nm) spectral region during stimulation by red light. Although our measurement setup is not optimized, dose detection was confirmed as low as ~3 mGy to over 20 Gy. Furthermore, we have successfully demonstrated that the SHAPAL AlN ceramic plate has great potential to be used as an imaging plate in radiography. Keywords: AlN; OSL; dosimeter; X-rays 1. Introduction Ionizing radiations such as X-rays, gamma-rays, alpha-rays, etc., may be measured using various techniques. One of the common techniques is to use phosphors [1]. The phosphor materials have functionalities to convert radiation to light so that one can indirectly measure the radiation using conventional photodetectors such as charge-coupled device (CCD), complementary metal-oxide semiconductor (CMOS), and photomultiplier tube (PMT) [1]. Rapid light emission upon the absorption of radiation can be typically used for real-time measurements, and such phosphors are so-called scintillators [2–4]. On the other hand, another type of radiation phosphor can accumulate and store the incident dose information in the form of trapped charges, which can be read out optically or thermally. Such phosphors are commonly referred to as so-called storage phosphors or dosimeters. The latter materials are used in practice in a wide range of areas such as medical imaging [5,6], radiation therapies [7], personnel dose monitoring [8] and so on. The mechanisms involved in dosimeter materials are typically but intuitively understood as follows. First, a numerous number of electrons and holes are generated upon the absorption of an incident of ionizing radiation, and then some of those charges are captured by localized trapping centers, and the captured carriers remain trapped over the experimental time scale; the stored charge is stable. These trapped charges can be detrapped by an external stimulation in the form of, in general, heat or light. As a result, the freed charges then recombine at luminescent centers followed by a light Photonics 2016, 3, 23; doi:10.3390/photonics3020023 www.mdpi.com/journal/photonics Photonics 2016, 3, 23 2 of 7 emission. Since the emitted light output is proportional to the incident radiation dose, the stimulated light emission may be indirectly used as a probe of the incident radiation dose. Dosimeters which use heat to stimulate the signal are so-called thermally stimulated luminescent detectors (TLD) [9], while the ones which rely on an optical stimulation are referred to as optically stimulated luminescent detectors (OSLD) [10–13]. Examples of dosimeter materials used in practice include LiF:Mg,Ti [14], Al O :C [15,16], and BeO [17]. These materials do indeed show excellent properties for dose detection 2 3 such as high sensitivity and stability of dose information during storage. In addition, it is a key requirement for dosimeter materials to consist of light chemical compositions that are equivalent to tissue in order for the radiation to interact with the dosimeter material in the same way as it does for tissue, and the dose measurement accuracy is maintained to be high. Aluminum nitride (AlN) is of our current interest to be studied as a dosimeter material due to its useful properties. It has a wide optical band-gap (~6.2 eV) and is used in UV-emitting LEDs [18]. Also, it has high thermal conductivity and it is an electrical insulator; it is practically used as a heat sink. In terms of the interactions with X-rays, AlN is considered to be equivalent to tissue as the effective atomic number of AlN (11.7) is considerably low and very similar to well-accepted Al O -based 2 3 dosimeters. Earlier, we reported that an AlN ceramic plate (SHAPAL , Tokuyama Corp., Yamaguchi, Japan), which is in fact a commercial product as a heat sink, shows thermally-stimulated luminescence (TSL) properties and it has a good potential to be used as a TLD dosimeter [19]. In this work, in contrast, we have newly discovered that the SHAPAL AlN ceramic plate also exhibits optically-stimulated luminescence (OSL) properties. The aim of this work is to characterize the AlN plate as an OSL dosimeter material and to show its ability in practical use such as personnel dose monitoring and dose distribution measurements. Our experimental results are provided in this paper. 2. Experimental Procedure AlN ceramic plates studied in this work were manufactured by Tokuyama Corp., Yamaguchi, Japan, and these are, in fact, commercially available as SHAPAL for heat sink applications. The samples were then characterized at the Nara Institute of Science and Technology (NAIST) towards OSL dosimeter applications. The typical sample size used in the investigations was 15  10  0.65 mm . The optical transmittance spectrum was measured using a spectrophotometer (SolidSpec-3700, Shimadzu, Kyoto, Japan). The OSL emission spectra were measured using the Hamamatsu Quantaurus-Tau (C11367, Shizuoka, Japan). A spectrofluometer (FP-8600, JASCO, Tokyo, Japan) was used to study the OSL stimulation spectra and the afterglow of OSL emission. During the measurements, a 340 nm band-pass filter was inserted at the detector side, and stimulation light was delivered through a 400 nm short-cut filter in order to avoid an interference of a second-order diffraction light. The samples were irradiated using an X-ray generator (Model XRBOP&N200X4550, Spellman, Hauppauge, NY, USA), which is equipped with a W anode target with the Be window. All the irradiations were carried out with the tube voltage of 40 kVp with the filament current of 5.2 mA. To demonstrate the AlN ceramic plate for use as an X-ray imaging plate, an original measurement setup was constructed in our laboratory. A large size AlN plate 59  38 mm was irradiated by X-rays through electronic integrated circuits. After the irradiation, the AlN plate was exposed to stimulation light at a wavelength of ~630  50 nm from a Xenon lamp (LAX-C100, Asahi Spectra, Tokyo, Japan), then the 2D OSL distribution was measured by a Peltier-cooled CCD camera (BK-54DUV, Bitran Corp., Saitama, Japan) equipped with an objective lens (PF10545MF-UV, Nikon, Tokyo, Japan). 