Eu-doped 6LiF–SrF2 eutectic scintillators for neutron detection

Optical Materials 34 (2012) 868–871

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Eu-doped 6LiF–SrF2 eutectic scintillators for neutron detection Takayuki Yanagida a,⇑, Kentaro Fukuda b, Yutaka Fujimoto d, Noriaki Kawaguchi b,d, Shunsuke Kurosawa d, Atsushi Yamazaki c, Kenichi Watanabe c, Yoshisuke Futami d, Yuui Yokota d, Jan Pejchal d, Akira Yoshikawa a,d, Akira Uritani c, Tetsuo Iguchi c a

New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Tokuyama Corporation Shibuya 3-chome, Shibuya-ku, Tokyo 150-8383, Japan Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan d Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b c

a r t i c l e

i n f o

Article history: Received 23 July 2011 Received in revised form 9 November 2011 Accepted 27 November 2011 Available online 16 December 2011 Keywords: Eutectic Scintillation detector Eu2+ Neutron Scintillator

a b s t r a c t Eu2+ 0.05%, 0.1%, and 0.2% activated LiF–SrF2 eutectic scintillators were prepared by the Bridgman method using 6Li enriched (95%) raw material. The a-ray-induced radio luminescence spectra showed intense emission peak at 430 nm due to an emission from Eu2+ 5d–4f transition in the Eu:SrF2 layers. When excited by 252Cf neutrons, all the samples exhibited almost the same light yields of 5000–7000 ph/n with a typical decay times of several hundreds ns. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Scintillator materials, which convert an energy of the ionizing radiation to thousands of UV/visible photons, have been playing a major role in many fields of radiation detection, including medical imaging, security, astrophysics, particle physics and oil logging. In these applications, scintillators for thermal neutron detection have recently attracted much attention due to diminishing resources of 3He gas. Up to now, most of the thermal neutron detectors are gas counters fulfilled with the 3He gas, because 3He has high thermal neutron cross section and low background c-ray sensitivity. However, the recent demand for 3He highly exceeds its supply because the usage of neutron detectors for security and oil logging applications increased. This huge discrepancy between the demand and the supply highly motivates us to develop novel thermal neutron scintillators which would replace the contemporary 3He-based systems. Prospective materials for such applications can be for example the materials containing 6Li, because 6Li has high interaction probability with thermal neutrons yielding suitable nuclear reaction 6Li (n, a) 3H with high Q-value of 4.8 MeV. Recently, we developed ⇑ Corresponding author. Present address: New Industry Creation Hatchery Center (NICHe), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-0812, Japan. Tel.: +81 22 217 5822; fax: +81 22 217 5102. E-mail address: [email protected] (T. Yanagida). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.11.022

Ce3+ and Eu2+ doped LiCaAlF6 scintillators which showed good scintillation properties under 252Cf neutron excitation [1–5]. In addition to these LiCaAlF6 scintillators, 6Li based eutectic materials are also considered for such applications. The LiF/CaF2 eutectic composite doped with Mn was originally proposed for dosimeter applications [6], and very recently it was studied as a neutron scintillator when doped with Eu [7]. Following these studies, we also grew 6LiF/CaF2:Eu using the Bridgman method and examined its neutron response [8]. In the neutron scintillator applications, the idea of using eutectic materials is schematically drawn in Fig. 1. In the LiF layers the incident neutrons are converted into secondary ionising particles by the reaction 6Li3 + 1n0 ? 3H1 + a and thereafter in CaF2 layer, their energy is converted to scintillation photons. Based on this idea, we can consider the CaF2 scintillator layers to be a luminescent volume excited by the charged particles. It is well known that Eu2+-doped CaF2 exhibits excellent scintillation properties [9–13]. In addition to the suitable CaF2 emission properties, the figure of merit of such a eutectic scintillator is high Li concentration (see Fig. 1 in [8]). Compared with conventional neutron scintillators, such as Li-glass (e.g., [14]), Eu:LiI (e.g., [15]), and LiF–ZnS (e.g., [16]), the macroscopic cross sections for thermal neutrons became larger. In the present study, we grew Eu2+ 0.05%, 0.1%, and 0.2% activated LiF–SrF2 eutectic scintillators, to examine the effect of Sr substitution for Ca, because for example, Eu doped LiCaAlF6 [5] and LiSrAlF6 [17] exhibit similar neutron responses. In all the samples,

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neutron 6Li(n,α)3H

LiF CaF2 LiF CaF2 LiF CaF2 LiF

α, 3H Few

several tens of μm

Fig. 1. Schematic drawing of the principle of eutectic scintillators.

252 Cf induced pulse height spectra and decay time profiles were systematically studied.

a-ray induced radio luminescence,

Fig. 3. SEM image of Eu 0.1% doped LiF–SrF2 prepared at the solidification rate of 10 mm/h.

