Rossi was enlisted to implement a method to test various weapon designs to arrive at one that produced an accurately symmetrical spherical implosion. The tests measured changes of the absorption of gamma rays in a metal sphere as it underwent implosive compression. The gamma rays were emitted by a pellet of the short-lived radioisotope Lanthanum-140 positioned in the center of the sphere. The term RaLa experiment is a contraction of Ra dioactive La nthanum. As compression progressed, the rapid increase in absorption was detected as a decrease of gamma ray intensity recorded outside of the assembly.