Andrzej J. Wojtowicz

Rare-earth-activated wide bandgap materials for scintillators

Abstract Open f-shell rare-earth (RE) ions in wide bandgap host materials are usually characterized by closely spaced electronic levels due to various electron configurations and charge states. These levels provide convenient luminescent transitions that can be excited by efficient recombination of charge carriers generated in the host material by ionizing radiation. Therefore, it is the area of ionizing radiation detectors, where search for new, fast and efficient scintillator materials for high-energy physics and nuclear medicine, has yielded much of the recent advances in the understanding of radioluminescence and scintillation mechanism in some solid state, UV and VUV luminescent, RE-activated materials. In this paper we shall present selected results of basic experiments such as radioluminescence, VUV spectroscopy, time profiles and thermoluminescence, on barium fluoride (activated with Ce, Pr, Nd, Tb) and two aluminum perovskites, YAlO3 and LuAlO3, activated with Ce. We shall demonstrate that these results point to consecutive carrier capture and recombination at RE ions as the basic mechanism of radioluminescence and scintillation in these materials, despite the strong self-trapping and poor charge transport properties. Consequently, various electron and/or hole traps that intercept and retain for some time the recombining charge carriers play an active role influencing both the scintillation light yield and time profiles of scintillation pulses in these and many other wide bandgap RE-activated luminescent materials.

Thermoluminescence and scintillation properties of rare earth oxyorthosilicate scintillators

In recent years the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln/sub 2/SiO/sub 5/:Ce where Ln = Y, La - Lu, have been reported and, in some cases, extensively studied. In addition, binary and ternary compounds such as (Lu,Y)/sub 2/SiO/sub 5/:Ce, (Lu,Gd)/sub 2/SiO/sub 5/ and (Lu,Y,Gd)/sub 2/SiO/sub 5/:Ce have been reported. All of these crystals have either monoclinic P or C structures with characteristic SiO/sub 4/ tetrahedra and trivalent cations occupying two unique crystallographic positions. The trivalent cerium activator ions are assumed to occupy the cation lattice sites and possibly interstitial positions as well. The excited 5d state of Ce/sup 3+/ is split into 3 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (/spl sim/3 eV) with a Stokes shift of /spl sim/ 0.5 eV. The band gap is about 6 eV, and the index of refraction is close to 1.8, with some variation according to crystallographic axes. Despite these similarities, important differences remain among the crystals including scintillation efficiency, decay time, rise time, and afterglow. In this paper, we report thermoluminescence measurements between 10K and 350K that allow the determination of trapping levels that may influence scintillation properties. The thermoluminescence data shows that the various scintillators compositions have surprisingly dissimilar sets of traps that may at least partially explain some of the differences in their scintillation properties.

Electron traps and scintillation mechanism in LuAlO3:Ce

In this paper we report measurements of thermoluminescence in the temperature range of 20–370 K, isothermal decays, pulsed vacuum ultraviolet and γ-excited luminescence time profiles at various temperatures on cerium-activated orthoaluminate (LuAlO3:Ce, LuAP), a new and promising scintillator material. We demonstrate that results of all these experiments can be consistently explained by assuming a recombination mechanism of scintillation light production in the LuAP scintillator. Using a simple first-order kinetic model that includes Ce3+ ions as recombination centres and a number of electron traps, we extract from experimental data the basic trap parameters (energy depths and frequency factors). Consequently we identify nine traps that are responsible for undesired features of the LuAP scintillator, such as a reduced scintillation light output, a relatively long scintillation rise time and slow scintillation components (afterglow) at room temperature. We demonstrate that some of these traps are responsible for large variations of the scintillation light yield with temperature as reported earlier. Although the deepest traps do not alter scintillation time profiles, they are responsible for a significant scintillation light loss and are, therefore, detrimental to scintillation performance of the material. We observe that there is an apparent correlation between trap depths and frequency factors for at least five of the traps that may fit some more general pattern involving various groupings of all the traps. This, in turn, would indicate that traps in LuAP are not unrelated and are due, most likely, to a series of native defects in the LuAP crystal structure. Although the specific identity of traps remains unknown, the performance of the LuAP scintillator is now, in practical terms, fully understood and can be described numerically at any temperature using a model and a set of parameters given in this paper. It is clear that any major improvement of the material would require that traps are eliminated or that their influence on the scintillation process is minimized.

The coupling of 4T2 and 2E states of the Cr3+ ion in solid state materials

Abstract In this paper we present and discuss two different models describing the coupling of 2 E and 4 T 2 states of octahedrally (or nearly octahedrally) coordinated Cr 3+ ions. Both the electron-lattice interaction and spin-orbit coupling of the doublet and quartet states are taken into account. The electron-lattice interaction is scaled by experimentally measured Stokes shifts whereas the spin-orbit coupling is used as a fitting parameter ( ∼ 200 cm -1 ). The results are then compared to experimental data for Cr 3+ -doped garnets, and kyanite (Al 2 O 3 :SiO 2 ). The agreement with experiment is much better for the model in which the spin-orbit interaction is first included in the electronic Hamiltonian with the lattice frozen at the point, characterizing the average lattice position in the vibronic state. In the case of kyanite different spin-orbit coupling constants (160 and 320 cm -1 ) have to be assigned to the split components of the 2 E level, contrary to the case of weakly split systems.

