Helium-3

Helium 3 is a rarely produced magnetometer, and is probably not produced for the commercial market at all. The helium nucleus' large mass allows it to precess for a very long time - often hours or even days - after polarization. This creates a nice continuous low frequency signal that can be sampled easily by inexpensive electronics. The drawback is that polarization requires large amounts of energy that must be supplied quickly to the sensor. Usually a dedicated high current battery is used that can be slowly recharged while the signal precesses. This makes a Helium-3 magnetometer very heavy, often quite large, and not suitable for portable applications.

Proton Precession

A proton precession magnetometer uses hydrogen the precession atom. Liquids such as kerosene and methanol are used because they offer very high densities of hydrogen, and they are not dangerous to handle. A standard proton precession magnetometer, such as the GSM-8, uses a high intensity artificial magnetic field around the sensor to polarize the liquid, which then provides signal for 1-2 seconds. The power required to polarize is much less than for Helium-3, but is still significant. Nevertheless, the standard proton precession magnetometer has long had a niche as a cheap portable magnetometer.

The Overhauser Effect

The Overhauser Effect is an enhancement to the proton precession principle, taking advantage of a quirk of physics that affects the Hydrogen atom. An Overhauser magnetometer uses RF power to excite the electrons of a small amount of specially bonded hydrogen atoms dissolved in the proton precession liquid. This excitation causes the rest of the liquid to become polarized, allowing the production of signal with very little power. Also, since the liquid can be polarized while the signal is being measured, Overhauser magnetometer have a much higher bandwidth and sensitivity than standard proton precession magnetoeters.

Overhauser magnetometers are without question the most energy efficient magnetometers available with sensitivities suitable for Earth field measurement. Power consumption can be optimized to be as low as 1W for continuous operation, yielding sensitivities between 0.1nT to 0.01nT, and sampling rates as high as 5Hz. Continuous Overhauser magnetometers, like the GSM-11, operates with continuous polarization, and offers sampling rates up to 10Hz, but requires more power to operate.

Scalar Magnetometers - Optically pumped (alkali vapour)

The following is a brief description of the theory of operation of optically pumped magnetometers. For a more detailed description, check out the GSMP-20 Technical Description in our documentation section.

Optically pumped magnetometers use alkali metals from the first column of the periodic table such as cesium and potassium, and they all operate on virtually the same principle. First, a cell containing the gaseous metal is polarized (or pumped) by exposure to light of a very specific wavelength. The light depopulates one electron energy level in the gas by pumping the electrons to a higher energy level . These electrons spontanously decay to lower energy levels, and eventually, a lower energy level is fully populated. Next, the cell is depolarized by shifting the electrons in the lower energy level back to their original position using lower wavelength RF power.

The energy required to repopulate this energy level varies with the ambient magnetic field, according a principle called the Zeeman effect. Therefore, the frequency of the depolarizing RF power corresponds to magnetic field. The Zeeman effect is not a unique energy difference in an alkali metal, however. All alkali metals possess several different Zeeman effect energy levels, each of which is proportional to magnetic field. These discrete energy levels are called spectral lines.

Because of the physics of the system, the intensity of the Zeeman effect is dependent on the direction of the ambient magnetic field with respect to the direction of applied light and RF power. This creates 'dead-zones' around the magnetometer sensor, which manifest as a loss of signal when the sensor is improperly oriented in the ambient field. The tolerance of this orientation is high, however - usually between 60 to 90 degrees of sensor orientation in each of three dimensions.

In addition, these magnetometers require the alkali metal to be gaseous to operate. That means that the cell containing the metal must be continuously heated to approximately 45C. The high power required to heat and operate optically pumped magnetometers typically does not allow them to be used in hand-held portable applications like proton magnetoemters.

Cesium

Cesium is the most widely available alkali vapour magnetometer. It offers good sensitivity and bandwidth (a few pT at 10Hz sampling) but has a few disadvantages that make it an expensive instrument to own and operate.
Cesium's spectral lines are quite wide, meaning that the electron energy levels associated with the Zeeman effect vary widely in magnitude over a population of Cesium atoms. Cesium's spectral lines are very wide and very strong, so that they in fact overlap, making it impossible to distinguish conclusively between them. This feature simplifies the design of a Cesium magnetometer, allowing it to work in a self-oscillating mode. However, when the ambient magnetic field changes with respect to the direction of applied light and RF power, the amplitude of the spectral lines will change, causing the measured signal to shift. In practive, this shift appears to the instrument as a change in the absolute value of the ambient field depending on the instruments orientation in space, also known as heading error. This error can be as high as 20nT, four orders of magnitude above the maximum sensitivity of the instrument, and can make cesium magnetometer data more difficult to process.

Because of this characteristic, the performance of a Cesium magnetometer depends heavily on the orientation of the applied light power with respect to the applied RF power. Small mechanical shifts within the sensor can cause large numerical shifts in the instrument's output. For this reason, a Cesium magnetometer must return to the factory periodically for calibration, or re-alignment of the respective power sources. The cost of this calibration can with time become greater than the purchase price of the instrument.

Potassium

Although potassium possesses the same amount of spectral lines as cesium, potassium's spectral lines are extremely narrow and they do not overlap. This makes the design of a potassium magnetometer more tricky than that of a cesium magnetometer, and is one of the reasons that GEM Systems is the world's only commercial supplier of potassium magnetometers.

The fact that potassium's spectral lines do not overlap means that potassium exhibits no heading error. It also means that a potassium magnetometer never requires calibration. The only component that wears out is the light source (the potassium lamp), which has a lifetime of thousands of hours, and costs almost nothing to replace.

Another benefit of narrow spectral lines is potassium's huge bandwidth and superb sensitivity. In the laboratory, special potassium magnetometers have shown noise levels of less than 0.05pT, and have tracked varying magnetic fields as high as 1000Hz. GEM Systems is currently developing a new potassium magnetometer that will take advantage of these benefits. See new potassium magnetometer in our news section.

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