Magnetometer
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A magnetometer is a scientific instrument used to measure the strength and/or direction of the magnetic field in the vicinity of the instrument.
Earth's magnetism varies from place to place and differences in the Earth's magnetic field (the magnetosphere) can be caused by two things:
- The differing nature of rocks
- The interaction between charged particles from the sun and the magnetosphere
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[edit] Uses
Magnetometers are used in geophysical surveys to find deposits of iron because they can measure the magnetic pull of iron. Magnetometers are also used to detect archaeological sites, shipwrecks and other buried or submerged objects. Magnetic anomaly detectors are used to detect submarines for military purposes.
A magnetometer can also be used by satellites like GOES to measure both the magnitude and direction of the earth's magnetic field.
They are used in directional drilling for oil or gas to detect the azimuth of the drilling tools near the drill bit. They are most often paired up with accelerometers in drilling tools so the both the inclination and azimuth of the drill bit can be found.
Magnetometers are very sensitive, and can give an indication of possible auroral activity before one can see the light from the aurora. A grid of magnetometers around the world constantly measures the effect of the solar wind on the earth's magnetic field.
[edit] 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. Examples of vector magnetometers are fluxgates and superconducting quantum interference devices, or SQUIDs. Some scalar magnetometers are discussed below.
A magnetograph is a special magnetometer that continuously records data.
[edit] 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.
[edit] Fluxgate magnetometer
A fluxgate magnetometer consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field, it will be more easily magnetised in alignment with that field and less easily magnetised in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field.
Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological prospection. In Britain the commonest such instruments to be used are the Geoscan FM series of instruments and the Bartington GRAD601. Both are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).
[edit] 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.
[edit] Cesium vapor magnetometer
A basic example of the workings of a magnetometer may be given by discussing the common "optically pumped cesium 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 cesium light emitter or lamp, an absorption chamber containing cesium vapour and a "buffer gas" through which the emitted photons pass, and a photon detector, arranged in that order.
[edit] Calibration
The basic principle that allows the device to operate is the fact that a cesium atom can exist in any of nine energy levels, which is the placement of electron atomic orbitals around the atomic nucleus. When a cesium 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 cesium 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 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.
[edit] 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 cesium 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 cesium 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 cesium vapour, it is easy to see how the system works.
[edit] Applications
When removed from an isolated environment, the cesium 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.
[edit] Spin-exchange-relaxation-free (SERF) atomic magnetometers
At sufficiently high atomic density, extremely high sensitivity can be achieved. Spin-exchange-relaxation-free atomic magnetometers containing potassium vapor operate similarly to the cesium magnetometers described above yet can reach sensitivities lower than 1 fT/√Hz [1]. Large volume detectors have achieved 200 aT/√Hz sensitivity. This technology has greater sensitivity per unit volume than SQUID detectors.
[edit] SQUID magnetometer
SQUIDs, or Superconducting Quantum Interference Devices, are used to measure extremely small magnetic fields; they are very sensitive vector magnetometers, with noise levels as low as 3 fT·Hz−0.5 in commercial instruments and 0.4 fT·Hz−0.5 in experimental devices. Until the advent of SERF atomic magnetometers in 2002, this level of sensitivity was unreachable otherwise.
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. SQUID magnetometers allow one to measure the magnetic fields produced by brain or heart activity (magnetoencephalography and magnetocardiography, respectively).
[edit] 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) [2]. It consisted of a permanent bar magnet suspended horizontally from a gold fibre [3]. A magnetometer is also called a gaussmeter.
