Proton magnetometer

A proton magnetometer (also: proton precession magnetometer ) is a measuring device for magnetic fields , which is based on the precession of the spin of protons .

Structure and functionality

A proton magnetometer consists of a non-magnetic vessel which is filled with a liquid containing protons and surrounded by a coil . A hydrocarbon such as B. Petroleum or water are used; it must be a substance that contains a lot of hydrogen , since the atomic nucleus of hydrogen is made up of one proton.

Portable proton magnetometer in field use, 1995

With the help of the coil, a strong magnetic field is applied that aligns the nuclear spin in the direction of the magnetic field. This strong magnetic field is then removed so that the protons are only exposed to the magnetic field to be measured, which is much weaker. Because the magnetic field to be measured does not point in the same direction as the coil magnetic field, the proton spins are brought to Larmor precession , thereby emitting electromagnetic radiation which induces a voltage in the coil. The precession frequency is proportional to the strength of the magnetic field ${\displaystyle B_{\rm {erd}}}$

${\displaystyle \omega _{L}=\gamma B_{\rm {erd}}\ ,}$

the constant factor is called the gyromagnetic ratio and has the value for protons . (For more detailed physical details see Larmor Precession , Nuclear Magnetic Resonance .) A proton magnetometer cannot measure continuously, but the proton spins must be aligned regularly; the precession decays exponentially within a few seconds after switching off the alignment field; measurements can only be made during this time. At the same time, this also determines the inertia of the instrument, because the precession frequency does not immediately adapt to a change in the magnetic field strength. ${\displaystyle \gamma }$ ${\displaystyle \gamma =2\pi \cdot 42,5\,{\tfrac {\rm {kHz}}{mT}}}$

With the proton magnetometer only the absolute magnetic field strength can be measured, but not the field direction - one speaks of a scalar magnetometer , in contrast to a vector magnetometer such as e.g. B. the Förster probe . Since frequencies can be measured very precisely, the measurement accuracy of a proton magnetometer is very high, around 0.5 nT.

The functional principle of the cesium magnetometer is similar ; however, the spin of electrons is used here. The alignment of the spin is done in a different way, namely by optical pumping .

A special type of proton magnetometer is a magnetic resonance tomograph - here, however, the magnetic field is not measured, but the protons are measured using a known magnetic field, in particular their presence and their interactions with neighboring atoms. The precession frequencies are excited by a transmitter in the radio range. Due to the ubiquity of water in the human body, this is a particularly versatile examination method in medicine.

application areas

• Archeology :
  • Metallic objects can be discovered.
  • The topsoil is usually more strongly magnetized than the subsoil, because magnetic minerals have formed through weathering. Therefore, trenches and depressions that have filled with topsoil over time are more strongly magnetized; On the other hand, excavation sites that were immediately filled in again (i.e. no topsoil could deposit there) are less magnetized than the surrounding area because the natural alignment of the minerals was disrupted during the excavation.
  • Pottery stands out clearly because the clay minerals it contains contain antiferromagnetic components that become ferromagnetic when burned and are permanently polarized when cooled in the earth's magnetic field.
• Geophysics :
  • Magnetic disruptive bodies ( e.g. ore deposits ) can be investigated
  • or rocks with strong magnetization (e.g. serpentine )
• Shipwrecks can be found in the water .