Where does stopping power collide?

In radiation physics, stopping power is the energy a charged particle loses per unit distance as it travels through matter, written as minus dE/dx, with the mass stopping power dividing that by the material's density. It is set mainly by ionisation and excitation of the medium's electrons, rising as the particle slows, and it determines how far radiation penetrates and where it deposits its dose. Bethe's 1930 Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie derived the quantum-mechanical formula for this electronic energy loss. The referent is a well-defined rate of energy deposition, measured in MeV per centimetre (or MeV·cm²/g), and it is a precise physical quantity quite unlike the loose terminal-ballistics notion of a bullet's power to stop a person, which borrows the same two words.[1]

Unit

Energy lost per unit path length, in MeV/cm (mass stopping power in MeV·cm²/g)

Recovery means

Not applicable

Measured by

Obtained from the Bethe formula and tabulated ranges, or measured by the energy loss of particles traversing known thicknesses

In terminal ballistics, stopping power is the popular construct of a cartridge's capacity to rapidly incapacitate a living target with few hits, often reduced to single-number indices built from bullet mass, velocity, and diameter. The wound-ballistics literature treats these indices with caution, because incapacitation depends on where and how tissue is disrupted rather than on a scalar of kinetic energy; Kneubuehl and colleagues' Wound ballistics: Basics and applications grounds the question in the actual mechanisms of wounding. The referent is an operational, contested measure of physiological effect on a target, and it is a different construct from the physicist's exactly defined energy-loss rate, which shares only the name.[2]

Unit

An operational index of incapacitating effect (no single agreed physical unit)

Recovery means

Not applicable

Measured by

Approached through wound-ballistic quantities such as penetration depth, temporary and permanent cavity, and energy transfer, rather than a single reliable index