Applied Radiation and Isotopes 86 (2014) 36–40
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Poly (ether sulfone) as a scintillation material for radiation detection Hidehito Nakamura a,b,n, Yoshiyuki Shirakawa b, Hisashi Kitamura b, Nobuhiro Sato a, Sentaro Takahashi a a b
Kyoto University, 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan
H I G H L I G H T S
PES PES The The The
is characterised as a scintillation material for radiation detection. has an emission maximum at 350 nm. effective refractive index for PES is 1.74 based on its emission spectrum. light yield of PES is 2.21 that of PET and 0.31 times that of PEN. PES response to 5486 (6118) keV alpha particles is 546781 (598 783) keV electron equivalents.
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2013 Received in revised form 13 December 2013 Accepted 14 December 2013 Available online 2 January 2014
Considerable attention has been drawn to the advantages of using aromatic ring polymers for scintillation materials in radiation detection. Thus, it is important to identify and characterise those with the best potential. Here, we characterise poly (ether sulfone) (PES), which is an amber-coloured transparent resin that possesses sulfur as a main component and has a density of 1.37 g/cm3. PES emits short-wavelength light with a 350-nm maximum. By taking into account its emission spectrum, we demonstrate that its effective refractive index is 1.74. Light yield distributions generated by 137Cs and 207 Bi radioactive sources were obtained. PES has a light yield that is 2.21 times that of poly (ethylene terephthalate), and 0.31 times that of poly (ethylene naphthalate). The energy response to 5486 keV alpha particles emitted from 241Am was 5467 81 keV electron equivalents (keVee), while the energy resolution was 17.070.1%. The energy response to 6118 keV alpha particles emitted from 252Cf was 598 783 keVee, while the energy resolution was 16.0 70.1%. Overall, PES has potential for use as a scintillation material in radiation detection. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Poly (ether sulfone) Aromatic ring polymer Refractive index Light yield Alpha response
1. Introduction Aromatic ring polymers have been used as organic scintillation materials for radiation detection for many years (Beringer, 2012, Knoll, 2010; Leo, 1992). In many cases, they have been doped with various fluorescent guest molecules. Important characteristics that determine the performance of a scintillation material for radiation detection are its density, emission and excitation wavelengths, refractive index, and light yield. The refractive index is particularly important because it directly affects light propagation to the detector. Thus, there is a considerable effort worldwide to develop organic scintillation materials that have favourable characteristics (Nakamura et al., 2012a, 2013a). In addition, advanced refining techniques have n Corresponding author at: Kyoto University, 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan. Tel./fax: þ 81 72 451 2463. E-mail address: [email protected] (H. Nakamura).
0969-8043/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.12.028
provided high-purity aromatic ring polymers (Nakamura et al., 2013b). In past few years, we have demonstrated that both undoped poly (ethylene terephthalate) (PET) and poly (ethylene naphthalate) (PEN) possess several properties suitable for radiation detection (Nakamura et al., 2010, 2011, 2012b). These results drew global attention to the advantages of undoped scintillation materials in radiation detectors (Kumar et al., 2012, Nakamura et al., 2013c, 2013d; Nagata et al., 2013a, 2013b; Sen et al., 2012). It is thus important identify potential scintillation materials by surveying the huge variety of aromatic ring polymers for suitable characteristics, and to summarise those characteristics. The performance of a survey metre using PEN has also been reported, as well as that of blended PET and PEN (Nakamura et al., 2013e; Nakamura et al., in press; Shirakawa et al., 2013a). PET has a benzene ring and PEN has a naphthalate ring in each repeat unit. Poly (ether sulfone) (PES) possesses sulfur as its main
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
37
components and the repeat unit is:
Here, we assess undoped PES for radiation detection by characterising its emission and excitation spectra, refractive index, and light yields. We also characterise its response to alpha particles. Overall, undoped PES is a candidate as a scintillation material for radiation detection.
2. Materials and methods A 31 31 5 mm PES plate (4100G; Sumitomo Chemical Co., Ltd.) was prepared. The amber-coloured PES emits slightly blue light as shown in Fig. 1. It is stable at high temperatures. The density of PES is 1.37 g/cm3, which is higher than that of either PET or PEN (both 1.33 g/cm3). Its emission and excitation spectra were obtained with a fluorescence spectrometer (F-2700; Hitachi HighTechnologies Co.). Refractive indices were determined with a refractometer (PR-2; Carl Zeiss, Jena, Germany) at the C line of a hydrogen lamp (656 nm), the D line of a sodium lamp (589 nm), the F line of a hydrogen lamp (486 nm), and the g line of a mercury lamp (436 nm).