3. Results and Discussion The AlN plate used in this research is illustrated in Figure 1 (left). These ceramics can be fabricated homogeneously over a relatively large area as can be confirmed in the photograph. The latter feature is of fundamental interest, especially for two-dimensional radiation imaging. The size of the AlN plate in the photograph is 59  38 mm with a thickness of 0.65 mm. As a commercial product (as a heat sink), it is currently available as large as ~120  120 mm but it is certainly possible to fabricate AlN in a larger Photonics 2016, 3, 23 3 of 7 Photonics 2016, 3, 23 3 of 7 area. The color of the AlN plate is light grey as provided. However, once it is irradiated by X-rays, it it is irradiated by X-rays, it turns into a yellowish color. The color change is, in fact, due to the turns into a yellowish color. The color change is, in fact, due to the appearance of photoabsorption appearance of photoabsorption bands which are strongly induced by the X-ray irradiation, and bands which are strongly induced by the X-ray irradiation, and appear in the UV-blue range (shorter appear in the UV-blue range (shorter than ~500 nm). The X-ray–induced absorbance spectra are than ~500 nm). The X-ray–induced absorbance spectra are illustrated in Figure 1 (right). A significant illustrated in Figure 1 (right). A significant absorption is observed in the range of 250–550 nm. The absorption is observed in the range of 250–550 nm. The induced absorbance increases as a function of induced absorbance increases as a function of the irradiated dose. We think that it is due to the the irradiated dose. We think that it is due to the common effect of radiation-induced color centers common effect of radiation-induced color centers created by the irradiation; however, detailed created by the irradiation; however, detailed studies are required in order to determine the origin of studies are required in order to determine the origin of these induced color centers. these induced color centers. Figure 1. (left) AlN ceramic plate (SHAPAL ) manufactured by Tokuyama Corp. (right) X-ray–induced Figure 1. (left) AlN ceramic plate (SHAPAL ) manufactured by Tokuyama Corp. (right) X-ray– absorbance as a function of X-ray dose irradiated. The inset is the integrated absorbance signal vs. the induced absorbance as a function of X-ray dose irradiated. The inset is the integrated absorbance corresponding dose. signal vs. the corresponding dose. After X-ray irradiation, an OSL emission is observed in the UV range when stimulated by red After X-ray irradiation, an OSL emission is observed in the UV range when stimulated by red light. The emission spectrum is illustrated in Figure 2. The irradiation dose was 20 Gy. The emission light. The emission spectrum is illustrated in Figure 2. The irradiation dose was 20 Gy. The emission signal has a broad spectral feature peaking around 360 nm, while the sample is stimulated by 630 nm. signal has a broad spectral feature peaking around 360 nm, while the sample is stimulated by 630 nm. Further, there seems to be a very small emission band with the peak around 505 nm, but the latter Further, there seems to be a very small emission band with the peak around 505 nm, but the latter emission is found to be negligibly small compared to the UV emission. It is instructive to mention emission is found to be negligibly small compared to the UV emission. It is instructive to mention here that, interestingly, the OSL was also observed after a UV (254 nm) irradiation. A similar emission here that, interestingly, the OSL was also observed after a UV (254 nm) irradiation. A similar emission was previously observed at ~340 nm in radioluminescence [19] and in cathodoluminescence [20–22]; was previously observed at ~340 nm in radioluminescence [19] and in cathodoluminescence [20–22]; however, as far as we are aware, the origin of this emission has not yet been fully identified. Figure however, as far as we are aware, the origin of this emission has not yet been fully identified. 2 also illustrates the stimulation spectrum measured while monitoring the 360 nm emission. The Figure 2 also illustrates the stimulation spectrum measured while monitoring the 360 nm emission. observed stimulation spectrum suggests that the OSL can be stimulated by light in a wide spectral The observed stimulation spectrum suggests that the OSL can be stimulated by light in a wide spectral range from approximately 550 nm to over 800 nm. The stimulated spectrum curve has some range from approximately 550 nm to over 800 nm. The stimulated spectrum curve has some noticeable noticeable features. Particularly around 800 nm, the OSL stimulation is the most efficient among the features. Particularly around 800 nm, the OSL stimulation is the most efficient among the spectral spectral region tested. This indicates that the AlN has a large number of trapping center groups with region tested. This indicates that the AlN has a large number of trapping center groups with different different trapping depths. In the previous work, considerably shallow trapping centers were trapping depths. In the previous work, considerably shallow trapping centers were presented by presented by TSL [19]; however, the OSL stimulation spectrum suggests a presence of deeper-but- TSL [19]; however, the OSL stimulation spectrum suggests a presence of deeper-but-stimulable charge stimulable charge trapping centers, especially those around 600 nm (~2.0 eV). trapping centers, especially those around 600 nm (~2.0 eV). Figure 3 shows a typical OSL decay curve of AlN ceramic, that is the transient OSL signal Figure 3 shows a typical OSL decay curve of AlN ceramic, that is the transient OSL signal intensity during stimulation as a function of time. The stimulation light used here is 610 nm, and the intensity during stimulation as a function of time. The stimulation light used here is 610 nm, and measured OSL emission signal is at 340 nm. The decay curve can be decomposed into a sum of two the measured OSL emission signal is at 340 nm. The decay curve can be decomposed into a sum exponential decay functions. The derived time constants are 20 and 155 s. Since the decay curve can of two exponential decay functions. The derived time constants are 20 and 155 s. Since the decay be decomposed well by only two exponential decay functions, this fact suggests that there are two curve can be decomposed well by only two exponential decay functions, this fact suggests that groups of charge trapping centers mainly responsible for the OSL process involved here. there are two groups of charge trapping centers