C. Dujardin

Measurements of the intrinsic rise times of common inorganic scintillators

The intrinsic rise times of a number of common inorganic scintillators are determined using ultrafast measurements of luminescence following pulsed X-ray excitation. A Ti-sapphire mode-locked laser and a light-excited X-ray tube are used to produce X-ray pulses with 60 ps fwhm. Fluorescence photons are detected with a microchannel phototube and the response of the phototube and electronics is 45 ps fwhm. Samples are either powders or thin crystals painted black on five sides to reduce delayed scattered photons. The intrinsic scintillators CeF/sub 3/, CdWO/sub 4/, Bi/sub 4/Ge/sub 3/O/sub 12/, and CsI have rise times /spl les/30 ps, indicating that electrons are promptly captured to form the excited states. The activated scintillators CaF/sub 2/:Eu, ZnO:Ga, and Lu/sub 2/SiO/sub 5/:Ce have rise times /spl les/40 ps, indicating that the luminescent centers are excited by rapid sequential hole capture- electron capture. The activated scintillators CsI:Tl and YAlO/sub 3/:Ce have slower rise times due to processes that delay the formation of excited states. It is shown that for practical scintillation detectors, internal reflections in the crystal can degrade observed rise times by hundreds of ps depending on size, reflector, and index of refraction.

Luminescence properties and scintillation mechanisms of cerium- and praseodymium-doped lutetium orthoaluminate

Absorption, reflection as well as luminescence emission, excitation, and decay curves for single crystals of and grown by the Bridgman technique have been measured at various temperatures. The fluorescence spectra photo-excited over a wide energy domain ranging from the UV to the x-ray region, and the kinetics are typical of the cerium and praseodymium ions. These experimental results show that the exciton transfer to the dopant occurs at around 8 eV, and the energy transfer via sequential hole and electron trapping is dominant at higher energy. This process must be considered as the main scintillation mechanism in this crystal. The high efficiency of this mechanism is explained by the small energy difference between the 4f level of the dopant and the top of the valence band, estimated from XPS measurements.

X-ray excited charge transfer luminescence of ytterbium-containing aluminium garnets

Abstract We report on measurements of X-ray excited charge transfer luminescence occurring in Yb 3+ -doped yttrium aluminium garnet (YAG:Yb) and ytterbium aluminium garnet (YbAG). The temperature dependence of luminescence was investigated in variously Yb 3+ -doped crystals. The fluorescence intensity and decay time drop drastically at low temperatures ( T

Elaboration and scintillation properties of Eu3+-doped Gd2O3 and Lu2O3 sol–gel films

Abstract High-resolution X-rays imaging requires thin films of dense materials with high light yield under X-ray excitation. Polycrystalline films of europium-doped gadolinium and lutetium oxide have been prepared by sol–gel method. The scintillation performances of these compounds for X-rays imaging are presented and discussed.

LuAG:Ce fibers for high energy calorimetry

The main objective of this contribution is to point out the potentialities of cerium doped LuAG single crystal as pixels and fibers. We first show that after optimization of growth conditions using Bridgman technology, this composition exhibits very good performances for scintillating applications (up to 26 000 photons/MeV). When grown with the micropulling down technology, fiber shapes can be obtained while the intrinsic performances are preserved. For the future high energy experiments requiring new detector concepts capable of delivering much richer informations about x- or gamma-ray energy deposition, unusual fiber shaped dense materials need to be developed. We demonstrate in this frame that cerium doped LuAG is a serious candidate for the next generation of ionizing radiation calorimeters.

Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol–gel method

Abstract Recently, there has been a growth of interest in the preparation of new phosphor films for high-resolution X-ray imaging systems. Europium activated gadolinium oxide is very interesting because of its scintillation properties especially as a red component. The sol–gel method has been used to synthesize europium-doped gadolinium oxide films. The films present waveguiding properties and this special feature is used to study their microstructure by Raman spectroscopy in waveguiding configuration. Structural results, confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations, show that the crystallization in the cubic phase occurs at 700 °C. Opto-geometrical parameters were determined with respect to the annealing temperature. After annealing at 1000 °C, very dense europium-doped gadolinium oxide films are obtained with a thickness of 390 nm and a refractive index of 1.88 at 632.8 nm. Spectroscopic results constituted by emission spectra (UV and X-ray excitation) and decay measurements are presented.

Optical and scintillation properties of large crystals

The system has been shown to be a promising scintillator for medical imaging devices. Recently, efforts were focused on the improvement of its scintillating properties. Several large crystals with various cerium concentrations were grown. Absorption and excitation spectra were measured in a range extending from the visible to the vacuum ultraviolet (VUV). Emission spectra, fluorescence decay times and light yields, both under -ray and x-ray excitation, were measured under various experimental conditions. A reabsorption process is shown to take place in this material. This process is responsible for the observed decrease of the light yield when increasing the size of the sample.

Observation of the gap blueshift on Gd2O3:Eu3+ nanoparticles

We report on the characterization of Gd2O3 nanocrystals of different sizes doped with 2.5 at. % Eu3+ ions. The particles have been synthesized by a sol-lyophilisation process. This method allows the synthesis of 7–100 nm diameter cubic-phase particles. The photoluminescence properties have been studied from visible to vacuum ultraviolet (VUV) wavelengths. Compared to the bulk material properties, some important changes on the luminescence are observed. In particular, some lines are strengthened when the particle’s size is diminished. In another article we had ascribed these bands to doping ions located on sites close to the surface. Both contributions of volume and surface states are observed. VUV spectroscopy performed selectively on the volumes states has allowed in sesquioxide nanoparticles to point out a gap blueshift due to quantum confinement.

EXTENSIVE STUDIES ON CEF3 CRYSTALS, A GOOD CANDIDATE FOR ELECTROMAGNETIC CALORIMETRY AT FUTURE ACCELERATORS

Abstract In the framework of its search for new heavy, fast and radiation hard scintillators for calorimetry at future colliders, the Crystal Clear Collaboration performed a systematic investigation of the properties and of the scintillation and radiation damage mechanisms of CeF 3 monocrystals. Many samples of various dimensions up to 3 × 3 × 28 cm 3 were produced by industry and characterised in the laboratories by different methods such as: optical transmission, light yield and decay time measurements, excitation and emission spectra, gamma and neutron irradiations. The results of these measurements are discussed. The measured light yield is compared to the theoretical expectations. Tests in high energy electron beams on a crystal matrix were also performed. The suitability of CeF 3 for calorimetry at high rate machines is confirmed. Production and economical considerations are discussed.

Luminescence and Scintillation Properties at the Nanoscale

This contribution is a review of the luminescence and scintillation properties of nanoparticles (NP), particularly doped insulators. Luminescence spectroscopy is an appropriate tool to probe matter at the nanoscale. Luminescence is also the last stage of the scintillation process. Specific surface and structural effects occurring in NP are reported. Their consequences on the NP luminescence properties are discussed. Parts of the effects are related to the preparation method. On the other hand some intrinsic properties of the nanostructures which can modify the optical properties are described: quantum confinement and dielectric confinement. The response under high-energy excitation is also discussed. It appears that their size can be used as a tool to describe the spatial distribution of electronic excitations induced by the relaxation process after the high-energy excitation. Finally, potentiality to grow transparent bulk materials based on small Nps agglomeration via soft chemistry route is presented. It is a promising approach toward the development of scintillating materials.