nuclearspin

Nuclear spin is the intrinsic angular momentum of atomic nuclei, arising from the quantum spins of protons and neutrons and, in some cases, their orbital motion. The total nuclear spin quantum number I results from the vector sum of nucleon spins and orbital angular momenta. In many nuclei with paired nucleons, spin contributions cancel, giving I = 0; nuclei with unpaired nucleons can have nonzero I, set by shell structure. Even-even nuclei typically have I = 0; odd-A and odd-odd nuclides may have various I values.

Nuclear spin is tied to a magnetic moment, μ = g_I μ_N I, with μ_N the nuclear magneton and g_I the nuclear g-factor. The moment interacts with magnetic fields, producing Zeeman splittings, and couples to electrons to create hyperfine structure in atomic spectra. Thus nuclear spin is central to techniques that probe nuclei or their environment, such as nuclear magnetic resonance and related spectroscopies.

Common examples include 1H (I = 1/2) and 13C (I = 1/2); 14N has I = 1, 15N I = 1/2, and 12C has I = 0. Deuterium (2H) has I = 1. The distribution of I among isotopes reflects shell-model occupancy and pairing effects. Transitions involving nuclear spin obey selection rules for changes in I and its projection m_I and are key to hyperfine structure and some decay processes.

Practical use of nuclear spin spans spectroscopy and imaging. NMR uses nuclear magnetic moments in radiofrequency fields to determine molecular structure and dynamics, while MRI applies NMR to image tissues. Nuclear spin also influences beta-decay selection rules and provides a platform for quantum information experiments using nuclei as qubits.

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