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Development of a flexible γ-ray detector using a liquid scintillation light guide (LSLG)
Applied Radiation and Isotopes 139 (2018) 12–19
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Development of a flexible γ-ray detector using a liquid scintillation light guide (LSLG)
T
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Kiyoshi Nomuraa, , Akira Yunokib, Masayuki Harac, Yuko Moritoa, Akira Fujishimaa a
Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku 162-8601, Japan National Institute of Advanced Industrial Science and Technology (AIST)/National Metrological Institute of Japan, Umesono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan c Tokyo Medical and Dental University (TMDU), Yushima1-5-45, Bunkyo-ku, Tokyo 113-8510, Japan b
H I GH L IG H T S
γ detector using liquid scintillation light guide (LSLG) was developed. • APHAflexible depends on diameter, length and scintillator concentration of LSLG, and distance of γ ray irradiation point from PMT head. • Countspectrum ratio of two divided channel regions in PHA decreases linearly as the distance increases. • Radiation dose rate can be estimated by setting an LSLG tube to circular shape. • A flexible and long LSLG detector using single PMT is useful for determination of dose rate and for detection of local contamination. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible detector for gamma ray Liquid scintillation light guide (LSLG) Long tube liquid scintillation counter Position sensitive detector Nondestructive survey meter. Dose rate meter
A flexible γ detector using a liquid scintillation light guide (LSLG) was developed. The analyzed pulse height (PHA) spectrum depended on the diameter, length and scintillator concentration of the LSLG, and the distance of a γ ray irradiation point from the head of photomultiplier tube (PMT). From the analysis of PHA spectrum, it was found that the count ratio of two divided channel regions linearly decreases as the distance from the PMT head increases. It was further found that the radiation dose rate can be estimated by setting the flexible LSLG tube to a circular shape since the count rate is proportional to the dose rate measured by a conventional NaI (Tl) scintillation detector. Therefore, a flexible and long LSLG detector using a single PMT is useful for determination of the dose rate and has a potential to detect local contaminations in a certain narrow space.
1. Introduction Radioactive nuclides sputtered widely by the accident of TEPCO's Fukushima Daiichi Nuclear Power Plant due to the Great East Japan Earthquake at 11th, March 2011, caused the radioactive contamination of soil surface, forests, house roofs, and so on. The contaminants exposed to weathering such as rain and wind flowed into foots of mountain and hill, side gutters, drains, lowlands and ponds, resulting in locally high radioactive contamination in the narrow spaces and the bottom sites of ponds. Conventional radiation counters cannot be applied to narrow spaces such as drain pipes and underground pipes in particular. Furthermore, the evaluation of the influence is particularly required in places where uniform radiation field does not appear. Here, a flexible detector would enable us to estimate easily the radiation dose and contamination distribution of radioactive materials especially when a nuclear accident and an emergency situation by a disaster occur.
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Corresponding author. E-mail address: [email protected] (K. Nomura).
https://doi.org/10.1016/j.apradiso.2018.04.018 Received 16 January 2018; Accepted 6 April 2018 Available online 10 April 2018 0969-8043/ © 2018 Published by Elsevier Ltd.
By the way, a liquid scintillation counter has been developed for detecting only low energy β particle emitter nuclides of a sample mixed with a liquid scintillator using the coincidence measurement system loaded with two photomultiplier tubes (PMT) in order to prevent from the high background counts. The radiation measurement using optical technique has been reviewed and the property of various scintillation fibers has been introduced (Nakazawa, 1994). On the other hand, a liquid light guide (LLG) has been developed for usage of ultraviolet and visual light transmission. LLG tubes with several meters in length are commercially available and useful for light transmission because of the large cross section for incident light although the light transmittance is not as good as glass fiber and plastic fiber guides. Glass and plastic fibers need to be bundled with many fibers because the fiber itself is too fine (less than 0.1 mm in diameters) to detect the radiation. Kawarabayashi et al. proposed a position sensitive detector with the
Applied Radiation and Isotopes 139 (2018) 12–19
K. Nomura et al.
Fig. 1. The typical relationship between refraction index of liquid (n1) and critical angle of incident light for total reflection when the refraction index of cladding material in tube (n2) is 1.34.
Here we report the fundamental property of LSLG tubes, the new PHA analysis as a position sensitive detector, and the application of a circular shape LSLG detector to a radiation dose meter.
LLG based on a time-of-flight (TOF) method with two PMTs for detection of high energy neutron using the photons created through Cerenkov Effect (Kawarabayashi et al., 2004). The position resolution was around 9.6 cm for an LLG tube with 5 mm in diameter and 2 m in length. For 60Co γ-ray beam with a diameter of 10 mm, light transmission loss was not observed against the integrated dose of 2.5 × 103 Gy, whereas the transmission loss of normal plastic fiber was large, that is, more than 90% at the same dose (Kawarabayashi et al., 2004). No light transmission degradation of LLG (5 mmØ, 10 m) tubes including liquid organic scintillator was further confirmed against a high dose of γ rays up to 1.5 × 104 Gy (Hayashi et al., 2008). Scintillation glass fibers (Naka et al., 2001) and optical plastic fibers (Imai et al., 1991) cannot be applied to a high-level radiation field because the loss of optical transmission is caused by radiation damage of solid state detectors. The concept of a long and flexible detector may be something innovational because the flexible detector would be useful for measuring the radioactivity of samples with any shape, and for detecting contamination inside narrow tubes such as sewer pipe, underground buried pipe used in radioisotope and nuclear facilities. Today, many facilities are in aging, and the restoration or reconstruction is required. A long β counter has been developed by covering with plastic scintillation film (43 cm) on a light guide rod for survey of contamination inside the pipes used in nuclear facility although it is not flexible (Maekawa et al., 2011). A light guide tube including liquid scintillator has higher detection efficiency, and more simplicity than a plastic scintillation fiber tube. The density of a liquid scintillator is so close to tissue equivalent that the flexible detector including liquid scintillator may be useful as an absorption dose meter. However, the attenuation length of the previous system was too short to monitor distributions of radioactive isotopes in nuclear facilities because commercial liquid scintillator cocktail was used as it was. Since then, there had been no extended study concerning the flexible light guide itself. Therefore, we have fabricated a flexible radiation detector with liquid scintillator as the core liquid of a light guide and a single PMT (Nomura et al., 2016). It is named as a liquid scintillation light guide (LSLG) detector. The new flexible counter with LSLG developed is expected to be applicable to the detection of γ rays in wide dynamic ranges because the decay time of liquid scintillator is several nanosecond orders. An LSLG counter itself has higher detection efficiency than detectors with plastic fiber and glass fiber guides. Since the light transmission of LSLG is depending on scintillator concentration and tube diameter as well as tube length, it is necessary to clarify the luminescence and transmission properties of various LSLG tubes.
