Adam C. Lindsey

High energy resolution scintillators for nuclear nonproliferation applications

The detection of ionizing radiation is important in numerous applications related to national security ranging from the detection and identification of fissile materials to the imaging of cargo containers. A key performance criterion is the ability to reliably identify the specific gamma-ray signatures of radioactive elements, and energy resolution approaching 2% at 662 keV is required for this task. In this work, we present discovery and development of new high energy resolution scintillators for gamma-ray detection. The new ternary halide scintillators belong to the following compositional families: AM2X5:Eu, AMX3, and A2MX4:Eu (A = Cs, K; M = Ca, Sr, Ba; X = Br, I) as well as mixed elpasolites Cs2NaREBr3I3:Ce (RE = La, Y). Using thermal analysis, we confirmed their congruent melting and determined crystallization and melting points. Using the Bridgman technique, we grew 6, 12 and 22 mm diameter single crystals and optimized the Eu concentration to obtain the best scintillation performance. Pulse-height spectra under gamma-ray excitation were recorded in order to measure scintillation light output, energy resolution and light output nonproportionality. The KSr2I5:Eu 4% showed the best combination of excellent crystal quality obtained at fast pulling rates and high light output of ~95,000 photons/MeV with energy resolution of 2.4% at 662 keV.

Growth of inch-sized KCa0.8Sr0.2I3:Eu2+ scintillating crystals and high performance for gamma-ray detection

A quaternary iodide KCa0.8Sr0.2I3:Eu2+ scintillator with 2.5% energy resolution at 662 keV in a 5 mm3 sample shows great potential for use in gamma-ray spectroscopy applications. In this work, we report the state-of-the-art growth of high quality 25, 38, and 50 mm diameter KCa0.8Sr0.2I3:Eu2+ single crystals by the vertical self-seeding Bridgman method. KCa0.8Sr0.2I3:Eu2+ with a size of ∅25 mm × 25 mm can achieve excellent energy resolutions of 3.15% at 662 keV and 6.8% at 122 keV irradiation, which are superior to that of a commercial NaI:Tl+ of the same size. The nonuniformity of light collection and production in ∅25 mm × 25 mm and ∅38 mm × 38 mm KCa0.8Sr0.2I3:Eu2+ crystals was evaluated by using a technique based on a collimated 137Cs source and coupling the crystal to a photomultiplier tube (PMT) in different directions. The performances of the packaged crystals for practical use were also measured.

Optical spectroscopy of rare-earth doped ternary lead based halides

Spectroscopic properties of Pr3+ and Er3+ -doped KPb2Br5 crystals were investigated for possible applications in eye-safe lasers as well as Ce3+-doped KPb2Cl5 and Eu2+-doped KPb2Cl5/KPb2Br5 for potential radiation detectors. The studied materials were synthesized through careful purification of starting materials including multi-pass zone-refinement and halogenation. The growth of the purified materials was then carried out through the vertical or horizontal Bridgman technique. Under resonant excitation, infrared (IR) emissions at ~1.5 μm and ~1.6 μm were observed from Er:KPb2Br5 and Pr:KPb2Br5 corresponding to the 4f-4f transitions of 4I13/2→4I15/2 and 3F4,3F3→3H4, respectively. Emission characteristics of the ~1.5 μm Er3+ and ~1.6 μm Pr3+ transitions including IR to visible upconversion emission studies were also discussed. Under xenon lamp excitation, spectroscopic results showed allowed 5d-4f Ce3+ emission centered at ~375 nm in Ce3+-doped KPb2Cl5. Fast photoluminescence decay time of ~30-50 ns was attained from Ce:KPb2Cl5, while X-ray excited emission at ~530 nm appeared to originate from the host KPb2Cl5 crystal. In addition, a commercial Ce:YAP (yttrium aluminum perovskite, YAlO3) crystal was included in this study for comparison. Eu2+ 5d-4f emissions were not observed from Eu2+-doped KPb2Cl5 and KPb2Br5 crystals.

Growth of CsCe 2 Cl 7 and Cs 3 CeCl 6 utilizing the Bridgman method

We present progress on the growth of single crystals of Cs3CeCl6 and CsCe2Cl7, two materials recently discovered to have good scintillation properties with light yields of 21,000 and 26,000 ph/MeV respectively. While still early in their development for potential use as X-ray and gamma-ray detectors, their growth as single crystals has been studied little, and in this work, we present growth of these materials using the vertical Bridgman method which has produced improved crystal size and quality over previous efforts from investigators in recent years.