Discovery of New Inorganic Metal Halide Scintillators for Homeland Security Applications

Discovery of new scintillation materials supports a number of applications related to public health and safety, like medical imaging and homeland security. Among several complementary homeland security technologies for radiation detection, high energy (0.5-9 MeV) X-ray radiography enables the high throughput and non-invasive screening of cargo entering the United States. Current systems primarily use CdWO₄ [cadmium tungstate] inorganic scintillation crystals, as well as CsI:Tl [cesium iodide doped with thallium]. These scintillators have performance and practical limitations that encourage research for alternative detector materials.

Due to interesting structure-property relationships and advancements in photoconversion efficiency, halide perovskite and related compositional families have recently garnered significant attention for photovoltaics as well as radiation detection. However, many of these compounds have only been synthesized as nanocrystals or thin films, and they suffer from chemical and functional stability. In radiation detection applications, several inorganic metal halide perovskite and perovskite-derivative scintillators containing cesium and potassium have shown excellent light yield, greater than 50,000 photons per MeV, and energy resolution, less than 4 percent at 662 keV, which makes them well-suited for gamma ray spectroscopy applications. However, there are few reports on rubidium-based halide perovskite compounds and scintillators, in addition to effects of radiation damage.

The goals of this work were to discover new inorganic metal halide perovskite and perovskite-like scintillators and investigate their suitability for homeland security applications. The compositional space focused on rubidium-based scintillators in RbBX₃, RbB₂X₅ and Rb₄BX₆ compositional families, where B²⁺ = Sr, Ca, Ba, Eu, Yb and X^- = Cl, Br, I, as well as the cubic perovskite CsCaCl₃(Eu, Tl) [cesium calcium chloride]. These new scintillator compositions were grown from the melt via the Vertical Bridgman method, and their physical and scintillation properties were tested. For the most promising scintillators, scale up of crystal growth was pursued (22 mm diameter) and the X-ray afterglow using a pulsed X-ray source was measured, as was the temperature stability of the light yield. Finally, the effects of radiation damage (10 Mrad with Co-60 and 9 MV X-rays) were investigated.