2. Experiment 2.1. Sample preparation

120

2.2. Scintillation properties Simulating the neutron irradiation where the charged particles generated from the 6Li (n, a) 3H reaction excite scintillators, the radio luminescence spectra were recorded by using Edinburgh FLS920 under 241Am a-ray excitation. The main purpose of the radioluminescence measurements was to characterize emission wavelength under a-ray excitation, because the emission intensity of this kind of integrated type measurement is not a quantitative value. The step in the spectra acquisition was set to be 1 nm at each measurement, and all the measurements were carried out at room

Eu 0.05%

Eu0.05 Eu0.1 Eu0.2

100

Intensity (a. u.)

The starting material was prepared from high-purity (99.99%) fluoride powders of LiF, SrF2, and EuF3 (Stella Chemifa Corporation) and placed into a graphite crucible. According to the phase diagram [18], LiF and SrF2 were mixed in the eutectic composition, 80% and 20% respectively, and EuF3 was added in molar ratio of 0.05%, 0.1%, and 0.2% to SrF2. The 6Li-enriched (95%) LiF was used to improve the neutron detection efficiency. The Bridgman method was employed to produce LiF/SrF2:Eu composites in order to achieve the ordered lamellar structure. In the unidirectional solidification processes such as Bridgman, Czochralski, or micro-pulling-down method [19], LiF and SrF2 phases deposit from the melt on their own formed solid phases. Thus, each phase grows along the solidification direction and the ordered structure can be achieved. The preheating treatment under vacuum was performed to eliminate water and oxygen traces from the starting materials. Subsequently, high-purity Ar gas was introduced into the furnace. Unidirectional solidification process was controlled by pulling down the crucible at various speeds. During the growth, the solidification rate was10 mm/h. After the preparation of the sample, the lamellar structure was confirmed by the backscattered electron image from an SEM (Hitachi, S3400N). The thickness of the LiF layers was measured for the composites cut across the solidification direction by the calibrated length scale of the SEM.

80 60 40 20 0

400

450

500

550

600

650

700

750

Wavelength (nm) Fig. 4. Radioluminescence spectra of Eu 0.05 (red), 0.1 (blue), and 0.2 (green)% doped LiF–SrF2 eutectic scintillators under 241Am a-ray excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

temperature. The detailed description of a-ray-induced radioluminescence was explained in previous paper [20]. In the neutron response measurements, the sample crystals were wrapped with several layers of Teflon tape and were coupled to the PMT R7600 (Hamamatsu) with optical grease (OKEN 6262A). The high voltage of 700 V was supplied (ORTEC 556), and signals were read out from the anode of the PMT. The neutron source was a 252Cf isotope enclosed in a polyethylene container with 43 mm thickness for thermalization of the fast neutrons. In order to cut the background c-rays, the sample was surrounded by 5 cm thick Pb blocks. Once a neutron from the 252Cf was detected, the signals were fed into a pre-amplifier (ORTEC 113), a shaping amplifier (ORTEC 572) with 6 ls shaping time. After being converted to digital signals by a multi-channel analyzer (Amptek, Pocket MCA 8000A) they were recorded in a computer. To evaluate the absolute

Eu 0.1%

Eu 0.2%

Fig. 2. Appearance of LiF–SrF2:Eu eutectic scintillators. From left to right, Eu 0.05%, 0.1%, and 0.2% doped LiF–SrF2.

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Fig. 2 illustrates the polished LiF–SrF2:Eu eutectics. Though the eutectics are made of two different phases, they showed a good transparency owing to the matching of the refractive indices of LiF (1.397 at 430 nm) and SrF2 (1.444 at 430 nm). In Fig. 3, SEM image of Eu 0.1% doped sample is shown. The well ordered lamellar structure was clearly observed along the solidification direction and the typical scale of the thickness of the layer was approximately 3 lm in this solidification rate.

400

Counts /channel

350

Li-glass Eu 0.05% Eu 0.1% Eu 0.2%

300 250 200 150 100

3.2. Scintillation properties

50 0

0

100

200

300

400

500

MCA channel Fig. 5. 252Cf neutron induced pulse height spectra of Li-glass (red), Eu 0.05 (blue), 0.1 (green), and 0.2 (black)% doped LiF–SrF2 eutectic scintillators. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Intensity (a.u)

100

10 3 - 1x10

0

3

3

3

3

3

3

1x10 2x10 3x10 4x10 5x10 6x10

Time (ns) Fig. 6. Decay time profile of Eu 0.2% doped LiF–SrF2 eutectic scintillator.

800

Decay time ( ns)

700

600

500

400

0

0.05

0.1

0.15

0.2

0.25

Eu concentration (%) Fig. 7. Decay time constants plotted against Eu concentration under neutron irradiation.

light yield, Li glass scintillator GS20 (6000 ph/n [21]) was used as a reference. At the same time, decay time profiles were recorded by WE7311 digital oscilloscope (Yokogawa). The obtained decay time profiles were averaged over 100 times.

