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[Dongsheng Yuan](https://orcid.org/0000-0001-9650-2272), [Encarnación G. Víllora](https://orcid.org/0000-0001-8868-0028), Daisuke Nakauchi, Takumi Kato, Noriaki Kawaguchi, Takayuki Yanagida, [Kiyoshi Shimamura](https://orcid.org/0000-0001-6502-8731)

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Scattering-free Ce:LYBO single crystals for thermal neutron detectionApplied Physics Express     LETTER • OPEN ACCESSScattering-free Ce:LYBO single crystals forthermal neutron detectionTo cite this article: Dongsheng Yuan et al 2024 Appl. Phys. Express 17 015502 View the article online for updates and enhancements.You may also likeWavelength down-conversion study ofBa3Y1-X (BO3)3: x Tb3+& Eu3+ [0.005  X 0.05] phosphor for solid state lightingapplicationsS P Hargunani, R M Chavan, R P Sonekaret al.-Rare Earth (RE) doped color tunablephosphors for white light emitting diodesSaniya Khan, Yatish R Parauha, DharmaK Halwar et al.-(Invited) High-Throughput DFTThermochemistry Applied to the Design ofCathode Coatings Forlithium-Ion BatteriesChris Wolverton-This content was downloaded from IP address 144.213.253.16 on 18/01/2024 at 13:52https://doi.org/10.35848/1882-0786/ad1892/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012021/article/10.1088/1742-6596/1913/1/012017/article/10.1088/1742-6596/1913/1/012017/article/10.1149/MA2016-01/4/432/article/10.1149/MA2016-01/4/432/article/10.1149/MA2016-01/4/432Scattering-free Ce:LYBO single crystals for thermal neutron detectionDongsheng Yuan1* , Encarnación G. Víllora1, Daisuke Nakauchi2 , Takumi Kato2 , Noriaki Kawaguchi2,Takayuki Yanagida2 , and Kiyoshi Shimamura11National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan2Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan*E-mail: yuan.dongsheng@nims.go.jpReceived December 18, 2023; revised December 21, 2023; accepted December 24, 2023; published online January 17, 2024Ce:Li6Y(BO3)3 (LYBO) is a well-known candidate for thermal neutron detection with a very high Li concentration (3.06 × 1022/cm3). So far, as-grown crystals exhibit a milky appearance that compromises their performance as scintillators. Current work demonstrates, for the first time, thegrowth of scattering-free undoped and Ce-doped LYBO by a thermal quenching process. The origin and features of the scattering centers areinvestigated in detail. Furthermore, the annealing treatment for the scintillation activation is studied, finding that the reduction in oxygen vacanciesis mandatory. Under thermal neutron irradiation, the annealed scattering-free Ce:LYBO single crystal achieves a record-high light yield of6200 ph/n in a single decay with a lifetime as short as 24 ns. © 2024 The Author(s). Published on behalf of The Japan Society of Applied Physicsby IOP Publishing LtdNeutron detectors are used for homeland security, oilwell-logging, radiation therapy, and fundamentalresearch.1–4) Among various kinds of detectors, theproportional counters based on gas tanks filled with 3He orBF3 are still the most common types. However, besides thedesire for more compact and handy detectors, all-solid-statealternatives are in high demand due to the worldwideshortage of 3He gas, on the one hand, and the corrosiveand toxic nature of BF3 gas, on the other. Among all-solid-state scintillators, Li-based materials have attracted muchattention due to their relatively high capture cross-section(σ = 940 barn) and the distinguishable reaction energy(∼ 4.8 MeV) after the interaction between a 6Li isotope anda neutron. The representative scintillators include Li-glass(with 95% 6Li enriched GS20 as the most representative one,i.e. a Li content of 1.43 × 1022 cm–3), Li-halides (Eu:LiI,Ce:Cs2LiYCl6, Eu:LiCaAlF6), and LiF/ZnS composites,etc.5–9) Though some of them have become commerciallyavailable, one or more critical drawbacks still exist, such as ahigh production cost, serious hygroscopicity, and a longscintillation decay.Ce-doped Li6Y(BO3)3 (LYBO) stands out as a promisingalternative.10,11) Compared with other candidates, it compro-mises a priori of all required basic material properties: a highLi content (3.06 × 1022 cm–3), good chemical stability, atrivalent site (Y3+) for efficient Ce3+ doping, cheap andabundant constituents, and the possibility to grow large singlecrystals for low-cost mass production. Despite these pro-mising characteristics, the poor crystalline and optical qualityof Ce:LYBO crystals has largely discouraged its develop-ment for scintillation applications so far.12–16) Recently, wehave demonstrated that a post-growth thermal treatment closeto Tm can drastically reduce the milky appearance of crystalswhile the light yield (LY) rises to 4650 