Tesla (unit) in the context of "Nuclear magnetic resonance"

The metric system is a system of measurement that standardises a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules governing the metric system have changed over time, the modern definition, the International System of Units (SI), defines the metric prefixes and seven base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).

An SI derived unit is a named combination of base units such as hertz (cycles per second), newton (kg⋅m/s), and tesla (1 kg⋅s⋅A) and in the case of Celsius a shifted scale from Kelvin. Certain units have been officially accepted for use with the SI. Some of these are decimalised, like the litre and electronvolt, and are considered "metric". Others, like the astronomical unit are not. Ancient non-metric but SI-accepted multiples of time, minute and hour, are base 60 (sexagesimal). Similarly, the angular measure degree and submultiples, arcminute, and arcsecond, are also sexagesimal and SI-accepted.

The magnetic field of the Moon is very weak in comparison to that of the Earth; the major difference is the Moon does not have a dipolar magnetic field currently (as would be generated by a geodynamo in its core), so that the magnetization present is varied (see picture) and its origin is almost entirely crustal in location; so it's difficult to compare as a percentage to Earth. But, one experiment discovered that lunar rocks formed 1 - 2.5 billion years ago were created in a field of about 5 microtesla (μT), compared to present day Earth's 50 μT. During the Apollo program several magnetic field strength readings were taken with readings ranging from a low of 6γ (6nT) at the Apollo 15 site to a maximum of 313γ (0.31μT) at the Apollo 16 site, note these readings were recorded in gammas(γ) a now outdated unit of magnetic flux density equivalent to 1nT.

One hypothesis holds that the crustal magnetizations were acquired early in lunar history when a geodynamo was still operating. An analysis of magnetized Moon rocks brought to Earth by Apollo astronauts showed that the Moon must have had a strong (above 110 μT) magnetic field at least 4.25 billion years ago, which then fell to 20 μT level in the 3.6 - 3.1 billion years BP period. The small size of the lunar core, however, is a potential obstacle to promoting that hypothesis to the status of theory. However, single silicate grains with magnetic inclusions from Apollo rocks formed at 3.9, 3.6, 3.3, and 3.2 billion years ago have been shown to be capable of recording strong magnetic fields but do not. This supports the alternative hypothesis that the Moon never had a long-lasting core dynamo, consistent with the lack of energy needed to sustain a field.

Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic field. This re-orientation occurs with absorption of electromagnetic radiation in the radio frequency region from roughly 4 to 900 MHz, which depends on the isotopic nature of the nucleus and increases proportionally to the strength of the external magnetic field. Notably, the resonance frequency of each NMR-active nucleus depends on its chemical environment. As a result, NMR spectra provide information about individual functional groups present in the sample, as well as about connections between nearby nuclei in the same molecule. As the NMR spectra are unique or highly characteristic to individual compounds and functional groups, NMR spectroscopy is one of the most important methods to identify molecular structures, particularly of organic compounds.

The principle of NMR usually involves three sequential steps:

A magnetar is a type of neutron star with an extremely powerful magnetic field (~10 to 10 T, ~10 to 10 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

The existence of magnetars was proposed in 1992 by Robert Duncan and Christopher Thompson following earlier work by Jonathan I. Katz on the Soft Gamma Repeater SGR 0525-66, then called a gamma-ray burst.

Magnetic resonance imaging (MRI) is a medical imaging technique mostly used in radiology and nuclear medicine in order to investigate the anatomy and physiology of the body, and to detect pathologies including tumors, inflammation, neurological conditions such as stroke, disorders of muscles and joints, and abnormalities in the heart and blood vessels among other things. Contrast agents may be injected intravenously or into a joint to enhance the image and facilitate diagnosis. Unlike CT and X-ray, MRI uses no ionizing radiation and is, therefore, a safe procedure suitable for diagnosis in children and repeated runs. Patients with specific non-ferromagnetic metal implants, cochlear implants, and cardiac pacemakers nowadays may also have an MRI in spite of effects of the strong magnetic fields. This does not apply on older devices, and details for medical professionals are provided by the device's manufacturer.

Certain atomic nuclei are able to absorb and emit radio frequency energy when placed in an external magnetic field. In clinical and research MRI, hydrogen atoms are most often used to generate a detectable radio-frequency signal that is received by antennas close to the anatomy being examined. Hydrogen atoms are naturally abundant in people and other biological organisms, particularly in water and fat. For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the signal in space. By varying the parameters of the pulse sequence, different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms therein.

The vacuum magnetic permeability (variously vacuum permeability, permeability of free space, permeability of vacuum, magnetic constant) is the magnetic permeability in a classical vacuum. It is a physical constant, conventionally written as μ₀ (pronounced "mu nought" or "mu zero"), approximately equal to 4π × 10 H/m (by the former definition of the ampere). It quantifies the strength of the magnetic field induced by an electric current. Expressed in terms of SI base units, it has the unit kg⋅m⋅s⋅A. It can be also expressed in terms of SI derived units, N⋅A, H·m, or T·m·A, which are all equivalent.

Since the revision of the SI in 2019 (when the values of e and h were fixed as defined quantities), μ₀ is an experimentally determined constant, its value being proportional to the dimensionless fine-structure constant, which is known to a relative uncertainty of 1.6×10, with no other dependencies with experimental uncertainty. Its value in SI units as recommended by CODATA is: