Perturbed-Angular-Correlation Spectroscopy: Renaissance of a Nuclear Technique

Abstract

During the past decade, perturbed-angular-correlation (PAC) spectroscopy has emerged as an important technique in several areas of materials science. PAC spectroscopy is used to measure the effects of local fields at well-defined lattice sites in a crystal. These measurements can provide unique information about the structures, kinetics, and energetics associated with point defects, the mechanisms of phase transitions, and the strengths and symmetries of chemical bonds of atoms on surfaces and at interfaces. In what follows, I describe the PAC technique in the context of several examples of these applications and I comment on the historical evolution of this spectroscopy.Hyperfine InteractionsThe field of hyperfine interactions encompasses spectroscopic techniques that use the electric and magnetic nuclear moments to measure the specific extranuclear environment of the nuclear moments. Examples of these techniques are nuclear-magnetic-resonance (NMR), nuclear-quadrupole-resonance (NQR), electron-paramagnetic-resonance (EPA), muon-spin-rotation (MSR), Mössbauer-effect (ME), and perturbed-angular-correlation (PAC) spectroscopies. Each of these techniques is well-suited to certain types of measurements but useless for others. For example, NMR spectroscopy can be particularly useful for measuring chemical shifts associated with nuclei of atoms that are major constituents of crystalline solids. However, nuclear electricquadrupole interactions associated with some NMR-active nuclei are not only difficult to measure but they often obscure much of the chemical-shift information by producing spectral-line broadening. PAC spectroscopy can be used to accurately measure the nuclear electric-quadrupole interactions at the sites of nuclei of atoms that are trace dopants in crystals. But the PAC technique is insensitive to the effects of local charge distributions that produce the NMR chemical shifts. ME spectroscopy can be used to measure both of these effects as well as nuclear magnetic-dipole interactions. However, ME measurements are often only practical on crystals that have one of several elements as a major constituent, that is, either Fe or Sn. In addition, the ME sensitivity depends on temperature, and the NMR and EPR sensitivities also depend on temperature. However, the PAC measurement is independent of temperature, which can be a great advantage for studying phenomena such as phase transitions.

The Potential of the Perturbed Angular Correlation Technique in Characterizing Semiconductors

“…For practical reasons (e.g., concerning radiochemistry, anisotropic cÀc cascade population, the half-life of the intermediate level, and the nuclear electromagnetic moments of the intermediate state), favorable PAC probes at present are very few and are summarized in Table I along with their relevant nuclear properties. A more detailed description of the PAC technique can be found elsewhere [1][2][3][4] ; here we restrict ourselves to a short and relevant outline of the technique.…”

“…The chemical bonding of this impurity with the host lattice is different from in the indigenous host lattice, which leads to the socalled impurity-atom effect, giving rise to different hyperfine parameters for different probes. 4 The impurity-atom effect is manifested in the following two ways:…”

Nuclear Spin Relaxation in Some Non-viscous Media Using the 181Hf Probe

“…In this case, no time dependent perturbation is observed as expected. A least squares fitting to the perturbation function [5] gives the quadrupole frequency w Q = 118 Mrad/s. Considering this as the average quadrupole frequency at room temperature, the rotational correlation time in H 2 O has been found to be t C = 15 ns.…”

“…A typical time resolution for the coincidences was found to be õ1 ns. The perturbation functions G 22 A 22 (t) have been found out from the four coincidence combinations (two at 180-and two at 90-) in the usual way [5].…”

A perturbed angular correlation spectrometer for material science studies

“…For example, ME spectroscopy requires ∼10 14 probe atoms for absorption experiments and NQR spectroscopy typically requires about 10 18 atoms. This technique has been applied in several areas of material science [1,2] and chemistry [3] and has some application in biology also. Considering the growing applications of PAC technique in different interdisciplinary areas, a fourdetector PAC spectrometer with ultra-fast BaF 2 scintillators has been developed in Saha Institute of Nuclear Physics, Kolkata.…”