Alexander Annenkov

Lead tungstate (PbWO4) scintillators for LHC EM calorimetry

Abstract This report describes the work carried out in order to analyse the properties of PbWO4 crystals as scintillators and to determine the perspectives of their use in calorimetry in Large Hadron Collider (LHC) experiments. The scintillation mechanism in PWO crystals is explained and the properties connected with their use as scintillators are analysed both for undoped and Nb doped crystals. The specific problems concerning the physical parameters in the case of large scale production of PWO scintillators are discussed.

Improved light yield of lead tungstate scintillators

Abstract The application at medium and low energies of lead tungstate scintillators, so far optimized for the ECAL calorimeter of CMS for the future LHC, is strongly limited by their poor light yield. Suitable dopants like molybdenum and terbium can help to overcome this problem. Concepts, results, advantages and drawbacks of this approach are discussed.

Progress in PbWO4 scintillating crystal

The latest results in improvement of scintillator properties with Nb doping of PbWO4 crystal are presented. The effects on the basic parameters of the crystal, such as radiation hardness, decay time and light yield, are evaluated.

Status on PWO crystals from Bogoroditsk after one year of preproduction for CMS–ECAL

Abstract In September 1998, the CMS electromagnetic calorimeter entered into its construction phase. Since that time, more than 2000 crystals have been produced by Bogoroditsk Techno-Chemical Plant (BTCP) in Russia and analysed at CERN. An overview about mechanical and optical properties as well as radiation hardness characteristics of these crystals will be presented.

Large Volume CaMoO

Several scintillation CaMoO4 crystals with size up to 28times28times220 mm3 were grown by the Czochralski method. Their scintillation properties have been evaluated. Light yield of full size crystals measured with a XP2020 PMT is about 4% relative to a small reference CsI(Tl) crystal. Radio luminescence spectrum under gamma-excitation contains single emission peak with maximum at 520 nm. Optical transmission spectra contain a weak absorption band around 420 nm, which has almost no influence on scintillation light. This allows to produce even larger scintillation elements without deteriorating the light yield. Scintillation kinetics was measured under gamma- and alpha-particle excitation both in fast (2000 ns) and slow (200 mus) time scales. Fast components - 12 ns, (0.1%); 200 ns (0.5%) were detected along with slow - 3.8 mus (3.4%); 20 mus (96%) - components. Difference in fast component contribution under gamma and alpha excitation allows to implement pulse-shape discrimination of alpha-radioactive background coming from impurities in the crystals.

_{4}

Abstract A production of radiation hard synthetic single crystals require well organised mass production of raw material. Modern technology of such production included purification of the ingredients and synthesis of the final raw material. We discuss here a peculiarity of the mass production of the raw materials to produce radiation hard lead tungstate crystals. Specifications and methods of the raw material cleaning also will be observed.

Scintillation Crystals

Several scintillation CaMoO4 crystals with size up to 28times28times220 mm3 were grown by the Czochralski method. Their scintillation properties have been evaluated. Light yield of full size crystals measured with a XP2020 PMT is about 4% relative to a small reference CsI(Tl) crystal. Radio luminescence spectrum under gamma-excitation contains single emission peak with maximum at 520 nm. Optical transmission spectra contain a weak absorption band around 420 nm, which has almost no influence on scintillation light. This allows to produce even larger scintillation elements without deteriorating the light yield. Scintillation kinetics was measured under gamma- and alpha-particle excitation both in fast (2000 ns) and slow (200 mus) time scales. Fast components - 12 ns, (0.1%); 200 ns (0.5%) were detected along with slow - 3.8 mus (3.4%); 20 mus (96%) - components. Difference in fast component contribution under gamma and alpha excitation allows to implement pulse-shape discrimination of alpha-radioactive background coming from impurities in the crystals.

Production of specified raw materials for mass manufacturing of radiation hard scintillation materials

This year an extensive R&D on lead tungstate crystals has entered into the pre-production phase at the Bogoroditsk Techno-Chemical Plant (BTCP). Laboratory small-scale PWO crystal growth technology, which has been tuned and optimised over the last years, is transforming now into an industrial technology of mass production. This mass production technology is based on a set of methods and instrumentation for crystal growth, machining, crystal quality control and certification. According to the specification on lead tungstate pre-production crystals, one of the most important categories of tolerance is the radiation hardness. Control of the PWO radiation hardness at the pre-production phase requires reliability and an easy to use measuring tool with a high productivity. A semi-automatic spectrometric setup for PWO radiation hardness monitoring has been developed and tested at the X5 CERN irradiation facility. After final crosschecks the setup was set into operation at the BTCP. Together with several other methods this setup covers a full range of radiation damage phenomena in PWO crystals.

Large volume Ca_Mo_O-4 scintillation crystals

Recently, lead tungstate PbWO/sub 4/, also called PWO crystals, have been considered as a promising material for precise electromagnetic calorimetry and the first tests have shown that energy resolution, with photomultiplier readout, better than 3%//spl radic/E/spl oplus/0.7 can already be achieved with a calorimeter prototype of 9 or 20 counter cells (20/spl times/20/spl times/180/spl divide/200 cm/sup 3/). Among the relevant properties of the crystal, its good radiation hardness has been specially mentioned as well as its short radiation length and its emission in the blue range of the spectrum. Further development in PWO technology to improve the transparency and uniformity of long crystals suitable for calorimetry has been undertaken. The usual growing conditions for normal PbWO/sub 4/ crystals have been tuned for a better control of stoichiometry, and investigation of pentavalent doping has been explored.<<ETX>>

Equipment and methods for rapid analysis of PWO full-sized scintillation crystal radiation hardness during mass production

This chapter introduces the basic definitions and gives the minimum necessary information about the phenomenon of scintillation and the mechanisms which have to be taken into account for the development of scintillation materials. It starts with an historical brief and describes the sequence of the processes leading to scintillation in a dielectric medium. Definitions are then given of the parameters related to the physical process of light production in the medium, and not dependant on the shape, surface state and optical quality of the scintillator block. After a survey of scintillation mechanisms it is shown that several self-activated scintillators show better scintillation properties when they are doped with appropriate ions. A description is given of the most important activators with a discussion about the conditions for the activator to be efficient in a host matrix. As an example the electron energy level structure of Ce3+ and Pr3+ is described. It is shown that these two ions are good activators with a bright and fast scintillation in many compounds. Several approaches to classify scintillation materials are discussed. This chapter is concluded with a list of the scintillation materials developed so far and of their most important properties.