Magnetometer

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A magnetometer is a scientific instrument used to measure the strength of magnetic fields.

Earth's magnetism varies from place to place and differences in the Earth's magnetic field can be caused by a couple of things:

  1. The differing nature of rocks
  2. The interaction between charged particles from the Sun and the magnetosphere and the ionosphere.

Uses

Magnetometers are used in geophysical surveys to find changes in concentration of magnetic minerals in geologic formations (and also deposits of iron because they can measure the magnetic pull of iron). Magnetometers are also used to detect archeological sites, shipwrecks and other buried or submerged objects.

A magnetometer can also be used by satellites like NASA's Magsat or German CHAMP satellites to measure both the magnitude and direction of the Earth's magnetic field.

Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can even see the light from the aurora. There is a grid of magnetometers around the world constantly measuring the effect of the solar wind on the Earth's magnetic field.

There are a variety of military applications for magnetometers. These include magnetic anomaly detectors for sensing submerged submarines from aircraft, unmanned remote detectors of the metal of vehicles and personnel, detonators that trigger when sensing a large metal mass (e.g., a ship), etc.

Types

Magnetometers can be divided into two basic types:

Scalar magnetometers

Measure the total strength of the magnetic field to which they are subjected, and

Vector magnetometers

Have the capability to measure the component of the magnetic field in a particular direction. The use of three orthogonal vector magnetometers allows the magnetic field strength, inclination and declination to be uniquely defined.

Magnetograph

A special magnetometer that continuously records data.

Examples of vector magnetometers are fluxgates and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.

Proton precession magnetometer

One type of magnetometer is the proton precession magnetometer, which operates on the principle that protons are spinning on an axis aligned with the magnetic field.

An inductor creates a strong magnetic field around a hydrogen-rich fluid, causing the protons to align themselves with the newly created field. The field is then interrupted, and as protons are realigned with Earth's magnetic field, spinning protons precess at a specific frequency. This produces a weak magnetic field that is picked up by the same inductor. The relationship between the frequency of the induced current and the strength of Earth's magnetic field is called the proton gyromagnetic ratio, and is equal to 0.042576 Hertz per nanotesla (Hz/nT).

Because the precession frequency depends only on atomic constants and the strength of the external magnetic field, the accuracy of this type of magnetometer is very good. Magnetic impurities in the sensor and errors in the measurement of the frequency are the two causes of errors in these magnetometers.

If several tens of watts are available to power the aligning process, these magnetometers can be moderately sensitive. Measuring once per second, standard deviations in the readings in the 0.01 nT to 0.1 nT range can be obtained.

Overhauser magnetometer

The Overhauser effect takes advantage of a quantum physics effect that applies to the hydrogen atom. This effect occurs when a special liquid (containing free, unpaired electrons) is combined with hydrogen atoms and then exposed to secondary polarization from a radio frequency (RF) magnetic field (i.e. generated from a RF source).

RF magnetic fields are ideal for use in magnetic devices because they are transparent to the Earth's DC magnetic field and the RF frequency is well out of the bandwidth of the precession signal (i.e. they do not contribute noise to the measuring system).

The unbound electrons in the special liquid transfer their excited state (i.e. energy) to the hydrogen nuclei (i.e. protons). This transfer of energy alters the spin state populations of the protons and polarizes the liquid – just like a proton precession magnetometer – but with much less power and to much greater extent.

The proportionality of the precession frequency and magnetic flux density is perfectly linear, independent of temperature and only slightly affected by shielding effects of hydrogen orbital electrons. The constant of proportionality is known to a high degree of accuracy and is identical to the proton precession gyromagnetic constant.

Overhauser magnetometers achieve some 0.01 nT/√Hz noise levels, depending on particulars of design, and they can operate in either pulsed or continuous mode.

Caesium vapor magnetometer

A basic example of the workings of a magnetometer may be given by discussing the common "optically pumped caesium vapour magnetometer" which is a highly sensitive (0.004 nT/√Hz) and accurate device used in a wide range of applications. Although it relies on some interesting quantum mechanics to operate, its basic principles are easily explained.

The device broadly consists of a photon emitter containing a caesium light emitter or lamp, an absorption chamber containing caesium vapour and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.

SQUID magnetometer

SQUIDs, or Superconducting Quantum Interference Devices, are used to measure extremely small magnetic fields; they are currently the most sensitive vector magnetometers known, with noise levels as low as 3 fT·Hz−0.5.

These magnetometers require cooling with liquid helium (4.2 K) or liquid nitrogen (77 K) to operate, hence the packaging requirements to use them are rather stringent both from a thermal-mechanical as well as magnetic standpoint.

Calibration

The basic principle that allows the device to operate is the fact that a caesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a caesium atom within the chamber encounters a photon from the lamp, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The caesium atom is 'sensitive' to the photons from the lamp in three of its nine energy states, and therefore eventually, assuming a closed system, all the atoms will fall into a state in which the all the photons from the lamp will pass through unhindered and be measured by the photon detector. At this stage the device can be said to be perfectly calibrated.

Detection

Given that this theoretically perfect magnetometer is now calibrated it can be exposed to the environment. It is easy to imagine that the environment is constantly emitting quanta of energy and that some of these will pass through the chamber. When they do, they may hit one of our caesium atoms and cause it to jump into a new energy state, which may in turn be one in which it can absorb a photon from our caesium emitter. If this is the case it will cause a decrease in the number of photons reaching our detector and this can be easily recorded. Scaling from this simple example to account for the vast number of energy transactions occurring within the caesium vapour, it is easy to see how the system works.

Applications

When removed from an isolated environment, the caesium vapour can never be 'perfectly' calibrated and the system is subject to environmental interference as are all scalar magnetometers. However, by the application of feedback systems and an averaging of the detection rates seen in a benign environment, the instrument can be calibrated sufficiently well in a real-world environment to make it accurate and useful for detection.

Early magnetometers

In 1833 Carl Friedrich Gauss, head of the Geomagnetic Observatory in Göttingen, published a paper entitled "On the intensity of the Earth's magnetic field expressed in absolute measure". It described a new instrument that Gauss called a "magnometer" (a term which is still occasionally used instead of magnetometer) [1]. It consisted of a permanent bar magnet suspended horizontally from a gold fibre [2]. A magnetometer is also called a gaussmeter.

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