The decay of m Sn to U1 ln via electron capture is employed for the first observation of a solid-state effect caused by nuclear recoil due to neutrino emission. The monoenergetic 1H In recoil atoms of 29 eV start from substitutional lattice sites in copper and produce isolated, single Frenkel pairs. The effect is traced by observation of yy perturbed angular correlations following the decay H1 In to m Cd.PACS numbers: 61.70.Bu, 76.80. + y Monoenergetic recoil atoms following /3-decay via electron capture (EC) observed more than thirty years ago served as an indirect proof for the existence of the neutrino. 1 The disintegration energy Q EC of such a decay is determined by the mass difference betweeen parent and daughter atom. This energy is essentially carried away by the neutrino, while the recoil energy E r of the daughter nucleus is only very small. Therefore, experiments aiming at a direct detection of the recoil atoms were performed on free atoms. 1 In the present experiment the recoil atoms start from substitutional lattice sites in a metal. The purpose is to produce isolated vacancy-interstitial pairs (Frenkel pairs) with the recoil atoms not only acting as primary knockon atoms (PKAs) of the given energy E n but also as nuclear probes providing a means for observing the effect on an atomic scale. Thus the fate of the PKA in single Frenkel-pair production can be investigated experimentally, while it could hitherto only be followed up in computer simulations. 2 A well-defined signal which can be definitely assigned to a monovacancy is obtained as a manifestation of the neutrino-recoil effect.As already verified by a great number of experiments, 3 single Frenkel-pair production in a metal sets in at a certain PKA energy E d , while above 2E d multiple defect production becomes possible. Thus, for exclusive single Frenkel-pair production we have to employ a decay for which the condition E d < E r < 2E d ( at a given E d ) is fulfilled. On the other hand, the use of the yy perturbed angular correlation (PAC) method imposes a restriction to suitable radioactive tracers. Both requirements are perfectly satisfied by utilizing the following decay via two successive ^-disintegrations (half-lives in parentheses):35min) (2.8d) , m Sn -in In -m Cd. 61% of the lu Sn nuclei decay via EC to the ground state of ni In. 4 For the recoil energy E r of the U1 ln atoms one obtains E r^( Q EC -E b ) 2 /2Mc 2 =29eV. QEC = 2465 (12) keV is the energy equivalent to the mass difference between the neutral lu Sn and in In atoms, 5 M denotes the mass of the H1 In atom, and E b = 29 keV is the binding energy of the m Sn K electron that dominates the capture process. Following the decay of nl In, the yy PAC is measured for the 171-245-keV y cascade of m Cd. Thus the interaction of the ul Cd nucleus in the isomeric -f state (half-life 85 ns) with an extranuclear electricfield gradient (EFG) can be observed. The interaction is described by the frequency VQ = e 2 Qq/h, where Q is the probe's nuclear electric quadrupole moment and q the lar...

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