Doped Beam Very Low Energy Particle Induced X-Ray Emission
- Publication Type:
- Thesis
- Issue Date:
- 2022
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Particle Induced X-Ray Emission (PIXE) is a spectroscopic technique where characteristic X-Rays are generated from a sample by the impact of high energy particles. PIXE is typically performed with protons in a particle accelerator at energies in excess of 1MeV and is used for the detection of trace elements due to the lower background compared to complementary techniques such as Scanning Electron Microscope (SEM) Energy Dispersive Spectroscopy (EDS). PIXE performed at energies of less than 1MeV is sometimes used to enhance sensitivity to light elements, however very low energy PIXE (VLE-PIXE) performed at energies available to a commercial focused ion beam microscope of ≤30keV was considered impossible due to the extremely low X-Ray production at these energies.
In this research, VLE-PIXE was made possible by doping a hydrogen focused ion beam with a small proportion of a heavier ion species such as Ar or Xe. The characteristic X-Ray signal was shown to increase dramatically, allowing trace element analysis in the low parts per million range, offering performance comparable to proton only PIXE performed at much higher energies. This thesis outlines the implementation, characterisation, and application of the doped beam VLEPIXE technique in a commercial focused ion beam microscope utilising available hardware and little to no modification to the instrument.
An investigation into the beam doping technique led to an interpretive model which considers various physical mechanisms which may be responsible for the increased performance which includes: the formation of quasi-molecules between the heavy projectile ion and the target atom, the suppression of non-radiative transitions, and vacancy lifetime modification due to multiple ionisation. These mechanisms may arise from the coincident impact of protons and a heavy ion species upon the same region of the sample. The ions backscattering from the surface during VLE-PIXE analysis were also analysed to provide additional information regarding the sample thickness and composition. This leads to the possibility of several new techniques such as simultaneous doped beam VLE-PIXE and backscattered ion spectroscopy for real-time tomography, or endpointing during Focused Ion Beam (FIB) milling.
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