Speakers: Dr. Maria Zhuravleva (Scintillation Materials Research Center, University of Tennessee. USA)
Place: Library of the Institute of Physics CAS, Cukrovarnicka 10, Praha 6
Presented in English
Organisers:
Department of Optical Materials
Our goal is to discover and develop low-cost, effective new scintillators that can be used for the detection of illicit radioactive materials. In order to unambiguously identify the specific gamma-ray signatures of radioactive elements, scintillator materials must possess energy resolution approaching 2% at 662 keV. Currently available radiation sensors have either inadequate energy resolution (NaI:Tl), unacceptably high cost (CZT and LaBr3:Ce), or limiting operational burden (HPGe).
We present a summary of crystal growth and scintillation properties of new high-resolution scintillators that were discovered by our group during the past two years. When activated with Eu2+, ternary compounds that belong to the KB2X5, K2BI4 and ABX3 (A = Cs, K; B = Ca, Sr, Ba; X = I, Br) or compositional families demonstrate excellent combinations of scintillation light yield and proportionality. Light yields up to ~95,000 photons/MeV and energy resolution as good as 2.4% at 662 keV were measured. Several of the new compounds display very proportional photon and electron responses. The effect of intrinsic radioactivity due to 40K on their performance in nuclear security applications was evaluated. Using the vertical Bridgman method, we have demonstrated practical crystal growth from the melt. Thermal gradients, pulling rate, and growth ampoule geometry were explored in order to overcome challenges such as solid-solid phase transitions, constitutional supercooling, and anisotropic thermal expansion. Growth form off-stoichiometric melts was carried out to suppress formation of inclusions and secondary phases due to volatilization of melt constituents during growth. Cation and anion substitutions resulted in phase stabilization and increase of the scintillation light yield. The advantages of vacuum-drying and zone-refining of raw materials as well as vacuum aging of the melt prior to growth were demonstrated to effectively remove oxygen-containing and other impurities. As a result, optical quality and gamma-ray energy resolution of these scintillator detectors were improved.
In order to demonstrate feasibility of producing high performance scintillators in large volume and lower facility costs, a prototype of a commercial multi-crystal growth furnace, the Multi-Ampoule-Growth-Station, was designed and constructed. To date, the simultaneous growth of five 1-inch diameter crystals has been successfully demonstrated for CsCaI3:Eu and KCaI3:Eu scintillators. The dopant distribution, compositional uniformity and gamma-ray spectroscopic performance as a function of growth parameters were studied for relatively large samples (~1 inch3).
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