Kernspin

Kernspin, in nuclear physics, is the intrinsic angular momentum of an atomic nucleus. It is a quantum property described by a spin quantum number I, which can take integer or half-integer values (for example I = 1/2 for 1H and 13C, I = 1 for 14N, I = 0 for many even-even nuclei). The total nuclear spin arises from the combination of the spins and orbital motions of protons and neutrons within the nucleus, as described by models such as the nuclear shell model. In many nuclei with even numbers of protons and neutrons, the spins couple to give I = 0, yielding no net magnetic moment, while others have nonzero spin and a corresponding magnetic moment.

Each nucleus with nonzero spin behaves like a tiny magnet. Its magnetic moment μ is related to I by μ = γħ I, where γ is the gyromagnetic ratio specific to the isotope. In an external magnetic field, the energy levels split (Zeeman splitting) and transitions between levels obey the selection rule Δm_I = ±1. The Larmor frequency ω0 = γB0 sets the resonance condition used in magnetic resonance techniques.

Nuclear spin affects spectroscopy and chemical shifts through hyperfine interactions and influences relaxation dynamics via spin-lattice and spin-spin relaxation times (T1 and T2). Practically, nonzero-spin nuclei enable nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). Many stable isotopes have I = 0 and are NMR-inactive, while others such as 1H, 13C, 15N, and 19F are widely used as nuclear probes.