femtometerscale

The femtometer scale refers to lengths on the order of 10^-15 meters. The unit, femtometer (fm), is widely used in nuclear and subnuclear physics to describe the sizes and structures of atomic nuclei and nucleons. Because the de Broglie wavelengths associated with typical nuclear processes fall near the femtometer scale, it provides a natural descriptor for these phenomena.

Nuclear sizes follow R ≈ r0 A^(1/3) with r0 about 1.2 fm, so most atomic nuclei have radii of a few femtometers and diameters up to around 12–14 fm for the heaviest stable nuclei. The intrinsic size of a nucleon (proton or neutron) is about 0.8–0.9 fm in terms of charge or matter radius. The deuteron, a bound proton–neutron pair, has a root-mean-square size around 2.1 fm.

Probing the femtometer scale requires high-energy scattering experiments. Elastic and inelastic electron scattering, as well as hadron–nucleus and heavy-ion collisions, reveal the distribution of charge, mass, and internal structure of nuclei and nucleons. The relevant momentum transfers are of order 1 fm^-1, corresponding to energies from hundreds of MeV to several GeV, where the associated wavelengths are comparable to nuclear dimensions. Observables include form factors, cross sections, and structure functions, which inform models of nuclear density distributions and nucleon substructure.

The femtometer scale sits within a broader context of scales used in physics to describe matter, bridging macroscopic nuclear physics and subnucleonic phenomena described by quantum chromodynamics.