Radiation resistance is a mandatory requirement for a major part of the active or passivecomponents of the particle detectors planned for the future high energy accelerator LHC.

Ulysse is a training reactor of the Institut National des Sciences et Techniques Nucl éaires (INSTN), in Saclay, and a very suitable tool for testing materials and devicesfor these future large detectors. Its radiation spectrum characteristics are very similarto those expected in these detectors, in terms of spectra of fast neutrons, thermal neutrons and gamma photons, and ratios between those types of radiation.

Results are presented on the neutron resistance of two scintillating crystals, candidatesfor electromagnetic calorimetry, cerium fluoride (CeF3) and lead tungstate (PbWO4), and on the behaviour under neutron irradiation of some avalanche photodiodes, currently under development for light detection of these crystals.

A low dose rate evaluation of several scintillators and wavelength shifters of interest to the CEBAF program was performed with a 60Co line source to a total dose of 160 Gray at a rate of 0.5 Gy/hr in a slowly flowing oxygen atmosphere. The samples included the following: (1) a 2 meter piece of Bicron BC412 scintillator being extensively used in the CEBAF Large Acceptance Spectrometer, (2) a 1 meter long piece of green scintillator doped with a uniquely fast green fluor (G2), (3) wavelength shifting fiber dopedalso with G2, and (4) grooved acrylic-based scintillator that is readout with the WLS fiber. The data indicates that the BC412 shows no loss in intrinsic light output, but does have an attenuation loss amounting to 10% at 1 meter and 16% at 2 meters. The fast green scintillator (and fibers) showed no damage. When read out by WLS fibers, the acrylic scintillator displayed a loss in intrinsic light output, but no change in attenuation.

Phosphors are ubiquitously present in our everyday environment. The most common uses are in fluorescent lamps and display devices; e.g., TV tubes and computer displays. In addition, phosphors are used in medical radiology to transform the x-ray energy to light that is recorded on film for diagnostic purposes. The types of phosphors, mechanisms of operation, and future needs for these major phosphor technologies will be surveyed. In addition, brief mention will be made of some of the minor uses of phosphor materials.

We have examined the optical bandgap, particle size distribution, photoluminescence spectra, and cathodoluminescent response of fifteen phosphors that contain transition-metal and main-group sensitizers. We determined luminance versus applied voltage curves for electron energies from 5 to 1000 eV. Seven phosphors exhibit threshold voltages less than 110 V. Y1.96Eu0.04O3 exhibits a threshold voltage of 13 V and, at 300 V, displays a luminance of 25 fL and a luminous efficiency of 0.02 lm/W.

Detailed properties of the defects and energy transfers responsible for afterglow in transparent sintered ceramic Y2O3:Eu and (Y,Gd)2O3:Eu based scintillators have been determined by transient thermoluminescence ('TL) and spectral measurements of afterglow emission. X-ray excited TTL spectra between 150 K and 350 K reveal 4 defect transitions for 3 mole percent Eu. Adequate modeling of this data requires for each defect transition a trap depth energy, an attempt-to-escape frequency, a broadened density of trap state energies, and a trap filling frequency. These defect parameters have been determined and can be used to predict afterglow dependence on time after excitation shut-off, exposure time, and temperature. The afterglow emission spectra at all temperatures are characteristic of Eu under S6 site-selective UV excitation of these rare-earth C-type structure scintillators. This indicates that states associated with the Eu activator on the S6 site are the terminal states upon thermal release of trapped charge carriers.

To test the potential of the heat exchange method for growing scintillator crystals, ingots of mono-, bi- and trivalent halides were prepared. Except for monovalent halides, the removal of oxygen compounds is indispensable. In the case of CaF2 and BaF2, the purification by reaction with NH4F and a superheating of the melt could be included in the crystal growth process. Clear, monocrystalline ingots of these substances were grown in 4f-heated glassy carbon crucibles without seed crystals. Oxygen-free starting material had to be prepared for the growth of inclusion free CeF3 crystals.

Chromium doped yttrium aluminum garnet (Y3Al5O12:Cr or YAG:Cr) phosphors were synthesized by hydrolysis of aqueous nitrate solutions. The resulting as-synthesized powder was amorphous and crystallized into the YAG structure at ~1000°C. The light output as a function of annealing temperature was found to increase with increasing temperature. Particle size, adsorbed surface species or residual impurities, lattice stability and chromium site occupation were investigated as possible explanations. It was found that there was a small decrease in lattice parameter upon annealing, the particle size increased roughly five times, and residual or adsorbed surface species were present for the low temperature annealed powders. There was no evidence that kinetic barriers to diffusion of Cr ions into the luminescent sites occurred.

Prospects for preparation of two-component dense optical materials for scintillators are shown, using data on phase diagrams of about 300 MFm - RFn (m, n ≤ 4) type systems, formed by metal fluorides. Primary characteristics (decay time and light output of luminescence, radiation hardness, etc.) of some multicomponent crystals are reported.

Zincselenide single crystals grown by the sublimation method are annealed in Zn or Se atmosphere. The annealing effects are examined by means of photoluminescence (PL) and reflection spectroscopy at 4.2 K. In the PL spectrum for the as-grown crystal, bound exciton lines (I2, I1) are observed. For the Zn-annealed crystal, the free exciton line is clearly observed. For the Se-annealed crystal, peak positions of all lines shift to the higher energies and all lines become sharp, compared with the spectra for the as-grown crystal. It is concluded that Sc-vacancies affect the bandgap energy more than Zn-vacancies do.

Results of a study of the luminescence of rare earth ions in crystalline thin films grown on reflective substrates will be presented. As an example, the luminescence of Sm2+:CaF2 films has been calculated as a function of the separation between the active layer and a silicon substrate. These calculations involve coupling the narrow resonance of the rare earth ions to the electromagnetic modes of the film. Results including the optimum thickness for enhanced luminescence will be presented. Experimental work on Sm2+:CaF2 films grown on Si by molecular beam epitaxy will also be discussed.

New machining and polishing techniques have been developed for large scintillating crystal arrays such as the Barium Fluoride Electromagnetic Calorimeter for the GEM Detector at SSCL, the Crystal Clear Collaboration's cerium fluoride or lead tungstenate calorimeter at the proposed LHC at CERN, the PHENIX Detector at RHIC (barium fluoride), and the cesium iodide Calorimeter for the BaBar Detector at PEP-I B Factory at SLAC. The machining and polishing methods to be presented in this paper provide crystalline surfaces without sub-surface damage or deformation as verified by Rutherford Back-scattering (RBS) analysis. Surface roughness of about 10-20 angstroms and sub-micron mechanical tolerances have been demonstrated on large barium fluoride crystal samples. Mass production techniques have also been developed for machining the proper angled surfaces and polishing up to five 50 cm long crystals at one time. These techniques utilize kinematic mount technology developed at LLNL to allow precision machining and polishing of complex surfaces. We will present this technology along with detailed surface studies of barium fluoride and cerium fluoride crystals polished with this technique.

Experiments on connecting scintillating crystals to form homogeneous pieces by means of plastical deformation are described. Optical transmission, radiation hardness, and scintillation spectra have been used for the evaluation of the quality of the bond. The results of experiments on fluorides of cerium, barium, and lead; and cesium iodide confirm that deformation welding can be used successfully for economic manufacturing of large homogeneous scintillators from several smaller parts.