2. Principle Since refraction is one of important factors for scintillation counters, we explain it simply. Refraction is the path bending of the light wave when it passes across the boundary separating two media (core liquid and cladding tube). Refraction is caused by a change in wave speed as the media changes. The principle of LLG follows Snell's law for light refraction:
n1 sin θ1 = n2 sin θ2 where, n1 and n2: refraction index of core liquid and cladding material, respectively, θ1 and θ2: incident and refracting angles from normal to the interface between core liquid and cladding material, respectively. Necessary conditions for reflection in tube are n1 > n2, and for total reflection θ2 ≥ 90°. By the way, when θ2 = 90°, the critical angle for total reflection, θ1, is expressed as sinθ1 = n2/n1. The relationship between liquid refraction index n1 and critical reflection angle calculated as refraction index n2 = 1.34 of cladding material is shown in Fig. 1. For example, the total reflection can be available at the incident angle ≥ 64° when n1 = 1.49. As another important factor for light transmission in a liquid light guide, there is the Lambert-Beer law for a certain light wavelength; Abs= −log T1/T0 =εCL. Where, Abs is absorbance, T0 and T1 are luminescence intensity of core liquid in a tube before and after absorption, respectively, ε is mole absorbance factor, C is concentration of luminescence reagent, and L is length of a light guide. Commercial liquid scintillator cocktails are normally used by mixing with the sample when β particles emitted nuclei such as 3H, 14C and 35S are detected by a liquid scintillation counter. Liquid scintillators contain at least two luminescence reagents because of wave length shift, and further a surfactant to make water sample soluble. The commercial liquid scintillators may be too concentrated to use as the core liquid of an LLG and so should be diluted with toluene and so on. The light absorption in an LSLG tube may not perfectly follow the Lambert-Beer law, and the characteristics of light transmission for LSLG tubes will be shown in a section of consideration.
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Fig. 2. a) Photograph of typical LSLG tube with length (L) = 3 m, inner diameter (D) = 8 mm, and the quartz window. b) The luminescent spectra of light with λ = 420–560 nm observed when LED (λ = 375 nm, 1 mW) light is irradiated to the window of LSLG tubes with D = 7 mm and L = 1 m.
3. Experimental The picture of typical LSLG prepared is shown in Fig. 2a). As the flexible plastic tube, fluorinated ethylene and propylene (FEP) copolymer was used and the inside was cleaned first of all. The refraction index (RI) of FEP is small (1.33–1.34), compatible to RI of water (1.333). The core liquid with the higher RI than FEP cladding is necessary for reflection. The commercial liquid scintillator cocktails such as Ultima Gold (UG: water soluble scintillator) and UGF (UGF: organic scintillator) (PerkinElmer Co.) were used and the luminescence and transmission properties are shown in Fig. 2b). The light transmittance of both LSLG tubes including UG and UGF was relatively lower than the LSLG with scintillator prepared by us although the reflection indexes of UG and UGF were large. It is the reason why the dilution by an organic solvent is necessary. We prepared liquid scintillations by dissolving with toluene using the following fluorescent reagents: Butyl PBD (2-(4-tert-Butyl Phenyl)5-(4-Biphenyl)-1,3,4- oxadiazole: emission wavelength: 363 nm, and absorption wavelength: 304 nm) as the first luminescent reagent, and Bis-MSB (1,4-bis[2-methylstyryl] benzene; emission wave length: 420 nm and absorption wave length: 346 nm) as the second reagent. The toluene solutions with different RI using Butyl PBD and Bis-MSB were adjusted: 1.4980 and 1.4975 at 20 °C. From the light transmittance properties as shown in Fig. 2b) it is clear that the larger RI, the larger transmittance in the case of a toluene solution of Butyl PBD and Bis-MSB. The concentration of Bis-MSB should be less than one tenth of Butyl PBD. An outer protective guide of an inner FEP flexible tube is composed of a flexible Al spiral tube and a poly vinyl chloride black tube. A quartz glass was used as a window material of the tube. One side of the tube was fixed to a photomultiplier (PMT: Hamamatsu: H6612), which was connected to a preamplifier, a linear amplifier (ORTEC: 572A, 590A) and a Multichannel analyzer (ORTEC: Easy-MCA-2K) to get a PHA spectrum. The several standard γ sources of 137Cs were used for the measurement of position sensitivity and spacial radiation.
Fig. 3. PHA spectra of LSLG with length (L): 1 m and diameter (D): 5 mm and 10 mm. Core liquid: toluene solution of Butyl PBD and Bis-MSB, Source: 220 kBq Cs-137, Position: 1.5 cm from center of tube. Point source of 137Cs is located at 5 cm and 95 cm from a PMT head. The displayed count rates were calculated as integral counts in the channel area of ≥ 45 channels because the peak in channel area of < 45 may contain the circuit noise and leakage light. HV: − 1.3 kV.
