Luminescence of doped lithium tetraborate single crystals and glass

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Radiation Measurements 38 (2004) 571 – 574 www.elsevier.com/locate/radmeas

Luminescence of doped lithium tetraborate single crystals and glass M. Ishiia;∗ , Y. Kuwanoa , S. Asabaa , T. Asaia , M. Kawamuraa , N. Senguttuvanb;1 , T. Hayashib , M. Koboyashic , M. Nikld , S. Hosoyae , K. Sakaif , T. Adachif , T. Okuf , H.M. Shimizuf a Dai-Ichi

Kiden Co., Ltd. Shimo-ishihara, Chofu, Tokyo 182-0034, Japan Institute of Technology, Fujisawa 251-8511, Japan c KEK, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan d Institute of Physics, AS CR, Cukrovarnicka 10, Prague 16200, Czech Republic e Yamanashi University, Takeda, Kofu 400-8510, Japan f RIKEN, The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan b Shonan

Received 24 November 2003; received in revised form 24 November 2003; accepted 16 March 2004

Abstract Lithium tetraborate single crystal doped with Cu seems to be a promising new material for neutron detection. With the presence of Li and B, both having large cross section for neutron capture, e7cient neutron detection is expected with high-energy deposits. In the present work, we chose 14 di:erent dopants from Ia, Ib, IIIa, IVa and Va groups in the periodic table besides the rare earth Ce ion. The crystals were grown to a size of 20 mm in diameter and 70 mm in length by vertical Bridgman method. The grown crystals were characterized by optical transmittance and excitation–emission studies. Transmittance and emission characteristics of Ce-doped lithium borate glass are also reported. c 2004 Elsevier Ltd. All rights reserved.  Keywords: Lithium tetraborate; Lithium borate glass; Neutron scintillator

1. Introduction Lithium tetraborate (Li2 B4 O7 , LBO) is a congruently melting compound with low melting point and small density (melting point: 916◦ C,  = 2:45 g=cm3 ) the single crystals of which are usually grown in bulk form by Czochralski and Bridgman methods. In our earlier work, we studied the crystal growth and properties of LBO doped with some impurities and conducted the preliminary research on the possibility of scintillator for neutron detection (Senguttuvan et al., 2002). Dopants such as Cu+=2+ , In3+ , and Ni2+ with ionic radius closer to Li+ , and Ce3+ , a well-known ∗ Corresponding author. Tel.: +81-424-883-312; fax: +81-424-883-420. E-mail address: [email protected] (M. Ishii). 1 Present address: Hitachi Chemical Co., Ltd., Tsukuba 300-4247, Japan.

c 2004 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter
doi:10.1016/j.radmeas.2004.03.017

activator for scintillators, were experimentally selected from a viewpoint of crystal chemistry. Emission under UV excitation was studied for all these crystals and it was found that Cu-doped LBO had the most intense emission. Short-decay characteristics were also observed under alpha and neutron irradiation (Kobayashi, 2002). Recently, studies on the luminescent properties of LBO have appeared in the literature. Polycrystalline Cu and In-doped LBO were studied for thermoluminescent detector (TLD) applications (Kamenskikh et al., 1997; Furetta et al., 2001). With the presence of Li and B in the crystal, e7cient neutron capture is expected with high-energy deposits (van Eijk, 1997); on the contrary, low e7ciency for (unwanted) gamma ray background detection can be expected due to low material density and low e:ective atomic number (Ze: = 7:3). However, search of a suitable dopant to LBO crystal to obtain e7cient scintillation is still incomplete.

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It is the aim of this paper to report on the absorption and luminescence of alkali metal, ns2 (Tanimizu, 1999), and transition metal ions doped LBO. In addition, Ce3+ was chosen, as it is a well-known activator for scintillation crystals. Investigation was also carried out on the Ce-doped LBO glass.

2. Experimental procedure

2.3. Characterization Evaluation of crystal was carried out by optical transmittance followed by excitation–emission measurement. Optical transmittance spectra were measured using Hitachi U3210 spectrophotometer, and excitation–emission spectra using Hitachi F4500 Luorescence spectrophotometer. In the case of Cu-doping, luminescence decay times were measured with spectroLuorometer 199S, Edinburgh Instrument.

