Winicjusz Drozdowski

Scintillation Properties of Praseodymium Activated

Scintillation properties of LuAG:Pr grown by Furukawa Co. Ltd., Japan, have been studied. The best crystals display light outputs up to 19000 ph/MeV and an energy resolution of 4.6% at 662 keV. The scintillation yield is found to be a function of size and temperature of the sample; it can be enhanced by 40% upon heating to 450 K. Radioluminescence spectra show both d- f and f-f transitions of Pr3+ ions; the contribution of the latter increases with temperature. The scintillation decays are complex, with a fast decay constant of 20 ns. The presence of 176Lu induces high background activity.

{\rm Lu}_{3}{\rm Al}_{5}{\rm O} _{12}

CeBr3 crystals have been studied to assess their utility as potential gamma ray spectrometers for future ESA planetary missions. Pulse height spectra, scintillation time profiles, X-ray excited emission spectra, and photoluminescence spectra have been recorded as a function of temperature between 78 and 600 K. In addition, the influence of exposing CeBr3 to various doses of gamma rays from a strong 60Co source on its scintillation performance has been investigated.

Single Crystals

In this paper we present the studies performed on a set of Lu3Al5O12:Pr (LuAG:Pr) crystals with praseodymium concentration between 1.5 and 10%, grown by the micro-pulling-down (muPD) technique. The research comprises the measurements of X-ray excited emission spectra and 137Cs gamma-ray pulse height spectra in a range from 78 to 600 K, and thermoluminescence glow curves. Based on experimental data we discuss the dependence of scintillation properties of Lu3Al5O12:Pr on praseodymium content and temperature. The main attention is focused on a distinct increase of scintillation yield with temperature, which we attribute to existence of shallow electron traps and their temperature-dependent contribution to scintillation of LuAG:Pr. An active role of traps is demonstrated by a novel experiment combining X-ray and laser excitation.

CeBr

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.

_{3}

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.

Scintillator Development for Possible Use in Space Missions

Current technologies for X-ray detection rely on scintillation from expensive inorganic crystals grown at high-temperature, which so far has hindered the development of large-area scintillator arrays. Thanks to the presence of heavy atoms, solution-grown hybrid lead halide perovskite single crystals exhibit short X-ray absorption length and excellent detection efficiency. Here we compare X-ray scintillator characteristics of three-dimensional (3D) MAPbI3 and MAPbBr3 and two-dimensional (2D) (EDBE)PbCl4 hybrid perovskite crystals. X-ray excited thermoluminescence measurements indicate the absence of deep traps and a very small density of shallow trap states, which lessens after-glow effects. All perovskite single crystals exhibit high X-ray excited luminescence yields of >120,000 photons/MeV at low temperature. Although thermal quenching is significant at room temperature, the large exciton binding energy of 2D (EDBE)PbCl4 significantly reduces thermal effects compared to 3D perovskites, and moderate light yield of 9,000 photons/MeV can be achieved even at room temperature. This highlights the potential of 2D metal halide perovskites for large-area and low-cost scintillator devices for medical, security and scientific applications.

Effect of Electron Traps on Scintillation of Praseodymium Activated Lu

Despite the large interest in the application of LaBr3:Ce3+, little is known yet about its optical properties, as measurements are hampered by the hygroscopicity of LaBr3:Ce3+ and because it is not trivial to produce crystals with optically polished surfaces. Here, the absorption and scattering lengths as well as the refractive index of LaBr3:5%Ce3+ are determined experimentally for the first time. The refractive index is found to vary from 2.25 to 2.40 in the Ce3+ emission wavelength region depending on crystal orientation. Furthermore, a model of the Ce3+ absorption and emission probability as a function of wavelength, Ce3+ concentration, and scintillation photon traveling distance is developed. This model is used in combination with the measured absorption and scattering lengths to obtain the intrinsic emission spectrum of LaBr3:Ce3+ from measured emission spectra. Additionally, the model is used to illustrate the importance of the investigated crystal properties for scintillation detector design. It is demonstrated that for crystals with dimensions in the order of a few centimeters, the fraction of scintillation photons undergoing scattering and/or absorption before reaching the photosensor can be several tens of percents depending on the Ce3+ concentration. Finally, it is shown that self-absorption and re-emission of the scintillation photons can have a non-negligible effect on the timing resolution of LaBr3:Ce3+ scintillator detectors.

_3

(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.

Al

Future planetary missions such as BepiColombo are resource limited in both mass and power. Due to the proximity of the spacecraft to the Sun, the instrumentation will encounter harsh environments as far as radiation levels and thermal loads are concerned. Only radiation hard detectors that need little or no cooling will be able to successfully operate after long cruise times and over the expected mission lifetimes. The next generation of lanthanum halide scintillators promises to provide sufficient resolution in the spectral range between 1 and 10 MeV where most of the elemental gamma-ray emission lines can be detected. In order to be suitable for planetary gamma-ray spectrometers with sufficient sensitivity it had to be proven that larger crystals of size 3 can be produced and that they maintain their resolution of 3% at 662 keV. For that purpose we have produced and characterized several larger crystals and assessed their radiation hardness by exposing the crystals to radiation doses that are representative of the expected conditions in the space environment. Systematic measurements on several crystals allowed the determination of the activation potential and the performance verification from which the consequences for instrument flight performance can be derived. From these investigations we conclude that these scintillators are well suited for planetary missions, with excellent and stable performance.

_5

Abstract Results of thermoluminescence (TL) studies on LuAlO 3 : Ce, a new fast and efficient scintillator, are reported. The simple model of TL is used for the glow-curve analysis, leading to the trap depth values of 0.7 and 1.6 eV. The influence of the traps on the scintillation process is also discussed.