Ion Beam Channelling (RBSc)
Scattering chamber dedicated for the performance of ion beam channelling experiments is placed at the 15-degree beamline of the 6.0 MV Tandem Van de Graaff accelerator. The same chamber can accept ions from the 1.0 MV Tandetron accelerator as well. Small divergence beams are collimated to the beam spot sizes of 1-3 mm, while the sample position is controlled by a motorized 6-axes goniometer. In addition, the sample can be heated during the measurements up to the 600 °C.
When a crystalline sample is exposed to the ion beam, while the rows of atoms in the lattice are aligned parallel to the incident ion beam, an ion channelling will occur. The bombarding ion beam will backscatter from the first few monolayers of the material at the same rate as a non-aligned sample, however the backscattering from buried atoms in the lattice will be drastically reduced, since these atoms are shielded from the incident ions by the atoms in the surface layers. By measuring the reduction in backscattering yield when a sample is channelled, it is possible quantitatively to measure and profile the crystal perfection of a sample and to determine its orientation. This technique is called RBS channelling (RBS/c). Channelled ions can also be used to observe the reduction in the yield of the emitted characteristic X-rays, in which case the technique is called PIXE channelling (PIXE/c).
For irradiation purposes, the beam from the 6.0 MV tandem or 1.0 MV tandem is collimated by circular interchangeable apertures of 1, 3- and 5-mm diameter. The scattering chamber is also equipped with an RBS detector and has the possibility of mounting an x-ray detector for PIXE.
Samples are positioned using the motorized XYZ stage and three-axis goniometer. The sample holder is circular, with a diameter of 2.5 cm. All samples have to be positioned within this holder and their maximum number depends therefore on the size of individual samples. Sample positioning is controlled by large view field long distance microscope.
Sample positioning and data acquisition:
A special version of the SPECTOR software is developed for the performance of channelling experiments. The user determines the detector to be used (either silicon SBD for RBS or silicon SDD for PIXE) and connects it to the data acquisition ADC. Software is also used to control the goniometer in order to perform initial imaging of crystal planes and to adjust the sample into the ‘channelling angle’. Once the channelling angle is adjusted, different measurements can be performed, e.g. degradation of the crystal due to irradiation.