mainly responsible for the OSL process involved Furthermore, the OSL curve in the shorter time range of 0–100 s, shown in the inset, clearly illustrates here. Furthermore, the OSL curve in the shorter time range of 0–100 s, shown in the inset, clearly a transient increase of OSL signal between 0–3 s, which is followed by an exponential decay. This illustrates a transient increase of OSL signal between 0–3 s, which is followed by an exponential decay. transition behavior suggests the presence of shallow trapping centers [11]. In fact, our observation agrees with a thermoluminescence study reported earlier [19] that there exists a glow peak located at Photonics 2016, 3, 23 4 of 7 This transition behavior suggests the presence of shallow trapping centers [11]. In fact, our observation Photonics 2016, 3, 23 4 of 7 Photonics 2016, 3, 23 4 of 7 agrees with a thermoluminescence study reported earlier [19] that there exists a glow peak located at a considerably a considerabl low y lotemperatur w tempera e, ture ~80 , ~C 80 . Detailed °C. Detaanalyses iled analar ys ees beyond are beyo the n scope d the ofsc this opepaper; of thihowever s paper;, a considerably low temperature, ~80 °C. Detailed analyses are beyond the scope of this paper; we howe plan veron , we further plan o investigations n further investi in ga order tionto s in understand order to un the dermechanism stand the minvolved. echanism involved. however, we plan on further investigations in order to understand the mechanism involved. Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation at the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation at 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. at 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at 610 nm Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at 610 nm Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at with a constant light intensity. The OSL curve can be approximated mainly by a sum of two with a constant light intensity. The OSL curve can be approximated mainly by a sum of two 610 nm with a constant light intensity. The OSL curve can be approximated mainly by a sum of exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after the exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after the two exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after stimulation started. stimulation started. the stimulation started. The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates the The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates the dose-response curve obtained when tested with X-rays. We have experimentally confirmed a linear The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates dose-response curve obtained when tested with X-rays. We have experimentally confirmed a linear response over four orders of magnitude without any saturation of the OSL signal. It is worth the dose-response curve obtained when tested with X-rays. We have experimentally confirmed a response over four orders of magnitude without any saturation of the OSL signal. It is worth mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper linear response over four orders of magnitude without any saturation of the OSL signal. It is worth mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully measure the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully measure measure the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean that the AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean that the that the AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the sensitivity of our experimental readout setup. The measurement capability of small doses is AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the sensitivity sensitivity of our experimental readout setup. The measurement capability of small doses is especially important for personnel dosimetry or medical imaging applications, while a dosimeter of our experimental readout setup. The measurement capability of small doses is especially important especially important for personnel dosimetry or medical imaging applications, while a dosimeter capable of large doses may find some applications such as radiation therapy [23–26]. capable of large doses may find some applications such as radiation therapy [23–26]. Photonics 2016, 3, 23 5 of 7 for personnel dosimetry or medical imaging applications, while a dosimeter capable of large doses may find some applications such as radiation therapy [23–26]. Photonics 2016, 3, 23 5 of 7 Figure 4. Experimental demonstrations of SHAPAL® AlN ceramic plate for use as an OSL dosimeter. Figure 4. Experimental demonstrations of SHAPAL AlN ceramic plate for use as an OSL dosimeter. (left) Dose response curve of OSL in an AlN ceramic plate. The stimulation light wavelength is 610 nm (left) Dose response curve of OSL in an AlN ceramic plate. The stimulation light wavelength is 610 nm while the emission light is 340 nm; (right) X-ray image of some electronic components taken by OSL of while the emission light is 340 nm; (right) X-ray image of some electronic components taken by OSL AlN ceramic plate. The stimulation light used here is 630 nm  50 nm. The electronic components are of AlN ceramic plate. The stimulation light used here is 630 nm ± 50 nm. The electronic components (from left to right): logic gate, field effect transistor, bipolar transistor, and operational amplifier. are (from left to right): logic gate, field effect transistor, bipolar transistor, and operational amplifier. Figure 4 (right) demonstrates an X-ray image taken on some electronic components using the Figure 4 (right) demonstrates an X-ray image taken on some electronic components using SHAPAL AlN ceramic plate. In the image, some electrodes and wiring hidden in the epoxy the SHAPAL AlN ceramic plate. In the image, some electrodes and wiring hidden in the epoxy encapsulations are clearly observed. Although the setup is not fully optimized for high resolution encapsulations are clearly observed. Although the setup is not fully optimized for high resolution imaging, our demonstration has shown that the legs of the bipolar transistor (the second part from imaging, our demonstration has shown that the legs of the bipolar transistor (the second part from the the right-hand side in the image), which has the width of 0.5 mm, are well resolved, and it indicates