Editing Magnetometer (section)

===Scalar magnetometers===

====Proton precession magnetometer====
{{Main|Proton magnetometer}}
''Proton precession magnetometer''s, also known as ''[[proton magnetometer]]s'', PPMs or simply mags, measure the resonance frequency of [[proton]]s (hydrogen nuclei) in the magnetic field to be measured, due to [[nuclear magnetic resonance]] (NMR). Because the precession frequency depends only on atomic constants and the strength of the ambient magnetic field, the accuracy of this type of magnetometer can reach 1 [[part per million|ppm]].<ref>Dr. Ivan Hrvoic, Ph.D., P.Eng. "[https://web.archive.org/web/20131212042401/http://info.orrvweb.com/wp-content/uploads/whitepaper/gemprotonaccuracy.pdf Requirements for obtaining high accuracy with proton magnetometers]". GEM Systems Inc., 11 January 2010.</ref>

A direct current flowing in a [[solenoid]] creates a strong magnetic field around a [[hydrogen]]-rich fluid ([[kerosene]] and [[decane]] are popular, and even water can be used), causing some of the protons to align themselves with that field. The current is then interrupted, and as protons realign themselves with the [[wikt:ambient|ambient]] magnetic field, they [[precession|precess]] at a frequency that is directly proportional to the magnetic field. This produces a weak rotating magnetic field that is picked up by a (sometimes separate) inductor, [[amplifier|amplified]] electronically, and fed to a digital frequency counter whose output is typically scaled and displayed directly as field strength or output as digital data.

For hand/backpack carried units, PPM sample rates are typically limited to less than one sample per second. Measurements are typically taken with the sensor held at fixed locations at approximately 10 metre increments.

Portable instruments are also limited by sensor volume (weight) and power consumption. PPMs work in field gradients up to 3,000 nT/m, which is adequate for most mineral exploration work. For higher gradient tolerance, such as mapping [[banded iron formation]]s and detecting large ferrous objects, [[#Overhauser effect magnetometer|Overhauser magnetometers]] can handle 10,000 nT/m, and [[#Caesium vapour magnetometer|caesium magnetometers]] can handle 30,000 nT/m.

They are relatively inexpensive (< US$8,000) and were once widely used in mineral exploration. Three manufacturers dominate the market: GEM Systems, Geometrics and Scintrex. Popular models include G-856/857, Smartmag, GSM-18, and GSM-19T.

For mineral exploration, they have been superseded by Overhauser, caesium, and potassium instruments, all of which are fast-cycling, and do not require the operator to pause between readings.

====Overhauser effect magnetometer====
The ''Overhauser effect magnetometer'' or ''Overhauser magnetometer'' uses the same fundamental effect as the ''proton precession magnetometer'' to take measurements. By adding [[free radical]]s to the measurement fluid, the [[nuclear Overhauser effect]] can be exploited to significantly improve upon the proton precession magnetometer. Rather than aligning the [[proton]]s using a solenoid, a low power radio-frequency field is used to align (polarise) the electron spin of the free radicals, which then couples to the protons via the Overhauser effect. This has two main advantages: driving the RF field takes a fraction of the energy (allowing lighter-weight batteries for portable units), and faster sampling as the electron-proton coupling can happen even as measurements are being taken. An Overhauser magnetometer produces readings with a 0.01 nT to 0.02 nT standard deviation while sampling once per second.

====Caesium vapour magnetometer====
The ''optically pumped [[caesium]] vapour magnetometer'' is a highly sensitive (300&nbsp;fT/Hz<sup>0.5</sup>) and accurate device used in a wide range of applications. It is one of a number of alkali vapours (including [[rubidium]] and [[potassium]]) that are used in this way.<ref name="snare" />

The device broadly consists of a [[photon]] emitter, such as a laser, an absorption chamber containing caesium vapour mixed with a "[[buffer gas]]" through which the emitted [[photon]]s pass, and a photon detector, arranged in that order. The buffer gas is usually [[helium]] or [[nitrogen]] and they are used to reduce collisions between the caesium vapour atoms.

The basic principle that allows the device to operate is the fact that a caesium atom can exist in any of nine [[energy level]]s, which can be informally thought of as the placement of [[electron]] [[atomic orbital]]s around the [[atomic nucleus]]. When a caesium atom within the chamber encounters a photon from the laser, it is excited to a higher energy state, emits a photon and falls to an indeterminate lower energy state. The caesium atom is "sensitive" to the photons from the laser in three of its nine energy states, and therefore, assuming a closed system, all the atoms eventually fall into a state in which all the photons from the laser pass through unhindered and are measured by the photon detector. The caesium vapour has become transparent. This process happens continuously to maintain as many of the electrons as possible in that state.

