Light Yield Proportionality Research Articles

Correlation of Nonproportionality and Scintillation Properties with Cerium Concentration in YAlO3:Ce

Cerium-doped YAlO3:Ce (YAP:Ce) is an interesting oxide scintillator that exhibits a wider range of light yield nonproportionality on a sample-to-sample basis than most other well-known oxide scintillators. In general, most oxide materials, such as Bi4Ge3O12 and Lu2SiO5:Ce, are thought to have an intrinsic proportional response that is nearly constant between samples and independent of growth conditions. Since light yield nonproportionality is responsible for degrading the achievable energy resolution of all known scintillators, it is important to understand what contributes to the behavior. In an attempt to understand if the phenomenon can be affected by growth parameters or by other means, seven samples of YAP:Ce were collected from various sources, and eight samples were grown in-house using the Czochralski method. Based on optical and scintillation measurement as well as direct measurement of the cerium concentration, it was determined that the light yield proportionality in YAP:Ce is strongly related to the cerium concentration. Samples that were found to have higher relative cerium concentration displayed a more proportional light yield response. In addition, it was determined that samples with higher cerium concentration also exhibit a faster decay time and an enhanced energy resolution when compared to samples with less cerium. It was also determined that growth in a reducing atmosphere can effectively suppress a parasitic optical absorption band.

Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI2: a key to high-performance scintillation

AbstractThe excellent light yield proportionality of europium-doped strontium iodide (SrI2:Eu) has resulted in state-of-the-art γ-ray detectors with remarkably high-energy resolution, far exceeding that of most halide compounds. In this class of materials, the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI2, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. We propose a rationale for the unexpected high-energy resolution of SrI2. We identify nine stable hole polaron configurations, which consist of dimerised iodine pairs with polaron-binding energies of up to 0.5 eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbour migration paths. Ab initio molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. Consequently, carriers created during γ-irradiation can quickly diffuse away reducing the chance for non-linear recombination, the primary culprit for non-proportionality and resolution reduction. We conclude that the flat, albeit complex, landscape for polaron migration in SrI2 is a key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid-state ionic conductors.

Effects of Na and K co-doping on growth and scintillation properties of Eu:SrI2 crystals

We grew Na and K co-doped Eu:SrI2 [Na,Eu:SrI2 and K,Eu:SrI2] crystals by a modified micro-pulling-down method to reveal the co-doping effects on the crystal growth and scintillation properties. The non-codoped, Na0.5%, Na1.0%, K0.5% and K1.0%,Eu:SrI2 crystals indicated high transparency while the milky parts were generated in the Na5.0% and K5.0%,Eu:SrI2 crystals. The light yields of Na,Eu:SrI2 and K,Eu:SrI2 crystals under γ-ray irradiation were decreased by the Na and K co-doping. On the other hand, there was a small change within 940–1020 ns in the decay times by the Na and K co-doping. In the light yield proportionality under γ-ray irradiation, the non-proportionality in the low energy region was improved by Na and K co-doping.

Scintillation Properties of Ce-Doped LuYAP Crystal for Gamma Ray Detection

In the present day, inorganic scintillating crystals become a main part in detection and spectroscopy of nuclear particles and high energy photons, more spectively in X/g-ray imaging. The good properties for the scintillating crystals used in these applications require high photon yield, high stopping power, good energy resolution, good light yield proportionality, and minimal afterglow. The main useful of Ce-doped Lu0.7Y0.3AlO3 (LuYAP(Ce)) are high stopping power and non-hygroscopic which are expected to be key ingredients for medical imaging. In this work, we studied the light yield non-proportionality and energy resolution of LuYAP(Ce) crystal with the energy range from 31 to 1,274.5 keV using photomultiplier tube (PMT) readout. The intrinsic resolution of the LuYAP(Ce) crystal has been determined after correcting the measured PMT resolution. The results showed that the non-proportional response of the crystals was strongly correlated with the intrinsic resolution of the crystals.