LSLG tubes with 5 mm and 10 mm in diameter. The detection mechanism is mainly due to Compton scattered electrons arising from the interaction between γ-rays and a core liquid of LSLG tubes. The PHA spectra were enhanced in the high energy channels when the γ-rays were irradiated at the point close to the PMT head because a light transmittance loss was small, but the attenuated spectra were observed when a γ-ray source was far from PMT. This is because luminescent absorption occurs according to the distance under a certain scintillator concentration, based on Lambert Beer's law. However, in the case of a flexible LSLG, the wavelength shift of light was observed due to reflection of light by the cladding tube and due to absorption of two fluorescent reagents. Therefore, it does not follow this rule perfectly, which will be shown in more detail in a discussion section. PHA spectra gave a significantly different shape although the count rate obtained from the total count in the channel regions of 45 or more was not as large as shown in Fig. 3. The peak in the low channel regions of less than 45 may contain circuit noise and leakage light, and was ignored in these spectra at the initial measurement. As a result, the detection efficiency of the 10 mm diameter LSLG was about 3.5 times the detection efficiency of the 5 mm diameter LSLG. The adjusted liquid scintillator of the LSLG detectors is a toluene solution of Butyl PBD +Bis-MSB. Liquid scintillator cocktails (UG and UGF) are available
4. Results 4.1. Properties of LSLG detectors PHA spectra obtained by placing a γ-ray source of 137Cs at a certain distance of LSLG with one meter in length from a PMT head are shown in Fig. 3. The spectra depend on the diameter and source position of 14
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4.2. Measuring dose rates using an LSLG detector Fig. 7 shows the PHA spectra obtained when the LSLG tubes with different lengths of 1 m and 3 m were arranged as a circle and three circles, respectively. The diameter of the tube circle is 30 cm. The count rates of LSLG (30% UG) detector with D = 8 mm and L = 1 m were measured by setting 137Cs source with different activities at the center position of the tube circle and by setting them far from the circle face. Each PHA spectrum in this arrangement is considered to be the average spectrum of PHA spectra measured at different positions of the γ source on the LSLG tube (in Fig. 4). The dose rates were measured using a NaI (Tl) scintillation survey meter (ALOKA: TCS-161). The count rates [(s−1)/(μSv/h)] for different applied high voltages (HV) were measured using a γ source of 137Cs (2 cmø) set at the center of the circle tube of LSLG as shown in Photograph 1. The counting efficiency ratios of 3 m LSLG tube to 1 m LSLG tube were 1.73 for HV= −1.3 kV, 1.63 for HV= =− 1.2 kV, and 1.45 for HV= −1.1 kV, respectively. The counting efficiency of 3 m LSLG tube was less than twice that of 1 m LSLG tube. The counting efficiency ratios decreased with decreasing the applied high voltages. The low efficiency of a long LSLG tube is due to the light transmittance process as described in Section 4.1. The relationship between the counting rate (Y) and the dose rate (X) for LSLG (30% UG, D = 8 mm, L = 1 m) was measured at HV= −1.1 kV by setting the γ source vertically far from one circle face. As shown in Fig. 8a), the following linear relationship was obtained:
Fig. 4. PHA spectra of LSLG (UG: 30% in toluene) detector with 3 m in length and 8 mm in diameter, measured by using a point source (137Cs) located at the displayed distance from a PMT head. HV: − 1.3 kV.
commercially, but they have so large light attenuation. The cocktail was diluted with toluene. PHA spectra were measured using an LSLG detector with an internal tube diameter (D) of 8 mm and a length (L) of 3 m including diluted 30% UG. The full PHA spectra are shown in Fig. 4. In order to survey a peak in the low channel region, PHA spectra in regions of 200 channels or less are shown on a linear scale in Fig. 5a). The peak observed in the low channel region was initially ignored. However, the PHA spectrum can be divided into two channel regions (A, B) as the peak counts are as A and B, respectively. When the count ratio of [B/(A+B)] is plotted as a function of the distance from the head of PMT, a linear relationship is obtained as shown in Fig. 5b). The count ratios of [B/(A+B)] using the data subtracting background counts show a linear relationship with the higher gradient. From the relationship, it was found that the position where a high radiation source exists could be estimated even if two PMT's were not used like a time-of-flight (TOF) method. An LSLG detector loaded with one PMT can be used as socalled a position sensitive detector. The PHA spectra of LSLG tubes with D = 10 mm, L = 1 m, and with D = 5 mm and L = 2 m are shown in Fig. 6a) and c), respectively. The core liquid of these LSLG tubes is a toluene solution of Butyl PBD and Bis-MSB. The transmittance is superior to LSLG tubes including UG. As shown in Fig. 3, the detection efficiency of the large diameter LSLG is high. The background counts of a long LSLG tube are larger than that of a short LSLG tube, especially in low channel regions. The count ratios of B/(A + B) gradually decreases as the distance from the source position on the LSLG tube to the PMT head increases, and the attenuation gradient for the long and small diameter tube is lower than that of the short and large diameter tube of LSLG as shown in Fig. 6b) and d).
Y = a∗X + b,
where a = 300.9 ± 1.4, b = 6.26 ± 26.3.
Therefore, from this relationship, the counting efficiency was 301 [(s−1)/(μSv/h)]. The counting efficiency is almost consistent with the value measured at a center of the circle as shown in Fig. 7. The linear relationship suggests that an LSLG detector with a single PMT can be used as a dose rate meter. In the case of 30% UG LSLG with D = 8 mm, L = 3 m and three circles, the relationship between dose rate and count rate is shown in Fig. 8b). The linear fitting is obtained as follows:
Y = a∗X + b,
where a = 463.3 ± 0.9, b = 100.7 ± 23.7.
The counting efficiency of 3 m LSLG with 3 circles was 463 [(s−1)/ (μSv/h)], which was 1.54 times that of 1 m LSLG with one circle. The counting efficiency ratio is almost consistent with that obtained by setting a source at the center of a LSLG circle. This result suggests that the dynamic range of counting is wide. In the case of 10% UG LSLG with D = 8 mm, L = 1 m and one circle, the following relationship was obtained as shown in Fig. 9:
Fig. 5. a) Low channel PHA spectra of LSLG (30% UG) with D= 8 mm and L= 3 m. BG: 0.06 μSv/h, ROI: 10–45 ch (count A) and > 45 ch. (count B), b) Relationship between count ratios and distances from a photomultiplier tube (PMT) head. 15
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Fig. 6. a) PHA spectra of LSLG with D = 10 mm and L = 1 m, and b) the relationship between distance and count ratio of B/(A + B) in two divided channel regions (A, B). c) PHA spectra of LSLG with D = 5 mm and L = 2 m, and d) the relationship between distance and count ratio of B/(A+B). The upper linear fitting lines after subtracting background counts are shown in b), and d), respectively. Core liquid: toluene solution of Butyl PBD and Bis-MSB, Distance: from PMT head to 137Cs source. HV: − 1.3 kV.
Y = a*X + b,
where a = 290.2 ± 7.9, b = 6.26 ± 26.3.
position sensitive detector and the diluted scintillator is used. The emission lifetime of the liquid scintillator is about several nanoseconds (ns), which is much shorter than other solid scintillators. An LSLG detector can be applied to the radiation measurement with high counting rates and wide dynamic ranges. It was confirmed previously that there was no loss of light transmittance up to 1.5 × 104 Gy by γ irradiation of 60 Co (Hayashi et al., 2008). In the high dose field of nuclear reactors, the solid scintillators themselves and electronic circuit parts are susceptible to radiation damage, but LSLG itself is less deteriorated by radiation. Further, since the electronic circuit parts are far from the LSLG detection parts, it is possible to avoid from radiation damage to the electronic circuit portion in the measurement of the high radiation field. As the scintillator concentration increases, the detection efficiency improves, but the attenuation rate of the PHA spectrum increases with increasing the distance between PMT head and point source. The optimum concentration varies depending on the length and diameter of the flexible tube. Conversely, it is easy to manufacture one with different attenuation factors of PHA spectra depending on the main purpose, such as whether it is for a dosimeter or a position sensitive detector. A flexible detector including such a liquid scintillator has another new application possibility. For example, arraying a number of flexible LSLG tubes enables to make a soft and flexible plate detector. It would be useful to inspect radiation as a flexible and flat detector covering the surface of human body and animals contaminated (Nomura, 2018).
