Emission channeling is a sensitive technique to measure the lattice location of radioactive impurities embedded in single crystals. It is based on the fact that charged particles from nuclear decay (alpha, beta-, beta+, conversion electrons, Auger electrons) experience channeling or blocking effects along major crystallographic axes and planes. The resulting anisotropic emission yield from the crystal surface characterizes the lattice site occupied by the probe atoms during decay and is measured using position-sensitive detectors. In particular we use
- Si pad detectors (developed at CERN for the Compton camera project) and Si pixel detectors (under consideration are MediPix, TimePix) for the detection of electrons in the energy range > 40 keV up to several MeV (beta-, beta+, conversion electrons);
- Charged Coupled Devices (CCDs) for the detection of very low-energy conversion electrons (< 40 keV) and Auger electrons;
- Si detectors working with the principle of resistive charge division for the detection of alpha particles > 1 MeV.
The main application of emission channeling is the lattice location of electrical, optical and magnetic dopants in semiconductors and oxides, e.g. electrical dopants in novel wide-band gap semiconductors such as ZnO, AlN, InN and diamond, transition metals and rare earths as magnetic impurities in spintronic materials, and rare earths as optical dopants in nitride semiconductors.
In order to suppress dechanneling, the radioisotopes used for emission channeling experiments must be incorporated at a depth smaller than a few thousand Å below the surface of the sample, which is usually accomplished by means of low-energy ion implantation. The technique hence relies on the availability of a wide range of pure beams of radioisotopes at relatively high intensities (> 10E6 ions/s) but low energies (< 100 keV), for which ISOLDE is a unique facility. In particular, such beams have not been foreseen to be developed at SPIRAL or FAIR.
Emission channeling experiments have successfully been demonstrated using ~65 different isotopes of a variety of elements including Li, Na, P, Ca, Fe, Cu, Ga, As, Se, Br, Rb, Sr, Ag, Cd, In, Sn, Sb, I, Xe, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tm, Yb, Lu, Hf, Au, Hg, Bi, Po, At, Rn, Fr and Ra.
The beam intensity upgrade and the new target and ion source developments foreseen at HIE-ISOLDE, in particular the upgrade of the resonant laser ion source (RILIS) along with new ionization schemes and improved techniques for reducing surface-ionized radioisotopes, will make available pure radioactive beams for a number of elements which have been unavailable so far, or of insufficient purity or intensity. Among the radioactive probes which are feasible at HIE-ISOLDE in the near future are, e.g., 27Mg (9.5 min), 31Si (2.6 h), 35S (87.5 d), 65Ni (2.5 h), 75Ge (83 min), 124Sb (60 d), or 198Au (2.7 d).
In the long run, we would like to encourage beam development of other promising emission channeling probes, in particular of the light elements, e.g., 8B (0.76 s), 15C (2.5 s), 16N (7.2 s), 19O (27 s), or 33P (25 d).