N4C1  Scintillators: Crystal Growth and Characterization

Thursday, Nov. 5  14:00-16:00  Town and Country

Session Chair:  Akira YOSHIKAWA, IMR, Tohoku University, Japan; Edgar van Loef, Radiation Monitoring Devices, Inc., United States

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(14:00) N4C1-1, Crystal Growth and Characterization of CsSrBr3: Eu Scintillator Gamma-Ray Detectors

S. S. Gokhale, L. Stand, M. Loyd, M. Zhuravleva, C. L. Melcher

Scintillation Materials Research Center, Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, USA
In an effort to develop promising scintillator materials for use in high energy resolution gamma-ray spectroscopy, we continue the investigation of europium doped cesium strontium bromide (CsSrBr3: Eu). The good performance makes CsSrBr3 a suitable scintillator detector for applications in medical imaging and national security. CsSrBr3 melts congruently at 758 °C, making it relatively easy to grow large crystals via the vertical Bridgman method from a molten mixture of binary metal halides. This work involves the optimization of the crystal growth parameters in order to grow large single crystals (1 inch diameter) of CsSrBr3 with improved crystal quality and detector performance. It was observed that CsSrBr3 undergoes a unique degradation mechanism when exposed to the atmosphere, where over time in addition to the full energy peak a secondary peak is recorded in a gamma-ray spectrum, while the peak position and resolution of the original full energy peak remains unaffected. This degradation process was found to be reversible by annealing the detectors. The detectors were annealed at a temperature of 400 °C under a vacuum of 10^-6 Torr. This degradation along with the effect of annealing on the detectors was investigated. Initial experiments show that CsSrBr3 doped with 5% Eu2+ is a promising scintillator material with a light output of 60000±2000 ph/MeV and an energy resolution of 6%, both measured for a detector with dimensions 15 × 15 × 4 mm under irradiation with 662 keV gamma-rays from a Cs-137 radiation source.

(14:20) N4C1-2, New Lead Tungstate Crystal Production for High-Energy Physics Experiments Based on the Czochralski Technique

R. W. Novotny1, V. Dormenev1, M. Korjik2, J. Houzvicka3, H.-G. Zaunick1

12nd Physics Institute, Justus-Liebig-University, Giessen, Germany
2Institute for Nuclear Problems, Minsk, Belarus
3Crytur, Turnov, Czech Republic
Presently, there is still and again a strong demand to consider and apply high quality lead tungstate scintillation crystals for electromagnetic calorimetry (EMC). Unfortunately, the mass production of lead tungstate using the Czochralski method was stopped after bankruptcy of the Bogoroditsk Technical Chemical Plant (Bogoroditsk, Russia). The producer CRYTUR (Turnov, Czech Republic) having a long time experience and the necessary technology in the development and mass production of oxide crystals expressed the interest in meeting PWO requirements for the high-energy physics community in particular the PANDA experiment at FAIR and calorimeter projects at JLab. The development of lead tungstate crystals was started by CRYTUR last year. Several series of samples were produced under different technical conditions. All test crystals are being characterized with respect to light yield, scintillation kinetics, photoluminescence, optical transmittance and radiation hardness studied by gamma-irradiation in laboratories at Giessen and Minsk. The obtained results confirmed that the technological approach of CRYTUR will allow to produce PWO crystals with properties very close to the PWO-II specifications of the PANDA experiment at FAIR (Darmstadt, Germany). A pre-production of full size crystals of 200 mm length of rectangular and tapered crystal shapes has been started.

(14:40) N4C1-3, The Impact of Cation, Anion, and Carbon Impurities on SrI2(Eu) Performance

S. Lam, S. E. Swider, A. Datta, S. Motakef

CapeSym, Inc., Natick, MA, USA
Despite the excellent light yield and energy resolution of SrI2(Eu), there remains a continuing drive for improved performance. To better identify and examine the influence of impurities on the scintillation performance, crystals of SrI2(4%Eu, 0.2%X), where X = {Ba2+, Ca2+, Cs+, Cu+, Fe2+, K+, Mg2+, Na+, Sn2+, O2-, S2-, C), were grown and characterized. Four crystals were grown at a time by the Bridgman technique using a multiple-ampoule holder. Final impurity concentrations in the grown crystals were measured by ICPMS. Light yield, energy resolution, and decay time were determined via pulse-height gamma ray spectroscopy while emission wavelengths were measured by emission spectroscopy. Some dopants proved beneficial, while others were detrimental to performance. Both Ba and Ca did not segregate well but both codoped crystals, especially the Ba-codoped crystal, exhibited better light yields than the uncodoped standard (4%Eu), but with a slight degradation in resolution. The Cs, Fe and Mg codoped crystals had excellent segregation and led to improvements in light yield—especially Mg. The crystals codoped with Na and Sn had the poorest segregation, and both of these impurities degraded scintillation performance, especially the resolution. In particular, the Sn-doped scintillator displayed a parasitic emission from 600-800 nm and a poor light yield. The results for K-codoped crystals will also be discussed. Finally, the performance of strontium iodide doped with the anions oxygen and sulfur, and with carbon (in the form of graphite) will be shown for the first time.

