K.L. Ovanesyan

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.

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.

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.

Potential of existing growth methods of LuAP and related scintillators

Abstract LuAP and LuYAP are promising single-crystal scintillators for use in the next generation of positron emission tomographs. The practical crystal composition in terms of major components and Ce-doping level is however under discussion yet. Bridgman and Czochralski melt techniques are used to develop various crystal compositions for scintillation studies and final composition choice. In this paper we compare performance of 30 LuAP, LuYAP and YAP variously Ce-doped single-crystals grown by Bridgman technique, which reflect potential of existing materials and technologies. In addition to available optical properties, the chemical composition of solid solutions is described in terms of variation of Ce concentration and of Lu/Y ratio.

Formation and properties of crystalline compounds in the Lu2O3-Al2O3 system

Supercooling of the melt is a necessary condition for the formation of letetium orthoaluminate structure. Unless supercooled, a melt of Lu2O3:Al2O3 solidifies yields lutetium aluminum garnet and lutetium oxide. The growth of garnets and orthoaluminates is described briefly, and some basic physical properties of the crystals are reported.

LuAP/LuYAP single crystals for PET scanners: effects of composition and growth history

In this paper we present results of optical and scintillation studies of three series of LuAP and LuYAP crystals differing in yttrium and cerium concentrations. Comparison of as-grown and UV-irradiated crystals has revealed two types of color centers located at 200–240 nm and 250–320 nm and giving rise to underlying absorption. The percentage of light emitted at 1 ls increases from 76% in LuYAP (Lu80%) to 85% in LuAP but is dependent on Ce concentration too. The light output for the best LuYAP and LuAP specimens of 2 � 2 � 10 mm 3 measured in the vertical geometry was 50% of LSO and 35% of LSO respectively.

Growth of LuAP/LuYAP single crystals for pet applications

The light yield and decay properties are the most crucial parameters of cerium-doped lutetium orthoaluminate (LuAlO 3 -Ce, LuAP) and yttrium containing solid solution (Lu 1 m x Y x AlO 3 -Ce, LuYAP), which will be used in Positron Emission Tomographs (PET). Both characteristics strongly depend on the optical quality and cerium concentration introduced into the crystal. Factors governing the crystal quality are commented on. A clear improvement of scintillation properties with increasing the doping concentration is demonstrated in a series of LuYAP (Lu=80 v at.%) crystals scanning the concentration range from 0.1 to 0.4 v at.%. Testing of relations is performed between the crystal doping level and low-temperature thermoluminescence spectra, as well as between the doping level and absorption spectra in UV range.

Comparison of Spectral and Scintillation Properties of LuAP:Ce and LuAP:Ce,Sc Single Crystals

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series grown by the Bridgman method were studied under excitation by γ-rays from a 137Cs source. Both series were prepared using the same quality of starting oxides and demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield in the present series of LuAP:Ce crystals measured in 0.2 cm × 0.2 cm × 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at.% 6448±322 ph/MeV and 9911±496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1±0.4% (l=0.2 cm), 9.5±0.5% (l=0.8 cm). Intentional co-doping with Sc3+ ions was tested and resulted in increase of the Ce distribution coefficient from 0.17 in LuAP:Ce to about 0.3 in LuAP:Ce,Sc. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at.%, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on the results of measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.

Preparation and investigation of rare earth magnesium hexaaluminate solid solutions

Conditions of preparation, by the method of solid state reactions, of rare-earth hexaaluminates RE1−x−yMxM’yMgAl11O19 (RE = La, Sm; M, Mt’ = Gd, Yb, Lu, Y, Sc; x, y = 0, 0.15, 0.3), were investigated. For a number of compositions, high-degree single-phase products were obtained applying multi-step heat treatments in Ar/H2 atmosphere at 1650–1690°C. Intense (107) and (114) diffraction lines typical for the hexaaluminate phase have been observed in X-ray diffraction patterns. Studies of microstructure and of elemental composition showed that magnesium deficiency on the sample surface may reach some 20%, while composition in the bulk is more homogeneous and close to stoichiometric. Our estimations of structural homogeneity and thermal conductivity show that lanthanum hexaaluminates La1−x−yMxM’yMgAl11O19 with pair additives Gd-Yb, Gd-Y, Y-Yb, Y-Lu, Y-Sc (x = y = 0.15) and samarium hexaaluminates Sm1−x−yMxM’yMgAl11O19 with pair additives Gd-Yb, Y-Yb (x = y = 0.15), as well as Sm0.7Yb0.3MgAl11O19, may present interest as thermal barrier coatings.

Comparison of spectral and scintillation properties of LuAP:Ce and LuAP:Ce, Sc single crystals

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series were studied under excitation by gamma-rays from a 137Cs source. Both series demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield of LuAP:Ce crystals measured in 0.2 cm times 0.2 cm times 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at. % 6448 plusmn 322 ph/MeV and 9911 plusmn 496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1 plusmn 0.4% (I = 0.2 cm), 9.5 plusmn 0.5% (I = 0.8 cm). Intentional co-doping with Sc + ions was tested and resulted in increase of the Ce distribution coefficient to about 0.3. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at. %, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.