## GENERAL IDEA |

A nuclear level with spin , in a magnetic field B, will split into 2

At a steady temperature the populations of these levels are given by the Boltzmann distribution, so they will be essentially equal. To have unequal populations, the exponential in the Boltzmann distribution must be different from one, which means that we need cool the nuclei to ~ 10mK.

B/T ~ 10

This is typically achieved by implanting nuclei of interest into ferromagnetichosts, where they can experience large hyperfine fields ( ~ 100T ), and cooling the ensemble to ~ 10mK in a ^{3}He / ^{4}He dilution refrigerator. Thus we get a very nonuniform (or anisotropic) angular distribution with the Boltzamann factors that can differ substantially from one another.