Photo-luminescence properties of Cu and Ag doped Li 2 B 4 O 7 single crystals at low temperatures G.D. Patra, Mohit Tyagi, D.G. Desai, Babita Tiwari, Shashwati Sen, S.C. Gadkari n Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India article info Article history: Received 15 July 2011 Received in revised form 17 November 2011 Accepted 1 December 2011 Available online 9 December 2011 Keywords: Single crystal Czochralski crystal pulling Dosimetry Photoluminescence abstract UV excited photo luminescence from Li 2 B 4 O 7 :Cu and Li 2 B 4 O 7 :Cu, Ag single crystals has been investigated in the temperature range from 77 K to 300 K. An excitation band having a doublet structure at 240 nm and 262 nm was observed for the emission at 370 nm that corresponds to 1 A 1g - 1 E g and 1 A 1g - 1 T 2g crystal field components of the 3d 10 -3d 9 4s 1 transition of Cu þ . The relative intensity of these components and their temperature dependence provide a measure of the off-center displacement of the Cu þ ground state in the crystal lattice site. The co-doped Ag plays a role of a sensitizer when doped with Cu and increases the overall emission as the emission between Ag states lies in the excitation region of Cu states. The 370 nm emission in both the crystals slightly decreases with temperature; however a sudden increase in the intensity around 264 K was observed. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Lithium tetraborate (Li 2 B 4 O 7 ), abbreviated as LTB, has been considered to be one of the promising scintillators for the neutron detection due to large thermal neutron capture cross-sections of Li and B [1]. On the other hand it has a low efficiency for unwanted gamma-ray background detection because of low effective atomic number (Z eff ) and low material density. The LTB based phosphors have also been used for the thermo-lumines- cence dosimetry (TLD) applications in the past [2]. An important advantage of this crystal is its transparency up to 160 nm (7.8 eV), which is remarkable even among other borates. The large band gap (Eg  9 eV) of these crystals provide a large transparent window energy range for dopants and hence the luminescent spectra have the signs of atomic or ionic emission spectra of the relevant dopant atoms. Its sensitivity to ionizing radiation can be increased substantially depending on the choice of the dopant type. Though doping of the material is difficult due to its compact structure, small doping of Cu makes LTB a scintillator suited for the neutron detection and can also be used as a tissue-equivalent thermoluminescent dosimeter with outstanding sensitivities exceeding that of the well-known LiF:Mg, Ti phosphor [3]. Doped LTB polycrystalline materials are used in dosimeters and also available commercially but single crystals are more advantageous due to the absence of grain boundaries, which makes them high transparent to visible light and very effective for the collection of emitted light from the inner parts of the sample. Also, the resistance to humidity due to its stoichiometric composition and easy handling compared with powder samples make the LTB single crystal ideal for scintillation and thermo luminescence dosimetry (TLD) applications when doped with an efficient activator. The Cu is a well known activator material for the LTB single crystal [3,4]. Emission studies of LTB crystals doped with different dopants have been reported by many researchers but most of them used polycrystalline or glassy samples (see [5,6] and references therein). These studies, however, have been confined to room temperature and a relatively narrow range of excitation energies insufficient to reveal the variety of impurity states, which might be involved due to the large band-gap of the material [7]. The efficiency and sensitivity of LTB:Cu can be increased by co-doping with other elements. For effective energy transfer by co-doping, the emission of one element should lie in the excitation range of the other. It was observed that emission between Ag þ states lies in the excitation region of Cu þ states and it should increase the overall light yield by an energy transfer mechanism. In this communication, we report the growth of Ag and Cu doped LTB single crystals using the Czochralski technique and temperature dependence of photoluminescence of LTB:Cu and LTB:Cu, Ag crystals in the temperature range from 77 K to 300 K. The temperature dependence of excitation spectra is explained based on a possible mechanism of an off-center displacement of the Cu þ ions in the lattice. 2. Experimental Li 2 B 4 O 7 crystals doped with Cu, Ag, In and co-doped with Cuþ Ag, Ag þ In and Cuþ In were grown using the Czochralski technique. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.12.005 n Corresponding author. Tel.: þ91 22 25595051. E-mail address: gadkari@barc.gov.in (S.C. Gadkari). Journal of Luminescence 132 (2012) 1101–1105