ph/n.11) Nevertheless,to date it is still unknown, on the one hand, what are theorigin and nature of the scattering centers, and on the otherhand, what is the annealing mechanism that improves thescintillation response.In this work, undoped and Ce-doped LYBO crystals weregrown as explained previously,17) and growth and coolingrates were varied to elucidate the origin and nature ofscattering centers that lead to the typical milky appearanceof LYBO crystals. In the ultimate case, the power generatorwas switched off after the crystal separation from melt. Thisnew thermal quenching procedure led systematically toscattering-free crystals. This was particularly interesting inCe-doped crystals since so far Ce-doping had been found toincrease the amount of scattering centers. Single crystalplates for characterization were cut along the b-plane.Annealing was carried out under air for 15 h at 805 °C whilemonitoring the sample temperature with a nearby thermo-couple. All the samples were fine-polished on both sidesbefore characterization.The microstructure of LYBO crystals was investigated byhigh-resolution FE-SEM (JSM-6500F) and AFM (AsylumMFP-3D Origin) on a slightly etched sample. Apart fromsurface scratches due to polishing, the color contrast ob-served in FE-SEM images stems from a compositional shift.Regions with a higher effective atomic number Zeff emit andreflect electrons more efficiently and therefore appear in abrighter color. In the following discussion, the Zeff of variouscompounds is estimated using by the equation:18)Z w Z 1i ieff44 ( )*å=where wi is the atomic weight fraction of the ith element andZi its corresponding atomic number. The density of scatteringcenters was evaluated visually with a green laser pointer(550 nm, 1 mW), and more in detail with a He-Ne laser(633 nm, 5 mW) and a microscope (OLYMPUS BX51).Optical transmittance measurements were recorded withJasco spectrometers. The direct transmission with a V-570and the total one (direct plus diffused) with a V-770 equippedwith an integrating sphere. The elemental analysis wasconducted by the inductively coupled plasma optical emis-sion spectroscopy (ICP-OES) Aligent 5800.X-ray radioluminescence (XRL) spectra and decay werecharacterized using homemade setups.19–21) The pulse-heightspectra (PHS) were measured under neutron irradiation from252Cf with a photomultiplier tube (PMT, R7600-U200,Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of thiswork must maintain attribution to the author(s) and the title of the work, journal citation and DOI.015502-1© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdApplied Physics Express 17, 015502 (2024) LETTERhttps://doi.org/10.35848/1882-0786/ad1892https://crossmark.crossref.org/dialog/?doi=10.35848/1882-0786/ad1892&domain=pdf&date_stamp=2024-01-17https://orcid.org/0000-0001-9650-2272https://orcid.org/0000-0001-9650-2272https://orcid.org/0000-0001-6686-4694https://orcid.org/0000-0001-6686-4694https://orcid.org/0000-0002-5650-4449https://orcid.org/0000-0002-5650-4449https://orcid.org/0000-0002-7326-5849https://orcid.org/0000-0002-7326-5849mailto:yuan.dongsheng@nims.go.jphttps://creativecommons.org/licenses/by/4.0/https://doi.org/10.35848/1882-0786/ad1892Hamamatsu) operating at 600 V (gain 100, shaping time0.5 μs). The thermal neutron LY in photons/neutron (ph/n)was calibrated with a commercial 6Li-glass GS20 sample(from Saint-Gobain S.A.), while weighing the results from themultichannel analyzer with the corresponding XRL spectraand the quantum efficiency of the photomultiplier detector.Scattering centers in as-grown Ce:LYBO crystals can beeasily visualized with laser pointers, for example, the light of agreen one (550 nm, 1mW) exhibits a strong scattering path evenin slightly milky crystals as shown in our recent work.11) Toinvestigate the nature of these scattering centers, the micro-structure of fine-polished b-plane LYBO crystals was investi-gated by FE-SEM and AFM, as shown in Figs. 1(a) and 1(b),respectively. In both images, elongated structures ∼12° tiltedfrom the (−102) cleavage plane are observed, with the long oneshaving a needle-like appearance of ∼1 um in length. Thesemicrostructures are observed for the first time, and it is assumedthat these are the scattering centers that cause the milkyappearance of crystals. As the crystalline nature and compositionof these could not be investigated by high-resolution transmis-sion electron microscope and energy-dispersive X-ray spectro-scopy, due to the quick sample degradation under the electronbeam, at present we cannot confirm the nature of the scatteringcenters. Nevertheless, from the light-color compositional contrastin FE-SEM and the protuberance in AFM images, it can bededuced that the