Radioactive isotopes in solid state physics

mass separation ISOL-principle ial. Then these isotopes can be either diffused or implanted into the material under study. The most versatile procedure is ion implantation: Depending on the implantation energy, the concentration of dopants, their lateral and their depth distribution can be controlled easily. Any unwanted co-doping by other elements is only determined by the purity of the ion beam and ion implantation is a process not limited by thermal equilibrium, therefore doping is possible beyond any solubilities. But, the energies used for implantation (keV to MeV) are much higher than typical binding energies of atoms in a crystal (eV) so that high concentrations of intrinsic defects (vacancies, interstitials, antisites, dislocations, even amorphous layers) are created. A thermal annealing treatment of the implanted crystal is required in order to remove these defects. The most versatile "isotope factory" is represented by an online isotope separator facility such as ISOLDE at CERN in Geneva [5]. Here, the production, the chemical separation, the mass separation and the implantation of radioactive isotopes are integrated into one device (figure 1). At ISOLDE the radioactive isotopes are produced by spallation, fragmentation or fission reactions in solid or liquid targets hit by an external high energy proton beam. More than 600 different isotopes of 70 elements can be produced. The big success of the on-line mass separation technology at ISOLDE in manyfields, nuclear and atomic physics, astrophysics and solid state physics, triggered worldwide interest in installing similar facilities for the production ofradioactive ion beams [6]. N uclear physics has developed a number of experimental techniques for detecting particles or y-radiation emitted during the decay of radioactive isotopes. The radioactive decay also opens the possibility to detect with high sensitivity the interaction of nuclear moments with external electromagnetic fields. Many of these techniques have successfully entered the field of condensed matter physics often labelled as "nuclear solid state physics" [1,2]. The first application of radioactive isotopes in solid-state physics research dates back to 1920, when radioactive Pb atoms were used by G. v. Hevesey to study self diffusion in lead [3]. Hevesey also first used radioactive atoms to study biologic systems by tracking the flow of radioactive tracers from plant roots to the leaves. The "radio tracer diffusion" technique was born. Nowadays it is a common method for investigating atomic diffusion processes in solids. An important advantage of employing radioactive nuclei is the ability of detecting signals from very small amounts of impurity atoms. This is particularly important for the characterization of semiconductors or surfaces where already a very low concentration of impurity atoms has a significant influence on the properties of the system. An especially useful tool represents the nuclear transmutation process caused by the ~-decayof radioactive atoms since this process effects a change of the chemical properties of the respective atoms in a solid on a well-known time scale determined by the decay constant and therefore all properties (i.e. conductivity, luminescence) connected to the chemical nature of the impurity atoms should also change. The choice ofa radioactive atom for a specific experiment is on the one hand determined by its chemical nature and on the other hand by its nuclear properties. The host system under study can be doped with these radioactive "probe" atoms either by diffusion, nuclear reaction or ion implantation. The probe atoms interact with their lattice surroundings and the information on these interactions is transmitted to the outside world by the emitted decay products and gives access to internal electric and magnetic fields in crystals, to lattice sites of the probe atoms, to diffusion processes, and to interactions between the probe atoms and other defects present in the crystal. The ongoing experiments in solid state physics using radioactive ion deal with a wide variety of problems involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, surfaces, interfaces, magnetic systems, metals, and ceramics. This article can highlight only a few examples to illustrate the potential of the use of radiaoctive isotopes for various problems in solid state physics. For more extensive reviews of the field see [2,4].