right-hand side in the image), which has the width of 0.5 mm, are well resolved, and it indicates the the spatial resolution is at least equivalent or better than 0.5 mm. In addition, the uniformity of the spatial resolution is at least equivalent or better than 0.5 mm. In addition, the uniformity of the image image was tested with an image of a uniformly irradiated sample. The standard deviation was was tested with an image of a uniformly irradiated sample. The standard deviation was calculated to calculated to be 23.90 counts. This low value is mainly because of insufficient sensitivity of the CCD be 23.90 counts. This low value is mainly because of insufficient sensitivity of the CCD camera in the camera in the present measurement. A technique such as a flying-spot reader [5] should improve the present measurement. A technique such as a flying-spot reader [5] should improve the signal-to-noise signal-to-noise ratio since a photomultiplier tube has much better sensitivity to UV light. Overall, we ratio since a photomultiplier tube has much better sensitivity to UV light. Overall, we think that the think that the AlN ceramic plate has good potential to be used as an imaging plate for X-ray imaging AlN ceramic plate has good potential to be used as an imaging plate for X-ray imaging applications. applications. 4. Conclusions 4. Conclusions In this research, we have discovered that an AlN ceramic plate (SHAPAL , Tokuyama Co, In this research, we have discovered that an AlN ceramic plate (SHAPAL , Tokuyama Co, Yamaguchi, Japan) shows OSL, and we have investigated the OSL properties. The AlN ceramic plate Yamaguchi, Japan) shows OSL, and we have investigated the OSL properties. The AlN ceramic plate is, in fact, designed and manufactured for heat sink applications; however, it has good potential to is, in fact, designed and manufactured for heat sink applications; however, it has good potential to be used as an OSL storage phosphor for one- and two-dimensional dosimetry applications. By X-ray be used as an OSL storage phosphor for one- and two-dimensional dosimetry applications. By X-ray irradiation, the AlN ceramic plate turns in from a grey to yellowish color due to an X-ray–induced irradiation, the AlN ceramic plate turns in from a grey to yellowish color due to an X-ray–induced absorbance band appearing in the UV-blue range. With optical stimulation, an OSL emission can absorbance band appearing in the UV-blue range. With optical stimulation, an OSL emission can be be observed in the UV region around 360 nm with a broad spectral feature. The OSL signal can be observed in the UV region around 360 nm with a broad spectral feature. The OSL signal can be stimulated by light over a wide range of the spectrum from 400 to over 700 nm, which is a distinct stimulated by light over a wide range of the spectrum from 400 to over 700 nm, which is a distinct advantage. The OSL decay curve is a double-exponential, yielding two time constants (20 and 155 s). advantage. The OSL decay curve is a double-exponential, yielding two time constants (20 and 155 s). However, a transient increase behavior is observed in the OSL immediately after the stimulation However, a transient increase behavior is observed in the OSL immediately after the stimulation is is given until ~3 s, followed by the exponential decay. This observation suggests the presence of given until ~3 s, followed by the exponential decay. This observation suggests the presence of shallow shallow trapping centers in addition to the main dosimetric traps. The OSL dose response in the AlN trapping centers in addition to the main dosimetric traps. The OSL dose response in the AlN ceramic ceramic plate has been demonstrated over 3 mGy–10 Gy. A two-dimensional dosimetry has also been plate has been demonstrated over 3 mGy–10 Gy. A two-dimensional dosimetry has also been successfully demonstrated with an imaging capability. successfully demonstrated with an imaging capability. Acknowledgments: This research was co-supported by a Grant-in-Aid for Scientific Research (A) (26249147), Grant-in-Aid for Research Activity start-up (15H06409), and Green Photonics Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). It is also partially supported by the Adaptable and Seamless Technology transfer Program (A-STEP) by the Japan Science and Photonics 2016, 3, 23 6 of 7 Acknowledgments: This research was co-supported by a Grant-in-Aid for Scientific Research (A) (26249147), Grant-in-Aid for Research Activity start-up (15H06409), and Green Photonics Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). 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Aluminum Nitride Ceramic as an Optically Stimulable Luminescence Dosimeter Plate

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10.3390/photonics3020023
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hv photonics Article Aluminum Nitride Ceramic as an Optically Stimulable Luminescence Dosimeter Plate 1 , 2 3 1 Go Okada *, Kentaro Fukuda , Safa Kasap and Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan; t-yanagida@ms.naist.jp Tokuyama Corporation, 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648, Japan; ken-fukuda@tokuyama.co.jp Department of Electrical and Computer Engineering, University of Saskatchewan, 57 Campus Dr., Saskatoon, SK S7N 5A9, Canada; safa.kasap@usask.ca * Correspondence: go-okada@ms.naist.jp; Tel.: +81-743-72-6144 Received: 4 April 2016; Accepted: 27 April 2016; Published: 30 April 2016 Abstract: Photostimulable storage phosphors have been used in a wide range of applications including radiation measurements in one- and two-dimensional spaces, called point dosimetry and radiography. In this work, we report that an aluminum nitride (AlN) ceramic plate, which is practically used as a heat sink (SHAPAL , Tokuyama Corp., Yamaguchi, Japan), shows good optically-stimulated luminescence (OSL) properties with sufficiently large signal and capability for imaging applications, and we have characterized the AlN plate for OSL applications. Upon interaction with X-rays, the sample color turns yellowish, due to a radiation-induced photoabsorption band in the UV-blue range below ~500 nm. After irradiating the sample with X-rays, an intense OSL emission can be observed in the UV (360 nm) spectral region during stimulation by red light. Although our measurement