Aftereffects following β-decay of 181Hf1

Time differential perturbed angular correlation spectra of 181Ta produced by beta decay of 181Hf in monocline Zro2 have been measured over a temperature range 10 K to 1300 K. If times near t=0 are excluded, G2(t) is well fitted by a standard single site model for nuclei subject to an interaction with a static electric field gradient. For high purity samples, the effective anisotropy A2 is equal to its expected value above 200deg C but decreases abruptly by approximately a factor of two for lower temperatures. This unusual decrease, which does not occur in natural crystals or in Nb-doped powders, is attributed to an aftereffect of the beta emission that populates with approximately 50% probability an electron trap located about one eV below the conduction band. At low temperatures, this trapped electron causes the Ta nucleus to relax rapidly and contributes to G2(t) only near t=0. At high temperatures or in doped samples the electron escapes quickly enough to have neglibible effect on G2(t). (orig.)