Detection of alpha particles with undoped poly (ethylene naphthalate)

Hidehito Nakamura a,b,n, Yoshiyuki Shirakawa b, Hisashi Kitamura b,Nobuhiro Sato a, Sentaro Takahashi a

a Kyoto University, 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japanb National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan

a r t i c l e i n f o

Article history:Received 18 September 2013Received in revised form22 November 2013Accepted 5 December 2013Available online 16 December 2013

Keywords:Poly (ethylene naphthalate)Aromatic ring polymerRefractive indexLight yieldAlpha response

a b s t r a c t

There has been recent interest in the use of undoped, aromatic-ring polymers as organic scintillationmaterials for radiation detectors. Here, we characterise the response of poly (ethylene naphthalate) (PEN)to alpha particles. The energy response to 5486 keV alpha particles emitted from 241Am was554745 keV electron equivalents (keVee), with an energy resolution of 11.270.1%. The energy responseto 6118 keV alpha particles emitted from 252Cf was 618745 keVee, with a resolution of 8.870.1%. It isalso important to characterise the refractive index because it determines how efficiently light propagatesin scintillation materials to the photodetector. By taking into account the PEN emission spectrum, it wasrevealed that its effective refractive index was 1.70. Overall, the results indicate that PEN has potential asa scintillation material for the detection of alpha particles.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Aromatic ring polymers doped with various fluorescent guestmolecules have been used for many years as organic scintillationmaterials in radiation detectors [1–3]. Doping is used to convertthe radiation-induced ultra-violet emission of the polymers intomore easily detectable visible light. Advanced photodetectors,however, now enable direct detection of short wavelength lightfrom undoped polymers, and previously unknown optical proper-ties of the polymers in this region are now being characterised forradiation detection purposes [4–6]. With better refining techni-ques available, there is now a considerable effort worldwide toidentify polymers with increasingly favourable optical character-istics for use as pure base substrates in scintillation materials [7–9]. Thus, it needs to characterise the optical properties of thesepolymers in the context of radiation detection. The refractive indexin particular is an important optical property that determines howefficiently light propagates in scintillation materials to thephotodetector.

We recently demonstrated that undoped poly (ethylene naph-thalate) (PEN) possesses optical properties that are suitable forradiation detection [10–12]. PEN has an emission maximum at 425nm. With oxygen as a main component, it has a density of 1.33 g/cm3, and is durable. These characteristics have attracted

considerable attention for the potential application of PEN inradiation detectors. The repeat unit structure of PEN is:

Previous reports have examined the basic performance of PENfor the detection of beta particles, gamma-rays, and neutrons, butthere have been few reports concerning detection of alphaparticles [13–17]. Thus we have characterised its refractive index,the light yield, energy response and energy resolution for alphaparticles. Overall, PEN has favourable characteristics for alphaparticle detection.

2. Materials and methods

A 31�31�5 mm PEN plate (Teijin Ltd.) was prepared byinjection molding. Refractive indices were determined with arefractometer (PR-2; Carl Zeiss, Jena, Germany) at the C line of ahydrogen 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 mercurylamp (436 nm). The experimental arrangement for measuringlight yields is shown in Fig. 1. One 31�31 mm face was interfacedwith a photomultiplier tube window (PMT, R878-SBA; HamamatsuPhotonics Co., Ltd.) via a very thin layer of optical grease (EJ-550;

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Nuclear Instruments and Methods inPhysics Research A

0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nima.2013.12.021

n Corresponding author at: Kyoto University, 2, Asashiro-Nishi, Kumatori-cho,Sennan-gun, Osaka, 590-0494 Japan. Tel./fax: þ81 72 451 2463.

E-mail address: hidehito@rri.kyoto-u.ac.jp (H. Nakamura).

Nuclear Instruments and Methods in Physics Research A 739 (2014) 6–9

Eljen Technology), while a radioactive source was positioned in thecenter of the opposite face. Output signals from the PMT weredirectly digitized with a charge-sensitive analogue-to-digital con-verter module (RPC022, REPIC Co.).

Two radioactive sources, 137Cs and 207Bi, both of which emitmonoenergetic internal conversion electrons, were used to deter-mine the relationship between the PEN light yield and the electronenergy. The PEN light yield for alpha particles was then evalu-ated with 241Am and 252Cf radioactive sources that have no back-ground beta particle or gamma-ray emission with energies nearthose energies at which the alpha responses were characterised.Because these alpha sources are simply vapour-deposited radio-active isotopes, the active regions can be directly positioned on thePEN face.