2.1. Preparation of doped LBO single crystals

2.2. Preparation of LBO:Ce glass Ce does not form a solid solution with LBO single crystal and hence the dopant mostly segregated out of LBO phase making the crystal opaque. Since LBO can be easily vitriKed due to the presence of B, we tried to dope Ce at higher concentration into LBO glass. In our experiment, LBO and CeO2 mixture was heated at 1000◦ C and the melt was quenched at a rate of 150◦ C=h to 500◦ C. The resulting glass was transparent but the presence of oxygen at high temperature in air atmosphere led to Ce4+ appearance, which turned it brownish. A sample of 10 × 10 × 5 mm3 was cut out of this glass and two opposite 10 × 10 mm2 surfaces were polished for the evaluation of optical transmittance and luminescence.

3. Results and discussion 3.1. Optical properties The optical transmission spectra of typical doped LBO crystals are shown in Fig. 1. As can be seen in the Kgure, cut-o: wavelength, which is 167 nm for the pure LBO, has shifted to about 200 nm in the case of doped crystals. Characteristic absorption peaks appear at speciKc wavelengths for some of the dopants. The overall transmittance becomes low when a crystal shows bubbles or inclusions due to light scattering. Although the electric charge of the elements in Ia group, Na+ , K + , Rb+ and Cs+ , is the same as that of Li+ , the ionic radius is increasing in the range 42–180%, which can decrease noticeably the segregation coe7cient. From the optical transmittance spectra an absorption peak was clearly observed at 235 nm for the crystals doped with Na and K.

100

80

Transmittance (%)

Doped LBO single crystals were grown by the vertical Bridgman method. The details of crystal growth are given in our previous publication (Ishii et al., 2003). Polycrystalline 99.99% purity Li2 B4 O7 in the form of disc obtained from Tomiyama chemicals was used as starting material. It was ground to granular form, added with the required dopants at a level ranging from 0.01 to 1 wt% and mixed before melting for homogenization. The melt was cast on a carbon die with a size  20 × 120 mm2 for making polycrystalline feed material with uniform composition for crystal growth. For the crystal growth, three platinum crucibles with a size  20 × 120 mm2 connected together are used, the advantage of which is three kinds of single crystals with three kinds/levels of dopants can be simultaneously grown in one experiment. Crystals were grown at a crucible-lowering rate of 0:5 mm=h using a 0 0 1 oriented pure LBO seed crystal. Crystal growth was performed in air except for Cu- and Ag-doped crystals, which were grown in a nitrogen atmosphere. The grown crystals were about 90 mm in length and 20 mm in diameter. Although some bubbles/inclusions were observed near the seed crystal, i.e., during initial crystallization, the crystals were mostly clear with fewer defects in the second half of crystal growth. Crystals grown with higher dopant concentration contained more defects. It seemed, however, that cellular structure defects related to the impurities were mostly seen.

60

pure 40

K Pb Ag

20

Cu

0 190

290

390

490

590

Wavelength (nm) Fig. 1. Optical transmission spectra of typical doped LBO single crystals.

M. Ishii et al. / Radiation Measurements 38 (2004) 571 – 574

280 LBO:Cu

LBO:Na

LBO:Pure

573

LBO:Ag

LBO:Ga

LBO:Pb

LBO:Bi

Excitation (nm)

280

Excitation (nm)

240

200 280

240 200 280

LBO:Sn

240 LBO:K

LBO:Cs

200 300 360

240

200 300

420 300 360 420 300 Emission (nm)

360 420

Fig. 3. Excitation–emission contour measured on the crystals grown with Ib, IVa and Va group elements.

360

420

300

360

420

Fig. 2. Excitation–emission contour measured on the crystals grown with Ia group elements.