−1
The counting efficiency of 10% UG LSLG was 290 [(s )/(μSv/h)], which was not so small as that of 30% UG LSLG (Fig. 8a) although the concentration of scintillator in the 10% UG LSLG was one third of that of the 30% UG LSLG. This may be because the luminescence transmittance of 10% UG LSLG is superior to that of 30% UG LSLG. 5. Considerations LED light with λ = 375 nm was vertically irradiated at different positions on the tube side of unshielded LSLG in dark room as shown in Fig. 10a), and the emission spectra were measured at the end of tube. As shown in Fig. 10b), it was found that the intensity of the emitted light varies depending on the distance from the light detector. As shown in Fig. 10c) and d), the light attenuation rate varies depending on the wave length of the emitted light. As the distance of source light increases, the wavelength of emission light shifts to the long wavelength and the intensity decreases at the short wavelength (λ = 426 nm) larger than at the long wavelength (λ = 456 nm). This is the reason why the transmission of emission light in the LSLG does not follow Lambert-Beer law perfectly. Regarding the temperature quenching of liquid scintillator, it has been reported that a quenching ratio of about 10% is observed for temperature changes from 0 °C to 30 °C (Katou, 2003). In the case of LSLG, it is considered that the influence of the quenching becomes small because the counting ratio of two channel regions is used as a 16
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Fig. 7. a) Arrangement of 3 m LSLG tube for measurement of radiation dose rate and PHA spectra of b) 1 m LSLG (30% UG, D=8 mm) with one circle, and c) 3 m LSLG (30% UG, D=8 mm) with 3 circles (after each background was subtracted). The counting efficiency ratios are 665.1/385.0 = 1.73, 573.1/350.8 = 1.63, and 445.9/308.2 = 1.45 for HV= −1.3, − 1.2, − 1.1 kV, respectively.
Fig. 8. The relationship between count rate of LSLG and dose rate by NaI (Tl) scintillation survey meter. a) 1 m LSLG (30% UG, D=8 mm) with one circle. b) 3 m LSLG (30% UG, D=8 mm) with three circles. The counting efficiency ratio of 3 m LSLG to 1 m LSLG is 463.3/300.9 = 1.54.
6. Conclusion
monitoring the contamination inside pipes or tubes used in nuclear and radioisotope facilities, the activity distribution of contaminated ground, the leaked γ-ray surrounding an accelerator and a reactor as a room monitor, and as a high radiation field monitor in destroyed reactors. Thus, an LSLG detector would be useful for an absorption dosimeter as well as a position sensitive detector for high radiation dose distribution, especially at the events such as a nuclear accident and a disaster emergency.
LSLG is so superior in flexibility, less damage and high efficiency as compared with a conventional solid scintillator fiber. It was found from analysis of PHA spectra that an LSLG detector can be used as a position sensitive detector using a single photomultiplier. It was further shown that an LSLG detector is able to estimate the dose rates of radiation by arranging the LSLG tube into a circle shape. An LSLG detector has large potential to apply to many fields because of its flexibility and easy fabrication. An LSLG detector would be expected to be used for
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Fig. 9. a) PHA spectra (after background was subtracted) and b) the relationship between counting rate of LSLG and dose rate by NaI(Tl) scintillation survey meter. LSLG: 10% UG in toluene, D = 8 mm, and L = 1 m.
Fig. 10. a) Arrangement of LSLG tube and LED light, b) Emission spectra of LSLG (30% UG) with D= 5 mm and L= 2 m, obtained when LED light (λ = 375 nm) were irradiated vertically at different positions of unshielded LSLG tube in dark room, c) Attenuation of emission intensity at λ = 426 nm, and d) Attenuation of emission intensity at λ = 456 nm, depending on distance from light detector (Hamamatsu Photonics Co.: PMA-12).
Acknowledgements
References
Authors would like to appreciate Prof. Chiaki Terashima in Photocatalytic International Research Center, Tokyo University of Science, Mr. Kazuo Hosoda and Mr. Daisuke Yokota in U-VIX Co. for helping to prepare LLG tubes, and Profs. Takayuki Terai and Hiroyuki Takahashi in School of Engineering, the University of Tokyo, for supporting the radiation experiments.
Hayashi, M., Kawarabayashi, J., Asai, K., Iwai, H., Nakagawa, Y., Iguchi, T., 2008. Position-sensitive radiation detector with flexible light guide and liquid organic scintillator to monitor distributions of radioactive isotopes. J. Nucl. Sci., Technol. Suppl. 6, 81–84. Imai, S., Soramoto, S., Mochiki, K., Iguchi, T., Nakazawa, M., 1991. New radiation detector of plastic scintillation fiber. Rev. Sci. Instrum. 64, 1093–1097. Katou, T., 2003. Influence of temperature to quenching on liquid scintillation measurement” (Japanese). Radioisotopes 52, 335–339. Kawarabayashi, J., Mizuno, R., Inui, D., Watanabe, K., Iguchi, T., 2004. Potential on liquid light guide as distributed radiation sensor, In: Proceedings of the IEEE Nuclear
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review). Radioisotopes 43, 423–431. Nomura, K., 2018. Recommendation on development of a flexible radiation detector including liquid scintillator(Japanese). FBNews 493, 8–12. Nomura, K., Yunoki, A., Hosoda, K., Yokota, D., Morito, Y., Terai, T., Terashima, C., Fujishima, A., 2016. A new flexible γ-ray detector using a liquid scintillator light guide (LSLG), In: Proceedings of the 12th International Workshop on Ionizing Radiation Monitoring, p-22, Hosoda Hall, Oarai, Dec, 3–4.
