text: An RBS instrument generally includes three essential components:
  * An ion source, usually alpha particles (He ions) or, less commonly, protons.
  * A linear particle accelerator capable of accelerating incident ions to high energies, usually in the range 1-3 MeV.
  * A detector capable of measuring the energies of backscattered ions over some range of angles.
  Two common source/acceleration arrangements are used in commercial RBS systems, working in either one or two stages.  One-stage systems consist of a He source connected to an acceleration tube with a high positive potential applied to the ion source, and the ground at the end of the acceleration tube.  This arrangement is simple and convenient, but it can be difficult to achieve energies of much more than 1 MeV due to the difficulty of applying very high voltages to the system.
  Two-stage systems, or "tandem accelerators", start with a source of He ions and position the positive terminal at the center of the acceleration tube. A stripper element included in the positive terminal removes electrons from ions which pass through, converting He ions to He ions.  The ions thus start out being attracted to the terminal, pass through and become positive, and are repelled until they exit the tube at ground. This arrangement, though more complex, has the advantage of achieving higher accelerations with lower applied voltages: a typical tandem accelerator with an applied voltage of 750 kV can achieve ion energies of over 2 MeV.
  Detectors to measure backscattered energy are usually silicon surface barrier detectors, a very thin layer (100 nm) of P-type silicon on an N-type substrate forming a p-n junction.  Ions which reach the detector lose some of their energy to inelastic scattering from the electrons, and some of these electrons gain enough energy to overcome the band gap between the semiconductor valence and conduction bands.  This means that each ion incident on the detector will produce some number of electron-hole pairs which is dependent on the energy of the ion. These pairs can be detected by applying a voltage across the detector and measuring the current, providing an effective measurement of the ion energy.  The relationship between ion energy and the number of electron-hole pairs produced will be dependent on the detector materials, the type of ion and the efficiency of the current measurement; energy resolution is dependent on thermal fluctuations. After one ion is incident on the detector, there will be some dead time before the electron-hole pairs recombine in which a second incident ion cannot be distinguished from the first.
  Angular dependence of detection can be achieved by using a movable detector, or more practically by separating the surface barrier detector into many independent cells which can be measured independently, covering some range of angles around direct (180 degrees) back-scattering.  Angular dependence of the incident beam is controlled by using a tiltable sample stage.