In order to control the energy of ion beams independent from accelerator adjustments we use a magnetic electron spectrometer which measures the momentum of electrons accompanying the projectiles. These electrons result from distant collisions of the projectiles with target atoms or from penetration through thin stripper foils. They just release the binding to the projectile and, therefore, travel along the projectile trajectory. Due to a small spectrometer acceptance angle under 0° those electrons are fully received whereas electrons with increasing velocity relative to the projectile are progressively suppressed resulting to a cusp shaped peak in the electron momentum spectrum.
The standard accuracy is  Δp/p = 10^-3.