Science Symposium Conference Record, Oct. 16–22 pp. 712–714. Maekawa, T., Makino, S., Sumita, A., Goto, T., 2011. Long scintillation detector using composite light guide for β-ray survey measurement. J. Nucl. Sci. Tech. 48, 50–59. Naka, R., Watanabe, K., Kawarabayashi, J., Uritani, A., Iguchi, T., Hayashi, N., Kojima, N., Yoshida, T., Kaneko, J., Takeuchi, H., Kakuta, T., 2001. Radiation distribution sensing with normal optical fiber. IEEE Trans. Nucl. Sci. 48 (6), 2348–2351. Nakazawa, M., 1994. Radiation Measurement Using Optical Techniques, (Japanese
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Development of a flexible γ-ray detector using a liquid scintillation light guide (LSLG)
T
⁎
Kiyoshi Nomuraa, , Akira Yunokib, Masayuki Harac, Yuko Moritoa, Akira Fujishimaa a
Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku 162-8601, Japan National Institute of Advanced Industrial Science and Technology (AIST)/National Metrological Institute of Japan, Umesono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan c Tokyo Medical and Dental University (TMDU), Yushima1-5-45, Bunkyo-ku, Tokyo 113-8510, Japan b
H I GH L IG H T S
γ detector using liquid scintillation light guide (LSLG) was developed. • APHAflexible depends on diameter, length and scintillator concentration of LSLG, and distance of γ ray irradiation point from PMT head. • Countspectrum ratio of two divided channel regions in PHA decreases linearly as the distance increases. • Radiation dose rate can be estimated by setting an LSLG tube to circular shape. • A flexible and long LSLG detector using single PMT is useful for determination of dose rate and for detection of local contamination. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible detector for gamma ray Liquid scintillation light guide (LSLG) Long tube liquid scintillation counter Position sensitive detector Nondestructive survey meter. Dose rate meter
A flexible γ detector using a liquid scintillation light guide (LSLG) was developed. The analyzed pulse height (PHA) spectrum depended on the diameter, length and scintillator concentration of the LSLG, and the distance of a γ ray irradiation point from the head of photomultiplier tube (PMT). From the analysis of PHA spectrum, it was found that the count ratio of two divided channel regions linearly decreases as the distance from the PMT head increases. It was further found that the radiation dose rate can be estimated by setting the flexible LSLG tube to a circular shape since the count rate is proportional to the dose rate measured by a conventional NaI (Tl) scintillation detector. Therefore, a flexible and long LSLG detector using a single PMT is useful for determination of the dose rate and has a potential to detect local contaminations in a certain narrow space.
1. Introduction Radioactive nuclides sputtered widely by the accident of TEPCO's Fukushima Daiichi Nuclear Power Plant due to the Great East Japan Earthquake at 11th, March 2011, caused the radioactive contamination of soil surface, forests, house roofs, and so on. The contaminants exposed to weathering such as rain and wind flowed into foots of mountain and hill, side gutters, drains, lowlands and ponds, resulting in locally high radioactive contamination in the narrow spaces and the bottom sites of ponds. Conventional radiation counters cannot be applied to narrow spaces such as drain pipes and underground pipes in particular. Furthermore, the evaluation of the influence is particularly required in places where uniform radiation field does not appear. Here, a flexible detector would enable us to estimate easily the radiation dose and contamination distribution of radioactive materials especially when a nuclear accident and an emergency situation by a disaster occur.
⁎
Corresponding author. E-mail address: [email protected] (K. Nomura).
https://doi.org/10.1016/j.apradiso.2018.04.018 Received 16 January 2018; Accepted 6 April 2018 Available online 10 April 2018 0969-8043/ © 2018 Published by Elsevier Ltd.
By the way, a liquid scintillation counter has been developed for detecting only low energy β particle emitter nuclides of a sample mixed with a liquid scintillator using the coincidence measurement system loaded with two photomultiplier tubes (PMT) in order to prevent from the high background counts. The radiation measurement using optical technique has been reviewed and the property of various scintillation fibers has been introduced (Nakazawa, 1994). On the other hand, a liquid light guide (LLG) has been developed for usage of ultraviolet and visual light transmission. LLG tubes with several meters in length are commercially available and useful for light transmission because of the large cross section for incident light although the light transmittance is not as good as glass fiber and plastic fiber guides. Glass and plastic fibers need to be bundled with many fibers because the fiber itself is too fine (less than 0.1 mm in diameters) to detect the radiation. Kawarabayashi et al. proposed a position sensitive detector with the
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Fig. 1. The typical relationship between refraction index of liquid (n1) and critical angle of incident light for total reflection when the refraction index of cladding material in tube (n2) is 1.34.
Here we report the fundamental property of LSLG tubes, the new PHA analysis as a position sensitive detector, and the application of a circular shape LSLG detector to a radiation dose meter.
LLG based on a time-of-flight (TOF) method with two PMTs for detection of high energy neutron using the photons created through Cerenkov Effect (Kawarabayashi et al., 2004). The position resolution was around 9.6 cm for an LLG tube with 5 mm in diameter and 2 m in length. For 60Co γ-ray beam with a diameter of 10 mm, light transmission loss was not observed against the integrated dose of 2.5 × 103 Gy, whereas the transmission loss of normal plastic fiber was large, that is, more than 90% at the same dose (Kawarabayashi et al., 2004). No light transmission degradation of LLG (5 mmØ, 10 m) tubes including liquid organic scintillator was further confirmed against a high dose of γ rays up to 1.5 × 104 Gy (Hayashi et al., 2008). Scintillation glass fibers (Naka et al., 2001) and optical plastic fibers (Imai et al., 1991) cannot be applied to a high-level radiation field because the loss of optical transmission is caused by radiation damage of solid state detectors. The concept of a long and flexible detector may be something innovational because the flexible detector would be useful for measuring the radioactivity of samples with any shape, and for detecting contamination inside narrow tubes such as sewer pipe, underground buried pipe used in radioisotope and nuclear facilities. Today, many facilities are in aging, and the restoration or reconstruction is required. A long β counter has been developed by covering with plastic scintillation film (43 cm) on a light guide rod for survey of contamination inside the pipes used in nuclear facility although it is not flexible (Maekawa et al., 2011). A light guide tube including liquid scintillator has higher detection efficiency, and more simplicity than a plastic scintillation fiber tube. The density of a liquid scintillator is so close to tissue equivalent that the flexible detector including liquid scintillator may be useful as an absorption dose meter. However, the attenuation length of the previous system was too short to monitor distributions of radioactive isotopes in nuclear facilities because commercial liquid scintillator cocktail was used as it was. Since then, there had been no extended study concerning the flexible light guide itself. Therefore, we have fabricated a flexible radiation detector with liquid scintillator as the core liquid of a light guide and a single PMT (Nomura et al., 2016). It is named as a liquid scintillation light guide (LSLG) detector. The new flexible counter with LSLG developed is expected to be applicable to the detection of γ rays in wide dynamic ranges because the decay time of liquid scintillator is several nanosecond orders. An LSLG counter itself has higher detection efficiency than detectors with plastic fiber and glass fiber guides. Since the light transmission of LSLG is depending on scintillator concentration and tube diameter as well as tube length, it is necessary to clarify the luminescence and transmission properties of various LSLG tubes.