3. Results and discussion

The refractive index (ND) of PEN at the 589-nm D line ofthe sodium lamp is 1.65. However, since PEN does not emit lightin this region, refractive indices were obtained as a function ofwavelength.

The results are plotted in Fig. 2 and follow the Sellmeierdispersion function [18]. We can then obtain an “effective”refractive index Neff¼1.70 by taking into account the emissionspectrum, rather than using ND¼1.65 at 589 nm [4,17].

Fig. 3 shows the light yield distributions in PEN generated by the137Cs radioactive source, where the sharp peak corresponds to624 keV conversion electrons. Counts in the low light-yield regionderive from 514 keV beta particles and Compton recoil electronsgenerated by 662 keV gamma-rays. Similarly, Fig. 4 shows the lightyield distributions generated by the 207Bi radioactive source, wherethe sharp and small peaks correspond to 976 keV and 482 keVinternal conversion electrons, respectively. Fig. 5 reveals the linearregression fit between the peak values in the two light yielddistributions and the energies of the internal conversion electrons.

The relationship is then used to characterise alpha particles. Forexample, Fig. 6 plots the light yield distribution excited by the241Am radioactive source. The peak is generated by 5486 keV alphaparticles, and the energy response (Fig. 5) was found to be554745 keV electron equivalents (keVee). In addition, the energyresolution (s) for the 5486 keV alpha particles was 11.270.1%.Similarly, Fig. 7 plots the light yield distribution for the alpha

particles emitted from the 252Cf radioactive source. The peak isgenerated by 6118 keV alpha particles, and the energy responsewas 618745 keVee, with s¼8.870.1%.

These results demonstrate that the light yield for alpha parti-cles per unit energy was 1/9.9. The energy of most alpha particlesemitted from radioisotopes is in the 4–6 MeV range, whichcoincides with the range for the PEN light yields presented here[1,2]. The data are summarised in Table 1 and demonstrate thatPEN can be used for the detection of alpha particles.

Fig. 1. Arrangement for measuring light yields in PEN.

Fig. 2. Refractive indices of PEN at various wavelengths. The highlighted region(light blue) shows that the emission wavelengths of PEN dominate. The emissionmaximum is 425 m. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Fig. 3. Light-yield distribution from PEN when excited by radiation from a 137Csradioactive source. The peak in the distribution is from 624 keV internal conversionelectrons. The counts for the low light-yield region are from 514 keV beta particlesand Compton recoil electrons generated by 662 keV gamma-rays.

H. Nakamura et al. / Nuclear Instruments and Methods in Physics Research A 739 (2014) 6–9 7

4. Conclusions

We have demonstrated that undoped PEN has potential as ascintillation material for the detection of alpha particles. PEN emitsblue light with an emission maximum at 425 nm; taking intoaccount its emission spectrum, an effective refractive index of 1.70was determined.

The energy response to 5486 keV alpha particles emitted froma 241Am radioactive source was 554745 keV keVee, with anenergy resolution of 11.270.1%. For 6118 keV alpha particlesemitted from a 252Cf radioactive source, the energy response was618745 keVee, with an energy resolution of 8.870.1%. In addi-tion, the linearity of the light yield in PEN was confirmed for the

energies of most alpha particles emitted from radioisotopes.Finally, it needs to note that under high-dose environments, dopedfluorescent guest molecules can lead to significant performancedegradation. Because PEN does not require doping, it should bemuch more stable in these environments. Thus, it is important atthis stage to mount PEN in a radiation detector to evaluate theoverall performance [19,20].

Acknowledgements

This research was supported by the Kyoto University and theNational Institute of Radiological Sciences. The authors thank the

207Bi137Cs

Fig. 5. Plot of the peak values in the light yield distributions vs. internal-conversionelectron energy.

Fig. 6. Light yield distribution of PEN when excited by radiation from a 241Amradioactive source. The peak in the distribution indicates 5486 keV alpha particles.

Fig. 7. Light yield distribution of PEN when excited by radiation from a 252Cfradioactive source. The peak in the distribution indicates 6118 keV alpha particles.

Fig. 4. Light-yield distribution from PEN when excited by radiation from a 207Biradioactive source. The peaks in the distribution are from 482 keV and 976 keVinternal conversion electrons.