Fig. 2. Schematic of the arrangement for measuring light yields in PES.
Fig. 3. Fluorescence from PES. The correlation between the emission and excitation wavelengths is shown. The maximum peaks in the emission and excitation spectra are denoted by white lines in each axis.
Fig. 1. A 31 31 5 mm poly (ether sulfone) plate. PES is an amber-coloured transparent resin (top). PES emits slightly blue light when excited by ultra-violet light (bottom).
The experimental arrangement for measuring light yields is shown in Fig. 2. A photomultiplier tube (PMT; R878-SBA, Hamamatsu Photonics Co., Ltd.) was used as a photodetector. One 31 31 mm PES face was interfaced with the PMT window via a very thin layer of optical grease (BC-630; Saint-Gobain Ceramics & Plastic Inc.), while a radioactive source was positioned in the centre of the opposite face. Two radioactive sources, 137Cs and 207 Bi, both of which emit monoenergetic internal conversion electrons, were used to obtain the relationship between the light yield in PES and the radiation energy. The light yield for alpha particles was then evaluated with 241Am and 252Cf radioactive sources that do not emit background beta particles or gamma-rays. A data acquisition system was constructed from several CAMAC and NIM modules. The output signals from the PMT were directly digitised by a charge-sensitive analogue-to-digital converter module (RPC022, REPIC Co.).
38
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
Emission Excitation
Fig. 4. Emission and excitation spectra for PES. The emission spectrum was acquired at the 315-nm excitation maximum, and the excitation spectrum was monitored at the 350-nm emission maximum. Fig. 6. Light-yield distribution of PES when excited by radiation from a 137Cs radioactive source. The peak in the distribution is from 624-keV internal conversion electrons. The counts for the low light-yield region are from 514 keV beta particles and Compton recoil electrons generated by 662 keV gamma-rays.
Fig. 5. Refractive indices of PES at various wavelengths. The highlighted region (light blue) shows that the emission wavelengths dominate. The emission maximum is 350 nm.
3. Results and discussion PES fluorescence spectra are shown in Fig. 3. The emission and excitation maxima are 350 nm and 315 nm, respectively, and the emission is compatible with the short-wavelength sensitivity of the PMT. The emission maximum is at a shorter wavelength than that for either PET or PEN. Fig. 4 shows the fluorescence spectrum at 315-nm excitation and the excitation spectrum monitored at 350 nm. The refractive index (ND) of PES at the D line of the sodium lamp is 1.65, which is the same as that of PEN but larger that of PET. However, PES does not emit light in the 589-nm region, as indicated in Fig. 3. Refractive indices of PES are plotted as a function of wavelength in Fig. 5 and follow the Sellmeier
Fig. 7. Light-yield distribution of PES when excited by radiation from a 207Bi radioactive source. The sharp peak is from 976-keV internal conversion electrons. The small peak of 482-keV internal conversion electrons is included in the shoulder in the low light-yield region.
dispersion function (Sellmeier, 1871). From these data, we obtain an effective refractive index Neff ¼1.74 for the wide range of light emitted by taking into account the emission spectrum, instead of simply ND ¼1.65 at 589 nm (Nakamura et al., 2013b). Fig. 6 plots the light yield distributions in PES generated by the 137 Cs radioactive source, where the peak corresponds to 624 keV conversion electrons. The counts for the low light-yield region are from 514 keV beta particles and Compton recoil electrons generated by 662 keV gamma-rays. Similarly, Fig. 7 plots the light-yield
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
39
Table 1 Comparison of PES characteristics with those of PET and PEN. Substrate Formula
PET PES PEN
Density (g/cm3)
1.33 C10H8O4 C12H8O3S1 1.37 C14H10O4 1.33
Emission maximum (nm)
Refractive index (ND)
Light yield (normalised)
385 350 425
1.57 1.65a 1.65
0.14 0.31 1
a The effective refractive index (Neff) taking the emission spectrum into account is 1.74.
