Sieverts are actually the equivalent dose, which has units of energy deposited per kg of tissue (J/kg).
To calculate equivalent dose from an external radiation source, you have to take the exposure, X (C/kg or R), and calculate the absorbed dose, D, (J/kg or Gy or rad). D depends on the energy of the energy of the incident radiation, the attenuation coefficient of tissue at that given energy, and a few other factors I think. To get from absorbed dose to equivalent dose, H, (J/kg or Sv or rem), you multiply by a unitless conversion factor Q which depends on the type of radiation being absorbed. H = D*Q. For gamma rays, Q is 1. For alpha particles, which are only a risk if the alpha emitter is already in your body, the Q factor is 20.