R. L. Rasera

Microcomputer‐controlled perturbed angular correlation spectrometer

A four‐detector time‐differential perturbed angular correlation spectrometer controlled by an inexpensive microcomputer is described. The use of twin‐single‐channel analyzers permits each detector to identify both gamma rays in the cascade. An improved gating logic system reduces the MCA/computer deadtime and allows larger sample activity and faster data accumulation. The twin‐single‐channel analyzer and gating circuit additions also permit simultaneous accumulation of two statistically independent sets of four‐channel spectra. The spectrometer provides on‐line display of raw data and the perturbation function derived from either set of four‐channel spectra.

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).

Zirconia Characterization by Perturbed Angular Correlation Spectroscopy

Perturbed Angular Correlation spectroscopy of 181 Ta nuclei occupying cation sites has been used to investigate microscopic structural properties of several pure and stabilized zirconia materials. The PAC spectra are used to derive the electric field gradients at the tracer nucleus. Thse are compared with calculations based on a simple point-ion model. Agreement between experiment and theory is good for stabilized material and qualitatively reasonable for pure zirconia.

Perturbed angular correlation studies of ferroelectrics

To explore the range of phenomena that could be measured via nuclear electric quadrupole interactions in ferroelectric ternary metal oxides, Perturbed Angular Correlation (PAC) measurements were made on LiNbO, and LiTaO, at laboratory and elevated temperatures, on Ca-doped barium titanate at laboratory and elevated temperatures, and on non-doped barium titanate at cryogenic temperatures. Since the measurements and the analyses are still in progress, a detailed report will follow.

Motional Correlation Time of Dilute 111 Cd Impurities in Se-Rich Liquid Se-Te Alloys

The motional correlation time, τ c , in liquid Se and Se-rich Se-Te alloys has been investigated between 500 and 900°C using time differential perturbed angular correlations of y-rays from dilute 111 Cd impurities. In all alloys we find τ c ∝ exp (E 0 /kT) at low T where E 0 = 0.36 eV. τ C deviates from this relation at high T. At low T, τ C is tentatively identified as the lifetime of a Cd to host molecule bond, and at high T as the average lifetime of bonds in the host molecule.