Bridgman bulk growth and scintillation measurements of SrI2:Eu2+

Large diameter Bridgman growth of europium activated strontium iodide SrI2:Eu2+ produces crystals with light yield of up to 115,000ph/MeV with an excellent light yield proportionality. SrI2:Eu2+ exhibits an outstanding energy resolution of better than 3% FWHM at 662keV. Its emission is centered at 435nm. The scintillation decays with a 1μs time constant for small samples and up to 5μs to larger crystals. This paper presents successful progress made in the vertical Bridgman crystal growth of SrI2:Eu2+ and its scintillator properties. Large diameter, crack-free and transparent SrI2:Eu2+single crystals with diameters of 1in., 1.3in., 1.5in. and 2in. were all successfully grown.

Nonproportionality and Scintillation Studies of ${\hbox{Eu:}} {\hbox{SrI}}_{\rm 2}$ From 295 to 5 K

The low temperature scintillation properties of 5 atomic % Eu:SrI2 from ambient temperature down to 5 K were studied for the first time. With decreasing temperature, a shift in emission wavelength and a shortening of decay time were observed. Light yield and energy resolution exhibited notable changes with temperature, and were maximized as temperature was decreased. A degradation of light yield proportionality with decreasing temperature was observed.

Scintillation and Optical Properties of ${\rm BaBrI}\!:\!{\rm Eu}^{2+}$ and ${\rm CsBa}_{2}{\rm I}_{5}\!:\!{\rm Eu}^{2+}$

This paper reports on the scintillation and optical properties of the recently discovered europium doped BaBrI and CsBa2I5 scintillators. Single crystals of BaBrI and CsBa2I5 were grown from the melt by the vertical Bridgman technique. Crystals were doped with 8% and 4% Eu2+ for BaBrI and CsBa2I5 , respectively. These crystals have higher optical quality and present improved scintillation performance compared to the previously reported values. An estimated light yield of 97,000±5,000 and 102,000±6,000 photons per MeV (ph/MeV) of absorbed gamma-ray energy is now measured for BaBrI:8%Eu2+ and CsBa2I5:4%Eu2+ , respectively. A better energy resolution has also been observed. BaBrI energy resolution for the 662 keV full absorption peak (FWHM over peak position) decreased to 3.4±0.2% and CsBa2I5:4%Eu2+ energy resolution decreased to 2.55±0.2% . Additional measurements show both scintillators with an excellent light yield proportionality. Finally, based on optical and x-ray excited emission spectra and decay curves, energy transfer mechanisms in BaBrI and CsBa2I5 scintillators are briefly discussed.

Effects of packaging SrI2(Eu) scintillator crystals

Recent renewed emphasis placed on gamma-ray detectors for national security purposes has motivated researchers to identify and develop new scintillator materials capable of high energy resolution and growable to large sizes. We have discovered that SrI2(Eu) has many desirable properties for gamma-ray detection and spectroscopy, including high light yield of ∼90,000 photons/MeV and excellent light yield proportionality. We have measured <2.7% FWHM at 662 keV with small detectors (<1 cm3) in direct contact with a photomultiplier tube, and ∼3% resolution at 662 keV is obtained for 1 in.3 crystals. Due to the hygroscopic nature of SrI2(Eu), similar to NaI(Tl), proper packaging is required for field use. This work describes a systematic study performed to determine the key factors in the packaging process to optimize performance. These factors include proper polishing of the surface, the geometry of the crystal, reflector materials and windows. A technique based on use of a collimated 137Cs source was developed to examine light collection uniformity. Employing this technique, we found that when the crystal is packaged properly, the variation in the pulse height at 662 keV from events near the bottom of the crystal compared to those near the top of the crystal could be reduced to <1%. This paper describes the design and engineering of our detector package in order to improve energy resolution of 1 in.3-scale SrI2(Eu) crystals.

Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators

Single crystals of SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Eu and SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Ce/Na were grown from anhydrous iodides by the vertical Bridgman technique in evacuated silica ampoules. Growth rates were of the order of 5-30 mm/day. Radioluminescence spectra of SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Eu and SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Ce/Na exhibit a broad band due to Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> emission, respectively. The maximum in the luminescence spectrum of SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Eu is found at 435 nm. The spectrum of SrI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :Ce/Na exhibits a doublet peaking at 404 and 435 nm attributed to Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> emission, while additional impurity-or defect-related emission is present at approximately 525 nm. The strontium iodide scintillators show very high light yields of up to 120 000 photons/MeV, have energy resolutions down to 3% at 662 keV (Full Width Half Maximum) and exhibit excellent light yield proportionality with a standard deviation of less than 5% between 6 and 460 keV.

Scintillators With Potential to Supersede Lanthanum Bromide

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.

Measurements of NaI(Tl) Electron Response: Comparison of Different Samples

This paper measures the sample to sample variation in the light yield proportionality of NaI(Tl), and so explores whether this is an invariant characteristic of the material or whether it depends on the chemical and physical properties of the tested samples. We report on the electron response of nine crystals of NaI(Tl), differing in shape, volume, age, manufacturer and quality. The proportionality has been measured at the SLYNCI facility in the energy range between 3.5 to 460 keV. We observe that while samples produced by the same manufacturer at approximately the same time have virtually identical electron response curves, there are significant sample to sample variations among crystals produced by different manufacturers or at different times. In an effort to correlate changes in the electron response with details of the scintillation mechanism, we characterized other scintillation properties, including the gamma response and the x-ray excited emission spectra and decay times, for the nine crystals. While sample to sample differences in these crystals were observed, we have been unable to identify the underlying fundamental mechanisms that are responsible for these differences.

Prospects for High Energy Resolution Gamma Ray Spectroscopy with Europium-Doped Strontium Iodide

AbstractEuropium-doped strontium iodide scintillators offer a light yield exceeding 100,000 photons/MeV and excellent light yield proportionality, while at the same time, SrI2 is readily grown in single crystal form. Thus far, our collaboration has demonstrated an energy resolution with strontium iodide of 2.6% at 662 keV and 7.6% at 60 keV, and we have grown single crystals surpassing 30 cm3 in size (with lower resolution). Our analysis indicates that SrI2(Eu) has the potential to offer 2% energy resolution at 662 keV with optimized material, optics, and read-out. In particular, improvements in feedstock purity may result in crystal structural and chemical homogeneity, leading to improved light yield uniformity throughout the crystal volume, and consequently, better energy resolution. Uniform, efficient light collection and detection, is also required to achieve the best energy resolution with a SrI2(Eu) scintillator device.

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields. SrI2(Eu) emits into the Eu2+ band, centered at 435nm, with a decay time of 1.2μs and a light yield of ∼90000photons∕MeV. It offers energy resolution better than 4% full width at half maximum at 662keV, and exhibits excellent light yield proportionality. BaI2(Eu) produces &amp;gt;30000photons∕MeV into the Eu2+ band at 420nm (&amp;lt;1μs decay). An additional broad impurity-mediated recombination band is present at 550nm (&amp;gt;3μs decay), unless high-purity feedstock is used.

Study of LaCl3:Ce light yield proportionality with a hybrid photomultiplier tube

Abstract In these years a great interest has been growing for the cerium-doped lanthanum crystals due to their high scintillation yield well-matched to present photocathodes, which makes them promising for nuclear medical imaging. In this paper we report on the LaCl3:Ce scintillation response, with the aim to evaluate its degree of non-proportionality. Light yields of LaCl3:Ce, expressed in photoelectron numbers, were measured for X-ray and γ -energies ranging between 13.4 and 1332 keV . In the same energy range, the energy resolution was also measured. The test was performed with a 25 × 25 × 3 mm 3 crystal mounted on a Hybrid Photomultiplier Tube (HPMT) PP0470 by DEP. We observed the presence of two different ranges of LaCl3:Ce light yield response, respectively, for energies higher than 22Na- 511 keV and for energies lower than 57Co- 122 keV . A sensible difference of light yield was observed in these two regions.