Geometrics, Inc. G-824A Magnetometer Manual
53
Because Cesium is an alkali metal, the outer most electron shell (orbit) has only one
electron. It is the presence of this single electron that makes the Cesium atom well-suited
for optical pumping and therefore magnetometry.
The Cesium atom has a
net
magnetic dipole moment
. This net dipole moment, termed
F
, is the sum of the
nuclear dipole moment
, called
I
, and the
electron's angular
momentum
, called
J
. In a Cesium atom:
I = 7/2
J = 1/2
and thus
F
can have two values depending on whether the electron's angular momentum
adds to or subtracts from the nuclear dipole moment. Therefore,
F
can have the value of
3
or
4
. These values are called the hyperfine energy levels of the ground state of Cesium.
Normally the net dipole moments are randomly distributed about the direction (vector
sum of the 3 axial components) of the ambient magnetic field (H
0
). Any
misalignment
between the net atomic dipole moment and the ambient field vector causes the Cesium
atom be at a higher energy level than if the vectors were aligned. These small differences
are called
Zeeman splitting
of the base energy level.
The laws of quantum electrodynamics limit the inhabitable atomic magnetic dipole
orientations and therefore the atomic excitation energy to several discreet levels: 9 levels
for the
F=4
state and 7 levels for the
F=3
state.
It is this variation in electron energy
level state that is measured to compute the ambient magnetic field strength.
When a photon of the infrared light strikes a Cesium atom in the absorption cell, it may
be captured and drive the atom from its present energy level to a higher energy level. To
be absorbed the photon must not only have the exact energy of the Cesium band gap
(therefore the narrow IR line) but must also have the correct spin orientation for that
atom.
There is a high probability that the atom will immediately decay back to the initial energy
level but its original orientation to the ambient field is lost and it assumes a random
orientation. An atom that returns to the base level aligned such that it can absorb another
photon, will be driven back to the higher state. Alternately, if the atom returns to the base
level with an orientation that does not allow it to absorb an incoming photon, then it will
remain at that level and in that orientation. Atoms will be repeatedly driven to the higher
state until they happen to fall into the orientation that cannot absorb a photon.
Consequently, the circularly polarized light will depopulate either the aligned or inverse
aligned energy states depending on the orientation (spin) of light polarization.
Remember that one side of the cell is right-hand polarized and the other left-hand
polarized to minimize sensor rotational light shifts and subsequent heading errors.
Once most of the Cesium atoms have absorbed photons and are in a state that does not
allow them to absorb another photon, the light absorption of the cell is greatly reduced,
