RBS Instrumentation: Stripper Elements

The He- ions have substantial kinetic energy (typically 750 keV) when they arrive at the positive terminal. High energy ions lose electrons in grazing collisions as they travel through any material. The ions also give up a small amount of kinetic energy for each lost electron. However, the lost energy is small compared to the kinetic energy of the negative ions as they pass the terminal.

Convenient electron stripper materials (located at the terminal) include thin foils and gases. A foil is easier to use since it simply hangs in the beam path. However, the thickness of the foil can increase by carbon deposition (from residual carbon containing gases in the vacuum system) or decrease by sputtering. Foil thickness affects stripper efficiency and beam energy. The foils must occasionally be replaced, which involves opening the tank and the beam line.

Because gas cells operate more predictably and avoid maintenance problems, they have become more common. Gas cells consist of concentric tubes, perhaps 0.3 m long. The ends are closed except for small holes through which the ion beam passes. The outer tube is connected to an extra vacuum pump and gas is leaked from an external source into the inner tube to produce a pressure of 1 to 10 millitorr. This differential pumping in the outer tube reduces the amount of gas leaking into the beam line.

Because of the pressures required to maintain a gas cell, pressures in the flight tube (measured at the end of the accelerator) rise into the 1 to 10 microtorr range. Lower pressures are possible with a stripper foil. However, the modest collisional cross section of high energy ions with gas molecules diminishes the requirement for high vacuum in RBS experiments.

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