Development of a Micro-PIXE facility for analysis of geological samples
This work involved the development of a collimated micro-PIXE facility for analysis of geological samples. The old facility operated very well but there were problems encountered during its operation. The ion beam spot was found to have increased by a factor of two relative to the expected size. This was a beam halo that was formed around the beam. The beam halo problem was attributed to the collimator slit scattering, the length of the beamline and the vacuum level in the beamline. The existing quadrupole lens system was very close to the target, therefore strong focussing had to be used to achieve the aimed beam spot of the size in the range of 50-80 microns at the target. A small variation of the focussing led to defocusing at the target, this was another factor leading to the observed beam halo. A new beam-line had to be developed to achieve a beam-spot size in the range of 50-80 microns at the target. This was achieved by making the beamline longer and using three sets of collimators so as to produce a more parallel ion beam. A pair of quadrupole magnets was inserted about 300 cm from the collimators to gain more flexible control of the ion beam. This helped to reduce the beam halo by a considerable amount. This work required the use of an ion accelerator, bending/analysing magnets, quadrupole magnets, vacuum system, collimators, detectors, software and electronic equipment. Quadrupole magnets were used to focus the beam before entering the collimators. When selecting collimators, careful consideration was devoted to the shape of the collimators, the X-ray background that might be produced and scattering cross-sections of the collimator materials. Vacuum pumps such as Turbo Molecular Pump, Rotary Vane Pump, Ion Getter Pump, and Oil Diffusion Pumps were used in the beamline to control vacuum levels. Various photon detectors were calibrated to select one with the best energy resolution for the use in this facility. A HPGe detector was found to be suitable with a better resolution as compared to Ge (Li) and Si (Li) detectors. After completion of the construction phase of this work, the beam facility was tested using a proton beam from the AEC Van de Graaff accelerator. The system resolution was investigated using different combinations of collimators. This required proper alignment of these collimators. To find the resolution of the beam, a Molybdenum wire was scanned across the beam and the emitted X-rays were counted. Beam profiles were produced by plotting X-ray counts against wire positions. The resolution of the beam was taken as Full Width Half Maximum (FWHM) of the profile. The best resolution for this facility was 90μm.