2. Principle Since refraction is one of important factors for scintillation counters, we explain it simply. Refraction is the path bending of the light wave when it passes across the boundary separating two media (core liquid and cladding tube). Refraction is caused by a change in wave speed as the media changes. The principle of LLG follows Snell's law for light refraction:
n1 sin θ1 = n2 sin θ2 where, n1 and n2: refraction index of core liquid and cladding material, respectively, θ1 and θ2: incident and refracting angles from normal to the interface between core liquid and cladding material, respectively. Necessary conditions for reflection in tube are n1 > n2, and for total reflection θ2 ≥ 90°. By the way, when θ2 = 90°, the critical angle for total reflection, θ1, is expressed as sinθ1 = n2/n1. The relationship between liquid refraction index n1 and critical reflection angle calculated as refraction index n2 = 1.34 of cladding material is shown in Fig. 1. For example, the total reflection can be available at the incident angle ≥ 64° when n1 = 1.49. As another important factor for light transmission in a liquid light guide, there is the Lambert-Beer law for a certain light wavelength; Abs= −log T1/T0 =εCL. Where, Abs is absorbance, T0 and T1 are luminescence intensity of core liquid in a tube before and after absorption, respectively, ε is mole absorbance factor, C is concentration of luminescence reagent, and L is length of a light guide. Commercial liquid scintillator cocktails are normally used by mixing with the sample when β particles emitted nuclei such as 3H, 14C and 35S are detected by a liquid scintillation counter. Liquid scintillators contain at least two luminescence reagents because of wave length shift, and further a surfactant to make water sample soluble. The commercial liquid scintillators may be too concentrated to use as the core liquid of an LLG and so should be diluted with toluene and so on. The light absorption in an LSLG tube may not perfectly follow the Lambert-Beer law, and the characteristics of light transmission for LSLG tubes will be shown in a section of consideration.
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Fig. 2. a) Photograph of typical LSLG tube with length (L) = 3 m, inner diameter (D) = 8 mm, and the quartz window. b) The luminescent spectra of light with λ = 420–560 nm observed when LED (λ = 375 nm, 1 mW) light is irradiated to the window of LSLG tubes with D = 7 mm and L = 1 m.
3. Experimental The picture of typical LSLG prepared is shown in Fig. 2a). As the flexible plastic tube, fluorinated ethylene and propylene (FEP) copolymer was used and the inside was cleaned first of all. The refraction index (RI) of FEP is small (1.33–1.34), compatible to RI of water (1.333). The core liquid with the higher RI than FEP cladding is necessary for reflection. The commercial liquid scintillator cocktails such as Ultima Gold (UG: water soluble scintillator) and UGF (UGF: organic scintillator) (PerkinElmer Co.) were used and the luminescence and transmission properties are shown in Fig. 2b). The light transmittance of both LSLG tubes including UG and UGF was relatively lower than the LSLG with scintillator prepared by us although the reflection indexes of UG and UGF were large. It is the reason why the dilution by an organic solvent is necessary. We prepared liquid scintillations by dissolving with toluene using the following fluorescent reagents: Butyl PBD (2-(4-tert-Butyl Phenyl)5-(4-Biphenyl)-1,3,4- oxadiazole: emission wavelength: 363 nm, and absorption wavelength: 304 nm) as the first luminescent reagent, and Bis-MSB (1,4-bis[2-methylstyryl] benzene; emission wave length: 420 nm and absorption wave length: 346 nm) as the second reagent. The toluene solutions with different RI using Butyl PBD and Bis-MSB were adjusted: 1.4980 and 1.4975 at 20 °C. From the light transmittance properties as shown in Fig. 2b) it is clear that the larger RI, the larger transmittance in the case of a toluene solution of Butyl PBD and Bis-MSB. The concentration of Bis-MSB should be less than one tenth of Butyl PBD. An outer protective guide of an inner FEP flexible tube is composed of a flexible Al spiral tube and a poly vinyl chloride black tube. A quartz glass was used as a window material of the tube. One side of the tube was fixed to a photomultiplier (PMT: Hamamatsu: H6612), which was connected to a preamplifier, a linear amplifier (ORTEC: 572A, 590A) and a Multichannel analyzer (ORTEC: Easy-MCA-2K) to get a PHA spectrum. The several standard γ sources of 137Cs were used for the measurement of position sensitivity and spacial radiation.
Fig. 3. PHA spectra of LSLG with length (L): 1 m and diameter (D): 5 mm and 10 mm. Core liquid: toluene solution of Butyl PBD and Bis-MSB, Source: 220 kBq Cs-137, Position: 1.5 cm from center of tube. Point source of 137Cs is located at 5 cm and 95 cm from a PMT head. The displayed count rates were calculated as integral counts in the channel area of ≥ 45 channels because the peak in channel area of < 45 may contain the circuit noise and leakage light. HV: − 1.3 kV.