setup is not optimized, dose detection was confirmed as low as ~3 mGy to over 20 Gy. Furthermore, we have successfully demonstrated that the SHAPAL AlN ceramic plate has great potential to be used as an imaging plate in radiography. Keywords: AlN; OSL; dosimeter; X-rays 1. Introduction Ionizing radiations such as X-rays, gamma-rays, alpha-rays, etc., may be measured using various techniques. One of the common techniques is to use phosphors [1]. The phosphor materials have functionalities to convert radiation to light so that one can indirectly measure the radiation using conventional photodetectors such as charge-coupled device (CCD), complementary metal-oxide semiconductor (CMOS), and photomultiplier tube (PMT) [1]. Rapid light emission upon the absorption of radiation can be typically used for real-time measurements, and such phosphors are so-called scintillators [2–4]. On the other hand, another type of radiation phosphor can accumulate and store the incident dose information in the form of trapped charges, which can be read out optically or thermally. Such phosphors are commonly referred to as so-called storage phosphors or dosimeters. The latter materials are used in practice in a wide range of areas such as medical imaging [5,6], radiation therapies [7], personnel dose monitoring [8] and so on. The mechanisms involved in dosimeter materials are typically but intuitively understood as follows. First, a numerous number of electrons and holes are generated upon the absorption of an incident of ionizing radiation, and then some of those charges are captured by localized trapping centers, and the captured carriers remain trapped over the experimental time scale; the stored charge is stable. These trapped charges can be detrapped by an external stimulation in the form of, in general, heat or light. As a result, the freed charges then recombine at luminescent centers followed by a light Photonics 2016, 3, 23; doi:10.3390/photonics3020023 www.mdpi.com/journal/photonics Photonics 2016, 3, 23 2 of 7 emission. Since the emitted light output is proportional to the incident radiation dose, the stimulated light emission may be indirectly used as a probe of the incident radiation dose. Dosimeters which use heat to stimulate the signal are so-called thermally stimulated luminescent detectors (TLD) [9], while the ones which rely on an optical stimulation are referred to as optically stimulated luminescent detectors (OSLD) [10–13]. Examples of dosimeter materials used in practice include LiF:Mg,Ti [14], Al O :C [15,16], and BeO [17]. These materials do indeed show excellent properties for dose detection 2 3 such as high sensitivity and stability of dose information during storage. In addition, it is a key requirement for dosimeter materials to consist of light chemical compositions that are equivalent to tissue in order for the radiation to interact with the dosimeter material in the same way as it does for tissue, and the dose measurement accuracy is maintained to be high. Aluminum nitride (AlN) is of our current interest to be studied as a dosimeter material due to its useful properties. It has a wide optical band-gap (~6.2 eV) and is used in UV-emitting LEDs [18]. Also, it has high thermal conductivity and it is an electrical insulator; it is practically used as a heat sink. In terms of the interactions with X-rays, AlN is considered to be equivalent to tissue as the effective atomic number of AlN (11.7) is considerably low and very similar to well-accepted Al O -based 2 3 dosimeters. Earlier, we reported that an AlN ceramic plate (SHAPAL , Tokuyama Corp., Yamaguchi, Japan), which is in fact a commercial product as a heat sink, shows thermally-stimulated luminescence (TSL) properties and it has a good potential to be used as a TLD dosimeter [19]. In this work, in contrast, we have newly discovered that the SHAPAL AlN ceramic plate also exhibits optically-stimulated luminescence (OSL) properties. The aim of this work is to characterize the AlN plate as an OSL dosimeter material and to show its ability in practical use such as personnel dose monitoring and dose distribution measurements. Our experimental results are provided in this paper. 2. Experimental Procedure AlN ceramic plates studied in this work were manufactured by Tokuyama Corp., Yamaguchi, Japan, and these are, in fact, commercially available as SHAPAL for heat sink applications. The samples were then characterized at the Nara Institute of Science and Technology (NAIST) towards OSL dosimeter applications. The typical sample size used in the investigations was 15  10  0.65 mm . The optical transmittance spectrum was measured using a spectrophotometer (SolidSpec-3700, Shimadzu, Kyoto, Japan). The OSL emission spectra were measured using the Hamamatsu Quantaurus-Tau (C11367, Shizuoka, Japan). A spectrofluometer (FP-8600, JASCO, Tokyo, Japan) was used to study the OSL stimulation spectra and the afterglow of OSL emission. During the measurements, a 340 nm band-pass filter was inserted at the detector side, and stimulation light was delivered through a 400 nm short-cut filter in order to avoid an interference of a second-order diffraction light. The samples were irradiated using an X-ray generator (Model XRBOP&N200X4550, Spellman, Hauppauge, NY, USA), which is equipped with a W anode target with the Be window. All the irradiations were carried out with the tube voltage of 40 kVp with the filament current of 5.2 mA. To demonstrate the AlN ceramic plate for use as an X-ray imaging plate, an original measurement setup was constructed in our laboratory. A large size AlN plate 59  38 mm was irradiated by X-rays through electronic integrated circuits. After the irradiation, the AlN plate was exposed to stimulation light at a wavelength of ~630  50 nm from a Xenon lamp (LAX-C100, Asahi Spectra, Tokyo, Japan), then the 2D OSL distribution was measured by a Peltier-cooled CCD camera (BK-54DUV, Bitran Corp., Saitama, Japan) equipped with an objective lens (PF10545MF-UV, Nikon, Tokyo, Japan). 