207
Bi
137
Cs
Fig. 9. Light-yield distribution of PES when excited by radiation from a 241Am radioactive source. The peak in the distribution indicates 5486 keV alpha particles.
Fig. 8. Plot of the peak values in the light-yield distributions vs. internal-conversion electron energy.
distributions generated by the 207Bi radioactive source, where the sharp peak corresponds to 976 keV internal conversion electrons. The small peak from 482 keV internal conversion electrons can be seen in the shoulder in the low light-yield region. The results indicate that the light yield of PES is 2.21 times that of PET and 0.31 times that of PEN. The characteristics for PES, PET, and PEN are summarised in Table 1. Fig. 8 displays the linear regression fit between the peak values in the two light-yield distributions and the energies of the internal conversion electrons. For organic scintillation materials, the light yield for charged particles per unit energy is less than that for electrons per unit energy (Knoll, 2010). Thus, the term “keV electron equivalent” (keVee) is a special nomenclature used to characterise the light yield for alpha particles in terms of electron energy. Fig. 9 plots the light-yield distribution excited by the 241 Am radioactive source. The peak is generated by 5486 keV alpha particles, and is found to be 546 781 keVee by the electron energy response in Fig. 8. The energy resolution (s) for the alpha particle peak was 17.0 70.1%. Similarly, Fig. 10 plots the light-yield distribution for the alpha particles emitted from the 252Cf radioactive source. The peak is generated by 6118 keV alpha particles, and the alpha response was 598 783 keVee, with s ¼16.0 70.1%. It is important to note that the light yields from PES are in the 4–6 MeV energy range of most alpha particles emitted from radioisotopes. The data are summarised in Table 2.
Fig. 10. Light-yield distribution of PES when excited by radiation from a 252Cf radioactive source. The peak in the distribution indicates 6118 keV alpha particles.
The blending of two types of polymers enhances the strength of each and mitigates their weaknesses (Nakamura et al., 2013e). For example, the high density of PES is advantageous and it is compatible with PEN because their refractive indices are similar. In addition, PES colouration could be mitigated, and its short emission wavelength and low light yield could be improved by blending. Blending of PES and PEN will be reported in detail elsewhere. Also, it is important to note that under high-dose environments, doped fluorescent guest molecules in organic scintillation materials can lead to significant performance degradation. Because PES does not require doping, it should be much more stable in these environments.
4. Conclusions We have characterised PES without the need to doping. PES has an emission maximum at 350 nm.
40
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
Table 2 Response of PES to alpha particles. Substrate
PES a b
Fluorescent
Undoped
241
Ama
252
Cfb
Energy response
Resolution (r)
Energy response
Resolution (r)
546 7 81 keVee
17.0 70.1%
598 7 83 keVee
16.0 7 0.1%
241
Primarily, 5486 keV alpha particles are emitted from the Am radioactive source. Primarily, 6118 keV alpha particles are emitted from the 252Cf radioactive source.
By taking into account its emission spectrum, its effective refractive index is 1.74. It has a light yield that is 2.21 times that of PET, and 0.31 times that of PEN. The linearity of the light yield in PES is within the energy region for most alpha particles emitted from radioisotopes. The energy response to 5486 keV alpha particles emitted from 241Am was 546781 keVee, while the energy resolution was 17.070.1%. The energy response to 6118 keV alpha particles emitted from 252Cf was 598783 keVee, while the energy resolution was 16.070.1%. We have shown that PES can be used in radiation detection, and could be a potential component for base substrate blending. Overall, this knowledge increases the number of available organic scintillation materials and should promote the application of PES in radiation detectors (Shirakawa, 2005; Shirakawa et al., 2013b).