scattering centers correspond to segregatedparticles with a higher effective atomic number Zeff and a higherchemical stability than LYBO, respectively. After disregardingLi-borates due to their low Zeff, the following binary and ternarypotential oxide compounds that are stable at atmosphericpressure can be considered as potential candidates: LiYO2,YBO3, YB3O6, LiY6O5(BO3)3, Li2YB5O10, Li3Y2(BO3)3, andLi3Y(BO3)2. From Zeff considerations in Table I, Li2YB5O10 canbe excluded due to its lower value than LYBO, while YB3O6,Li3Y2(BO3)3, and Li3Y(BO3)2 are improbable due to theirsimilar ones. Consequently, only three potential candidatesremain: LiYO2, YBO3, and LiY6O5(BO3)3. The latter is foundto segregate at the crucible bottom during the LYBO growth bythe Czochralski technique,17) and at first sight, it was plausible tohypothesize that traces of it present in the melt segregate in thematrix during the growth of LYBO. As will be demonstratedlater, this is not the case because scattering centers appear afterthe LYBO solidification. Furthermore, as the LiY6O5(BO3)3segregation upon cooling would involve a non-negligiblediffusion of Y atoms through the lattice to increase theirconcentration according to the chemical formula, this compoundis tentatively disregarded. Consequently, the simpler LiYO2 and/or YBO3 remain the most probable compounds to segregate andto be the origin of light scattering. In the later discussion,however, it will be shown that YBO3 may be disregarded. Fornow, and before discussing the moment at which the segregationtakes place, the concentration of scattering centers in as-growncrystals and annealed ones is considered in the following.In a precedent publication, it was shown that the scintilla-tion properties of LYBO can be drastically improved after airannealing close to the Tm of LYBO.11) Though this treatmentcauses a remarkable reduction in the concentration ofscattering centers in LYBO, the latter do not represent themain source of non-radiative recombination, as will be seenlater in the case of scattering-free crystals. In order tovisualize the remaining scattering after annealing, a He-Nelaser with a five times higher power than a laser pointer wasused. Figure 2 shows the images of the light path under theoptical microscope before and after annealing, taken with a100x objective under the same exposure conditions. As-grown LYBO crystals possess such a high concentration ofscattering centers, even in less milky ones, that scatteringspots largely overlap and a density value cannot be given. Inany case, the scattering centers seem to distribute more or lesshomogeneously along the crystal. After annealing, thescattering path image becomes quite dark and single spotscan be discerned, suggesting a reduction in the scatteringcenter concentration of more than 1000 times. A roughestimation was carried out with the software ImageJ,28)obtaining a value of ∼1 × 107 spots cm–2. Despite variousefforts varying annealing temperature and time, no furtherdecrease in the concentration of scattering centers could beachieved. Therefore, we next aimed at the growth ofcompletely scattering-free crystals.(a)(b)Fig. 1. (a) FE-SEM and (b) AFM images of as-grown b-plane LYBOcrystals with scattering centers. The horizontal edge corresponds with the(−102) cleavage plane.Table I. List of stable candidates and their effective atomic numbers.Compounds ZeffLiYO222) 35.6YBO323) 34.4YB3O624) 31.2LiY6O5(BO3)325) 35.3Li2YB5O10 28.4Li3Y2(BO3)326) 32.4Li3Y(BO3)227) 30.9LYBO 28.6015502-2© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 015502 (2024) D. Yuan et al.As mentioned above, the first assumption for the formationof scattering centers was that, in the same way asLiY6O5(BO3)3 was gradually nucleating and growing onthe crucible bottom, traces of it were dispersed in the meltand continuously segregated at the interface of the growingcrystal. Under this hypothesis, it was reasonable to expect adecrease in the scattering center concentration with lowerpulling speeds, since the segregation coefficient dependssensitively on the growth rate. It was found, however, that theamount of scattering centers did not depend on this para-meter, which seemed to be odd. Subsequently, we focused onthe cooling rate and found that the amount of scatteringcenters was apparently larger for a crystal that was cooleddown much more slowly, which seemed to be counter-intuitive. Following this finding, instead of a gradual coolingrate, the heating of the hot zone was stopped after crystalseparation, leading to a relatively quick decrease in crystaltemperature. By this “quenching” method, the growth ofscattering-free LYBO crystals was achieved for the first time,while demonstrating that