N3C1  Scintillation Processes

Wednesday, Nov. 4  14:00-15:50  Town and Country

Session Chair:  Nerine Cherepy, LLNL, United States; Takayuki Yanagida, Nara Institute of Science and Technology, Japan

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(14:00) N3C1-1, Transport and Rate Equation Modeling of Experiments on Proportionality of Decay Time Components in CsI:Tl

R. T. Williams1, X. Lu1, A. Syntfeld-Kazuch2, L. Swiderski2, M. Moszynski2, A. V. Gektin3

1Dept. of Physics, Wake Forest University, Winston-Salem, NC, USA
2National Centre for Nuclear Research, Otwock-Swierk, Poland
3Institute for Scintillation Materials, Kharkov, Ukraine
Recent measurements of the proportionality of gamma response for different decay components of scintillation in CsI:Tl [1] together with temperature dependence [1,2] open a new level of detail that can in principle provide new insight on the causes of intrinsic nonproportionality and possible ways to improve energy resolution. Nonproportionality of light yield in a high-energy particle track is a complex nonlinear phenomenon in a very inhomogeneous distribution of excitations whose evolution is in principle dependent on more than 20 transport and rate coefficients. To analyze the new detail made available by experiments such as in Refs. [1,2], it is essential to employ the structured context of a physically-based computational model. After several developmental stages [3-6], a set of coupled transport and rate equations taking into account spatial inhomogeneity of cylindrical tracks, related carrier transport of hot and thermalized carriers, and rate equations up to 3rd order in carrier density has been recently tested in the specific system of undoped CsI at 295 K and 100 K and Tl-doped CsI at 295 K.[7] An important feature of the model is that in addition to calculating proportionality and light yield of the full scintillation response, one can extract plots of spatial distributions of carriers, excitons, charge states of traps, and light emission at any instant of time after excitation. Such plots provided considerable insight on the origins of nonproportional response and variation with temperature and doping in the initial tests of the model.[7] By extension to the richer data set of the measurements now available in [1], such combined spatial-, temporal-, and energy-resolved analysis provides opportunity to better understand variables affecting proportionality as well as to refine and further validate the numerical model itself, moving closer to a computational tool for scintillation materials engineering.

(14:20) N3C1-2, Preferential Eu Site Occupation and its Consequences in Ternary Luminescent Halides AB2I5:Eu2+ (A=Li-Cs; B=Sr, Ba)

K. Biswas, C. M. Fang, A. Burger, S. Mukhopadhyay

Department of Chemistry & Physics, Arkansas State University, State University, AR, USA
Several rare-earth doped, heavy metal halides have been recently identified as potential next-generation luminescent materials with high efficiency at low cost. AB2I5:Eu2+ (A=Li-Cs; B=Sr, Ba) is one such family of halides. Its members, such as CsBa2I5:Eu2+ and KSr2I5:Eu2+, along with a few other binary halides (e.g., LaBr3:Ce and SrI2:Eu) are currently being investigated as high performance scintillator materials with improved sensitivity, light yield, and energy resolution less than 3% at 662 keV. Within the AB2I5 family, our first-principles based calculations reveal two remarkably different trends in Eu site occupation. The Sr-containing crystals have the substitutional Eu ions occupying both eight-coordinated B1(VIII) and the seven-coordinated B2(VII) sites. However, in the Ba-containing crystals, Eu strongly prefer the B2(VII) sites. This random versus preferential distribution of Eu affects their electronic properties. The calculations also suggest that in the Ba-containing compounds one can expect formation of Eu-rich domains. These results provide atomistic insight into recent experimental observations about the concentration and temperature effects in Eu-doped CsBa2I5. We discuss the implications of our results with respect to luminescent properties and applications. We also hypothesize Sr, Ba-mixed quaternary iodides, ABaVIII(Sr,Eu)VIII5 as scintillators having enhanced homogeneity and electronic properties.

