Geometrics Inc. G-882 Cesium Marine Magnetometer Page 104
it. Both light beams exit the cell and pass to a second lens. This lens focuses the
light onto an infrared photo-detector.
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
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