Acknowledgements This research was supported by the Kyoto University and the National Institute of Radiological Sciences. The authors thank the KUR Research Program for the Scientific Basis of Nuclear Safety for partial support at this work. The authors are grateful to Dr. T. Murata, Dr. T. Fukunaga, Dr. H. Yamana, Dr. T. Arima, Dr. S. Kobayashi and Ms. M. Yasaku for their cooperation. References Beringer, J., 2012. Particle data group. Phys. Rev. D 86 (010001). Knoll, G.F., 2010. Radiation Detection and Measurement, fourth ed. Wiley New York. Kumar, V., Ali, Y., Sonkawade, R.G., Dhaliwal, A.S., 2012. Nucl. Instrum. Method Phys. Res. B 287, 10. Leo, W.R., 1992. Techniques for Nuclear and Particle Physics Experiments: A How-to Approach, second ed. Springer-Verlag, Berlin and Heidelberg. Nakamura, H., Kitamura, H., Shinji, O., Saito, K., Shirakawa, Y., Takahashi, S., 2012a. Development of polystyrene-based scintillation materials and its mechanisms. Appl. Phys. Lett. 101, 261110, http://dx.doi.org/10.1063/1.4773298. Nakamura, H., Shirakawa, Y., Kitamura, H., Sato, N., Shinji, O., Saito, K., Takahashi, S., 2013a. Mechanism of wavelength conversion in polystyrene doped with
benzoxanthene: emergence of a complex. Sci. Rep. 3, 2502, http://dx.doi.org/ 10.1038/srep02502. Nakamura, H., Shirakawa, Y., Kitamura, H., Sato, N., Shinji, O., Saito, K., Takahashi, S., 2013b. Light propagation characteristics of high-purity polystyrene. Appl. Phys. Lett. 103, 161111, http://dx.doi.org/10.1063/1.4824467. Nakamura, H., Kitamura, H., Hazama, R., 2010. Radiation measurements with heatproof polyethylene terephthalate bottles. Proc. R. Soc. A 466, 2847, http://dx. doi.org/10.1098/rspa.2010.0118. Nakamura, H., Shirakawa, Y., Takahashi, S., Shimizu, H., 2011. Evidence of deep-blue photon emission at high efficiency by common plastic. EPL (Europhys. Lett.) 95 (2), 22001, http://dx.doi.org/10.1209/0295-5075/95/22001. Nakamura, H., Shirakawa, Y., Takahashi, S., Yamano, T., Kobayashi, Y., Hazama, R., Takagi, C., Hasebe, O., 2012b. Cheap educational materials for understanding radiation. Phys. Educ. 47, 17, http://dx.doi.org/10.1088/0031-9120/47/1/F07. Nakamura, H., Yamada, T., Shirakawa, Y., Kitamura, H., Shidara, Z., Yokozuka, T., Nguyen, P., Kanayama, M., Takahashi, S., 2013c. Optimized mounting of a polyethylene naphthalate scintillation material in a radiation detector. Appl. Radiat. Isot. 80, 84, http://dx.doi.org/10.1016/j.apradiso.2013.06.011. Nakamura, H., Shirakawa, Y., Yamada, T., Nguyen, P., Takahashi, S., 2013d. Senses alone cannot detect different properties. Phys. Educ. 48, 556, http://dx.doi.org/ 10.1088/0031-9120/48/5/F02. Nakamura, H., Shirakawa, Y., Kitamura, H., Yamada, T., Shidara, Z., Yokozuka, T., Nguyen, P., Takahashi, T., Takahashi, S., 2013e. Blended polyethylene terephthalate and polyethylene naphthalate polymers for scintillation base substrates. Radiat. Meas. 59, 172, http://dx.doi.org/10.1016/j.radmeas.2013.06.006. Nakamura, H., Shirakawa, Y., Sato, N., Takahashi, S., 2014. Characterizing radiation spectra with stacked plastic sheets. Phys. Educ.. (in press). Nagata, S., Katsui, H., Hoshi, K., Tsuchiya, B., Toh, K., Zhao, M., Shikama, T., Hodgson, E.R., 2013a. Recent research activities on functional ceramics for insulator, breeder and optical sensing systems in fusion reactors. J. Nucl. Mater. 442 (Supplement 1), S501. Nagata, S., Mitsuzuka, M., Onodera, S., Yaegashi, T., Hoshi, K., Zhao, M., Shikama, T., 2013b. Damage and recovery processes for the luminescence of irradiated PEN films. Nucl. Instrum. Method Phys. Res. B 315, 157. Sellmeier, W., 1871. Zur Erklärung der abnormen Farbenfolge im Spectrum einiger Substanzen. Ann. Phys. 219, 272. Sen, I., Urffer, M., Penumadu, D., Young, S.A., Miller, L.F., Mabe, A.N., 2012. Polyester composite thermal neutron scintillation films. IEEE Trans. Nucl. Sci. 59 (4), 1781. Shirakawa, Y., 2005. Quick response of a survey meter in static condition. Radioisotopes 54, 199, http://dx.doi.org/10.3769/radioisotopes.54.199. Shirakawa, Y., Nakamura, H., Kamata, T., Watai, K., Mitsunaga, M., Shidara, Z., Murakawa, F., 2013a. Radiation counting characteristics on surface-modified polyethylene naphthalate scintillators. Radioisotopes 62, 879, http://dx.doi.org/ 10.3769/radioisotopes.62.879. Shirakawa, Y., Nakamura, H., Kamata, T., Watai, K., 2013b. A fast response radiation detector based on a response prediction method for decontamination. Radiat. Meas. 49, 115, http://dx.doi.org/10.1016/j.radmeas.2012.12.001.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Poly (ether sulfone) as a scintillation material for radiation detection Hidehito Nakamura a,b,n, Yoshiyuki Shirakawa b, Hisashi Kitamura b, Nobuhiro Sato a, Sentaro Takahashi a a b