3. Results and Discussion The AlN plate used in this research is illustrated in Figure 1 (left). These ceramics can be fabricated homogeneously over a relatively large area as can be confirmed in the photograph. The latter feature is of fundamental interest, especially for two-dimensional radiation imaging. The size of the AlN plate in the photograph is 59  38 mm with a thickness of 0.65 mm. As a commercial product (as a heat sink), it is currently available as large as ~120  120 mm but it is certainly possible to fabricate AlN in a larger Photonics 2016, 3, 23 3 of 7 Photonics 2016, 3, 23 3 of 7 area. The color of the AlN plate is light grey as provided. However, once it is irradiated by X-rays, it it is irradiated by X-rays, it turns into a yellowish color. The color change is, in fact, due to the turns into a yellowish color. The color change is, in fact, due to the appearance of photoabsorption appearance of photoabsorption bands which are strongly induced by the X-ray irradiation, and bands which are strongly induced by the X-ray irradiation, and appear in the UV-blue range (shorter appear in the UV-blue range (shorter than ~500 nm). The X-ray–induced absorbance spectra are than ~500 nm). The X-ray–induced absorbance spectra are illustrated in Figure 1 (right). A significant illustrated in Figure 1 (right). A significant absorption is observed in the range of 250–550 nm. The absorption is observed in the range of 250–550 nm. The induced absorbance increases as a function of induced absorbance increases as a function of the irradiated dose. We think that it is due to the the irradiated dose. We think that it is due to the common effect of radiation-induced color centers common effect of radiation-induced color centers created by the irradiation; however, detailed created by the irradiation; however, detailed studies are required in order to determine the origin of studies are required in order to determine the origin of these induced color centers. these induced color centers. Figure 1. (left) AlN ceramic plate (SHAPAL ) manufactured by Tokuyama Corp. (right) X-ray–induced Figure 1. (left) AlN ceramic plate (SHAPAL ) manufactured by Tokuyama Corp. (right) X-ray– absorbance as a function of X-ray dose irradiated. The inset is the integrated absorbance signal vs. the induced absorbance as a function of X-ray dose irradiated. The inset is the integrated absorbance corresponding dose. signal vs. the corresponding dose. After X-ray irradiation, an OSL emission is observed in the UV range when stimulated by red After X-ray irradiation, an OSL emission is observed in the UV range when stimulated by red light. The emission spectrum is illustrated in Figure 2. The irradiation dose was 20 Gy. The emission light. The emission spectrum is illustrated in Figure 2. The irradiation dose was 20 Gy. The emission signal has a broad spectral feature peaking around 360 nm, while the sample is stimulated by 630 nm. signal has a broad spectral feature peaking around 360 nm, while the sample is stimulated by 630 nm. Further, there seems to be a very small emission band with the peak around 505 nm, but the latter Further, there seems to be a very small emission band with the peak around 505 nm, but the latter emission is found to be negligibly small compared to the UV emission. It is instructive to mention emission is found to be negligibly small compared to the UV emission. It is instructive to mention here that, interestingly, the OSL was also observed after a UV (254 nm) irradiation. A similar emission here that, interestingly, the OSL was also observed after a UV (254 nm) irradiation. A similar emission was previously observed at ~340 nm in radioluminescence [19] and in cathodoluminescence [20–22]; was previously observed at ~340 nm in radioluminescence [19] and in cathodoluminescence [20–22]; however, as far as we are aware, the origin of this emission has not yet been fully identified. Figure however, as far as we are aware, the origin of this emission has not yet been fully identified. 2 also illustrates the stimulation spectrum measured while monitoring the 360 nm emission. The Figure 2 also illustrates the stimulation spectrum measured while monitoring the 360 nm emission. observed stimulation spectrum suggests that the OSL can be stimulated by light in a wide spectral The observed stimulation spectrum suggests that the OSL can be stimulated by light in a wide spectral range from approximately 550 nm to over 800 nm. The stimulated spectrum curve has some range from approximately 550 nm to over 800 nm. The stimulated spectrum curve has some noticeable noticeable features. Particularly around 800 nm, the OSL stimulation is the most efficient among the features. Particularly around 800 nm, the OSL stimulation is the most efficient among the spectral spectral region tested. This indicates that the AlN has a large number of trapping center groups with region tested. This indicates that the AlN has a large number of trapping center groups with different different trapping depths. In the previous work, considerably shallow trapping centers were trapping depths. In the previous work, considerably shallow trapping centers were presented by presented by TSL [19]; however, the OSL stimulation spectrum suggests a presence of deeper-but- TSL [19]; however, the OSL stimulation spectrum suggests a presence of deeper-but-stimulable charge stimulable charge trapping centers, especially those around 600 nm (~2.0 eV). trapping centers, especially those around 600 nm (~2.0 eV). Figure 3 shows a typical OSL decay curve of AlN ceramic, that is the transient OSL signal Figure 3 shows a typical OSL decay curve of AlN ceramic, that is the transient OSL signal intensity during stimulation as a function of time. The stimulation light used here is 610 nm, and the intensity during stimulation as a function of time. The stimulation light used here is 610 nm, and measured OSL emission signal is at 340 nm. The decay curve can be decomposed into a sum of two the measured OSL emission signal is at 340 nm. The decay curve can be decomposed into a sum exponential decay functions. The derived time constants are 20 and 155 s. Since the decay curve can of two exponential decay functions. The derived time constants are 20 and 155 s. Since the decay be decomposed well by only two exponential decay functions, this fact suggests that there are two curve can be decomposed well by only two exponential decay functions, this fact suggests that groups of charge trapping centers mainly responsible for the OSL process involved here. there are two groups of charge trapping centers mainly responsible for the OSL process involved Furthermore, the OSL curve in the