Penetrability

Penetrability is the degree to which a barrier, medium, or interface can be penetrated by an external agent, such as a particle, photon, molecule, or field. The specific meaning varies with discipline, but all uses describe how easily penetration occurs under given conditions of energy, composition, and geometry.

In quantum mechanics, penetrability is often described by a transmission probability for a particle encountering a potential barrier. Even when a particle’s energy is below the barrier, tunneling yields a finite penetrability. For a simple one-dimensional barrier, the transmission coefficient T depends on barrier height, width, particle mass, and energy; in the WKB approximation, T is proportional to exp(-2∫κ(x) dx) with κ(x) = sqrt(2m(V(x) − E))/ħ.

In radiation physics and shielding, penetrability denotes how readily radiation or particles pass through a material. It depends on the energy and type of penetrant, material composition, density, and thickness; it is commonly described by attenuation coefficients and half-value layers. In diffusion and membrane science, penetrability describes the ability of molecules to cross a barrier or membrane, influenced by solubility, diffusivity, and thickness.

Measurement of penetrability is context-specific, often expressed as transmission flux, fraction, or probability, and may involve experimental transmission data, dosimetry, or diffusion experiments. Penetrability is not a universal property; it reflects the interaction between the penetrant and the host medium and is related to, but distinct from, concepts such as permeability and absorptivity.