Vladimir Ouspenski

Evidence and Consequences of Ce

Lu_2(1-x)Y_2xSiO₅:Ce (10 at% Y) single crystals co-doped with Ca²⁺ and Mg²⁺ were prepared by the Czochralski technique. It is shown that co-doping leads to significant improvements of the scintillation performances. Afterglow following X-ray excitation is reduced down to 200 ppm after 20 ms and light yield is increased from 28,000 ph/MeV up to 34,000 ph/MeV under ¹³⁷Cs-662 keV excitation. X-ray Absorption Near Edge Spectroscopy (XANES) was used to demonstrate that a significant part of the Ce ions are stabilized in the Ce⁴⁺ oxidation state in co-doped crystals. A new scintillation mechanism involving Ce⁴⁺ is proposed.

^{4+}

Commercially available LaBr3:5% Ce3+ scintillators show with photomultiplier tube readout about 2.7% energy resolution for the detection of 662 keV γ-rays. Here we will show that by co-doping LaBr3:Ce3+ with Sr2+ or Ca2+ the resolution is improved to 2.0%. Such an improvement is attributed to a strong reduction of the scintillation light losses that are due to radiationless recombination of free electrons and holes during the earliest stages (1–10 ps) inside the high free charge carrier density parts of the ionization track.

in LYSO:Ce,Ca and LYSO:Ce,Mg Single Crystals for Medical Imaging Applications

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.

Improvement of γ-ray energy resolution of LaBr3:Ce3+ scintillation detectors by Sr2+ and Ca2+ co-doping

The effect of high dose 60Co gamma irradiation on photoelectron yield, energy resolution, optical transmission, scintillation time profiles and thermoluminescence glow curves of LaBr3:5%Ce and LaCl3:10%Ce crystals has been investigated. Both materials show lower yields and deteriorated resolutions after exposure to a strong 60Co source, however their transmission, yield proportionality, scintillation decay, and thermoluminescence remain unaffected. Initial scintillation characteristics can be retrieved neither by spontaneous recovery nor upon heating in vacuum.

Development and Characterization of Large La-Halide Gamma-Ray Scintillators for Future Planetary Missions

LaBr3:Ce crystal scintillator can be co-doped with various alkaline earth metals to improve light output and energy resolution of the basic scintillator. Another benefit is improvement of alpha/gamma discrimination via pulse shape analysis. LaBr3:Ce contains a low level of actinium contamination, which produces an alpha particle background. This background is difficult to discriminate from gamma rays. Conversely, the addition of co-dopant into the crystal makes the alpha response much easier to distinguish. LaBr3:Ce,Sr, for example, produces a second, longer decay component in the scintillation pulse when excited by radiation. The amplitude of this second decay component changes in response to a gamma ray versus a heavy charged particle. The change in pulse shape is used to eliminate the alpha background and enable detection of neutron reaction products.

Gamma-Ray Induced Radiation Damage in

LaBr3:Ce crystal scintillator can be co-doped with various alkaline earth metals to improve light output and energy resolution of the basic scintillator. Another benefit is improvement of alpha/gamma discrimination via pulse shape analysis. LaBr3:Ce contains a low level of actinium contamination, which produces an alpha particle background. This background is difficult to discriminate from gamma rays. Conversely, the addition of co-dopant into the crystal makes the alpha response much easier to distinguish. LaBr3:Ce,Sr, for example, produces a second, longer decay component in the scintillation pulse when excited by radiation. The amplitude of this second decay component changes in response to a gamma ray versus a heavy charged particle. The change in pulse shape is used to eliminate the alpha background and enable detection of neutron reaction products.

{\rm LaBr} _{3}{:}5\char"25{\rm Ce}

Last generation medical imaging equipments require materials which possess outstanding performances. For scintillators in the high energy imaging field (PET), crystals with high light yields allow a decrease of the irradiation dose received by the patients during medical application and a more accurate diagnostic. Thermally stimulated luminescence (TSL) data provides the depth of hole or electron traps which can limit the efficiency and increase the kinetic. If these traps are due to lanthanide ions, the level schemes can predict the depth values. Thanks to comparison between TSL glow curves and energy diagrams, the traps inside oxide-based-hosts can be identified. Two examples are proposed here, first, the scintillation in the Ce:LYSO crystals which can be improved by thermal annealing and where divalent cations are used for charge compensation and traps removal and second, optical imaging using a new approach where persistent luminescent nanoparticles are used for in-vivo imaging. In both cases, traps depth should be carefully controlled.

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Cs2LiLaBr6(Ce) (CLLB) crystal scintillator shows great potential as a radiation detection material with excellent energy resolution for gammas, sensitivity to neutrons, and the ability to separate the two using pulse shape discrimination (PSD). Experiments have been performed testing this material using silicon photomultipliers for creation of compact, easily-portable detectors for dual gamma ray spectroscopy and neutron detection. Pulse shape discrimination in CLLB is achieved by analyzing the scintillation pulse decay on the time scales of >1 μs. This feature enables the silicon photomultipliers with low afterpulsing to achieve suitable discrimination. Experiments have been conducted attempting to find a suitable cost/efficiency compromise that maximizes performance by varying crystal cerium concentration along with silicon photomultiplier type and placement position. A disk of CLLB (diameter = 52 mm, thickness = 6 mm, 6Li enriched) coupled to a 6×6 mm2 silicon photomultiplier can achieve 4.4% gamma ray energy resolution at 1275 keV, and 74% thermal neutron detection efficiency with a high pulse shape discrimination figure-of-merit of 1.9.

{\rm LaCl} _{3}{:}10\char"25{\rm Ce}

This report presents the scintillation properties and temperature dependent neutron and gamma responses of Cs2LiLaBr6 elpasolite crystals with 0.5, 2, and 3.5% Ce doping. Cs2LiLaBr6 has excellent scintillation light output proportionality, high light output, and good energy resolution. It also shows good light output temperature stability. Pulse shape differences between neutron and gamma excited pulses are analyzed as a function of temperature. Neutron-gamma pulse shape discrimination is possible in a wide temperature range from -10°C up to at least 140 °C.

Scintillators

The performance improvement of large size (φ = 60 mm, ℓ = 80 mm) Sr²⁺ and Br²⁺ co-doped LaBr₃:Ce³⁺ scintillation crystals is reported. The scintillation light output of both Sr²⁺ and Br²⁺ co-doped crystals are significantly improved. Compared to 70,000 ph/MeV (at 662 keV) for Ce³⁺ only LaBr₃, Sr²⁺ and Ba²⁺ co-doping increase the light output by ~25% to 88,000 ph/MeV and 89,000 ph/MeV, respectively. The energy resolutions of both Sr²⁺ and Ba²⁺ co-doped crystals are improved over a wide energy range as well. The scintillation decay time is slightly lengthened with co-doping. No secondary slow component is observed. Co-doped crystals exhibit very similar emission and excitation characteristics to the Ce³⁺ only crystal. Both Sr²⁺ and Ba²⁺ show improved light output proportionality in the low energy range.