Improved Lithium Iodide neutron scintillator with Eu ${}^{2+}$ activation II: Activator zoning and concentration effects in Bridgman-grown crystals

Journal Article · 01 September 2018 · Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

DOI: https://doi.org/10.1016/j.nima.2018.06.057 · OSTI ID:1470866

Boatner, Lynn A.  ^[1]; Comer, Eleanor P.  ^[2]; Wright, Gomez W. ^[3]; Ramey, Joanne Oxendine ^[1]; Riedel, Richard A. ^[4]; Jellison Jr, Gerald Earle ^[1]; Kolopus, James A.  ^[1]

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1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Materials Science & Technology Division
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Materials Science & Technology; Univ. of Tennessee, Knoxville, TN (United States). Nuclear Engineering Department and Scintillation Materials Research Center
3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Nuclear Security and Isotope Technology Division
4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Instrument and Source Division, Neutron Sciences Directorate

We have previously reported on the formation of Suzuki Phase precipitate particles as a result of the addition of the divalent activator ion Eu²⁺ to the monovalent alkali halide host LiI. [Boatner et al. (2017)]. These precipitates form during Bridgman or other melt-growth processes, even at low Eu²⁺ concentrations (e.g., 0.1% EuI₂ doping), and scatter the scintillation light reducing the optical transparency of the scintillator and adversely affecting its radiation-detection performance. In our prior work, we developed a two-stage thermal-treatment method for the post-growth removal of the Suzuki Phase particles and the realization of a significant improvement in the optical transparency and associated neutron-detection of LiI:Eu²⁺ scintillators. These improvements resulted in neutron-detection performance that is superior to GS-20 glass and that allows for the application of pulse height gamma-ray discrimination over a wide range of gamma ray energies as opposed to pulse shape discrimination. Here in this paper, we apply the two-stage thermal-processing method for the removal of Suzuki phase precipitates and carry out an in-depth study, first, of the neutron scintillator performance versus the Eu²⁺ activator-ion-concentration spatial variation as a result of zoning effects during the Bridgman growth of LiI:Eu and, second, of the effects of varying the initial Eu²⁺ activator ion concentration prior to crystal growth. The Eu²⁺ zoning variation results allow one to identify and select the most efficient location of the scintillation performance in a directionally solidified single-crystal boule. The present study of the initial activator concentration levels shows that there are, in fact, two distinct types of luminescence centers with varying performance properties — one that occurs only at low EuI₂ addition levels (e.g., 0.01 to 0.06 %EuI₂) and that is quickly replaced by a second luminescing center with increasing Eu content (e.g., at 0.1% EuI2). The light yield for the luminescing center formed using a Eu activator in LiI is a critical function of the Eu concentration in the range of 0.01 to 0.1 % EuI₂, and a high light yield of 100,000 photons/neutron is observed at the 0.06 %EuIadditive level prior to thermal processing.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1470866
• Alternate ID(s): OSTI ID: 1495637
• Journal Information: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 903, Issue C; ISSN 0168-9002
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English