Kyoto University, 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan
H I G H L I G H T S
PES PES The The The
is characterised as a scintillation material for radiation detection. has an emission maximum at 350 nm. effective refractive index for PES is 1.74 based on its emission spectrum. light yield of PES is 2.21 that of PET and 0.31 times that of PEN. PES response to 5486 (6118) keV alpha particles is 546781 (598 783) keV electron equivalents.
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2013 Received in revised form 13 December 2013 Accepted 14 December 2013 Available online 2 January 2014
Considerable attention has been drawn to the advantages of using aromatic ring polymers for scintillation materials in radiation detection. Thus, it is important to identify and characterise those with the best potential. Here, we characterise poly (ether sulfone) (PES), which is an amber-coloured transparent resin that possesses sulfur as a main component and has a density of 1.37 g/cm3. PES emits short-wavelength light with a 350-nm maximum. By taking into account its emission spectrum, we demonstrate that its effective refractive index is 1.74. Light yield distributions generated by 137Cs and 207 Bi radioactive sources were obtained. PES has a light yield that is 2.21 times that of poly (ethylene terephthalate), and 0.31 times that of poly (ethylene naphthalate). The energy response to 5486 keV alpha particles emitted from 241Am was 5467 81 keV electron equivalents (keVee), while the energy resolution was 17.070.1%. The energy response to 6118 keV alpha particles emitted from 252Cf was 598 783 keVee, while the energy resolution was 16.0 70.1%. Overall, PES has potential for use as a scintillation material in radiation detection. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Poly (ether sulfone) Aromatic ring polymer Refractive index Light yield Alpha response
1. Introduction Aromatic ring polymers have been used as organic scintillation materials for radiation detection for many years (Beringer, 2012, Knoll, 2010; Leo, 1992). In many cases, they have been doped with various fluorescent guest molecules. Important characteristics that determine the performance of a scintillation material for radiation detection are its density, emission and excitation wavelengths, refractive index, and light yield. The refractive index is particularly important because it directly affects light propagation to the detector. Thus, there is a considerable effort worldwide to develop organic scintillation materials that have favourable characteristics (Nakamura et al., 2012a, 2013a). In addition, advanced refining techniques have n Corresponding author at: Kyoto University, 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan. Tel./fax: þ 81 72 451 2463. E-mail address: [email protected] (H. Nakamura).
0969-8043/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.12.028
provided high-purity aromatic ring polymers (Nakamura et al., 2013b). In past few years, we have demonstrated that both undoped poly (ethylene terephthalate) (PET) and poly (ethylene naphthalate) (PEN) possess several properties suitable for radiation detection (Nakamura et al., 2010, 2011, 2012b). These results drew global attention to the advantages of undoped scintillation materials in radiation detectors (Kumar et al., 2012, Nakamura et al., 2013c, 2013d; Nagata et al., 2013a, 2013b; Sen et al., 2012). It is thus important identify potential scintillation materials by surveying the huge variety of aromatic ring polymers for suitable characteristics, and to summarise those characteristics. The performance of a survey metre using PEN has also been reported, as well as that of blended PET and PEN (Nakamura et al., 2013e; Nakamura et al., in press; Shirakawa et al., 2013a). PET has a benzene ring and PEN has a naphthalate ring in each repeat unit. Poly (ether sulfone) (PES) possesses sulfur as its main
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
37
components and the repeat unit is:
Here, we assess undoped PES for radiation detection by characterising its emission and excitation spectra, refractive index, and light yields. We also characterise its response to alpha particles. Overall, undoped PES is a candidate as a scintillation material for radiation detection.
