  Based on large-scale shell model calculations we have determined the electron
capture, positron capture and beta-decay rates on more than 100 nuclei in the
mass range A=45-65. The rates are given for densities \rho Y_e =10^7-10^{10}
mol/cm^3 and temperatures T=10^9-10^{10} K and hence are relevant for both
types of supernovae (Type Ia and Type II). The shell model electron capture
rates are significantly smaller than currently assumed. For proton-to-baryon
ratios Y_e=0.42-0.46 mol/g, the beta-decay rates are faster than the electron
capture rates during the core collapse of a massive star.
