LITERATURE REVIEW
Nuclear Spin-Lattice Relaxation in Liquids
In a typical magnetic resonance experiment energy is absorbed
from a radio-frequency (rf) source by a system of nuclear spins
immersed in a magnetic field, Ho, as a result of transitions among
the energy levels of the spin system. For each of N non-interacting
spins, characterized by I and [, the maximum z components of angu-
lar momentum and magnetic moment, respectively, there are 21+1 levels
spaced in energy by p H/I. Upon application of an oscillating mag-
netic field, transitions corresponding to stimulated emission and
absorption occur. If there is a net absorption of energy from the rf
source, there must be an initial surplus of spins in the lower energy
levels. This condition will be attained in time if there is some way
for the spin system to interact with its surroundings and come to
thermal equilibrium at a finite temperature. At equilibrium the popu-
lations of the 21+1 levels will be governed by the Boltzmann factor,
and there will be the necessary surolus of spins in the lower states.
The exposure of the system to radiation, with subsequent
absorption of energy, tends to uoset the equilibrium state previously
attained by equalizing the populations of the various levels. The new
equilibrium state in the presence of the rf field represents a balance
between the processes of absorption of energy by the spins from the