Biomedical Engineering Reference
In-Depth Information
8
Interaction of Photons with Matter
8.1
Interaction Mechanisms
Unlike charged particles, photons are electrically neutral and do not steadily lose
energy as they penetrate matter. Instead, they can travel some distance before inter-
acting with an atom. How far a given photon will penetrate is governed statistically
by a probability of interaction per unit distance traveled, which depends on the
specific medium traversed and on the photon energy. When the photon interacts,
it might be absorbed and disappear or it might be scattered, changing its direction
of travel, with or without loss of energy.
Thomson and Raleigh scattering are two processes by which photons interact
with matter without appreciable transfer of energy. In Thomson scattering an elec-
tron, assumed to be free, oscillates classically in response to the electric vector of
a passing electromagnetic wave. The oscillating electron promptly emits radiation
(photons) of the same frequency as the incident wave. The net effect of Thomson
scattering, which is elastic, is the redirection of some incident photons with no
transfer of energy to the medium. In the modern, quantum-mechanical theory of
photon-electron interactions, Thomson scattering represents the low-energy limit
of Compton scattering, as the incident photon energy approaches zero.
Raleigh scattering of a photon results from the combined, coherent action of an
atom as a whole. The scattering angle is usually very small. There is no appreciable
loss of energy by the photon to the atom, which, however, does “recoil” enough to
conserve momentum. We shall not consider Thomson or Raleigh scattering fur-
ther.
The principal mechanisms of energy deposition by photons in matter are pho-
toelectric absorption, Compton scattering, pair production, and photonuclear reac-
tions. We treat these processes in some detail.
