Aftereffects following β-decay of¹⁸¹Hf¹

Time differential perturbed angular correlation spectra of¹⁸¹Ta produced by beta decay of¹⁸¹Hf in monoclinic ZrO₂ have been measured over a temperature range 10K to 1300K. 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 200°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 negligible effect on G₂(t).