Temperature dependence of radiation hardness of lead tungstate (PWO) scintillation crystals S. Burachas a , M. Ippolitov a , V. Manko a , S. Nikulin a , A. Vasiliev a , A. Apanasenko b , A. Vasiliev c , A. Uzunian c , G. Tamulaitis d, * a RRC Kurchatov Institute, Moscow, Russia b Kharkov University, Kharkov, Ukraine c Institute for High Energy Physics, Protvino, Russia d Institute of Applied Research, Vilnius University, Sauletekio 9-III, 10222 Vilnius, Lithuania article info Article history: Received 20 March 2009 Received in revised form 10 November 2009 Accepted 25 November 2009 Keywords: Scintillators Lead tungstate Radiation detectors Radiation hardness abstract Influence of irradiation on the light yield of PWO (lead tungstate, PbWO 4 ) scintillation crystals was studied in the temperature range from 25  C to þ60  C. Light output and optical transmittance were simultaneously measured as a function of time under irradiation in PWO single crystals grown in first and second recrystallization cycle of raw material, doped with different lanthanides and annealed in different conditions. Increased sensitivity to irradiation and slower recovery of the initial light yield were observed at decreased temperatures. The model of tungsten oxide clusters in a regular PWO lattice is used to interpret the experimental results, and dynamics of the light yield under irradiation at different temperatures are qualitatively explained using rate equations describing composition changes in the clusters. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Lead tungstate (PWO) crystals have been selected as the mate- rial of choice in radiation detectors for several high-energy physics experiments at CERN. Radiation hardness is one of the key issues in application of PWO scintillators. The radiation hardness of PWO crystals at room temperature has been considerably improved by recrystallization (growth of crystals by using previously grown crystals as a raw material) and by doping the PWO crystals with lanthanides (La, Y, Gd, etc.) (Kobayashi et al., 1997, 1998). Mean- while, quantum yield is another important parameter of any scin- tillation material. The quantum yield of PWO can be improved by lowering the operating temperature of the radiation detector. Temperature of 25  C is currently considered as an optimal temperature for practical applications of PWO detectors in CERN projects ALICE (Aleksandrov et al., 2005) and PANDA (Novotny, 2004). However, lowering the operating temperature also decreases the radiation hardness of the PWO crystals (Semenov et al., 2007; Novotny et al., 2007). The dependence of PWO prop- erties on temperature attracted considerable attention only a few years ago and needs a further study and interpretation (Semenov et al., 2007; Novotny et al., 2007). It is generally accepted that the irradiation has no considerable influence on the light emission mechanism of PWO (Zhu, 1996), but rather affects the quantum yield by inducing additional absorption in the spectral region of the emission of the PWO scintillator crystal (Zhu, 1997, 1998a,b). The initial transparency spontaneously recovers after termination of the irradiation, and two components, fast and slow, are observed in the recovery kinetics (Annenkov et al., 1997). The recovery rate is enhanced by higher crystal temperature and illumination at certain wavelengths in visible spectrum (Bondar et al., 1998) and depends on conditions of growth and after-growth annealing (Zhu, 1998a,b; Burachas et al., 2003). Interpretation of the experimental results has usually been based on the model of point defects (Annenkov et al., 2002). However, this approach was insufficient to explain all the experimentally obtained data (see Nikl, 2000 for review). An alternative interpretation is based on assumption that deviations from stoichiometry during PWO growth leads to formation of clusters of tungsten oxide WO 3x in the regular PbWO 4 lattice (Burachas et al., 2002, 2007). Here, WO 3x denotes oxides of tungsten with different valency ranging from four at x ¼ 1 to six at x ¼ 0. These clusters introduce additional absorption and, thus, influence the external quantum yield of the PWO crystal. Due to a variable tungsten valency, the composition and, consequently, the absorption of the clusters might be changed by thermal treatment or irradiation. These changes in composition of the clusters result in * Corresponding author. Tel.: þ370 52366071; fax: þ370 52366059. E-mail address: gintautas.tamulaitis@ff.vu.lt (G. Tamulaitis). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.11.038 Radiation Measurements 45 (2010) 83–88