LBO crystals doped with Ib elements, Cu and Ag, were grown in N2 atmosphere with the expectation that Cu+ and Ag+ will replace Li+ and form solid solution. Optical transmittance showed distinctive absorption peaks at 205 nm for Ag doped crystal, at 240 and 265 nm for Cu-doped crystals and at 242 nm for Pb-doped crystal. The crystals grown with other dopants such as Ga, In, Tl, Sb and Bi did not show any absorption in our studies. 3.2. Emission under the UV excitation The grown crystals were excited with UV light source in the wavelength region 190–300 and the luminescence (emission) spectra were measured. Fig. 2 shows the excitation– emission contour plot. For the undoped crystals, negligibly weak emission was noticed around 365 nm with the excitation at 240 nm. Na- and K-doped crystals also showed such weak emission, but those grown with Rb and Cs doping did not show any emission at all. The emission intensity was higher for the crystals grown with Ib group dopants such as Ag and Cu (Fig. 3). The excitation and the emission wavelengths were 210 and 260 nm for Ag-doped crystals and 260 and 365 nm for Cu-doped crystals, respectively. Comparing the emission intensity of these two crystals, Cu-doped crystals showed the most intense emission. Luminescence decay time of the 365 nm emission (ex = 260 nm) of Cu+ was constant within 0–150◦ C and of 28
s value, which Kts well the literature results (McClure and Weaver, 1991). Among the crystals grown with the doped ns2 ions from IVa elements (Sn and Pb) and Va elements (Sb and Bi), the former showed weak

Excitation (nm)

Emission (nm) 360

(a)

(b)

(c)

280 200 300

360 420 300 360 420 300 Emission (nm)

360 420

Fig. 4. Emission spectra of LBO:Ce single crystals with a CeO2 concentration of (a) 0:1 wt%, (b) 0:5 wt% and (c) 1:0 wt%.

but broad emission while the later showed no observable emission. The crystals grown with the elements from IIIa group (Ga, In and Tl) also did not show noticeable emission. 3.3. Properties of LBO:Ce crystal and glass The emission spectra of LBO:Ce single crystals with a Ce concentration of 0:1–1:0 wt% are shown in Fig. 4. Although a very weak absorption band was observed in the optical transmittance measurement due to the fact that Ce could not enter into LBO matrix, the emission was very intense as can be seen in the Kgure. The crystals showed the peak emission at 365 nm for the excitation at 260 and 330 nm. The emission intensity is the maximum for a CeO2 concentration of 0:5 wt%. However, due to poor crystal quality, the transmittance and the emission intensity decreased when the Ce concentration is increased to 1:0 wt%. Assuming that the emission from LBO:Ce crystal is actually from some form of Li–B–O–Ce compound, we prepared LBO:Ce glass by quenching its melt at su7ciently fast rate and in a reduction atmosphere (H2 in argon) to prevent Ce4+ formation and obtained colorless and clear LBO:Ce glass containing Ce3+ . Transmittance data of pure and Ce-doped (5 mol%) LBO glass are compared with that

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4. Summary

100

Doped single crystals of LBO with Li and B having large absorption cross section for neutron capture were grown by the vertical Bridgman method. The crystals grown were characterized by the optical transmittance and luminescence measurements. Strong luminescence and characteristic optical absorption were observed for the crystals grown with the dopants as Ag, Cu, Pb, and Ce. In addition, LBO:Ce glass was also prepared with higher Ce concentration and showed even more intense emission.

Transmittance (%)

80

60 LBO Crystal

40

LBO Glass LBO:Ce Glass 20

Acknowledgements

0 200

300

400

500

600

Wavelength (nm) Fig. 5. Transmittance spectra of pure and Ce-doped LBO glass and pure LBO crystal.

Authors want to appreciate M. Shiroi and Y. Itoh of Dai-ichi Kiden Co., Ltd. for supporting this research. The present work was partly supported by Grant-in Aid from Japan Science and Technology Corporation and Czech Ministry of Education, KONTAKT, ME519 Project.

Excitation (nm)

References

360

(a)

(b)

(c)

280 200 300

360

420

300 360 420 Emission (nm)

300

360

420

Fig. 6. Emission spectra of LBO glass with di:erent CeO2 concentration: (a) 0:5 wt%, (b) 1:0 wt% and (c) 5:0 wt%.

of pure LBO crystal in Fig. 5. As can be seen from the Kgure, the cut-o: (absorption edge) shifts to longer wavelength side when the Ce concentration is increased in the LBO glass. The emission spectra of LBO glass containing 0.5, 1.0 and 5:0 wt% CeO2 are shown in Fig. 6. As in the case of LBO:Ce single crystals, the emission of LBO:Ce glass is strong, peaking at 350 nm, and most e7ciently excited between 240 and 350 nm. Such luminescence characteristics are similar to the Ce3+ 4f–5d luminescence in other weak-Keld oxide crystals, and the obtained strong emission suggests the possibility that Li–B–glass may be a new neutron scintillator.

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