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Interaction of X-rays with matter

When an X-ray interacts with matter four possible effects may arise:

1. PHOTOELECTRIC EFFECT: the X-ray interacts with an electron of an inner shell of

the atom causing its ejection from the atom. The electron ejected is called Fast Electron

(i.e. which moves at a relativistic velocity) and it has a kinetic energy equal to .As a

consequence of the presence of a low energy empty state (the one previously occupied by

the electron ejected) a high-energy electron will fall to fill the vacuum releasing an

amount of energy equal to the energy jump made. This energy can cause two different

phenomena:

1.1FLUORESCENCE X-ray: the energy released from the falling electron is transformed

into a photon whose energy is equal to the energy lose by the electron (The term

fluorescence is applied to phenomena in which the absorption of radiation of a specific

energy results in the reemission of radiation of a different energy).

1.2 AUGER EFFECT: the energy released from the falling atom can be transferred to

another electron, which is ejected from the atom and is named Auger electron.

( )

where is the work function (orbinding energy: the energy required to remove an electron

from an atom) of the electron.

2. COMPTON SCATTERING(incoherent scattering): the X-ray interact with an electron of

an outer shell of the atom losing some energy to cause its ejection from the atom. The

energy possessed by the X-ray after the interaction depends on the angle between its

previous trajectory and the new one.

The Klein-Nishina formula gives the differential cross section (the scattering cross section is

an area which describes the likelihood of light or other radiation being scattered by a

particle. In the simplest case, the differential scattering cross section refers to the way a

single particle is scattered on a single scattering center.)of photons scattered from a single

free electron.

3. RAYLEIGH SCATTERING(coherent elastic scattering): is the elastic scattering of light

by particles much smaller than the wavelength of the light. The X-ray interact with an

electron of the atom without losing any energy being only deflected from its originaltrajectory.

4. PAIR PRODUCTION: is the creation of an elementary particle and its antiparticle, for

example, an electron and a positron. Usually it occurs when a photon interacts with a

nucleus. For this process to occur the energy of the photon must be sufficiently high, higher

than two times the rest energy of the electron (0.511Mev).

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