A nuclear level with spin in a magnetic field will divide into magnetic sub-levels with an energy spacing. The populations of these levels are determined by the Boltzmann distribution at a steady temperature and will essentially be equal. The exponential in the Boltzmann distribution should not be equal to 1 to obtain unequal populations. To achieve this, cooling to a temperature of around 10 millikelvin is needed. Typically, this is achieved by implanting the nuclei of interest into ferromagnetic hosts.