Condensed matter and biology

In condensed-matter physics and biology, techniques like 8Li β-NMR and emission channelling could profit from the available long beam times or from the frequent access to short beam times. Systematic measurements allow the investigation of a larger range of samples with varying parameters, which give rise to a more detailed study.

8Li β-NMR

The progress in applications (especially in the electronics industry) is firmly rooted in fundamental studies of nanostructured materials, where mesoscopic effects become very pronounced due to the fact that the system size is comparable to relevant physical length scales. One of the key features in understanding the physics of nanostructured materials is to have microscopic information about the local electrical and magnetic field within the sample. Very often these fields may vary significantly as function of, for instance, the depth within the sample.

In β-NMR , a polarized radioactive beam (e.g., 8Li) is implanted in the sample. The implantation depth is controlled by varying the electrostatic potential on the sample, which defines the beam energy. The spin rotation of the implanted radioactive isotope depends on the local hyperfine field. By measuring the β assymetry and its disappearance at the RF resonances, one can therefore extract information on the local magnetic and electric fields. Since several years, β-NMR is used very successfully for condensed-matter studies at the TRIUMF facility in Vancouver [1,2].

Schematic of the ISAC beta-NMR spectrometer

Schematic of the ISAC β-NMR spectrometer (picture from [3])

[1]  K.H. Chow et al., Physica B 340-342 (2003) 1151
[2] M. Xu et al., Journal of Magnetic Resonance 191 (2008) 47
[3] G.D. Morris et al., Phys. Rev. Lett. 93 (2004) 157601

 

Emission channeling

The presence of impurity atoms can drastically change the electrical, magnetic and optical properties of semiconductors. To understand the effect of impurities on the properties of the material, it is important to know the exact lattice location (in other words the local crystal field) of these impurities, and how the lattice site depends on the presence of defects in the semiconductor. Whereas methods such as infrared spectroscopy, photoluminescence spectroscopy, X-ray absorption fine structure or electron paramagnetic resonance characterize the local symmetry surrounding the impurity (hence providing indirect information on the lattice site), approaches relying on the channeling of charged particles provide direct information on the lattice site of impurity atoms.

In emission-channeling experiments, radioactive probe atoms – acting as emitters of (conversion) electrons, positrons, or α particles – are implanted in the single crystalline host. During the subsequent decay, the emitted charged particles can channel along a major symmetry direction (lattice axis or plane) or can be blocked, resulting in anisotropic emission patterns. The yield of charged particles emitted from the radioactive impurity atoms inside the sample is recorded with a two-dimensional position sensitive detector for different emission directions around major crystallographic axes and planes, where channeling or blocking effects are experienced.

Emission-channeling latice location of radioisotopes

Picture taken from U. Wahl, presented at BriX workshop 2008, SCK•CEN, Mol, Belgium