FTP, name this element with a Greek name meaning "hidden thing," the fourth noble gas.
A calorimeter being used for the NA48 experiments at CERN contains 27 tons of this element in liquid form.  Often used in photography due the white light it gives off when ionized, between 1960 and 1983 the orange-red spectral line of one of its isotopes was used to define the length of the meter.  Along with barium-141, the 92 isotope of this element is one of the products of uranium-235 fission, and although it is generally inert, it forms compounds with fluorine.  FTP, name this element with a Greek name meaning "hidden thing," the fourth noble gas.
A calorimeter being used for the NA48 experiments at CERN contains 27 tons of this element in liquid form.  
Krypton