At this point, the sample (or population) is said to have been optically pumped and ready for measurement to take place. When an external field is applied it disrupts this state and causes atoms to move to different states which makes the vapour less transparent. The photo detector can measure this change and therefore measure the magnitude of the magnetic field.

In the most common type of caesium magnetometer, a very small AC magnetic field is applied to the cell. Since the difference in the energy levels of the electrons is determined by the external magnetic field, there is a frequency at which this small AC field makes the electrons change states. In this new state, the electrons once again can absorb a photon of light. This causes a signal on a photo detector that measures the light passing through the cell. The associated electronics use this fact to create a signal exactly at the frequency that corresponds to the external field.

Another type of caesium magnetometer modulates the light applied to the cell. This is referred to as a Bell-Bloom magnetometer, after the two scientists who first investigated the effect. If the light is turned on and off at the frequency corresponding to the Earth's field,{{clarify|date=July 2013}} there is a change in the signal seen at the photo detector. Again, the associated electronics use this to create a signal exactly at the frequency that corresponds to the external field. Both methods lead to high performance magnetometers.

====Potassium vapour magnetometer====
Potassium is the only optically pumped magnetometer that operates on a single, narrow electron spin resonance (ESR) line in contrast to other alkali vapour magnetometers that use irregular, composite and wide spectral lines and helium with the inherently wide spectral line.<ref>Hrvoic I (2008) Development of a new high sensitivity Potassium magnetometer for geophysical mapping, First Break 26:81–85</ref>

====Metastable helium-4 scalar magnetometer====
Magnetometers based on [[helium-4]] excited to its metastable triplet state thanks to a plasma discharge have been developed in the 1960s and 70s by [[Texas Instruments]], then by its spinoff Polatomic,<ref>{{Cite web |title=Polatomic - Welcome |url=http://www.polatomic.com/ |access-date=2022-05-11 |website=www.polatomic.com}}</ref> and from late 1980s by [[CEA-Leti: Laboratoire d'électronique des technologies de l'information|CEA-Leti]]. The latter pioneered a configuration which cancels the dead-zones,<ref>{{Cite journal |last1=Leger |first1=Jean-Michel |last2=Bertrand |first2=François |last3=Jager |first3=Thomas |last4=Le Prado |first4=Matthieu |last5=Fratter |first5=Isabelle |last6=Lalaurie |first6=Jean-Claude |date=2009-09-01 |title=Swarm Absolute Scalar and Vector Magnetometer Based on Helium 4 Optical Pumping |journal=Procedia Chemistry |series=Proceedings of the Eurosensors XXIII conference |language=en |volume=1 |issue=1 |pages=634–637 |doi=10.1016/j.proche.2009.07.158 |issn=1876-6196|doi-access=free }}</ref> which are a recurrent problem of atomic magnetometers. This configuration was demonstrated to show an accuracy of 50 pT in orbit operation. The [[European Space Agency|ESA]] chose this technology for the [[Swarm (spacecraft)|Swarm mission]], which was launched in 2013. An experimental vector mode, which could compete with fluxgate magnetometers was tested in this mission with overall success.<ref>{{Cite journal |last1=Léger |first1=Jean-Michel |last2=Jager |first2=Thomas |last3=Bertrand |first3=François |last4=Hulot |first4=Gauthier |last5=Brocco |first5=Laura |last6=Vigneron |first6=Pierre |last7=Lalanne |first7=Xavier |last8=Chulliat |first8=Arnaud |last9=Fratter |first9=Isabelle |date=2015-04-25 |title=In-flight performance of the Absolute Scalar Magnetometer vector mode on board the Swarm satellites |journal=Earth, Planets and Space |language=en |volume=67 |issue=1 |pages=57 |doi=10.1186/s40623-015-0231-1 |bibcode=2015EP&S...67...57L |s2cid=55990684 |issn=1880-5981|doi-access=free }}</ref>

====Applications====
The caesium and potassium magnetometers are typically used where a higher performance magnetometer than the proton magnetometer is needed. In archaeology and geophysics, where the sensor sweeps through an area and many accurate magnetic field measurements are often needed, caesium and potassium magnetometers have advantages over the proton magnetometer.

The caesium and potassium magnetometer's faster measurement rate allows the sensor to be moved through the area more quickly for a given number of data points. Caesium and potassium magnetometers are insensitive to rotation of the sensor while the measurement is being made.

The lower noise of caesium and potassium magnetometers allow those measurements to more accurately show the variations in the field with position.