shorter time range of 0–100 s, shown in the inset, clearly illustrates here. Furthermore, the OSL curve in the shorter time range of 0–100 s, shown in the inset, clearly a transient increase of OSL signal between 0–3 s, which is followed by an exponential decay. This illustrates a transient increase of OSL signal between 0–3 s, which is followed by an exponential decay. transition behavior suggests the presence of shallow trapping centers [11]. In fact, our observation agrees with a thermoluminescence study reported earlier [19] that there exists a glow peak located at Photonics 2016, 3, 23 4 of 7 This transition behavior suggests the presence of shallow trapping centers [11]. In fact, our observation Photonics 2016, 3, 23 4 of 7 Photonics 2016, 3, 23 4 of 7 agrees with a thermoluminescence study reported earlier [19] that there exists a glow peak located at a considerably a considerabl low y lotemperatur w tempera e, ture ~80 , ~C 80 . Detailed °C. Detaanalyses iled analar ys ees beyond are beyo the n scope d the ofsc this opepaper; of thihowever s paper;, a considerably low temperature, ~80 °C. Detailed analyses are beyond the scope of this paper; we howe plan veron , we further plan o investigations n further investi in ga order tionto s in understand order to un the dermechanism stand the minvolved. echanism involved. however, we plan on further investigations in order to understand the mechanism involved. Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after Figure 2. OSL emission and stimulation spectra of AlN ceramic. These spectra were measured after the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation at the sample was irradiated with 10 Gy of X-rays. The emission spectra were obtained by stimulation at 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. at 630 nm whereas the stimulation spectra were obtained by monitoring the emission at 360 nm. Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at 610 nm Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at 610 nm Figure 3. OSL decay curve. The OSL emission monitored is at 340 nm during a stimulation at with a constant light intensity. The OSL curve can be approximated mainly by a sum of two with a constant light intensity. The OSL curve can be approximated mainly by a sum of two 610 nm with a constant light intensity. The OSL curve can be approximated mainly by a sum of exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after the exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after the two exponential decay functions. The inset shows the OSL curve in a shorter time range 0–25 s after stimulation started. stimulation started. the stimulation started. The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates the The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates the dose-response curve obtained when tested with X-rays. We have experimentally confirmed a linear The OSL signal intensity increases with the X-ray dose irradiated. Figure 4 (left) illustrates dose-response curve obtained when tested with X-rays. We have experimentally confirmed a linear response over four orders of magnitude without any saturation of the OSL signal. It is worth the dose-response curve obtained when tested with X-rays. We have experimentally confirmed a response over four orders of magnitude without any saturation of the OSL signal. It is worth mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper linear response over four orders of magnitude without any saturation of the OSL signal. It is worth mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully mentioning here that the dose response could not be tested for doses above 20 Gy due to the upper limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully measure the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean limit capability of our X-ray source. For lower doses, below 3 mGy, we could not successfully measure measure the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean that the AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the the OSL signal. It is instructive to mention here that the latter fact does not necessarily mean that the that the AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the sensitivity of our experimental readout setup. The measurement capability of small doses is AlN ceramic plate does not have sufficient sensitivity below 3 mGy, but it can be due to the sensitivity sensitivity of our experimental readout setup. The measurement capability of small doses is especially important for personnel dosimetry or medical imaging applications, while a dosimeter of our experimental readout setup. The measurement capability of small doses is especially important especially important for personnel dosimetry or medical imaging applications, while a dosimeter capable of large doses may find some applications such as radiation therapy [23–26]. capable of large doses may find some applications such as radiation therapy [23–26]. Photonics 2016, 3, 23 5 of 7 for personnel dosimetry or medical imaging applications, while a dosimeter capable of large doses may find some applications such as radiation therapy [23–26]. Photonics 2016, 3, 23 5 of 7 Figure 4. Experimental demonstrations of SHAPAL® AlN ceramic plate for use as an OSL dosimeter. Figure 4. Experimental demonstrations of SHAPAL AlN ceramic plate for use as an OSL dosimeter. (left) Dose response curve of OSL in an AlN ceramic plate. The stimulation light wavelength is 610 nm (left) Dose response curve of OSL in an AlN ceramic plate. The stimulation light wavelength is 610 nm while the emission light is 340 nm; (right) X-ray image of some electronic components taken by OSL of while the emission light is 340 nm; (right) X-ray image of some electronic components taken by OSL AlN ceramic plate. The stimulation light used here is 630 nm  50 nm. The electronic components are of AlN ceramic plate. The stimulation light used here is 630 nm ± 50 nm. The electronic components (from left to right): logic gate, field effect transistor, bipolar transistor, and operational amplifier. are (from left to right): logic gate, field effect transistor, bipolar transistor, and operational amplifier. Figure 4 (right) demonstrates an X-ray image taken on some electronic components using the Figure 4 (right) demonstrates an X-ray image taken on some electronic components using SHAPAL AlN ceramic plate. In the image, some electrodes and wiring hidden in the epoxy the SHAPAL AlN ceramic plate. In the image, some electrodes and wiring hidden in the epoxy encapsulations are clearly observed. Although the setup is not fully optimized for high resolution encapsulations are clearly observed. Although the setup is not fully optimized for high resolution imaging, our demonstration has shown that the legs of the bipolar transistor (the second part from imaging, our demonstration has shown that the legs of the bipolar transistor (the second part from the the right-hand side in the image), which has the width of 0.5 mm, are well resolved, and it indicates right-hand side in the image), which has the width of 0.5 mm, are well resolved, and it indicates the the spatial resolution is at least equivalent or better than 0.5 mm. In addition, the uniformity of the spatial resolution is at least equivalent or better than 0.5 mm. In addition, the uniformity of the image image was tested with an image of a uniformly irradiated sample. The standard deviation was was tested with an image of a uniformly irradiated sample. The standard deviation was calculated to calculated to be 23.90 counts. This low value is mainly because of insufficient sensitivity of the CCD be 23.90 counts. This low value is mainly because of insufficient sensitivity of the CCD camera in the camera in the present measurement. A technique such as a flying-spot reader [5] should improve the present measurement. A technique such as a flying-spot reader [5] should improve the signal-to-noise signal-to-noise ratio since a photomultiplier tube has much better sensitivity to UV light. Overall, we ratio since a photomultiplier tube has much better sensitivity to UV light. Overall, we think that the think that the AlN ceramic plate has good potential to be used as an imaging plate for X-ray imaging AlN ceramic plate has good potential to be used as an imaging plate for X-ray imaging applications. applications. 4. Conclusions 4. Conclusions In this research, we have discovered that an AlN ceramic plate (SHAPAL , Tokuyama Co, In this research, we have discovered that an AlN ceramic plate (SHAPAL , Tokuyama Co, Yamaguchi, Japan) shows OSL, and we have investigated the OSL properties. The AlN ceramic plate Yamaguchi, Japan) shows OSL, and we have investigated the OSL properties. The AlN ceramic plate is, in fact, designed and manufactured for heat sink applications; however, it has good potential to is, in fact, designed and manufactured for heat sink applications; however, it has good potential to be used as an OSL storage phosphor for one- and two-dimensional dosimetry applications. By X-ray be used as an OSL storage phosphor for one- and two-dimensional dosimetry applications. By X-ray irradiation, the AlN ceramic plate turns in from a grey to yellowish color due to an X-ray–induced irradiation, the AlN ceramic plate turns in from a grey to yellowish color due to an X-ray–induced absorbance band appearing in the UV-blue range. With optical stimulation, an OSL emission can absorbance band appearing in the UV-blue range. With optical stimulation, an OSL emission can be be observed in the UV region around 360 nm with a broad spectral feature. The OSL signal can be observed in the UV region around 360 nm with a broad spectral feature. The OSL signal can be stimulated by light over a wide range of the spectrum from 400 to over 700 nm, which is a distinct stimulated by light over a wide range of the spectrum from 400 to over 700 nm, which is a distinct advantage. The OSL decay curve is a double-exponential, yielding two time constants (20 and 155 s). advantage. The OSL decay curve is a double-exponential, yielding two time constants (20 and 155 s). However, a transient increase behavior is observed in the OSL immediately after the stimulation However, a transient increase behavior is observed in the OSL immediately after the stimulation is is given until ~3 s, followed by the exponential decay. This observation suggests the presence of given until ~3 s, followed by the exponential decay. This observation suggests the presence of shallow shallow trapping centers in addition to the main dosimetric traps. The OSL dose response in the AlN trapping centers in addition to the main dosimetric traps. The OSL dose response in the AlN ceramic ceramic plate has been demonstrated over 3 mGy–10 Gy. A two-dimensional dosimetry has also been plate has been demonstrated over 3 mGy–10 Gy. A two-dimensional dosimetry has also been successfully demonstrated with an imaging capability. successfully demonstrated with an imaging capability. Acknowledgments: This research was co-supported by a Grant-in-Aid for Scientific Research (A) (26249147), Grant-in-Aid for Research Activity start-up (15H06409), and Green Photonics Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). It is also partially supported by the Adaptable and Seamless Technology transfer Program (A-STEP) by the Japan Science and Photonics 2016, 3, 23 6 of 7 Acknowledgments: This research was co-supported by a Grant-in-Aid for Scientific Research (A) (26249147), Grant-in-Aid for Research Activity start-up (15H06409), and Green Photonics Research from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). It is also partially supported by the Adaptable and Seamless Technology transfer Program (A-STEP) by the Japan Science and Technology (JST) Agency, the Murata Science Foundation, and a cooperative research project of the Research Institute of Electronics, Shizuoka University. Author Contributions: e.g., Go Okada, Kentaro Fukuda, Safa Kasap, and Takayuki Yanagida conceived and designed the experiments; Kentaro Fukuda contributed the materials; Go Okada performed the experiments, analyzed the data, and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. References 1. Knoll, G.F. Radiation Detection and Measurement, 4th ed.; Wiley: New York, NY, USA, 2010. 2. Yanagida, T. Study of rare-earth-doped scintillators. Opt. Mater. 2013, 35, 1987–1992. [CrossRef] 3. Seeley, Z.; Cherepy, N.; Payne, S. Two-step sintering of Gd Lu Eu O transparent ceramic scintillator. 03 16 01 3 Opt. Mater. Express 2013, 3, 908–912. [CrossRef] 4. 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Published: Apr 30, 2016

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