Potassium Vapour Magnetometers – A Short Summary (Part 3)

Broad Line Versus Narrow Line Spectra

Potassium and Rubidium have 6 spectral lines of various intensities, Cesium 133 14 and Helium 4 just one but very wide. Width of the spectral line depends on many parameters such as the size of cells, collision of the atoms with the walls of the cells, collision with buffer gas, spin exchange, etc.

Contemporary Cs and Rb magnetometers have wide overlapping spectral lines. A composite spectral line is not symmetrical but the position of its peak depends on the geometry of the sensor and the applied magnetic field. There is a large shift in precession frequency when we change the orientation of the sensor in steady magnetic field.

This weakness is largely corrected by applying a split beam technique that symmetrizes the shape of the strong but wide spectral single line.

Advantaged of the strong single line are:

• Very high tolerance to gradients of magnetic field
• Simplicity, since the cesium magnetometer can self-oscillate its amplified signal is used to create a rotating magnetic field around the sensor, causing self-oscillations

Weaknesses are:

• Reduced sensitivity
• Poor absolute accuracy
• Pronounced tilt or heading error

Potassium spectral lines can be made very narrow and completely separated from each other. Self-oscillation is now not suitable, as it would result in a beating of several individual frequencies. Instead an auxiliary oscillator is used to create rotating field around the sensor for one spectral line only. Signal generated from that operation is then used to frequency lock the auxiliary oscillator’s frequency.

Technically this is more complex than self-oscillations. Advantages are:

• A maximum of the resolution
• Very high absolute accuracy
• “Heading error” due to varying geometry between the sensor axis and the magnetic field is very much reduced

The disadvantage is also a limited tolerance to gradients – as gradients widen the spectral lines.

The sensors of Potassium magnetometer need to be larger size than Cesium in order to achieve narrow spectral lines. In practice we use 70mm dia cells to achieve about 1nT line width and 120mm cells will give about 0.15nT.

Standard and Super-Resolution K-Sensors and Systems

We have built the observatory like testing site at Georgina Island in the Lake Simcoe, Ontario to test our “supergradiometer”. Latest results show about 0.1pTpp noise gradient mode and 1 second measuring interval. This is somewhat more than 10 fT rms per channel. Our standard gradiometers are about 5 times less sensitive.

As an illustration of our latest testing of the supergradiometer, enclosed is an anomaly of about 0.4pT registered by the supergradiometer.

Geometrical restrictions of potassium are very similar to those of cesium: right angles and collinear orientation related to the magnetic field directions are forbidden. Whether you define operating angles from 2-880 or 10 – 800 is really irrelevant – physics of it stays the same.

Future Directions

Current research is aimed at reducing the sensor size thereby reducing sensitivity to gradients while maintaining a relatively high sensitivity in comparison with other commercial instrumentation.

GEM Systems continues to advance its research and development which is leading to the next generation of gradient-tolerant ground systems based on new sensor technology as described above and high sensitivity airborne systems. Airborne systems currently in the air or being developed include high sensitivity single sensor systems as well as and multi-sensor airborne gradiometers.