(15:00) N4C1-4, The Impact of Oxygen and Sulfur Impurities in Alkali-Halide Scintillators

S. E. Swider, S. Lam, A. Datta, K. Becla, S. Motakef

CapeSym, Inc., Natick, MA, US
Alkali- and alkali earth-halide scintillators are known to be deliquescent and reactive with oxygen impurities. These impurities, in turn, form oxyhalides that become a source of ampoule adhesions, fracture, and yield loss in the final crystal. Combustion analysis may be used to analyze the quantity of oxygen in the alkali-halide precursors, or the final crystal. In this method, the assay is reacted with graphite at high temperature (~3000 oC), and the resulting CO2 is measured in an IR cell. We acquired such a combustion analysis system and installed it in a glove box at < 1 ppm moisture. Analysis of 1” diameter, 4” long Bridgman-grown SrI2:Eu crystals revealed that a chemical getter swept oxygen to the tail end effectively, but a non-zero amount of oxygen remained diffused evenly throughout. In addition, despite the use of melt fritting, dehydration, and getters, grown crystals were found to have similar oxygen values as the original precursor beads. The tolerance for oxygen was a function of volume – small SrI2 crystals (1 cm diameter) were shown to tolerate 0.25% oxygen content without fracture. In parallel study, a 1-cm diameter SrI2 crystal was doped with 0.20% sulfur in the form of SrS, and displayed severe fracturing. Therefore oxygen cannot be the sole consideration when purifying for crystal growth. We will discuss oxygen and sulfur measurement, removal, and methods for overcoming solubility limits.

(15:20) N4C1-5, Observing Dislocation Motion Induced by Laser Shock Peening in KI

D. Onken1, S. Gridin1, J. L. Drewery1, K. B. Ucer1, R. T. Williams1, E. Rowe2, E. Tupitsyn2, M. Groza2, P. Bhattacharya2, A. Burger2

1Dept. of Physics, Wake Forest University, Winston-Salem, NC, USA
2Dept. of Physics, Fisk University, Nashville, TN, USA
Laser shock peening (LSP) is applied to potassium iodide (KI) crystals. A selective etching technique is used to reveal the dislocation structure induced by the LSP process, confirming that LSP multiplies and moves dislocations in an alkali halide. Increasing the crystal temperature during LSP is shown to eliminate cracking along cleavage-planes caused by the LSP process and increases the range of dislocation motion in the crystal. LSP has long been used to increase surface toughness and fatigue life in metals. Having shown that LSP affects the mechanical properties in the ionic crystal KI, we propose studying the possible benefit of LSP in other halide crystals, including scintillators. One potential application might be reducing crack initiation and propagation at surfaces during Bridgman growth.

(15:40) N4C1-6, Neutron Imaging to Probe the Shape of Liquid-Solid Interface During Crystal Growth

A. S. Tremsin1, E. D. Bourret-Courchesne2, G. A. Bizarri2, D. Perrodin2, S. V. Vogel3, A. Losko1, J. J. Derby4, W. B. Feller5, I. Khodyuk2, T. Shinohara6

1University of California at Berkeley, Berkeley, USA
2Lawrence Berkeley National Laboratory, Berkeley, USA
3Los Alamos National Laboratory, Berkeley, USA
4University of Minnesota, Minneapolis, USA
5NOVA Scientific, Sturbridge, USA
6Japan Atomic Energy Agency, Naka-gun Ibaraki, Japan
When crystals are grown from the melt, the quality and yield of single crystals are intimately dependent upon the shape of the solid/liquid interface during the growth process. The incorporation of defects in the solid phase, the formation of facets on the solid/liquid interface, which give rise to strained areas within the crystals, and the uniformity of the dopant distribution are all closely dependent upon the shape of the solid/liquid interface. The imaging of that shape in real-time opens opportunities for continuous control throughout the growth process. Real-time control of the interface is inherently coupled to technical challenges. While X-ray techniques are typically limited in their penetration capability, especially for the crystals designed for X-ray and gamma-ray detection, neutrons offer a unique probing tool capable of in-situ studies of the interface profile and for some materials the properties of crystal and melt adjacent to it. In this paper we demonstrate with ex-situ experiments the feasibility to image and study the liquid-solid interface and defect formation through neutron imaging. Features of scintillator samples have been determined using several neutron imaging modalities and using the ability to detect neutrons in a wide range of energies. Structural features and composition variation are being imaged. Of particular importance is the visualization in three dimension of dopant segregation. These techniques can potentially be implemented for a real-time feedback control of the thermal and structural profiles, with image accumulation time on the scale of several minutes, which is acceptable considering a typical growth procedure lasting much longer than that.