page_title: Elastic recoil detection
section_title: Ion Source and Interactions
hierarchical_section_title: Elastic recoil detection / Instrumentation / Ion Source and Interactions
caption_attribution_description: English: Image of a Van de Graaff generator integrated with a particle accelerator. This generates high fields (megavolt range) through which the ions are accelerated.
context_page_description: Elastic Recoil Detection Analysis, also referred to as forward recoil scattering, is an Ion Beam Analysis technique in materials science to obtain elemental concentration depth profiles in thin films. This technique is known by several different names. These names are listed below. In the technique of ERDA, an energetic ion beam is directed at a sample to be characterized and there is an elastic nuclear interaction between the ions of beam and the atoms of the target sample. Such interactions are commonly of Coulomb nature. Depending on the kinetics of the ions, cross section area, and the loss of energy of the ions in the matter, Elastic Recoil Detection Analysis helps determine the quantification of the elemental analysis. It also provides information about the depth profile of the sample.
  The incident energetic ions can have a wide range of energy from 2 MeV to 200 MeV. The energy of the beam depends on the sample to be studied. The energy of the beam should be enough to kick out the atoms of the sample. Thus, ERD usually employs appropriate source and detectors to detect recoiled atoms.
context_section_description: Particle accelerators, such as a magnetron or cyclotron, implement electromagnetic fields to achieve acceleration of elements. Atoms must be electrically charged (ionized) before they can be accelerated. Ionization involves the removal of electrons from the target atoms. A magnetron can be used to produce H- ions. Van de Graaff generators have also been integrated with particle accelerators, shown in figure 3, for light ion beam generation.
  For heavier ion production, for example, an electron cyclotron resonance (ECR) source can be used. Figure 4 shows a schematic diagram of an ECR. At the National Superconducting Cyclotron Laboratory, neutral atoms have their electrons removed using an ECR ion source. ECR works by ionizing the vapor of a desired element such as chlorine and iodine.  Further, utilizing this technique metals (Au, Ag, etc.) can also be ionized using a small oven to achieve a vapor phase. The vapor is maintained within a magnetic field long enough for the atoms to be ionized by collisions with electrons. Microwaves are applied to the chamber as to keep the electrons in motion.
  The vapor is introduced via injection directly into the “magnetic bottle” or the magnetic field. Circular coils provide the shape for the magnetic bottle. The coils are found at the top and bottom of the chamber with a hexapole magnet around the sides. A hexapole magnet consists of permanent magnets or superconducting coils. The plasma is contained within the magnetic trap that is formed from electric current flowing in solenoids located on the sides of the chamber. A radial magnetic field, exerted by the hexapole magnetic, is applied to the system that also confines the plasma. Acceleration of the electrons is achieved using resonance. For this to occur, the electrons must pass through a resonance zone. In this zone, their gyrofrequency or cyclotron frequency is equal to the frequency of the microwave injected into the plasma chamber. Cyclotron frequency is defined as the frequency of a charged particle moving perpendicular to the direction of a uniform magnetic field B. Since the motion is always circular, cyclotron frequency-ω in radians/second-can be described by the following equation:
  (13)    = ω
  where m is the mass of the particle, its charge is q, and the velocity is v. Ionization is a step-by-step process from collisions of the accelerated electrons with the desired vapor atoms. The gyrofrequency of an electron is calculated to be 1.76x107 Brad/second.
  Now that the vapor of the desired has been ionized, they must be removed from the magnetic bottle. To do this, a high voltage is between the hexapoles applied to pull out the ions from the magnetic field. The extraction of the ions, from the chamber, is carried out using an electrode system through a hole in a positively biased plasma chamber. Once the ions have been extracted from the chamber, they are then sent to the cyclotron for acceleration.
  It is very important that the ion source used is optimal for the experiment being carried out. To perform an experiment in a practical amount of time, the ions provided from the accelerator complex should have the correct desired energy. The quality and stability of the ion beam needs to be considered carefully, due to the fact that only the ions with the correct flight trajectory can be injected in the cyclotron and accelerated to the desired energy.
  During ERDA, the idea is to place an ion beam source at a grazing angle to the sample. In this set up, the angle is calculated as to allow the incident ions to scatter off of the sample so that there is no contact made with the detector. The physical basis that has given the method its name stems from the elastic scattering of incident ions on a sample surface and detecting the recoiling sample atoms while the incident ions backscatter at such an angle, that they do not reach the detector; this is typically in reflection geometry, illustrated in figure shown:
  Another method for preventing incident ions from making contact with the detector is to use an absorber foil. During analysis of t
caption: Figure 3: Image of a Van de Graaff generator coupled with a particle accelerator[22]
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