M. Montecchi

Radiation induced formation of color centers in PbWO4 single crystals

The changes in the absorption spectra induced by Co60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O−, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co60 irradiation induced absorption changes is discussed.

Influence of La3+-Doping on Radiation Hardness and Thermoluminescence Characteristics of PbWO4

(a) ENEA-INN/TEC, Via Anguillarese 301, S.Maria di Galeria, 00060 Roma, Italy (b) Institute of Physics, Cukrovarnicka 10, 16200 Prague, Czech Republic (c) INFN, Dept. of Physics, University of Roma 1, P. le A. Moro 2, Roma, Italy (d) SIT, Shonan Institute of Technology, Fujisawa 251, Japan (e) CRYTUR Preciosa a.s., Palackeho 175, 51119 Turnov, Czech Republic (f) KEK, National Laboratory for High Energy Physics, Tsukuba 305, Japan (g) INFM and Dept. of Physics, University of Milano, Via Celoria 16, 20133 Milano, Italy (h) Furukawa Co., Yoshima, Iwaki 970-11, Japan

Optical characterization of low-energy electron-beam-colored LiF crystals by spectral transmittance measurements

The complex refractive index of a LiF crystal surface layer irradiated by low-energy electrons is modified by the stable formation of color centers embedded in it. A simplified dipole-electromagnetic field interaction model has been adopted in order to estimate the dispersion curves of colored LiF from a single optical transmittance measurement. The excellent agreement with the corresponding experimental curves (obtained by means of spectrophotometry and ellipsometry) demonstrates this to be a promising approach for LiF-based optical waveguide characterization.

Characterisation of antireflective TiO2//SiO2 coatings by complementary techniques

Abstract Reactive magnetron sputtering deposition processes based on rotating or double cathodes allow the deposition of multiple TiO2//SiO2 layers on glass for obtaining antireflective coatings for large area applications with a good homogeneity and high resistance against environmental attacks. By increasing the number of TiO2//SiO2 layers, the luminous reflectance of the coated glass decreases but the calculation of angular daylight and energy parameters requires the use of an increasing number of complementary analytical techniques due to the complexity of the coating. In this paper we show how Rutherford back-scattering, neutron reflectivity and spectrophotometry can be successfully utilised to perform a quite complete characterisation of commercial antireflective coatings based on three TiO2//SiO2 layers.

Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces

Inhomogeneity of a thin film, rough interfaces of the film with air and substrate and non-parallelism of these interfaces severely affect reflectance and transmittance. Neglecting these film imperfections could give rise to erroneous optical characterisation of the film from measured spectra. Each one of these imperfections, considered alone, has its own characteristic influence on the interference fringes of the reflectance and transmittance spectra. In this work, a model of an inhomogeneous thin film bounded by rough, unparallel interfaces is introduced for calculating reflectance and transmittance at normal incidence. The effects on the spectra due to roughness as predicted by the proposed model are compared with those evaluated by an approach involving the effective-medium approximation (EMA). In this latter approach, a thin transition layer replaces the rough interface, but it proves inadequate for reproducing the transmittance spectrum. Finally, the model is successfully applied to the characterisation of a LiF film thermally evaporated on a glass substrate.

An automatic device for the quality control of large-scale crystal's production

Abstract In 1999, the construction of the electromagnetic calorimeter of the Compact Muon Solenoid (CMS) experiment started. Half of the barrel calorimeter made of 61200 lead tungstate (PWO) crystals will be assembled and tested in the Regional Centre of INFN-ENEA in Rome, Italy. Before assembling, all 30600 PWO crystals will be qualified for scintillation and radiation hardness characteristics by a specially built Automatic Crystal Control System. The measuring techniques for crystal qualification and performances of the automatic system will be discussed in this work.

Investigation of the avalanche photodiodes for the CMS electromagnetic calorimeter operated at high gain

Abstract Avalanche Photodiodes (APD) with improved characteristics were developed by Hamamatsu Photonics for the Electromagnetic Calorimeter of the CMS experiment. This report presents measurements of the latest generation of APDs, which are capable to operate at high gains (∼2000).

Influence of LiF film growth conditions on electron induced color center formation

Abstract The formation of laser active color centers in LiF films irradiated with low energy electrons has been investigated as a function of the film growth parameters. Spectroscopic measurements have shown that the formation efficiency of F-aggregate centers is higher in polycrystalline films than in single crystals and can depend on the film growth conditions, which in turn influence the surface to volume ratio, the void presence and the preferred orientation of the crystallites.

CATRAM: an apparatus for the optical characterization of advanced transparent materials

At the Transparent Materials Laboratory of the Italian Agency for Energy, New Technologies and Environment, the characterization of advanced transparent materials (CATRAM) facility permits the optical characteristics of transparent advanced materials to be evaluated, a procedure for which spectrophotometers are not adequate. The CATRAM facility is based mainly on a large-diameter integrating sphere to perform transmittance, reflectance, and absorptance measurements. We describe the measurement procedures and verify the reliability of the facility by comparing the results obtained for several samples with those from other laboratories. Furthermore, some system implementations for improvement of accuracy are discussed.

Effect of La Doping on Calcium Tungstate (CaWO4) Crystals Radiation Hardness

The induced absorption spectra were measured for equally grown CaWO 4 and CaWO 4 :La single crystals after 60 Co radioisotope irradiation by doses within 1 to 230 Gy. La doping increases considerably the radiation hardness of CaWO 4 material similarly as was observed for isostructural PbWO 4 single crystals recently. Transmission of as grown undoped CaWO 4 shows a noticeable decrease below 450 nm, which gives the evidence of deep trapping states. These trapping centres are effectively diminished by La doping as well. Induced absorption bands are discussed in the light of known and hypothesized color centers in CaWO 4 and PbWO 4 single crystals, which were reported earlier in the literature.