Radio luminescence spectra are demonstrated in Fig. 4. A broad and intense emission peak appeared at 430 nm due to Eu2+ 4f65d– 4f7 transition and Eu3+ 4f–4f related lines were observed mainly at 590 and 700 nm. The shape of the spectrum resembles that of Eu:CaF2. Although the intensities of the main peak at 430 nm were almost constant with Eu concentration, the intensities of 4f–4f lines became larger when Eu concentration increased. It seems that the absorbed energy could be easily transferred to 4f–4f transitions with increasing Eu concentration. In the previous study, because only X-ray induced radio luminescence spectrum was reported [7] and it focused on 5d–4f emission, we could not compare the present system with LiF–CaF2 system. It was confirmed that the present LiF–SrF2:Eu system acts as a scintillator under charged particle excitation. Fig. 5 shows pulse height spectra of LiF–SrF2:Eu eutectic scintillators under 252Cf neutron excitation compared with that of Liglass scintillator. As can be clearly seen, neutron peaks were detected in all the samples. The quantum efficiencies at 380 nm (emission of Li-glass) and LiF–SrF2:Eu are at the same level (approximately 40%), therefore we can directly compare their light yield from 252Cf neutron peak channels. Among the eutectic samples, Eu 0.1% doped one exhibited the highest light yield which was at the same level as that of Li-glass (6000 ph/n). When we introduce more Eu to SrF2 phase, the light yield decreased to 3500 ph/n for the Eu 0.2% doped sample. Thus, the optimum Eu concentration is about 0.1% which was the same as for Eu:CaF2. Previous study revealed that Eu:LiF–CaF2 ceramic showed approximately 10,000 ph/n under 252Cf irradiation, and SrF2 based systems showed less light yield than CaF2-based system. Fig. 6 exemplifies the decay time profile of Eu 0.1% doped sample. When we consider a single component exponential function for fitting, the experimental results of all the sample crystals can be well approximated by this function. The decay time constants of LiF–SrF2:Eu eutectic scintillators are plotted as a function of Eu concentration, as displayed in Fig. 7. The obtained main decay time constants of Eu 0.05%, 0.1%, and 0.2% scintillators were 611, 410, and 432 ns, respectively. In the same figure, we also plotted the data of Eu 0.1% doped CaF2 fabricated by our Bridgman furnace and the decay time was determined to be 585 ns. Compared with Eu:CaF2, decay times of the present samples became faster. In previous studies [7,8], decay time profiles were not reported at all, so that we plotted the decay time of Eu 0.1% doped LiF–CaF2 sample which was the brightest one in the previous study [8]. The decay of the LiF–CaF2 sample was determined to be 754 ns and was longer than the values related to the Eu:LiF–CaF2 systems. 4. Conclusion

3. Results and discussion 3.1. Preparation of eutectics The obtained eutectics were cut and polished to u14 mm  0.5 mm and used for measurements of scintillation properties.

Eu2+ activated LiF–SrF2 eutectic scintillators were successfully grown by the vertical Bridgman method to investigate the scintillation response of this material system. In 241Am a-ray-induced radioluminescence spectra, they showed Eu2+ 5d–4f emission at 430 nm and many Eu3+ 4f–4f lines at 590–700 nm. The Eu 0.1% doped one exibited the highest light yield among these samples

T. Yanagida et al. / Optical Materials 34 (2012) 868–871

with a leading decay time of 410 ns under 252Cf neutron exposure. As a result, although the scintillation decay times of LiF–SrF2 samples were faster than those of LiF–CaF2 samples when doped with Eu2+, the light yield was inferior to LiF–CaF2 system. However, the light yield can be improved by the optimization of the synthesis procedures. Acknowledgments This work was mainly supported by JST Sentan (TY) and partially by a Grant in Aid for Young Scientists (A)-23686135 (TY), and Challenging Exploratory Research-23656584 (TY) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). Partial assistance from the Yazaki Memorial Foundation for Science and Technology, Japan Science Society, Sumitomo Foundation, and Iketani Science and Technology Foundation are also gratefully acknowledged. References [1] A. Yoshikawa, T. Yanagida, Y. Yokota, N. Kawaguchi, S. Ishizu, K. Fukuda, T. Suyama, K.J. Kim, M. Nikl, M. Miyake, M. Baba, IEEE. Nucl. Trans. Sci. 56 (2009) 3796–3799. [2] T. Yanagida, A. Yoshikawa, Y. Yokota, S. Maeo, N. Kawaguchi, S. Ishizu, K. Fukuda, T. Suyama, Opt. Mater. 32 (2009) 311–314. [3] A. Yamazaki, K. Watanabe, A. Uritani, T. Iguchi, N. Kawaguchi, T. Yanagida, Y. Fujimoto, Y. Yokota, K. Kamada, K. Fukuda, T. Suyama, A. Yoshikawa, Nucl. Instr. Methods A 652 (2011) 435–438.

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