Journal of Luminescence 144 (2013) 212–216

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Journal of Luminescence

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Scintillation properties of LiF–SrF2 and LiF–CaF2 eutectic

Takayuki Yanagida a,n, Noriaki Kawaguchi b, Yutaka Fujimoto a, Kentaro Fukuda b,Kenichi Watanabe c, Atsushi Yamazaki c, Akira Uritani c

a Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japanb Tokuyama Corporation, 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648, Japanc Quantum Science and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

a r t i c l e i n f o

Article history:Received 27 February 2013Received in revised form20 June 2013Accepted 10 July 2013Available online 19 July 2013

Keywords:Eutectic scintillatorsLiF and SrF2 layersLiF and CaF2 layersSelf-trapped excitonNeutron detection

13/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jlumin.2013.07.016

esponding author. Tel./fax: +81 93 695 6049.ail address: yanagida@lsse.kyutech.ac.jp (T. Ya

a b s t r a c t

Dopant free eutectic scintillators 6LiF–SrF2 and 6LiF–CaF2 were developed by the vertical Bridgemanmethod for the purpose of thermal neutron detection. The molar ratio of LiF and Ca/SrF2 was 4:1 on itseutectic composition. The α-ray induced radioluminescence spectra of the scintillators showed intenseemission peak at 300 nm due to the emission from the self-trapped exciton in Ca/SrF2 layers. When thesamples were irradiated with 252Cf neutrons, 6LiF–SrF2 and 6LiF–CaF2 exhibited the light yields of 4700and 9400 ph/n, respectively. Scintillation decay times of 6LiF–SrF2 and 6LiF–CaF2 were accepted forscintillation detectors, 90 and 250 ns, respectively.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Scintillation detectors, which convert a single photon (X- andγ-ray) or other high-energy particles (charged particles andneutrons) into hundreds of visible–ultraviolet photons [1], havebeen playing a major role in many fields of radiation detection,including medical imaging [2], security [3], astrophysics [4],particle physics [5] and well logging [6]. In these applications,scintillators for thermal neutron detection have recently attractedmuch attention because of deficit of 3He gas [7,8]. Up to now, mostof the thermal neutron detectors were 3He gas proportionalcounters, because 3He has high cross section to thermal neutronsand low background γ-ray sensitivity [9]. The natural abundance of3He is approximately 10�6 to 4He, and it can be generated by thedecay of tritium which is no longer generated by nuclear experi-ments. The huge disproportion between the demand and thesupply of the 3He gas highly motivates academia and industry todevelop novel inorganic scintillators suitable for the thermalneutron detectors to replace existing 3He based systems.

One of the candidates for such application is 6Li containingmaterial, because 6Li has a high probability (940 barn at 25 meV) ofinteraction with neutrons based on 6Li(n, α)3H reaction with highQ-value of 4.8 MeV [10]. In the recent past, 6Li based scintillators,including Ce3+ and Eu2+ doped 6LiCaAlF6 were developed, and they

ll rights reserved.

nagida).

demonstrated acceptable scintillation responses under 252Cf neutronirradiation [11–15]. Elpasolite scintillators which have a chemicalcomposition of A2LiMX6:Ce (A¼¼Cs, Rb; M¼¼Y, La, Sc; X¼¼Cl,Br, I) have been also studied extensively recently [16–18]. Althoughelpasolites show good scintillation efficiency, huge hygroscopicity is abig problem for practical applications.

In addition to LiCaAlF6 and elpasolite scintillators, 6Li-based eutec-tics are now considered as appropriate materials for neutron detectors.LiF-containing eutectics are examples of such materials, and LiF–CaF2eutectic composite doped with Mn was first proposed for dosimeterapplications [19]. Then, Eu-doped LiF–CaF2 was recently studied forneutron scintillator [20]. Following these reports, evaluation of theneutron responses of 6LiF–CaF2:Eu [21] and 6LiF–SrF2:Eu [22] withdifferent Eu concentrations was also performed. This eutectic system isespecially promising, because molar ratio at the eutectic compositionis 81.5:18.5 [23] that results nearly identical volumetric fraction of56:44. Compared to conventional neutron scintillators, such as Li-glass[24], single crystal Eu:LiI [25], and LiF–ZnS powder sintered ceramic[26], the macroscopic cross section to thermal neutrons of the aboveeutectics is considerably high (see Fig. 1 in Ref. [20]). In addition, lowsensitivity to background γ-rays due to low density is anotheradvantage of this system.

