The neutron decay leads to emission of β -particles and:
57Co decays to 57Fe by β+ emission. The resulting 57Fe is in its excited state and comes to the ground state by emitting γ rays. The half-life of β+ decay is 270 days and that of the γ emission is 10−6 s. A sample of 57Co gives 5.0×109 gamma rays per second. How much time will elapse before the emission rate of gamma rays drops to 2.5 ×109