Brief history of PAC

Who gave birth to PAC ?

In 1940, D.R. Hamilton published a paper [1] in which he treated the theory of the directional correlation of a gamma-gamma cascade. It took seven years before Brady and Deutsch successfully measured such a correlation [2]. They had to work with Geiger counters that combine low detection efficiency with a very bad time resolution and no energy resolution at all, and it is really amazing that a positive result was obtained with such rather primitive equipment. The experimental conditions improved considerably when scintillation detectors became available. From then on gamma-gamma angular correlation experiments became a standard technique to determine spins and parities of nuclear states.

It was soon realized that extranuclear fields may perturb - and sometimes completely wipe out - the angular correlation [3]. As a matter of fact, this property permitted the determination of the nuclear g-factor [4] and of the nuclear quadrupole moment [5], and offered a tool to investigate solid state properties [6]. Theory as well as experimental approach were improved step by step, by many scientists, and before long the first report on a time-differential perturbed angular correlation (PAC) measurement was presented [7].

The early period

The first PAC experiments were intended to measure hyperfine fields, either magnetic or electric. In these experiments a suitable nuclear probe was introduced into the sample, usually being a chemical compound or a dilute alloy. One of the aimes of the experiments of that early period was to collect in a systematic way data on magnetic hyperfine fields in ferromagnetic materials like iron, cobalt, nickel and gadolinium, and on electric field gradients in chemical compounds and noncubic metals.

The samples for those experiments were prepared by means of chemical and metallurgical methods. In the mid-sixties, ion implantation was added to the arsenal of preparative techniques [8]. Although people were aware of the fact that ion implantation can change the local environment of the implanted probe atom, the results of the first implantation experiments did not seem to yield evidence that lattice damage was an important effect. Indeed, in the first PAC study of lattice defects, carried out as early as 1963, Hinman et al. did not implant their samples but they bombarded them with 40 MeV alpha particles [9].

A few years later, point defects in metals had become a fashionable field of applications of the PAC technique. By that time, PAC had evolved into a method that is still widely used to study a large variety of solid state problems at an atomic scale.

First-generation defect studies

During the next period, point defects - and in particular vacancies - were observed in nearly all the cubic and noncubic metals. Ion implantation was the most frequently applied method to introduce the radioactive probe atoms and, at the same time, to create the point defects. Typical issues that were addressed concerned the nature of the observed elementary defect (vacancy vs. interstitial), the number of vacancies in a particular defect cluster, and the geometrical arrangement of these vacancies. After single-crystal samples had been introduced [10], it became possible to deduce valuable information about the symmetry of the defect cluster.

Second-generation defect studies

In modern PAC experiments not the elementary defect itself is the object of investigation, but one uses the vacancy clusters, which are now routinely prepared, to trap other mobile species such as interstitial atoms. In these second-generation experiments it is the trapping ability of the vacancy that keeps the interstitial atom so close to the nuclear probe that it can be studied through the hyperfine interaction, just like the study of vacancy clusters had become possible thanks to the trapping ability of the ¹¹¹In probe atom.

1. D.R. Hamilton, Phys. Rev. 58 (1940) 122
2. E.L. Brady and M. Deutsch, Phys. Rev. 72 (1947) 870
3. G. Goertzel, Phys. Rev. 70 (1946) 897
4. H. Aeppli, H. Albers-Schönberg, A.S. Bishop, H. Frauenfelder, and H. Heer, Phys. Rev. 84 (1951) 370
5. H. Albers-Schönberg, K. Alder, O. Braun, E. Heer, and T.B. Novey, Phys. Rev. 91 (1953) 1287
6. E. Heer and T.B. Novey, Solid State Physics, Vol. 9, ed. F. Seitz and D. Turnbull (Academic Press, New York, 1959) p. 200
7. P. Lehmann and J. Miller, Comptes Rendus 240 (1955) 298
8. H. de Waard and S.A. Drentje, Phys. Lett. 20 (1966) 38
9. G.W. Hinman, G.R. Hoy, J.K. Lees, and J.C. Serio, Phys. Rev. 135 (1964) A218
10. F. Pleiter, Hyperfine Interact. 5 (1977) 109