Effects of nuclear spins on the quantum relaxation of the magnetization for the molecular nanomagnet Fe8

strong influence of nuclear spins on resonant quantum tunneling in the molecular cluster Fe-8 is demonstrated for the first time by comparing the relaxation rate of the standard Fee sample with two isotopic modified samples: (i) Fe-56 is replaced by Fe-57, and (ii) a fraction of H-1 is replaced by H-2. By using a recently developed "hole digging" method, we measured an intrinsic broadening which is driven by the hyperfine fields. Our measurements are in good agreement with numerical hyperfine calculations. For T > 1.5 K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.