In the present work, non-doped LiF–CaF2 and LiF–SrF2 eutecticscintillators are fabricated and evaluated on their scintillationresponses. Unlike other eutectics scintillators, our aim is to examinedopant-free neutron scintillators. Although most of scintillators userare earth elements as the emission center, the price of rare earthelements increases and the suppression of the usage amount of rare

T. Yanagida et al. / Journal of Luminescence 144 (2013) 212–216 213

earth is highly required. In addition, dopant-free materials are freefrom non-uniformity of emission properties in bulk form because thedopant segregation cannot be avoided in the melt growth. Generally,manufacturers pay their effort to fabricate uniform materials and suchdopant-free scintillators are ideal for mass production. Through thisstudy, optical transmittance, α-ray and X-ray induced radio lumines-cence, neutron induced light yield, and scintillation decay lifetimesunder neutron and X-ray excitation were systematically studied.

2. Experimental procedure

High-purity (99.99%) fluoride powders of 6LiF (95% enriched), CaF2,and SrF2, (Stella Chemifa Corporation) were used as starting materials.The LiF and Ca/SrF2 powders were mixed in 80:20 M ratio thatcorrespond to the eutectic composition. The mixtures were put intographite crucibles, and the micro-Bridgman method [27] was used toproduce LiF–CaF2 and LiF–SrF2 eutectics. The crucibles were set in thestainless chamber and enclosed by the carbon resist heater. Most partsof hot zone were made of high purity carbon. After the setting ofcrucibles and hot zone, the chamber was evacuated up to 10�4 Torr.Then, the crucible was heated up to 400 1C and kept for about 8 h atthis temperature in order to remove water and oxygen traces. Afterthe baking, the chamber was filled with high purity Ar gas (99.999%)and CF4 gas (99.999%) until ambient pressure. The ratio of Ar and CF4was 9:1. Then, the crucible was heated up to 800 1C and kept for30 min. Finally, the furnace was cooled to the room temperature witha cooling rate of 5 1C/min. After the fabrication of eutectics, they werecut and polished to the sizes of 1�2�4�8mm3 to investigateoptical and scintillation properties. Backscattered electron imageswere observed by using scanning electron microscope (SEM) toobserve lamellar structures.

Transmittance spectra were recorded by using JASCO V670 spec-trometer. If undesirable impurities are contaminated, sharp (due torare earth) or broad (due to transitional metal) absorption bands willbe observed. In addition, if the water is contaminated, absorptionbands around 2700 nm will appear. Practically, the scintillators areexcited by the neutron irradiation with charged particles producedfrom the 6Li(n,α)3H nuclear reaction. To simulate this process inlaboratory conditions, the 241Am 5.5 MeV α-ray induced radio lumi-nescence spectra were recorded using JASCO FP8600 fluorescence

Fig. 1. Photograph of LiF–SrF2 (left) and LiF–CaF2 (right).

Fig. 2. SEM images of LiF–CaF2 (le

spectrometer at room temperature. The main purpose of the radioluminescence measurements was to detect emission wavelengthunder α-ray excitation. Because the emission intensity of this kind ofintegrated type measurement is not a quantitative value, we cannotdiscuss the light yield of scintillators by radio luminescence intensities.The detailed description and geometry of the α-ray induced radioluminescence measurements were reported previously [28]. Then,X-ray-induced radioluminescence spectra were also measured to becompared with the α-ray-excited ones. The excitation source was ouroriginal instrument fabricated by OURSTEX Corporation. The X-raytube (W target) was supplied with 70 kV and 1mA. The emissionspectrawere recorded using Andor DU-420-BU2 CCD spectrometer. ItsCCD-based detector (cooled down to 188 K by a Peltier module) wascoupled with a monochromator SR163 (Andor, 1200 grooves/mm,300 nm blaze wavelength). The geometry of the set-up was describedpreviously [29].

In order to evaluate neutron induced absolute light yield, thepulse height measurements were performed. The eutectic sampleswrapped with several layers of Teflon tape to collect scintillationphotons were coupled with photomultiplier tube (PMT) R7600-200 produced by Hamamatsu with optical grease (OKEN 6262A).The high voltage of �700 V was supplied (ORTEC 556), and thesignals were read out from the anode of the PMT. The neutronsource of 252Cf was enclosed in a polyethylene container of 43 mmthickness for thermalization of fast neutrons. In order to cut thebackground γ-rays, the samples were surrounded by 5 cm thick Pbblocks. Once a neutron from the 252Cf was detected, the signalswere fed into the pre-amplifier (ORTEC 113) and then to theshaping amplifier (ORTEC 572) with 2 μs shaping time. Afterconverting to digital signals by a multi channel analyzer (Amptek,Pocket MCA 8000A), they were recorded to a computer. Toevaluate the absolute light yield, Li glass scintillator GS20(7200 ph/n [24]) was used as a standard.

