Therefore, these processes are also known as Umklapp (U) processes and can only occur when phonons with sufficiently large "q"-vectors are excited, because unless the sum of q2 and q3 points outside of the Brillouin zone the momentum is conserved and the process is normal scattering (N-process). The probability of a phonon to have energy "E" is given by the Boltzmann distribution formula_115. To U-process to occur the decaying phonon to have a wave vector q1 that is roughly half of the diameter of the Brillouin zone, because otherwise quasimomentum would not be conserved.