RBS Instrumentation: Instruments
The three main components of an RBS instrument are a source of helium ions, an accelerator to convert them to high energy alpha particles, and a detector to measure the energies of the backscattered ions. The type of accelerator determines the configuration of the other components. Single-ended accelerators have the ion source floating at high voltage. Electrical isolation of the megavolt potentials is achieved by housing the terminal in a tank filled with an insulating gas, usually SF6. One disadvantage of locating the ion source within the tank is that it is difficult to change or replenish the source.
The tandem accelerator is a clever innovation. The tandem accelerator uses a positive terminal located in the center of the device. Negatively charged particles are injected into the accelerator and attracted to the terminal where a stripper element removes two or more electrons from each particle. The positive terminal repels the resulting positive ion back toward ground. Thus the particle acquires energy both before and after the terminal.
The tandem configuration has two important advantages over a single-stage setup. First, lower terminal voltages are required, and second, both the source and the ion exit operate near ground potential. The main disadvantage is that inefficiencies of He- production and charge stripping lower He++ beam current to about 100 nA for a tandem versus 1 mA for a single-ended accelerator. Fortunately, most RBS experiments can only use about 100 nA because of detector limitations. A typical RBS installation uses a tandem accelerator, producing a 2.25 MeV He++ beam by removing three electrons from He- at the + 750 KV terminal.