Sulfur-31 is a ?+ emitter with a half-life of about 2.57 s. Find the amount of sulfur-31 required to produce a power output of 0.001 mW due to the positron-electron annihilations that occur. Recall that the energy released from one positron-electron annihilation is about twice the rest mass energy of an electron or positron, that is, about 1.022 MeV.