RBS Instrumentation: Related Techniques

By adding accessories to the sample chamber, or by changing the operating procedures, several other experiments can piggy back onto the RBS analysis. For example, measurement of the X-rays produced by ion bombardment is called particle induced X-ray emission (PIXE). Common RBS accessories include detectors for these X-rays, which are always produced, but not always measured. Hydrogen forward scattering (HFS) only requires the addition of a thin foil to separate forward scattered hydrogen from forward scattered primary He⁺⁺ ions. Heavy ion backscattering spectrometry (HIBS) is the same as RBS, except that heavy ions are used instead of He⁺⁺. Primary He⁺⁺ ions with higher energies than the usual 2.2 MeV scatter, but the nuclear interactions become more complicated. In spite of these complications, called resonance effects, higher beam energies often prove useful. In some cases, the incident ions are captured by a target isotope and a different particle is promptly emitted. Measurement of the energies of these reemitted particles is called nuclear reaction analysis (NRA). Finally, charged particle activation analysis (CPAA) uses the accelerator to produce new radioactive nuclides which are measured with the same instruments used in neutron activation analysis.

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Particle Induced X-Ray Emission

PIXE stands for Particle Induced X-Ray Emission. Several kinds of excitation beams produce X-rays with energies characteristic of the target elements. Thus, photon excitation (by X-rays) gives rise to X-ray fluorescence spectroscopy. Electron excitation in a scanning electron microscope or an electron microprobe provides energy dispersive or wavelength dispersive X-ray spectroscopy (depending on the X-ray detector). Charged particle beams of He⁺⁺, or more commonly H⁺, afford PIXE spectroscopy. In all three cases, the excitation beam removes a core electron and X-rays are emitted with specific energies when outer shell electrons change state to inner shell. The X-ray energies are independent of the excitation process. The PIXE accessory is useful for heavy element identification on RBS instruments. These heavy elements have small differences in RBS energies, but distinct differences in PIXE spectra. Dedicated PIXE instruments usually use H⁺ rather than He⁺⁺ because H⁺ provides higher sensitivity. If helium and hydrogen are mixed in the source, He⁺⁺-based RBS and H⁺-based PIXE can be performed with the same accelerator.

PIXE has several advantages as an analytic technique. It offers signal levels similar to its electron beam counterparts, but better signal-to-background ratios. The background in electron spectroscopy arises from bremsstrahlung which is largely absent in PIXE because He++ or H⁺ ions, even at PIXE energies, have much lower velocities than electrons. Another advantage of PIXE over electron induced spectroscopy is that, like RBS, PIXE works with insulating samples. Finally, a proton beam can pass through a thin window and penetrate several centimeters through air. Because of this, unusual samples, such as valuable art work, need not be subjected to the rigors of a vacuum. PIXE finds applications in geology, archaeology, and art conservation.
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Hydrogen Forward Scattering

HFS stands for Hydrogen Forward Scattering. An HFS experiment uses essentially the same apparatus as standard RBS. The analytical information obtained by HFS consists of hydrogen concentration versus depth. The sample is tilted so that the He⁺⁺ beam strikes at a grazing angle. Helium is heavier than hydrogen, so there is no backscattering of He⁺⁺ from hydrogen. Instead, hydrogen (H⁺ and Ho) is knocked forward with significant energy after being struck by He⁺⁺.

The He⁺⁺ also scatters toward the detector because of heavy elements in the sample. The number of He⁺⁺ ions scattering at this low angle is large relative to forward scattered hydrogen. The He⁺⁺ signal would swamp the H⁺ signal except that a thin foil (about 8 microns) positioned in front of the detector separates the interfering He⁺⁺ from the H⁺. Carbon, mylar, and aluminum foils are commonly used. The foils cause significant energy loss and straggling in the forward scattered hydrogen. Although these effects limit the HFS depth profile resolution to about 50 nanometer, hydrogen (or deuterium) quantitation at surfaces is possible with 5 % accuracy down to 0.01 % detection limits.
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Heavy Ion Backscattering Spectrometry

HIBS stands for Heavy Ion Backscattering Spectrometry. This technique uses ions heavier than He++. Collision cross sections are higher for heavy primary ions, and there are no resonance effects at available energies. These heavy ion beams provide advantages in trace heavy element determinations of light element samples. The matrix elements are all scattered forward and cannot contribute interference signals. The forward scattered analyte elements can also be useful for analysis. Measurement of analyte ions scattered forward by heavier primary ions is a general phenomenon called Elastic Recoil Detection (ERD). (HFS) is a special case of ERD.

The accelerators and detectors are the same for HIBS and RBS, but different ion sources are required. A typical heavy ion source uses a sputtering Cs⁺ beam impinging on a heavy element such as silicon from which negative ions (Si^- in this example) are extracted. The charge stripper removes a variable number of electrons from Si^-, leaving ions with mixed charge states and energies. The primary ion beam must be separated into components, usually in a magnetic field, before the beam strikes the sample.
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Nuclear Reaction Analysis

NRA stands for Nuclear Reaction Analysis. Helium ions with less than 2.2 MeV energy undergo elastic recoil with all elements. Elastic recoil, like colliding billiard balls, can be described by classical mechanics. For ease of interpretation, most RBS analyses use less than 2.2 MeV He⁺⁺ beams. Higher energy He⁺⁺ ions collide non-elastically with analyte nuclei. This means that the collision cross sections can be much higher for certain (resonance) energies. At resonance energies, the analyte nucleus seems to absorb and reemit the He⁺⁺ ion rather than simply scatter it. Quantum mechanics are required to explain these interactions.

An example is the well known resonance effect at 3.045 MeV for alpha particles scattered from ¹⁶O. The process is indistinguishable from elastic recoil except that the cross section is many times greater. Although variable cross sections resulting from higher beam energies complicate the analysis, the higher energies are useful in specific cases such as oxygen present as a single impurity in a thin layer.

At some resonance energies, the primary ion is absorbed by the analyte nucleus and a different particle (usually a proton, neutron, alpha particle, or gamma ray) is promptly emitted. Among the light elements, there are several potentially useful reactions such as ¹⁹F(p,alpha)¹⁶O. In this case a ¹⁹F nucleus absorbs a 1.25 MeV proton and promptly emits an 8.114 MeV alpha particle. Measurement of the alpha energy indicates the depth from which it arose. This is the nuclear reaction analysis technique.
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