A. G. Petrosyan

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

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 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.

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Temperature dependence of the charge transfer and f?f luminescence of Yb3+ in garnets and YAP

For single crystals of YAG–15 mol% Yb, LuAG–15 mol% Yb and YAP–8 mol% Yb the temperature dependence of the charge-transfer (CT) and f–f luminescence of Yb3+ excited in the CT absorption band in the temperature range 7–300 K has been studied. Simulation of the experimental data taking into consideration three processes, photoionization of the CT state and radiative and non-radiative energy transfer to the 4f levels, has demonstrated that for adequate description of the relaxation of the CT state it is essential to get information on the temperature dependence of the direct excitation efficiency of the f–f luminescence as well as to study the role of intrinsic luminescence in the energy transfer to ytterbium ions. Excitation spectra of the CT and IR luminescence in the energy region 4–20 eV at RT and 10 K are presented and discussed.

Ytterbium-based scintillators, a new class of inorganic scintillators for solar neutrino spectroscopy

Abstract The observed deficit of the solar neutrino flux is now well established. This puzzling problem of todays particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using 176 Yb as a target for low-energy solar neutrino spectroscopy, an intense R&D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves.

4f2 to 4f5d excited state absorption in Pr3+:YAlO3

Excited-state absorption spectra, from the 3PJ -1I6 or 1D2 manifold to 4f5d levels, recorded in the 23 000-42 000 cm 1 range and calibrated in units of cross sections, are reported and discussed in the case of Pr 3+ -doped YAlO3. The 4f5d! 4f 2 Pr 3+ emission between 30 000 and 44 000 cm 1 is then observed, showing the possibility of UV laser operation following two-step excitation pumping using near-UV-visible pump photons.

Growth and light yield performance of dense Ce3+-doped (Lu,Y)AlO3 solid solution crystals

Many Ce-doped (Lu,Y)AlO 3 solid solution single crystals of various Lu/Y ratio and dopant concentration have been grown using the vertical Bridgman and Czochralski processes. Light yields of around 200% of BGO have been measured under gamma-ray excitation. The as-grown and gamma-ray-induced color centers have been compared in crystals of various composition. The absorption properties were characterized in as-grown crystals and after exposure to 1 Mrad of 60 Co. The extent of radiation-induced optical absorption was found to be dependent on the initial light transmission properties of the crystals.

Single crystalline LuAG fibers for homogeneous dual-readout calorimeters

For the next generation of calorimeters, designed to improve the energy resolution of hadrons and jets measurements, there is a need for highly granular detectors requiring peculiar geometries. Heavy inorganic scintillators allow compact homogeneous calorimeter designs with excellent energy resolution and dual-readout abilities. These scintillators are however not usually suited for geometries with a high aspect ratio because of the important losses observed during the light propagation. Elongated single crystals (fibers) of Lutetium Aluminium garnet (LuAG, Lu3Al5O12) were successfully grown with the micropulling-down technique. We present here the results obtained with the recent fiber production and we discuss how the light propagation could be enhanced to reach attenuation lengths in the fibers better than 0.5 m.