Introduction to Ion beam methods



Ion beam methods

Complementary X-ray methods


Physical backgrounds

Ion beam methods are based on the physical interaction of ion beam of a few MeV energy and matter. Usually light ions (H+, D+, He+, Li+) produced by medium energy particle accelerators (Van de Graaff/Singletron/Tandetron) are applied. They interact with the atomic nuclei and/or electrons of the atoms exciting electromagnetic radiation (light, X-rays and gamma-rays), producing emitted particles (electron, proton, alpha, etc.), scattered primary particles, furthermore, can modify chemical state or structure. The depth of interaction (probing depth) is typically in the micrometers - tenth of micrometers range.

The ion beam analytical methods (IBA) utilize that the energy of the particles or radiation is characteristic for the atomic number (isotopic number, in nuclear reactions of light elements) of the interacting atom. The intensities of the emitted radiations or particles can be related to the elemental (isotopic) composition.

The ion beam induced radiation damage (change in chemical state or structure) can be utilized for the modification of the chemical and physical characteristics of materials. One of its applications is proton beam writing (PBW), or micromachining.

Ion beam induced effects can also be utilized for testing various physical parameters of materials. For example, semiconductor radiation detectors can be tested by the so called ion beam induced current (IBIC) technique.


Technical backgrounds

Ion beam systems consist of an accelerator, beam transport system and measurement/irradiation facilities (experimental set-ups). There are macro beam facilities using collimated ion beam (the beam diameter is typically a few mm2) and modern (Scanning) Nuclear Microprobes (SNM) facilities. SNMs focus the ion beam into the micrometer (or sub micrometer) size and can scan it across the surface of a sample thus allow to measure or modify materials on the microscopic scale.

Elements of experimental set-ups: measuring chamber; sample manipulator equipment (XYZ-stage, goniometer, etc.); particle, X-ray, gamma-ray detectors (Si, Si(Li), HPGe, Clover-Ge-BGO, etc.); nuclear electronic units (power supplies, preamplifiers, amplifiers, Analogue-to-Digital Converters (ADCs), Digital Signal Processors (DSPs), multichannel analyzers (MCAs); Data Acquisition system (DAQ); analytical software.


Methods

IBA: Ion Beam Analytical methods are nondestructive, simultaneous, complementer analytical methods, can be used in macro and micro mode (macro-IBA/ micro(µ)-IBA) for the characterization of bulk and microscopic sized samples. Using SNMs, the distribution of elements can be visualized across a small surface (a few mm2 area) with ~1 µm2 lateral resolution (mapping). This capability is similar to that of scanning electron microprobes. Nevertheless, IBA provides lower Detection Limits (DL=1-100 µg/g) and capable for depth profiling of elements with minimum 2-20 nm resolution. It is qualitative and quantitative from H to U for major, minor and trace elements. IBA methods:

  • PIXE: Proton Induced X-ray Emission (PIXE) is used to determine the elemental composition of samples in the Carbon (Boron) - Uranium range. The method is based on the measurement of characteristic X-ray radiation induced by protons impinging on a sample. This technique is especially suitable for the detailed characterization of archaeological, atmospheric aerosol, biological, geological, industrial, etc. samples.
  • RBS: Rutherford Backscattering Spectrometry (RBS) is used to determine the structure and composition of materials. This method is based on the measurement of the elastically scattered primary ions from the sample. This technique is especially used for the characterization of surface topography, thin films, multilayers, depth and lateral distribution of elements in various samples. It is sensitive for the middle and high atomic number elements, applicable from Boron to Uranium.
  • NRA/PIGE/DIGE: Nuclear Reaction Analysis (NRA), especially proton/deuteron induced gamma ray emission analysis (PIGE/DIGE) are used for the detection of light elements in the Li-P atomic number range with some possibility for isotope detection. They are based on the measurement of particles or characteristic gamma-rays induced by proton beam in nuclear reactions. These methods are advantageous e.g. for depth profiling of some elements (e.g. Oxygen), in archaeometry, geological research, etc.
  • ERDA: Elastic Recoil Detection Analysis (ERDA) is applied for the quantitative measurement of Hydrogen. It is based on the measurement of elastically recoiled atoms by He+ (or heavier) ion beam.
  • STIM: The Scanning Transmission Ion Microscopy (STIM) technique measures the energy loss of ions penetrating through the sample, allowing the determination of the areal density across the sample surface with high (~100 nm) lateral resolution.

PBW: Modification of materials by ion beam, or the so called proton beam writing (PBW) is based on the physical interaction of a few MeV ion beam and matter. The radiation damage induced by light ions (e.g. H+) is used for the modification of the physical and chemical characteristics of special materials, like porous silicon, polymer resists poly(methyl-metacrylate), etc. Exploiting the high penetration depth of MeV protons into materials (~50 µm), so called high aspect ratio (3D) micro devices can be created, e.g. Si micro-turbine, microstructured chemical reactor, microcapillaries, micro wave guides, etc.

IBIC: Ion Beam Induced Charge/Current (IBIC) is used for the characterization of semiconductor detectors, e.g. radiation hardness, the variation of charge collection efficiency across the surface of a PIN diode, etc.


XRF: The principle of conventional X-ray Fluorescence (XRF) technique is similar to that of PIXE except that, instead of ion beam, X-rays are used for the excitation of samples. X-rays are emitted by radio isotope sources or X-ray tubes. In our laboratory, XRF is used as a complementary analytical technique to PIXE. It is especially sensitive for the middle and high atomic number elements.

SR-XRF, XANES: X-ray Fluorescence (XRF) and X-ray Absorption Near Edge Structure (XANES) methods are based on focussed synchrotron radiation. They are used as complementary analytical technique to PIXE. SR-XRF is especially sensitive for the middle and high atomic number elements. XANES provide information on the atomic organisation and chemical bonding around an absorbing atom. It provides a tool to differentiate among oxidation states, mineral phases, or molecular species.