EI,SE'~qER UCLEAR PHYSIC5 Nuclear Physics B (Proc. Suppl.) 61B (1998) 66-70 PROCEEDINGS SUPPLEMENTS INVESTIGATION OF LEAD TUNGSTATE (PbWO4) CRYSTAL PROPERTIES S. Baccar&, B. Borgia b, A. Cecilia =, I. Dafinei b, M. Diemoz b, P. Fabeni ~, M. Nikl d, M. Martini", M. MontecchP, G. Pazzi ", G. Spinoloe, A. Vedda ~ =ENEA/INN, Casaccia, Via Anguillarese 301,00060 S.Maria di Galeria (Roma), Italy blNFN, Sez. Roma, Univ. "La Sapienza", P.le Aldo Mort 2, 00185 Roma, Italy qROE- CNR, Via Panciatichi 64, 1-50127 Firenze, Italy dlnstitute of Physics, Cukrovarnicka I 0, 16200 Prague, Czech Republic qstituto Nazionale di Fisica della Materia and Dipartimento di Fisica dell'Universita' di Milano, Via Celoria 16, 1-20133 Milano, Italy Abstract - Single crystals of PbWO4 have been studied in connection with their application for high energy physics experiments. A comprehensive characterization of PbWO4 by means of transmission, thermoluminescence and emission spectra measurements will be described. Annealing procedures in the air and vacuum atmospheres at high temperatures were used to study induced changes in the transmission. The gamma radiation induced changes in the absorption spectra of several PbWO4 crystals of different origin, and intentionally doped will be illustrated. 1. INTRODUCTION Lead tungstate PbWO4 is a birefrigent scheelite structure with tetragonal unit cell [1]. The structure of Czochralski grown PbWO4 consists of WO4 tetrahedra linked by Pb ions. The first coordination sphere of Pb ions is created by eigth oxygen ions in a distorted cube-like arrangement. The scintillation and luminescence properties of PbWO4 have been intensively studied in the past years [2-4]. At present, there is a strongly increased interest on this material induced by the fact that it was found to satisfy the requirements for modern scintillation detectors in high energy physics [5,6,7]. Recently, PbWO4 has been chosen as a scintillating medium for a crystal calorimeter of a new generation for the Large Hadron Collider (LHC) project at CERN [8]. Position and shape of the peaks in scintillation spectra, decay kinetics, light yield and radiation resistance are the important parameters for any scintillation material. Radiation resistance is particularly important, because high energy physics applications create rather severe environment with high level of radiation as a result of particle collisions. Under recent deeper investigations the crystals of PbWO 4 show a rather complicated behaviour both as for emission properties [9], influence of trap centres on the decay kinetics [10], transmission spectrum and radiation damage [1 la,b]. The aim of this work is to obtain a comprehensive experimental characterization of PbWO4 single crystals for a better understanding of the scintillation and radiation hardness properties in particular for LHC application .thus creating the premise for a reproducible production of a PbWO 4 scintillators in the mass scale. The complex character of PbW04 emission characteristics is given by the fact that both regular lattice 0NO4) 2" and a defect based centre (WO3), contribute to the emission spectrum and the concentration of these defects is strongly influenced by the raw material and the technology used for the crystal growth. These two types of centres give rise to blue (420 nm, 2.9 eV) and green ( 520 nm, 2.38 eV) emissions, respectively. Red emission component peaking at about 650 nm (1.9 eV) is also present and it strongly depends on the purity of the powder and on the growth technology. As a matter of fact this peak is minimum at the top (seed) and maximum at the bottom of the crystal and therefore it was ascribed at Pb 3÷ [5,7]. In previous studies [5], only a fast component (of the order of ns) was mentioned in the room temperature scintillation decay, but recently very slow components in microsec-msec time scale were also revealed, and attributed mainly to the defect-based green-emission [9]. The investigation concerning the origin of these components is particularly important, due to the fact that slow processes are potentially harmful for high-repetition rate applications. To improve the radiation resistance of crystals usually they are doped with a small amount (10s-10 "4 of the mass) of Nb ions (PbWO4:Nb) [llc]. In this case the green luminescence intensity is slightly diminished, the red luminescence band mentioned above disappears, while a new luminescence band with a maximum at 520 nm and an excitation of nearly 340 nm is observed. A decrease of green luminescence intensity is noted with the increase of Nb content while no effect is seen on the blue luminescence emission. This is in agreement with 0920-5632/98/$1900 © 1998 Elsevier Science B.V. All rights reserved. Pll S0920-5632(97)00540-9