Improved Lithium Iodide neutron scintillator with Eu~(2+) activation Ⅱ: Activator zoning and concentration effects in Bridgman-grown crystals

摘要

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% EuI2doping), 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, 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:Eu2+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 EuI2addition levels (e.g., 0.01 to 0.06 %EuI2) and that is quickly replaced by a second luminescing center with increasing Eu2+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 photonseutron is observed at the 0.06 %EuI2additive level prior to thermal processing.