LSLG tubes with 5 mm and 10 mm in diameter. The detection mechanism is mainly due to Compton scattered electrons arising from the interaction between γ-rays and a core liquid of LSLG tubes. The PHA spectra were enhanced in the high energy channels when the γ-rays were irradiated at the point close to the PMT head because a light transmittance loss was small, but the attenuated spectra were observed when a γ-ray source was far from PMT. This is because luminescent absorption occurs according to the distance under a certain scintillator concentration, based on Lambert Beer's law. However, in the case of a flexible LSLG, the wavelength shift of light was observed due to reflection of light by the cladding tube and due to absorption of two fluorescent reagents. Therefore, it does not follow this rule perfectly, which will be shown in more detail in a discussion section. PHA spectra gave a significantly different shape although the count rate obtained from the total count in the channel regions of 45 or more was not as large as shown in Fig. 3. The peak in the low channel regions of less than 45 may contain circuit noise and leakage light, and was ignored in these spectra at the initial measurement. As a result, the detection efficiency of the 10 mm diameter LSLG was about 3.5 times the detection efficiency of the 5 mm diameter LSLG. The adjusted liquid scintillator of the LSLG detectors is a toluene solution of Butyl PBD +Bis-MSB. Liquid scintillator cocktails (UG and UGF) are available
4. Results 4.1. Properties of LSLG detectors PHA spectra obtained by placing a γ-ray source of 137Cs at a certain distance of LSLG with one meter in length from a PMT head are shown in Fig. 3. The spectra depend on the diameter and source position of 14
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4.2. Measuring dose rates using an LSLG detector Fig. 7 shows the PHA spectra obtained when the LSLG tubes with different lengths of 1 m and 3 m were arranged as a circle and three circles, respectively. The diameter of the tube circle is 30 cm. The count rates of LSLG (30% UG) detector with D = 8 mm and L = 1 m were measured by setting 137Cs source with different activities at the center position of the tube circle and by setting them far from the circle face. Each PHA spectrum in this arrangement is considered to be the average spectrum of PHA spectra measured at different positions of the γ source on the LSLG tube (in Fig. 4). The dose rates were measured using a NaI (Tl) scintillation survey meter (ALOKA: TCS-161). The count rates [(s−1)/(μSv/h)] for different applied high voltages (HV) were measured using a γ source of 137Cs (2 cmø) set at the center of the circle tube of LSLG as shown in Photograph 1. The counting efficiency ratios of 3 m LSLG tube to 1 m LSLG tube were 1.73 for HV= −1.3 kV, 1.63 for HV= =− 1.2 kV, and 1.45 for HV= −1.1 kV, respectively. The counting efficiency of 3 m LSLG tube was less than twice that of 1 m LSLG tube. The counting efficiency ratios decreased with decreasing the applied high voltages. The low efficiency of a long LSLG tube is due to the light transmittance process as described in Section 4.1. The relationship between the counting rate (Y) and the dose rate (X) for LSLG (30% UG, D = 8 mm, L = 1 m) was measured at HV= −1.1 kV by setting the γ source vertically far from one circle face. As shown in Fig. 8a), the following linear relationship was obtained:
Fig. 4. PHA spectra of LSLG (UG: 30% in toluene) detector with 3 m in length and 8 mm in diameter, measured by using a point source (137Cs) located at the displayed distance from a PMT head. HV: − 1.3 kV.
commercially, but they have so large light attenuation. The cocktail was diluted with toluene. PHA spectra were measured using an LSLG detector with an internal tube diameter (D) of 8 mm and a length (L) of 3 m including diluted 30% UG. The full PHA spectra are shown in Fig. 4. In order to survey a peak in the low channel region, PHA spectra in regions of 200 channels or less are shown on a linear scale in Fig. 5a). The peak observed in the low channel region was initially ignored. However, the PHA spectrum can be divided into two channel regions (A, B) as the peak counts are as A and B, respectively. When the count ratio of [B/(A+B)] is plotted as a function of the distance from the head of PMT, a linear relationship is obtained as shown in Fig. 5b). The count ratios of [B/(A+B)] using the data subtracting background counts show a linear relationship with the higher gradient. From the relationship, it was found that the position where a high radiation source exists could be estimated even if two PMT's were not used like a time-of-flight (TOF) method. An LSLG detector loaded with one PMT can be used as socalled a position sensitive detector. The PHA spectra of LSLG tubes with D = 10 mm, L = 1 m, and with D = 5 mm and L = 2 m are shown in Fig. 6a) and c), respectively. The core liquid of these LSLG tubes is a toluene solution of Butyl PBD and Bis-MSB. The transmittance is superior to LSLG tubes including UG. As shown in Fig. 3, the detection efficiency of the large diameter LSLG is high. The background counts of a long LSLG tube are larger than that of a short LSLG tube, especially in low channel regions. The count ratios of B/(A + B) gradually decreases as the distance from the source position on the LSLG tube to the PMT head increases, and the attenuation gradient for the long and small diameter tube is lower than that of the short and large diameter tube of LSLG as shown in Fig. 6b) and d).
Y = a∗X + b,
where a = 300.9 ± 1.4, b = 6.26 ± 26.3.
Therefore, from this relationship, the counting efficiency was 301 [(s−1)/(μSv/h)]. The counting efficiency is almost consistent with the value measured at a center of the circle as shown in Fig. 7. The linear relationship suggests that an LSLG detector with a single PMT can be used as a dose rate meter. In the case of 30% UG LSLG with D = 8 mm, L = 3 m and three circles, the relationship between dose rate and count rate is shown in Fig. 8b). The linear fitting is obtained as follows:
Y = a∗X + b,
where a = 463.3 ± 0.9, b = 100.7 ± 23.7.
The counting efficiency of 3 m LSLG with 3 circles was 463 [(s−1)/ (μSv/h)], which was 1.54 times that of 1 m LSLG with one circle. The counting efficiency ratio is almost consistent with that obtained by setting a source at the center of a LSLG circle. This result suggests that the dynamic range of counting is wide. In the case of 10% UG LSLG with D = 8 mm, L = 1 m and one circle, the following relationship was obtained as shown in Fig. 9:
Fig. 5. a) Low channel PHA spectra of LSLG (30% UG) with D= 8 mm and L= 3 m. BG: 0.06 μSv/h, ROI: 10–45 ch (count A) and > 45 ch. (count B), b) Relationship between count ratios and distances from a photomultiplier tube (PMT) head. 15
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Fig. 6. a) PHA spectra of LSLG with D = 10 mm and L = 1 m, and b) the relationship between distance and count ratio of B/(A + B) in two divided channel regions (A, B). c) PHA spectra of LSLG with D = 5 mm and L = 2 m, and d) the relationship between distance and count ratio of B/(A+B). The upper linear fitting lines after subtracting background counts are shown in b), and d), respectively. Core liquid: toluene solution of Butyl PBD and Bis-MSB, Distance: from PMT head to 137Cs source. HV: − 1.3 kV.
Y = a*X + b,
where a = 290.2 ± 7.9, b = 6.26 ± 26.3.
position sensitive detector and the diluted scintillator is used. The emission lifetime of the liquid scintillator is about several nanoseconds (ns), which is much shorter than other solid scintillators. An LSLG detector can be applied to the radiation measurement with high counting rates and wide dynamic ranges. It was confirmed previously that there was no loss of light transmittance up to 1.5 × 104 Gy by γ irradiation of 60 Co (Hayashi et al., 2008). In the high dose field of nuclear reactors, the solid scintillators themselves and electronic circuit parts are susceptible to radiation damage, but LSLG itself is less deteriorated by radiation. Further, since the electronic circuit parts are far from the LSLG detection parts, it is possible to avoid from radiation damage to the electronic circuit portion in the measurement of the high radiation field. As the scintillator concentration increases, the detection efficiency improves, but the attenuation rate of the PHA spectrum increases with increasing the distance between PMT head and point source. The optimum concentration varies depending on the length and diameter of the flexible tube. Conversely, it is easy to manufacture one with different attenuation factors of PHA spectra depending on the main purpose, such as whether it is for a dosimeter or a position sensitive detector. A flexible detector including such a liquid scintillator has another new application possibility. For example, arraying a number of flexible LSLG tubes enables to make a soft and flexible plate detector. It would be useful to inspect radiation as a flexible and flat detector covering the surface of human body and animals contaminated (Nomura, 2018).
