Potassium Vapour Magnetometers – A Short Summary (Part 2)

By Dr. Ivan Hrvoic, President, GEM Systems, Inc.

Optical Pumping of Alkali Vapours

Only unpaired and free electrons exhibit spin with the features described above. Vapours of the alkali group of elements have a single, unpaired electron in their valence shell and they can be readily used as sources of electrons with spins. Helium gas in the other hand needs to be ionized in order to eliminate one electron from the valance shell; the remaining electron then behaves as an unpaired electron.

In ground state 2 S1/2 the electron has 2 energy levels, or -1/2 or +1/2 spins. To polarize it we need to depopulate one level and overpopulate the other. This is done by applying a light beam with special characteristics. Gas discharge lamps of the elements in question are used as sources of polarizing light. Photons of two spectral lines D1 and D2 can lift the electrons from either energy level of the background state into metastable state. There will be very little in polarization if we allow both D1 and D2 to act their polarizations are opposite and we need to eliminate or suppress one. This is done by an interference filter.

Next we need to circularly polarize the D1 light. Then, only electrons with – ½ spin will be able to absorb the quantum of light and be lifted into metastable 2 P1/2. There is a natural decay from metastable levels back into background levels, but eventually the – ½ spin level will be depleted, and the sensor will become more transparent (not absorbing photons any more).

If we now apply rotating magnetic field around the sample and in the plane perpendicular to the applied magnetic field, there will be a precession of the magnetization due to electron spins.

Depending on the phase of this precession the ability of the spins to absorb protons of D1 light will vary i.e. the intensity of light passing through the sample of spins will be modulated in synchronization with the preceding magnetization.

We can detect this modulation; amplify it and measure its frequency and compute the value of the applied magnetic field from it.

In reality, the situation is somewhat different. Due to magnetic properties of the nucleus of the alkali metals, there is a whole mini-spectrum of spectral lines instead of a single one.