At present, research in particle physics is mostly carried out by the use of proton synchrotrons. In these, protons are accelerated by an alternating electric field and guided by a powerful magnetic field at right angles to it. By this means the particles are accelerated along a circular path from which they may be extracted by kicking magnets for use in various experiments. The picture on page 507 gives a view inside the tunnel containing the super proton synchronous at CERN (European Organization for Nuclear Research). This tunnel has a diameter of 2.2 km and in it protons are accelerated to energies of 400 GeV.
The unit of energy used in particle physics is the electron volt (eV), which is defined as the energy acquired by an electron (or a proton) in moving through a potential difference of 1 volt. Other units are:
1 keV = 1 thousand (103) electron volts
1 MeV = 1 million (l06) electron volts
1 GeV = 1 thousand million (109) electron volts
With the very high-energy particles obtained from these machines, atomic nuclei in selected targets can be disintegrated into fragments and particles, the nature of which are investigated by the bubble chambers mentioned earlier on page 551. See Fig. 47.11 and 47.12.