The complex decay scheme shown below indicates that Mo-99 decays by b- emission to Tc-99m, the metastable (excited) state of Tc. The half-life of the Mo-99 is 67 hr and the half-life of Tc-99m is 6.02 hr. From there it decays by isomeric transition to the ground state, Tc-99, which has a half-life of 220,000 yr. The final step is decay of Tc-99 by b- emission to Ru-99, which is not radioactive.