Neutron induced decay lifetime profiles were recorded byTDS3052C digital oscilloscope (500 MHz and 5 Gs/s, Tektronix).As same as the radioluminescense spectra, decay time profilesunder X-ray excitation were investigated by pulse X-ray streakcamera system [30–31]. Because scintillation decay times underhigh energy photon (X- and γ-rays) and charged particles some-times differ due to the difference of the excitation density, tocompare neutron and X-ray induced decay times is important.

3. Results and discussion

The cut and polished LiF–CaF2 and LiF–SrF2 eutectic specimenshad typical dimensions of 1�2–4�8 mm3. Fig. 1 illustrates viewof the specimens ready for the characterizations. Their appear-ances looked semi-transparent due to the eutectic structure.

To observe details of the lamellar structure of the eutectic, themicroscopic images were taken using SEM. Fig. 2 demonstratessuch images made for LiF–CaF2 and LiF–SrF2. Bright fractions of the

ft) and LiF–SrF2 (right).

2000

4000

60008000

104

5 104

3 104

200 300 400 500 600 700

LiF-CaF2LiF-SrF2LiF-ZnS:AgLi-glassLiI:Eu

Wavelength (nm)

1

10

200 300 400 500 600 700 800 900 1000

LiF-CaF2LiF-SrF2LiF-ZnS:AgLi-glass

Wavelength (nm)

Fig. 4. α-Ray (a) and X-ray (b) induced radioluminescence spectra.

150

LiF-CaF2LiF-SrF2

T. Yanagida et al. / Journal of Luminescence 144 (2013) 212–216214

images represent substances containing heavier elements. Thus,the black and gray colored phases correspond to the LiF andCa/SrF2, respectively. The layer thicknesses of present eutecticswere typically 2 μm. LiF and Ca/SrF2 phases were clearly separatedin the all SEM images.

Transmittance spectra of eutectics are presented in Fig. 3. Noparticular absorption bands were not observed in 190–2700 nm.Thus, no contamination was included in samples. The discontinu-ity around 850 nm was ascribed to the change of the detector andworse transmittance around 190 nm was due to the sensitivity ofthe instrument. The transmittance of both samples resulted 1–10%and it was consistent with their semi-transparent appearances.

α-Ray induced radio luminescence spectra are demonstrated inFig. 4(a). Broad and intense emission peaks were detected at300 nm wavelengths as a result of self-trapped exciton lumines-cence in both LiF–CaF2 and LiF–SrF2 eutectics. In this figure, otherconventional neutron scintillators, such as LiF–ZnS:Ag and Li-glass(GS20), are overlaid. Because sample surface areas of LiF–ZnS:Ag(10�10 mm2) and Li-glass (10�10 mm2) were larger than thoseof eutectic scintillators, emission intensities were stronger. Inthese spectra of eutectics, self-trapped exciton luminescence fromLiF layer (300–400 nm) was also included [32]. Then, radiolumi-nescence spectra under X-ray excitation are demonstrated in Fig. 4(b). In this figure, another conventional neutron scintillator, Eu:LiIwas superposed. Eu:LiI was highly hygroscopic so that it wascovered by Al case same as NaI:Tl and we could not evaluate α-rayinduced radioluminescence. Although samples were same as theα-ray induced ones, emission strengths of eutectics were superiorto that of conventional Li-glass scintillator possibly due to thedifference of the effective detector areas. In α-ray induced raiolu-minescence, the photodetector was PMT with large effective area.On the other hand, scintillation photons were accumulated via1 mm diameter optical fiber in X-ray induced radioluminescence.Based on these experiments, we decided to use PMT R7600-–200which had the highest quantum efficiency of 37% at 300 nm incommercially available PMTs.