(14:40) N3C1-3, Scintillation Properties of Single-Crystal and Ceramic GGAG(Ce) and Ceramic GYGAG(Ce) at Temperatures up to 200?C

O. Philip1, G. Gunow2, I. Shestakova1, M. Berheide3, C. Stoller1, N. Cherepy4

1Princeton Technology Center, Schlumberger, Princeton Junction, NJ, USA
2Massachusetts Institute of Technology, Cambridge, MA, USA
3Doll Research, Schlumberger, Cambridge, MA, USA
4Lawrence Livermore National Laboratory, Livermore, CA, USA
Many gamma ray measurements in the oil field require fast scintillators with high stopping power that are capable of surviving the hostile downhole environment of high temperatures and elevated levels of shock and vibration. Two recently developed Ce-doped fast scintillator materials with high stopping power are ceramic (Gd,Y)3(Al,Ga)5O12 (GYGAG, LLNL) and single-crystal Gd3Ga3Al2O12 (GGAG, Furukawa Corp.). They were evaluated with respect to their performance at temperatures up to 200°C to determine whether they would be suitable for use in downhole applications. The light emission spectra for both samples were measured at temperatures from 25°C to 200°C. While the samples were heated, 137Cs nuclear spectra were continuously acquired. This made it possible to measure light output and spectral resolution as a function of temperature. Although both samples exhibit promising nuclear performance at room temperature in terms of light output and energy resolution, their scintillation properties quickly degrade with temperature. The measurement techniques and results will be presented.

(15:00) N3C1-4, Estimation of Fano factor in Inorganic Scintillators from Time Correlations

V. Bora1, H. H. Barrett1, D. Fastje1, E. Clarkson1, L. Furenlid1, K. Shah2, J. Glodo2

1College of Optical Sciences, University of Arizona, Tucson, AZ, USA
2Radiation Monitoring Devices, Watertown, MA, USA
For a given energy deposited, the Fano factor of a scintillator is defined as the ratio of the variance of the number of scintillation photons to the mean number of scintillation photons. Correlations in time between the signals from two photomultiplier tubes collecting light from the same scintillation event were used to estimate the Fano factor of scintillators. At 662 KeV, LaB3:Ce was found to be sub-Poisson, while YAP:Ce was found to be close to Poisson.

(15:20) N3C1-5, invited, Combinatorial Approach to Bulk Detector Material Engineering: Application to Rapid Scintillators Performance Improvement via Multi-Parameter Optimization Strategy

I. V. Khodyuk, D. Perrodin, S. E. Derenzo, E. D. Bourret, G. A. Bizarri

Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Historically, the discovery and optimization of doped bulk materials has been predominantly developed through an Edisonian approach. While successful and despite the constant progress in fundamental understanding of detector materials physics, the process has been restricted by its inherent slow pace and low success rate. This poor throughput owes largely to the considerable compositional space that needs to be accounted for to fully comprehend complex material/performance relationship. Here, we present a combinatorial approach where bulk scintillator materials can be rapidly optimized for their properties through concurrent multi-parameter optimization strategies. The concept relies on a three-step process: (i) experimental planning and the application of design of experiment (DoE), (ii) material synthesis and characterization with the use of rapid single crystal growth and evaluation techniques, and (iii) data analysis leveraging response surface and multivariable regression analysis methods. The DoE revolves around a Taguchi method that is particularly well adapted to simultaneously study multiple factors influence on a targeted output parameter. The arrangement of the experimental set, an orthogonal array, is designed to explore and optimize the material performance in a multi-dimensional space using the least possible number of experiments. This framework was coupled to the LBNL high-throughput synthesis and characterization facility to rapidly produce and evaluate single crystalline samples. The approach was successfully applied to the optimization of the light output and energy resolution of NaI as a function of multiple elements doping/co-doping strategies. The results show a drastic improvement of both properties. Optimized sample shows an improvement of its energy resolution down to 4.9% at 662 keV and a light output up to 52,000 ph/MeV. The same combinatorial approach is now being applied to other materials, objectives and factors.