scattering centers are not caused bythe incorporation of secondary phases in the LYBO matrixduring the crystal growth. The comparison between crystalswith and without scattering centers, before and after an-nealing, in terms of light scattering of the green laser pointerand optical transmittance is given in Fig. 3. As can beexpected, the scattering-free crystal exhibits the highest directtransmittance (A1), free of absorption in the whole UV-visible wavelength region, while the crystal with scatteringcenters presents a decreasing transmittance towards shorterwavelengths (B1). After annealing, however, both crystalswith (B2) and without (A2) initial scattering appear to havevery similar transmission spectra, and at the same timedifferent from as-grown crystals. In the ∼200–350 nm range,the direct transmission [Fig. 3(b)] shows a slight linear droptowards short wavelengths, which could be caused byabsorption and/or scattering. As shown in the case ofscattering-free crystals of Fig. 3(c), total transmittancemeasurements with an integrating sphere indicate that thedecrease in UV transparency by annealing is caused by a newabsorption, whose origin needs to be investigated in moredetail. Furthermore, it should be noted that below 200 nm,i.e., close to the absorption cutoff, a sharp increase in(a)(b)(c)Fig. 3. (a) Scattering paths of a green laser pointer (550 nm, 1 mW) inLYBO single crystals. As-grown scattering-free crystals before (A1) andafter annealing (A2); as-grown scattering crystals before (B1) and afterannealing (B2). (b) Optical direct transmittance spectra of these LYBOcrystals. (c) Total transmittance spectra of as-grown scattering-free LYBOcrystal before and after annealing. The small peak located at 396 nm is ameasurement artifact.(a)(b)Fig. 2. Optical microscope images of the scattering path of a He-Ne laser(5 mW, 633 nm) in as-grown (a) and annealed (b) LYBO crystals.015502-3© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 015502 (2024) D. Yuan et al.transmittance is observed. This indicates a better crystallinityafter annealing despite the newly induced UV absorbance,and as will be seen below, it’s closely related to thesuppression of non-radiative recombination centers.Beyond the growth of undoped scattering-free LYBOcrystals demonstrated so far, the “quenching” techniquewas successfully applied as well for the case of dopedcrystals. If previously Ce-doping was found to promote theformation of scattering centers, with the new coolingtechnique scattering-free Ce:LYBO single crystals could besystematically grown. In the following, the scintillationproperties of the best crystal are presented.In analogy with previous findings on scattering Ce:LYBOcrystals, scattering-free ones need to be annealed under air tomeasure their scintillation performance.11) The remarkabledecrease in non-radiative recombination centers after an-nealing is attributed to a reduction in oxygen vacanciespresent in as-grown crystals. This hypothesis is furthersupported by the enhanced transmittance close to theabsorption cutoff (see Fig. 3) since the valence band inLYBO crystal is defined by oxygen atoms.29) Consideringthat as-grown crystals are oxygen deficient, some sort ofcompensating defects need to be present to keep crystalelectroneutrality. Due to the large difference in cationic radii,anti-site occupations LiY and/or LiB can be disregarded, andtherefore only vacancies of volatile Li and B are plausible.Since boron oxide has a higher vapor pressure than lithiumoxide, the predominance of VB is more probable. Thisassumption is in good accordance with the ICP results givenin Table II, which indicate that the Li/B ratio in LYBOcrystal is higher than the theoretical value of two. In any case,by the reduction or even elimination of VO after annealing,the defects at the atomic level (VLi and VB) have anotherlocal environment that leads to different optical features, assuggested by the slight absorption in the UV. One possibilityis the formation of lithium and boron-depleted clusters withLi5YB2O7, Li3YB2O6, or even Li3Y2(BO3)3 compositions. Inthe particular case of as-grown scattering crystals, annealingadditionally promotes the redissolution of segregated phasesduring the cooling process. From the two potential phasesthat were considered for segregation at the beginning, namelyLiYO2 and YBO3, the latter is more improbable for a boron-deficient crystal. Furthermore, it is feasible that during thecooling process the local decomposition at the atomic level isfavored by the presence of VLi and VB according to thebreakdown reaction:Li Y BO V V2V “Li YB O ” LiYO Li B O 26 3 3 Li BO 5 2 7 2 4 2 5( )( )+ ++   +whereas the first phase represents the simplest configuration ofneighboring VLi and VB in LYBO, the second LiYO2 is thephase visualized in FE-SEM and AFM images (Fig. 2) thanksto its relatively higher Zeff and chemical stability, and the thirdthe high-temperature stable Li4B2O5, which would furtherdissociate into Li3BO3 and Li6B4O9 upon cooling below∼600 °C according to the Li2O-B2O3 phase diagram.30,31)The scintillation characteristics of a nominal 2% Ce:LYBOcrystal, which has an actual Ce concentration of 0.4% relativeto Y according to the ICP-OES measurement, are shown inFig. 