2. Materials and methods A 31 31 5 mm PES plate (4100G; Sumitomo Chemical Co., Ltd.) was prepared. The amber-coloured PES emits slightly blue light as shown in Fig. 1. It is stable at high temperatures. The density of PES is 1.37 g/cm3, which is higher than that of either PET or PEN (both 1.33 g/cm3). Its emission and excitation spectra were obtained with a fluorescence spectrometer (F-2700; Hitachi HighTechnologies Co.). Refractive indices were determined with a refractometer (PR-2; Carl Zeiss, Jena, Germany) at the C line of a hydrogen lamp (656 nm), the D line of a sodium lamp (589 nm), the F line of a hydrogen lamp (486 nm), and the g line of a mercury lamp (436 nm).
Fig. 2. Schematic of the arrangement for measuring light yields in PES.
Fig. 3. Fluorescence from PES. The correlation between the emission and excitation wavelengths is shown. The maximum peaks in the emission and excitation spectra are denoted by white lines in each axis.
Fig. 1. A 31 31 5 mm poly (ether sulfone) plate. PES is an amber-coloured transparent resin (top). PES emits slightly blue light when excited by ultra-violet light (bottom).
The experimental arrangement for measuring light yields is shown in Fig. 2. A photomultiplier tube (PMT; R878-SBA, Hamamatsu Photonics Co., Ltd.) was used as a photodetector. One 31 31 mm PES face was interfaced with the PMT window via a very thin layer of optical grease (BC-630; Saint-Gobain Ceramics & Plastic Inc.), while a radioactive source was positioned in the centre of the opposite face. Two radioactive sources, 137Cs and 207 Bi, both of which emit monoenergetic internal conversion electrons, were used to obtain the relationship between the light yield in PES and the radiation energy. The light yield for alpha particles was then evaluated with 241Am and 252Cf radioactive sources that do not emit background beta particles or gamma-rays. A data acquisition system was constructed from several CAMAC and NIM modules. The output signals from the PMT were directly digitised by a charge-sensitive analogue-to-digital converter module (RPC022, REPIC Co.).
38
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
Emission Excitation
Fig. 4. Emission and excitation spectra for PES. The emission spectrum was acquired at the 315-nm excitation maximum, and the excitation spectrum was monitored at the 350-nm emission maximum. Fig. 6. Light-yield distribution of PES when excited by radiation from a 137Cs radioactive source. The peak in the distribution is from 624-keV internal conversion electrons. The counts for the low light-yield region are from 514 keV beta particles and Compton recoil electrons generated by 662 keV gamma-rays.
Fig. 5. Refractive indices of PES at various wavelengths. The highlighted region (light blue) shows that the emission wavelengths dominate. The emission maximum is 350 nm.
3. Results and discussion PES fluorescence spectra are shown in Fig. 3. The emission and excitation maxima are 350 nm and 315 nm, respectively, and the emission is compatible with the short-wavelength sensitivity of the PMT. The emission maximum is at a shorter wavelength than that for either PET or PEN. Fig. 4 shows the fluorescence spectrum at 315-nm excitation and the excitation spectrum monitored at 350 nm. The refractive index (ND) of PES at the D line of the sodium lamp is 1.65, which is the same as that of PEN but larger that of PET. However, PES does not emit light in the 589-nm region, as indicated in Fig. 3. Refractive indices of PES are plotted as a function of wavelength in Fig. 5 and follow the Sellmeier
Fig. 7. Light-yield distribution of PES when excited by radiation from a 207Bi radioactive source. The sharp peak is from 976-keV internal conversion electrons. The small peak of 482-keV internal conversion electrons is included in the shoulder in the low light-yield region.
dispersion function (Sellmeier, 1871). From these data, we obtain an effective refractive index Neff ¼1.74 for the wide range of light emitted by taking into account the emission spectrum, instead of simply ND ¼1.65 at 589 nm (Nakamura et al., 2013b). Fig. 6 plots the light yield distributions in PES generated by the 137 Cs radioactive source, where the peak corresponds to 624 keV conversion electrons. The counts for the low light-yield region are from 514 keV beta particles and Compton recoil electrons generated by 662 keV gamma-rays. Similarly, Fig. 7 plots the light-yield
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
39
Table 1 Comparison of PES characteristics with those of PET and PEN. Substrate Formula
PET PES PEN
Density (g/cm3)
1.33 C10H8O4 C12H8O3S1 1.37 C14H10O4 1.33
Emission maximum (nm)
Refractive index (ND)
Light yield (normalised)
385 350 425
1.57 1.65a 1.65
0.14 0.31 1
a The effective refractive index (Neff) taking the emission spectrum into account is 1.74.
