Uranium-235
Enriched uranium is best known for its use in the bombing of Hiroshima on August 6, 1945.  The bomb called Little Boy used Uranium-235. Uranium-235 can be used for nuclear fission.  This fission is about 504.81 barns for slow thermal neutrons and 1 barn for fast thermal neutrons.  Uranium-235 was discovered in 1935 by Arthur Jeffrey Dempster.