Fig. 5 illustrates pulse height spectra of LiF–CaF2 and LiF–SrF2eutectic scintillators recorded under 252Cf neutron irradiation andcompared with that of conventional neutron scintillator, Li-glassGS20 [24]. The thermal neutron peaks were clearly detected in allthe samples. The quantum efficiencies at emission peaks of Li-glass (395 nm) and LiF–Ca/SrF2 (300 nm) were similar, 40% and37%, respectively. Therefore based on the absolute light yield of7200 ph/MeV of Li-glass [24], the neutron induced absolute lightyields of LiF–CaF2 and LiF–SrF2 were 4700 and 9400 ph/n,

0.001

0.01

0.1

1

10

100

500 1000 1500 2000 2500

LiF-SrF2LiF-CaF2

Wavelength (nm)

Fig. 3. Transmittance of LiF–CaF2 and LiF–SrF2.

0

50

100

0 100 200 300 400 500 600

Li-glass

MCA channel

Fig. 5. 252Cf neutron excited pulse height spectra of LiF–CaF2, LiF–SrF2, and Li-glass.

respectively. The absolute scintillation light yield LiF–SrF2exceeded conventional Li-glass scintillator. Energy resolutions ofeutectics were worse than that of Li-glass due to the opacity.Compared with Eu2+ doped LiF–CaF2 [22] and Ce3+ doped LiF–SrF2[23] systems, nondoped LiF–SrF2 system had higher light yield.

0.2

0.3

0.4

0.5

0.60.70.8

1 2 3 4 5 6 7 8 9

I = 0.51 exp(-t/639 ns) + 0.08 exp(-t/2409 ns) + 0.22

1 2 3 4 5 6 7 8 9

0.3

0.4

0.5

0.6

0.70.80.9

I = 0.53 exp(-t/330 ns) + 0.07 exp(-t/2026 ns) + 0.33

Fig. 7. X-ray induced decay time profiles of LiF–CaF2 (a) and LiF–SrF2 (b). Dottedline represents double exponential fitting function.

0.1

0.2

0.3

0.5

0.9

-5000 0 5000 10000 15000 20000

I = 0.87exp(-t/260 ns) + 0.01exp(-t/1157 ns) + 0.12

1

-5000 0 5000 10000 15000 20000

0.1

I = 0.78exp(-t/90 ns) + 0.01exp(-t/1013 ns) + 0.19

Fig. 6. Neutron induced decay time profiles of LiF–CaF2 (a) and LiF–SrF2 (b). Dottedline represents double exponential fitting function.

T. Yanagida et al. / Journal of Luminescence 144 (2013) 212–216 215

Fig. 6 shows the decay lifetime profile of LiF–CaF2 (a) and LiF–SrF2 (b) under 252Cf neutron excitation. In both eutectics, decaylifetime profiles were well reproduced by the double exponentialfunction. The main decay lifetime component of LiF–CaF2 and LiF–SrF2 were 260 ns and 90 ns, respectively. In addition to these fastprimary decay times, they had slower component around 1 μs.Therefore, the present nondoped LiF–SrF2 system was faster thanpreviously reported Ce3+-doped eutectic system [23].

X-ray induced decay time profiles are shown in Fig. 7. As clearlyseen, X-ray induced decay times were slower than those ofneutron induced ones. The primary decay time of LiF–CaF2 and

LiF–SrF2 were 639 and 330 ns, respectively, with typically �2 μsslower component. The difference will be attributed to the excita-tion density and it is a general phenomenon that higher excitationdensity causes faster scintillation decay time. Based on theseobserved results, LiF–SrF2 shows better performance for neutrondetection applications.

4. Conclusion

LiF–CaF2 and LiF–SrF2 eutectic scintillators were fabricated bythe micro-Bridgeman method. In α- and X-rays induced radiolu-minescence spectra, they showed intense emission peaks at300 nm due to the self-trapped exciton. When they were irra-diated by 252Cf neutrons, absolute light yields of LiF–CaF2 and LiF–SrF2 eutectic scintillators were 4700 and 9400 ph/n, respectively.Therefore, LiF–SrF2 exhibited better scintillation performance thanpreviously reported Eu2+ or Ce3+ doped systems.

Acknowledgments

This work was mainly supported by JST Sentan and partially bya Grant in Aid for Young Scientists (A)—23686135, and ChallengingExploratory Research—23656584 from the Ministry of Education,Culture, Sports, Science and Technology of the Japanese govern-ment (MEXT). Partial assistance from the Yazaki Memorial Foun-dation for Science and Technology, Shimazu Sci. Foundation, KatoFoundation for Promotion of Science, and Nippon Sheet GlassFoundation for Materials Science and Engineering, are also grate-fully acknowledged.

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