 Lanthanum-140 was chosen for having the right emitted gamma ray energy (1.60 megaelectronvolts ( MeV), with fraction of 0.49 MeV ) and a sufficient radiation intensity to provide satisfying signal from the ionization chambers, combined with a rapid radioactive decay into stable Cerium-140 (reducing the radiation hazard for the operators after a period of several half lives). It was also potentially available in larger quantities because its parent nuclide barium-140 is an abundant fission product of uranium. As a consequence, Lanthanum-140 samples contained small amounts of barium-140, caesium-140, and especially strontium-90, which still presents a radioactive contamination problem in the area of the tests. Lanthanum-140 has a specific activity of 5.57 × 105 Ci / g (20.6 PBq / g); a 1,000 Ci (37 TBq) La-140 source therefore equals about 1.8 mg of lanthanum.
