A chemical element in gaseous form. It is called a noble gas since it is inert and rarely reacts with other chemicals. Several radioactive isotopes of this chemical element can only be produced by a nuclear reaction and are therefore measured to detect clandestine underground nuclear explosions. These isotopes are xenon-131m, xenon-133m, xenon-133 and xenon-135. See also Radionuclide monitoring and noble gas.