H. Nakamura et al. / Nuclear Instruments and Methods in Physics Research A 739 (2014) 6–98

KUR Research Program for the Scientific Basis of Nuclear Safety forpartial support at this work. The authors are grateful to Dr. T. Murata,Dr. T. Fukunaga, Dr. H. Yamana, Dr. T. Arima, Dr. S. Kobayashi,Mr. Z. Shidara, Mr. F. Murakawa, Mr. Y. Okamoto and Ms. M. Yasakufor their cooperation.

References

[1] G. Knoll, Radiation Detection and Measurement, fourth ed., Wiley, New York,2010.

[2] W.R. Leo, Techniques for Nuclear and Particle Physics Experiments: A How-toApproach, second ed., Springer-Verlag, Berlin and Heidelberg, 1992.

[3] J. Beringer, Physical Review D 86 (2012) 010001.[4] H. Nakamura, Y. Shirakawa, H. Kitamura, N. Sato, O. Shinji, K. Saito,

S. Takahashi, Applied Physics Letters 103 (2013) 161111.[5] H. Nakamura, Y. Shirakawa, H. Kitamura, N. Sato, O. Shinji, K. Saito,

S. Takahashi, Scientific Reports 3 (2013) 2502.[6] H. Nakamura, H. Kitamura, O. Shinji, K. Saito, Y. Shirakawa, S. Takahashi,

Applied Physics Letters 101 (2012) 261110.[7] H. Nakamura, H. Kitamura, R. Hazama, Proceedings of the Royal Society A 466

(2010) 2847.[8] H. Nakamura, Y. Shirakawa, S. Takahashi, T. Yamano, Y. Kobayashi, R. Hazama,

C. Takagi, O. Hasebe, Physics Education 47 (2012) 17.

[9] V. Kumar, Y. Ali, R.G. Sonkawade, A.S. Dhaliwal, Nuclear Instruments andMethods in Physics Research B 287 (2012) 10.

[10] H. Nakamura, Y. Shirakawa, S. Takahashi, H. Shimizu, EPL (Europhysics Letters)95 (2) (2011) 22001.

[11] H. Nakamura, T. Yamada, Y. Shirakawa, H. Kitamura, Z. Shidara, T. Yokozuka,P. Nguyen, M. Kanayama, S. Takahashi, Applied Radiation and Isotopes80 (2013) 84.

[12] H. Nakamura, Y. Shirakawa, N. Sato, S. Takahashi, Physics Education 48 (2013)556.

[13] I. Sen, M. Urffer, D. Penumadu, S.A. Young, L.F. Miller, A.N. Mabe, IEEETransactions on Nuclear Science NS 59 (4) (2012) 1781.

[14] S. Nagata, M. Mitsuzuka, S. Onodera, T. Yaegashi, K. Hoshi, M. Zhao, T. Shikama,Nuclear Instruments and Methods in Physics Research B 315 (2013) 157.

[15] S. Nagata, H. Katsui, K. Hoshi, B. Tsuchiya, K. Toh, M. Zhao, T. Shikama,E.R. Hodgson, Journal of Nuclear Materials 442 (2013) S501.

[16] Y. Shirakawa, H. Nakamura, T. Kamata, K. Watai, M. Mitsunaga, Z. Shidara,F. Murakawa, Radioisotopes 62 (2013) 879.

[17] H. Nakamura, Y. Shirakawa, H. Kitamura, T. Yamada, Z. Shidara, T. Yokozuka,P. Nguyen, T. Takahashi, S. Takahashi, Radiation Measurements 59 (2013) 172.

[18] W. Sellmeier, Annalen der Physik 219 (1871) 272.[19] Y. Shirakawa, Radioisotopes 54 (2005) 199.[20] Y. Shirakawa, H. Nakamura, T. Kamata, K. Watai, Radiation Measurements

49 (2013) 115.

Table 1Response of PEN to alpha particles.

Substrate Fluorescent a241Am b252Cf

Energy response Resolution (r) Energy response Resolution (r)

PEN Undoped 554745 keVee 11.270.1% 618745 keVee 8.870.1%

a Primarily, 5486 keV alpha particles are emitted from the 241Am radioactive source.b Primarily, 6118 keV alpha particles are emitted from the 252Cf radioactive source.

H. Nakamura et al. / Nuclear Instruments and Methods in Physics Research A 739 (2014) 6–9 9