Dependence of the nuclear recoil motion on the phonon spectrum of solids investigated by nuclear resonance fluorescence

The time dependence of the recoil velocity of a140Ce nucleus in a La and a LaF3 lattice after ß emission is studied via the Doppler shift of the 1.596 MeV ? transition in140Ce. Evidence is obtained that the nucleus does not leave the lattice site even at a recoil energy of about 20 eV but carries out a damped oscillation. The quantitative comparison of the recoil velocity with the predictions of lattice theory leads to partial agreement.