This element’s liquid form is used as a scintillation target to look for WIMPs in the LZ experiment. The EXO experiment is searching for evidence of one isotope of this element undergoing neutrinoless double beta decay. An isotope of this element is the most powerful neutron absorber known, and its buildup in reactors can lead to a namesake “poisoning” (10[1]-5[1])that contributed to the Chernobyl meltdown. The NSTAR and NEXT ion engines use this element as propellant. The first excimer lasers used dimers of this element, which is the most common gas ionized by arc lamps. Neil Bartlett reacted this element with platinum hexafluoride to create the (10[1])first known noble gas compound. Beta decay of iodine yields, for 10 points, what noble gas with atomic number (10[1])54, which lies below krypton? ■END■ (10[1])