We report the observation of a /?-decay-induced lattice site change after the decay of 90 Nb (Ti/2 -14.6 h) to 90 Zr in Fe. Samples of 90 Nb£> were prepared by recoil implantation and cooled to temperatures of -10 mK. The lattice location before and after the p decay was monitored via nuclear magnetic resonance on oriented nuclei (NMR-ON) on 90 Nb and 90m Zr (ri/ 2 =0.8 s), respectively. With double-resonance NMR-ON measurements it could be shown that for a fraction of -0.14(3) of 90m Zr a /?-decay-induced lattice site change must have occurred.PACS numbers: 61.72.Hh, 76.70.~r, 76.80.+y Radioactive isotopes as dilute impurities in a ferromagnetic host lattice such as Fe, Ni, Co, or Gd are subject to a (large) magnetic hyperfine interaction. It is characterized by the hyperfine splitting frequency v^, VA/H^/V^HFAL (1) where g is the nuclear g factor and Z?HF is the magnetic hyperfine field. In this context the lattice location of the impurity atoms is important since the hyperfine fields of impurity atoms at substitutional sites (with an undisturbed surrounding), interstitial sites, or sites with a (well-defined) defect neighborhood are different. Usually when hyperfine fields are discussed, lattice location on substitutional lattice sites is tacitly assumed. Meanwhile, for Fe and Ni as host lattice, the hyperfine fields of most elements (in substitutional lattice sites) are known experimentally with good precision. The hyperfine fields are highly element specific; the order of magnitude is 1-100 T; the typical accuracy is ;S 10~2-10 -3 . With nuclear-orientation techniques the hyperfine splitting frequencies can be measured, and, with the known hyperfine fields, nuclear magnetic moments of radioactive nuclei can be determined. Currently there is a strong interest in the determination of nuclear moments in regions far off stability, for which the half-lives become so short that on-line techniques have to be used. Often radioactive precursors are implanted and the isotope of interest is studied after being populated in the decay chain. For the interpretation of such experiments the exact knowledge of the lattice location of the impurity nuclei is absolutely necessary. Until now it has been disregarded that, going away from the valley of stability to nuclei far off stability, the /?-decay energies increase from a few hundred keV to several MeV. Then, the recoil energy of the impurity atoms may be so large that a lattice site change may occur.With Cu as host matrix, neutrino-recoil-induced single-Frenkel-pair production had been reported in the literature [1]. In that experiment, the y-y perturbed angular correlation (PAC) for the 171-245 keV y cascade of m Cd was measured, which follows the decay of m In (ri/2 = 2.8 d). For the case that m In was produced in situ via the decay of m Sn (ri/2 -35 m) at 7=4.2 K, the PAC spectrum showed a quadrupole modulation which was attributed to a lll In defect configuration produced by the neutrino recoil after the m Sn electron capture (EC) decay (recoil energy 29 eV). Pure l...