−1
The counting efficiency of 10% UG LSLG was 290 [(s )/(μSv/h)], which was not so small as that of 30% UG LSLG (Fig. 8a) although the concentration of scintillator in the 10% UG LSLG was one third of that of the 30% UG LSLG. This may be because the luminescence transmittance of 10% UG LSLG is superior to that of 30% UG LSLG. 5. Considerations LED light with λ = 375 nm was vertically irradiated at different positions on the tube side of unshielded LSLG in dark room as shown in Fig. 10a), and the emission spectra were measured at the end of tube. As shown in Fig. 10b), it was found that the intensity of the emitted light varies depending on the distance from the light detector. As shown in Fig. 10c) and d), the light attenuation rate varies depending on the wave length of the emitted light. As the distance of source light increases, the wavelength of emission light shifts to the long wavelength and the intensity decreases at the short wavelength (λ = 426 nm) larger than at the long wavelength (λ = 456 nm). This is the reason why the transmission of emission light in the LSLG does not follow Lambert-Beer law perfectly. Regarding the temperature quenching of liquid scintillator, it has been reported that a quenching ratio of about 10% is observed for temperature changes from 0 °C to 30 °C (Katou, 2003). In the case of LSLG, it is considered that the influence of the quenching becomes small because the counting ratio of two channel regions is used as a 16
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Fig. 7. a) Arrangement of 3 m LSLG tube for measurement of radiation dose rate and PHA spectra of b) 1 m LSLG (30% UG, D=8 mm) with one circle, and c) 3 m LSLG (30% UG, D=8 mm) with 3 circles (after each background was subtracted). The counting efficiency ratios are 665.1/385.0 = 1.73, 573.1/350.8 = 1.63, and 445.9/308.2 = 1.45 for HV= −1.3, − 1.2, − 1.1 kV, respectively.
Fig. 8. The relationship between count rate of LSLG and dose rate by NaI (Tl) scintillation survey meter. a) 1 m LSLG (30% UG, D=8 mm) with one circle. b) 3 m LSLG (30% UG, D=8 mm) with three circles. The counting efficiency ratio of 3 m LSLG to 1 m LSLG is 463.3/300.9 = 1.54.
6. Conclusion
monitoring the contamination inside pipes or tubes used in nuclear and radioisotope facilities, the activity distribution of contaminated ground, the leaked γ-ray surrounding an accelerator and a reactor as a room monitor, and as a high radiation field monitor in destroyed reactors. Thus, an LSLG detector would be useful for an absorption dosimeter as well as a position sensitive detector for high radiation dose distribution, especially at the events such as a nuclear accident and a disaster emergency.
LSLG is so superior in flexibility, less damage and high efficiency as compared with a conventional solid scintillator fiber. It was found from analysis of PHA spectra that an LSLG detector can be used as a position sensitive detector using a single photomultiplier. It was further shown that an LSLG detector is able to estimate the dose rates of radiation by arranging the LSLG tube into a circle shape. An LSLG detector has large potential to apply to many fields because of its flexibility and easy fabrication. An LSLG detector would be expected to be used for
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Fig. 9. a) PHA spectra (after background was subtracted) and b) the relationship between counting rate of LSLG and dose rate by NaI(Tl) scintillation survey meter. LSLG: 10% UG in toluene, D = 8 mm, and L = 1 m.
Fig. 10. a) Arrangement of LSLG tube and LED light, b) Emission spectra of LSLG (30% UG) with D= 5 mm and L= 2 m, obtained when LED light (λ = 375 nm) were irradiated vertically at different positions of unshielded LSLG tube in dark room, c) Attenuation of emission intensity at λ = 426 nm, and d) Attenuation of emission intensity at λ = 456 nm, depending on distance from light detector (Hamamatsu Photonics Co.: PMA-12).
Acknowledgements
References
Authors would like to appreciate Prof. Chiaki Terashima in Photocatalytic International Research Center, Tokyo University of Science, Mr. Kazuo Hosoda and Mr. Daisuke Yokota in U-VIX Co. for helping to prepare LLG tubes, and Profs. Takayuki Terai and Hiroyuki Takahashi in School of Engineering, the University of Tokyo, for supporting the radiation experiments.
Hayashi, M., Kawarabayashi, J., Asai, K., Iwai, H., Nakagawa, Y., Iguchi, T., 2008. Position-sensitive radiation detector with flexible light guide and liquid organic scintillator to monitor distributions of radioactive isotopes. J. Nucl. Sci., Technol. Suppl. 6, 81–84. Imai, S., Soramoto, S., Mochiki, K., Iguchi, T., Nakazawa, M., 1991. New radiation detector of plastic scintillation fiber. Rev. Sci. Instrum. 64, 1093–1097. Katou, T., 2003. Influence of temperature to quenching on liquid scintillation measurement” (Japanese). Radioisotopes 52, 335–339. Kawarabayashi, J., Mizuno, R., Inui, D., Watanabe, K., Iguchi, T., 2004. Potential on liquid light guide as distributed radiation sensor, In: Proceedings of the IEEE Nuclear
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review). Radioisotopes 43, 423–431. Nomura, K., 2018. Recommendation on development of a flexible radiation detector including liquid scintillator(Japanese). FBNews 493, 8–12. Nomura, K., Yunoki, A., Hosoda, K., Yokota, D., Morito, Y., Terai, T., Terashima, C., Fujishima, A., 2016. A new flexible γ-ray detector using a liquid scintillator light guide (LSLG), In: Proceedings of the 12th International Workshop on Ionizing Radiation Monitoring, p-22, Hosoda Hall, Oarai, Dec, 3–4.
Science Symposium Conference Record, Oct. 16–22 pp. 712–714. Maekawa, T., Makino, S., Sumita, A., Goto, T., 2011. Long scintillation detector using composite light guide for β-ray survey measurement. J. Nucl. Sci. Tech. 48, 50–59. Naka, R., Watanabe, K., Kawarabayashi, J., Uritani, A., Iguchi, T., Hayashi, N., Kojima, N., Yoshida, T., Kaneko, J., Takeuchi, H., Kakuta, T., 2001. Radiation distribution sensing with normal optical fiber. IEEE Trans. Nucl. Sci. 48 (6), 2348–2351. Nakazawa, M., 1994. Radiation Measurement Using Optical Techniques, (Japanese
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