The nucleon magnetic moments are the intrinsic magnetic dipole moments of the proton and neutron, symbols μp and μn . The nucleus of an atom comprises protons and neutrons, both nucleons that behave as small magnets. Their magnetic strengths are measured by their magnetic moments. The nucleons interact with normal matter through either the nuclear force or their magnetic moments, with the charged proton also interacting by the Coulomb force. The proton's magnetic moment was directly measured in 1933 by Otto Stern team in University of Hamburg. While the neutron was determined to have a magnetic moment by indirect methods in the mid-1930s, Luis Alvarez and Felix Bloch made the first accurate, direct measurement of the neutron's magnetic moment in 1940. The proton's magnetic moment is exploited to make measurements of molecules by proton nuclear magnetic resonance. The neutron's magnetic moment is exploited to probe the atomic structure of materials using scattering methods and to manipulate the properties of neutron beams in particle accelerators. The existence of the neutron's magnetic moment and the large value for the proton magnetic moment indicate that nucleons are not elementary particles. For an elementary particle to have an intrinsic magnetic moment, it must have both spin and electric charge. The nucleons have spin 1 2 {\displaystyle \ {\tfrac {\ \!1\ \!}{2}}} ħ , but the neutron has no net charge. Their magnetic moments were puzzling and defied a valid explanation until the quark model for hadron particles was developed in the 1960s. The nucleons are composed of three quarks, and the magnetic moments of these elementary particles combine to give the nucleons their magnetic moments.
Description
The CODATA recommended value for the magnetic moment of the proton is μp = 2.79284734463(82) μN = 0.00152103220230(45) μB . The best available measurement for the value of the magnetic moment of the neutron is μn = −1.91304276(45) μN . Here, μN is the nuclear magneton, a standard unit of measure for the magnetic moments of nuclear components, and μB is the Bohr magneton, an alternate unit from spectroscopy, both being physical constants. In SI units, these values are μp = 1.41060679545(60)×10−26 J⋅T−1 and μn = −9.6623653(23)×10−27 J⋅T−1. A magnetic moment is a vector quantity, and the direction of the nucleon's magnetic moment is determined by its spin. The torque on the neutron that results from an external magnetic field is towards aligning the neutron's spin vector opposite to the magnetic field vector. The nuclear magneton is the spin magnetic moment of a Dirac particle, a charged, spin 1 / 2 elementary particle, with a proton's mass mp, in which anomalous corrections are ignored. The nuclear magneton is
μ N = e ℏ 2 m p , {\displaystyle \mu _{\mathsf {N}}={\frac {\ e\ \hbar \ }{2\ m_{\mathsf {p}}}}\ ,}
where e is the elementary charge, and ħ is the reduced Planck constant. The magnetic moment of such a particle is parallel to its spin. Since the neutron has no charge, it should have no magnetic moment by the analogous expression. The non-zero magnetic moment of the neutron thus indicates that it is not an elementary particle. The sign of the neutron's magnetic moment is that of a negatively charged particle. Similarly, that the magnetic moment of the proton, μp/μN ≈ 2.793 is not almost equal to 1 indicates that it too is not an elementary particle. Protons and neutrons are composed of quarks, and the magnetic moments of the quarks can be used to compute the magnetic moments of the nucleons. Although the nucleons interact with normal matter through magnetic forces, the magnetic interactions are many orders of magnitude weaker than the nuclear interactions. The influence of the neutron's magnetic moment is therefore only apparent for low energy, or slow, neutrons. Because the value for the magnetic moment is inversely proportional to particle mass, the nuclear magneton is about 1/2000 as large as the Bohr magneton. The magnetic moment of the electron is therefore about 1000 times larger than that of the nucleons. The magnetic moments of the antiproton and antineutron have the same magnitudes as their antiparticles, the proton and neutron, but they have opposite sign.
Measurement
Proton The magnetic moment of the proton was discovered in 1933 by Otto Stern, Otto Robert Frisch and Immanuel Estermann at the University of Hamburg. The proton's magnetic moment was determined by measuring the deflection of a beam of molecular hydrogen by a magnetic field. Stern won the Nobel Prize in Physics in 1943 for this discovery.
Neutron
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