207
Bi
137
Cs
Fig. 9. Light-yield distribution of PES when excited by radiation from a 241Am radioactive source. The peak in the distribution indicates 5486 keV alpha particles.
Fig. 8. Plot of the peak values in the light-yield distributions vs. internal-conversion electron energy.
distributions generated by the 207Bi radioactive source, where the sharp peak corresponds to 976 keV internal conversion electrons. The small peak from 482 keV internal conversion electrons can be seen in the shoulder in the low light-yield region. The results indicate that the light yield of PES is 2.21 times that of PET and 0.31 times that of PEN. The characteristics for PES, PET, and PEN are summarised in Table 1. Fig. 8 displays the linear regression fit between the peak values in the two light-yield distributions and the energies of the internal conversion electrons. For organic scintillation materials, the light yield for charged particles per unit energy is less than that for electrons per unit energy (Knoll, 2010). Thus, the term “keV electron equivalent” (keVee) is a special nomenclature used to characterise the light yield for alpha particles in terms of electron energy. Fig. 9 plots the light-yield distribution excited by the 241 Am radioactive source. The peak is generated by 5486 keV alpha particles, and is found to be 546 781 keVee by the electron energy response in Fig. 8. The energy resolution (s) for the alpha particle peak was 17.0 70.1%. Similarly, Fig. 10 plots the light-yield distribution for the alpha particles emitted from the 252Cf radioactive source. The peak is generated by 6118 keV alpha particles, and the alpha response was 598 783 keVee, with s ¼16.0 70.1%. It is important to note that the light yields from PES are in the 4–6 MeV energy range of most alpha particles emitted from radioisotopes. The data are summarised in Table 2.
Fig. 10. Light-yield distribution of PES when excited by radiation from a 252Cf radioactive source. The peak in the distribution indicates 6118 keV alpha particles.
The blending of two types of polymers enhances the strength of each and mitigates their weaknesses (Nakamura et al., 2013e). For example, the high density of PES is advantageous and it is compatible with PEN because their refractive indices are similar. In addition, PES colouration could be mitigated, and its short emission wavelength and low light yield could be improved by blending. Blending of PES and PEN will be reported in detail elsewhere. Also, it is important to note that under high-dose environments, doped fluorescent guest molecules in organic scintillation materials can lead to significant performance degradation. Because PES does not require doping, it should be much more stable in these environments.
4. Conclusions We have characterised PES without the need to doping. PES has an emission maximum at 350 nm.
40
H. Nakamura et al. / Applied Radiation and Isotopes 86 (2014) 36–40
Table 2 Response of PES to alpha particles. Substrate
PES a b
Fluorescent
Undoped
241
Ama
252
Cfb
Energy response
Resolution (r)
Energy response
Resolution (r)
546 7 81 keVee
17.0 70.1%
598 7 83 keVee
16.0 7 0.1%
241
Primarily, 5486 keV alpha particles are emitted from the Am radioactive source. Primarily, 6118 keV alpha particles are emitted from the 252Cf radioactive source.
By taking into account its emission spectrum, its effective refractive index is 1.74. It has a light yield that is 2.21 times that of PET, and 0.31 times that of PEN. The linearity of the light yield in PES is within the energy region for most alpha particles emitted from radioisotopes. The energy response to 5486 keV alpha particles emitted from 241Am was 546781 keVee, while the energy resolution was 17.070.1%. The energy response to 6118 keV alpha particles emitted from 252Cf was 598783 keVee, while the energy resolution was 16.070.1%. We have shown that PES can be used in radiation detection, and could be a potential component for base substrate blending. Overall, this knowledge increases the number of available organic scintillation materials and should promote the application of PES in radiation detectors (Shirakawa, 2005; Shirakawa et al., 2013b).
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