Eliminating Fractures in Large Lanthanum Bromide Crystal Packages

Abstract

Cerium-activated lanthanum bromide scintillator crystals have low thermal diffusivity and low fracture strength under tension. This combination makes LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce) susceptible to fracturing while cooling from high temperatures ( ≥ 175°C). Large crystals with low surface area-to-volume ratios are especially susceptible. When the cooling rate cannot be controlled, large temperature gradients can form between the crystal core and the crystal surface. The resulting tensile stress is often enough to fracture the crystal. Eliminating such fractures is important for applications involving large and rapid thermal excursions, such as oil well logging and space flight. Fractures can be mitigated through proper design of detector packaging. Measurements were taken to quantify the variability in fracture strength and fracture toughness of LaBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Ce). Corresponding packaging experiments exploring the tradeoffs in detector performance versus required surface feature size and compression have been performed. When external compression (>1.3 MPa) is combined with small surface features (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> <; 20 μm), the thermal stress intensity can be reduced below the fracture toughness limit and allow even large crystals (> 200 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) to survive rapid thermal ramps ( <; -2°C/min.).