When the particles have reached their maximum energy, they are steered out of the synchrotron and aimed at a target. Early synchrotrons used stationary targets. In order to double the energy of a collision, physicists in the 1970s began to collide two particle beams travelling in the opposite direction instead of just one beam and a stationary target. To have two beams that are travelling in the synchrontron in opposite directions, they used particles with the same mass but with the opposite sign. For example, electrons and positrons or protons and antiprotons.