Injection into the accelerator of carbon ions is achieved by means of an ion source in which carbon from the sample forms the target. In the tandem accelerator usually employed the negative ions so produced are accelerated by electrostatic attraction to a terminal at around 2 million V positive. Within this terminal the ions pass through a gas "stripper"; this causes an ion to lose several electrons and hence it emerges positively charged; it is now repelled by the positive voltage on the terminal and in returning to a region of zero volts (i.e. "earth") it gains further energy.