Radiation damage studies on NaBi(WO 4 ) 2 single crystals through oxygen related defects S.G. Singh n , M. Tyagi, D.G. Desai, A.K. Singh, S.C. Gadkari Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai-400085, India article info Article history: Received 9 February 2010 Received in revised form 6 April 2010 Accepted 7 April 2010 Available online 13 April 2010 Keywords: Double tungstate Cherenkov radiation Radiation hardness Annealing Glow curve abstract Single crystals of NaBi(WO 4 ) 2 grown under different conditions were studied for their radiation hardness to high gamma radiation doses (10 5 and 10 6 Gy) employing high dose rates ( 2 Gy/s). Annealing studies in oxygen as well as in vacuum were carried out and a relation between oxygen defects and irradiation induced damages has been proposed. Induced absorption spectra of irradiated and annealed samples were calculated and 5 individual peaks around 3.1, 2.8, 2.6, 2 and 1.7 eV related to different defect centers have been identified. Recovery of the transmission characteristics after the heavy irradiation was monitored at room temperatures (20 72 1C) and the corresponding lifetime has been found to be about 50 days. The thermoluminescence glow curve has been recorded and the activation energy of the defect centers was calculated as 0.9 eV using an initial rising method. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Currently ultra radiation hard high density scintillator crystals are in demand for applications in the high energy particle detection and as electromagnetic calorimeters at the end cap region of a beam. In these applications, due to a high level of charge background, Cherenkov crystals are better suited com- pared to scintillator crystals [1,2]. NaBi(WO 4 ) 2 (NBW) is a Cherenkov radiator that has all the required properties in its defect free state though a weak scintillation in the green region has also been reported in the literature [3–5]. Efforts on the development of large size NBW crystals gained momentum after the successful testing in HERMES experiments (DESY, Hamburg) as electromagnetic calorimeters [6]. Some of the important characteristics of NBW are given in Table 1. It is possible to grow large size NBW crystals as it congruently melts at relatively low temperatures (  935 1C) and does not undergo any phase transition during the cooling. The single crystals of NBW are generally grown by the Czochralski technique [2,7,8] though growth by Bridgman technique has also been reported [9].A continuous lowering of the crystallization temperature during the melt growth, which is related to compositional changes, is however reported to pose considerable problems in the growth of large single crystals [1,7,10]. During the crystal growth there are many processes and conditions such as differential evaporation, growth ambient, etc. that may introduce defects in the crystal. These defects invariably affect optical quality and radiation hardness of the crystal. While growth related literature for NBW is widely available [2,7–11], defect centers, their origin and effect on radiation hardness of the crystal have not been studied extensively [12]. Further, applica- tions of NBW crystals in the high energy physics will require an understanding of the nature of defects centers formed due to the irradiation. In addition, knowledge of the relation between defect centers and growth/annealing conditions and stability of crystals at room temperature will be useful. In the present paper gamma radiation damage and formation of induced color centers in the stoichiometric NBW crystals are reported at high irradiation doses (10 5 and 10 6 Gy) using a 60 Co gamma-ray source (at an accelerated rate of 2 Gy/s). The induced absorption pattern was deconvoluted assuming a Gaussian shape of absorption peaks. The samples were annealed in oxygen as well as in vacuum. The induced absorption due to annealing under these two conditions was analyzed in order to find out the role of oxygen in the defect formation and its relation to the radiation induced defect centers. Decay of induced absorption with time was also recorded at the peak absorption (445 nm) for 30 days near the room temperature. An average decay lifetime was determined by fitting the data to three decay components. The thermoluminescence glow curve of the material was recorded in order to find out the trap depth and its stability against temperature. 2. Experimental Single crystals of NBW were grown by the Czochralski technique employing an inductive heating mechanism and a Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.030 n Corresponding author. Tel.: + 91 22 25591665. E-mail address: shivgovind_singh143@yahoo.com (S.G. Singh). Nuclear Instruments and Methods in Physics Research A 621 (2010) 111–115