4 in comparison with those of the reference material forthermal neutron detection, namely the commercial Li-glassTable II. ICP results of LYBO single crystals.Element Mass (mg) Mass percentage (%)Atomic ratio relative toYLi 1.969 13.2 5.91Y 4.264 28.6 1B 1.507 10.1 2.91(a)(b)(c)Fig. 4. Scintillation performance of nominal 2% Ce:LYBO annealedcrystal in comparison with commercial Li-glass GS20. (a) Normalized XRLspectra, (b) thermal neutron pulse-height spectra, and (c) pulsed X-ray decaycurves.015502-4© 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing LtdAppl. Phys. Express 17, 015502 (2024) D. Yuan et al.GS20. The XRL profile of Ce:LYBO is very similar to that ofGS20,32) thus for practical applications it matches well withstandard scintillator photomultipliers, and optimum energyconversion efficiencies can be achieved. The LY as a thermalneutron scintillator was determined by PHS under 252Cfirradiation. Both 10B- and 6Li-peaks appear at well-differ-entiated channels, and a value of 6200 ph/n is estimated aftercorresponding corrections based on PMT quantum efficiencyand XRL spectra. This represents an increase of ∼800% withrespect to the non-annealed value of the same crystal, thanksto the reduction in VO, and a 36% rise in comparison with ourprevious record value of 4560 ph/n.11) Therefore, the LY ofCe:LYBO is already equal to that of GS20 reference, whilefurther improvements are expected upon optimization ofother parameters such as dopant concentration. The scintilla-tion decay upon pulsed X-ray excitation is described by asingle decay with a lifetime as short as 24 ns. This value issmaller than those of most common Ce-dopedscintillators33,34) and can therefore be advantageous fortime-resolved measurements.35) On the contrary, the decaycurve of GS20 is much more complex, and a three exponen-tial fitting leads to the following lifetimes (ratios): 7 ns (22%),34 ns (52%), and 92 ns (26%).To conclude, scattering-free LYBO and Ce:LYBO singlecrystals are successfully grown for the first time. Thecommonly observed milky appearance in LYBO is notcaused by the segregation of secondary phases during thegrowth, but by the decomposition of local Li- and B-deficientLYBO at the atomic level during the cooling process. Thisphenomenon can be prevented by thermal quenching uponcooling. The scattering centers are likely due to the participa-tion of simple stable compounds LiYO2 and Li4B2O5, whichtend to form aligned needle-like microstructures of up to2 μm in length. As-grown crystals, with and without scat-tering, present oxygen vacancies that decrease transmittanceand LY. These can be efficiently suppressed by annealingunder air. Scattering-free Ce:LYBO crystals exhibit a higherscintillation performance, reaching for the first time a LY of6200 ph/n, which is comparable to commercial Li-glassGS20 reference. In contrast to the complex and slow decayof the latter, Ce:LYBO exhibits a single decay with a lifetimeas short as 24 ns. Current results demonstrate the potential ofCe:LYBO single crystals for thermal neutron detectionapplications.Acknowledgments The authors would like to sincerely thank Mr. SatoshiYamamoto for his kind support with crystal cutting and polishing. This work waspartially supported by the Grant-in-Aid for User Facility Service in NIMS, as wellas by the “Advanced Research Infrastructure for Materials and Nanotechnology inJapan (ARIM)” of the Ministry of Education, Culture, Sports, Science andTechnology (JPMXP12 23NM5337).ORCID iDs Dongsheng Yuan https://orcid.org/0000-0001-9650-2272Daisuke Nakauchi https://orcid.org/0000-0001-6686-4694Takumi Kato https://orcid.org/0000-0002-5650-4449Takayuki Yanagida https://orcid.org/0000-0002-7326-58491) T. Yanagida, T. Kato, D. Nakauchi, and N. Kawaguchi, Jpn. J. Appl. Phys.62, 010508 (2023).2) R. T. Kouzes, A. T. Lintereur, and E. R. Siciliano, Nucl. Instrum. MethodsPhys. Res., Sect. A 784, 172 (2015).3) F. Sacchetti et al., Eur. Phys. J. Plus 130, 53 (2015).4) D. G. Chica, Y. He, K. M. McCall, D. Y. Chung, R. O. Pak, G. Trimarchi,Z. Liu, P. M. De Lurgio, B. W. Wessels, and M. G. Kanatzidis, Nature 577,346 (2020).5) V. Popov and P. 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