What is Barium fluoride?

  Barium fluoride is transparent from the ultraviolet to the infrared, from 150–200 nm to 11–11.5 µm, and can be used as a material to make optical components such as lenses. It is used in windows for infrared spectroscopy, in particular in the field of fuel oil analysis. Its transmittance at 200 nm is relatively low (0.60), but at 500 nm it goes up to 0.96–0.97 and stays at that level until 9 µm, then it starts falling off (0.85 for 10 µm and 0.42 for 12 µm). The refractive index is about 1.46 from 700 nm to 5 µm.
  Barium fluoride  is also a common, very fast (one of the fastest) scintillators for the detection of X-rays, gamma rays or other high energy particles. One of its applications is the detection of 511 keV gamma photons in positron emission tomography. It responds also to alpha and beta particles, but, unlike most scintillators, it does not glow in ultraviolet light.It can be also used for detection of high-energy (10–150 MeV) neutrons, and use pulse shape discrimination techniques to separate them from simultaneously occurring gamma photons.