The Crystal Ball was a hermetic particle detector used initially with the SPEAR particle accelerator at the Stanford Linear Accelerator Center beginning in 1979. It was designed to detect neutral particles and was used to discover the η c meson. Its central section was a spark chamber surrounded by a nearly-complete sphere of scintillating crystals (NaI(Tl)), for which it was named. With the addition of endcaps of similar construction, the detector covered 98% of the solid angle around the interaction point.