33000 photons per MeV from mixed (Lu0.75Y0.25)3Al5O12:Pr scintillator crystals

(LuxY1-x)3Al5O12:Pr (x = 0.25, 0.50, 0.75) crystals have been grown by the Czochralski method and their scintillation properties have been examined. Compared to the well-respected LuAG:Pr scintillator, which has so extensively been studied in the recent years, the new mixed LuYAG:Pr crystals display markedly higher light yields, regardless of the value of x. In particular, (Lu0.75Y0.25)3Al5O12:0.2%Pr characterized by a yield of 33000 ph/MeV, an energy resolution of 4.4% (at 662 keV), and a density of 6.2 g/cm3, seems to be an ideal candidate to supercede Lu3Al5O12:0.2%Pr (19000 ph/MeV, 4.6%, 6.7 g/cm3) in various applications. The observed enhancement of light output following the partial substitution of lutetium by yttrium is most probably related to some specific differences in distributions of shallow traps in particular materials.

VUV scintillation of LuPO/sub 4/:Nd and YPO/sub 4/:Nd

LuPO/sub 4/:Nd and YPO/sub 4/:Nd represent new and very fast potential scintillator materials. For most types of ionizing excitation, the luminescence of these materials is dominated by an emission band whose maximum intensity occurs at about 192 nm. The origin of this band lies in the fast 5d-4f transitions of the Nd/sup 3+/ ions. An additional contribution due to host-defect emission varies for different samples. These defects appear to be responsible for the limited light yield of LuPO/sub 4/:Nd.

Radio- and VUV - Excited Luminescence of YAP:Ce, YAP:Pr and YAG:Pr

In this communication we report initial results of studies on x-ray and VUV excited luminescence of YAP and YAG crystals activated with Pr3+ and YAP activated with Ce3+. Excitation and luminescence spectra of Pr3+ and Ce3+ d-f and Pr3+ f-f emissions and luminescence time profiles under pulsed synchrotron excitation are presented and analyzed in order to identify and characterize various host-to-ion energy transfer channels. The results support the notion that direct and trap mediated capture and recombination of holes and electrons via Pr3+ or Ce3+ ions provide the dominant mechanism of radio luminescence production in both YAP and YAG crystals.

Cerium-activated rare-earth orthophosphate and double-phosphate scintillators for x- and gamma-ray detection

When activated with an appropriate rare-earth ion (e.g., Ce or Nd), rare-earth orthophosphates of the form REPO4 (where RE = a rare-earth cation) and alkali rare-earth double phosphates of the form A3RE(PO4)2 (where A = K, Rb, or Cs) are characterized by light yields and decay times that make these materials of interest for radiation-detection applications. Crystals of the compound Rb3Lu(PO4)2 when activated with ~0.1 mol % Ce exhibit a light yield that is ~250% that of BGO with a decay time on the order of ~40 nsec. The cerium-activated rare-earth orthophosphate LuPO4:Ce is also characterized by a high light yield and a relatively fast decay time of ~25 nsec. Additionally, the rare-earth orthophosphates are extremely chemically, physically, and thermally durable hosts that recover easily from radiation damage effects. The properties of the rare-earth orthophosphates and double phosphates that pertain to their use as X- and gamma-ray detectors are reviewed. This review includes information related to the use of Nd-doped LuPO4 as a scintillator with a sufficiently energetic, short-wavelength output (λ=90 nm) so that it can be used in conjunction with appropriately activated proportional counters. Information is presented on the details of the synthesis, structure, and luminescence properties of lanthanide double phosphates that, when activated with cerium, are efficient scintillators with output wavelengths that are sufficiently long to be well matched to the response of silicon photodiode detectors.

X-Ray Study of Nd: YAG on (111)-Oriented Si Obtained by Pulsed Laser Deposition

Yttrium aluminium garnet thin films doped with neodymium have been prepared by Pulsed Laser Deposition method on (111)-oriented Si substrates. The substrate was heated up to temperature in the range between 200 and 600 degrees C. Obtained films were then characterized both by x-ray diffraction method using Siemens D5000 diffracto-meter and radioluminescence spectroscopy.

A deeper insight into (Lu,Y)AG:Pr scintillator crystals

Interior of Czochralski-grown (Lu,Y)AG:Pr crystals has been examined by means of several techniques, such as X-Ray Photoelectron Spectroscopy, X-Ray Diffraction, Time-of-Flight Secondary Ion Mass Spectrometry, and magnetic susceptibility measurements. Additionally, their luminescence has been monitored at various combinations of a double-beam (X-ray/IR) excitation.