Basic Interactions Between X-Rays And Matter

BASIC INTERACTIONS BETWEEN X-RAYS AND MATTER

BASIC INTERACTIONS BETWEEN X-RAYS AND MATTER

Based on the work of Rutherford and Bohr a simple mode of an atom may be visualized as a massive positively charged nucleus surrounded by electrons in orbits of specific diameter Atoms are bonded to the molecules by electrons in the outermost shell.

X-ray photons may interact with either orbital electrons or with the nucleus of atoms but in the diagnostic energy range, the interactions are always with orbital electrons.

Only two interactions are important in diagnostic radiology ; the Photoelectric Effect and Compton scattering

INTERACTIONS OF X RAYS WITH MATTER.

• There are five basic ways that an X-ray photon can interact with matter,these are
• Coherent scattering.
• Photoelectric effect.
• Compton scattering.
• Pair production.
• Photodisintegration.

COHERENT SCATTERING

The name coherent scattering is given to that interaction in which radiation undergoes a change in direction without a change in wavelength.

The type of interaction between X-rays and matter that does not cause ionization No energy is transferred and no ionization occurs with coherent scattering the only effect is to change the direction of the incident radiation.

Only 5% of the radiation that undergoes coherent scattering compared to other basic interactions It produces scattered radiation contributing to film fog but the total quantity is too small to be important in Diagnostic Radiology.

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PHOTOELECTRIC EFFECT

The photoelectric effect occurs when an incident photon with little more energy than the binding energy of a k-shell electron encounters one of these electrons and ejects it from its orbit.

The photon disappears giving up all its energy to the electrons (Most of the photon’s energy is needed to overcome the binding energy of the electron and the excess gives the electron kinetic energy).

The electron which is now free of its energy debt, flies off into space as a photoelectron As an electron drops into the k-shell it gives up energy in the form of an X-ray photon

The amount of energy is characteristic of each element

• The photoelectric effect always yields three end products
• Negative ion (Photoelectron)
• characteristic radiation
• 3 . A positive ion (an atom deficient in one electron)

The probability of occurrence

1 . The incident photon must have sufficient energy to overcome the electron’s binding energy.

• A photoelectric reaction is most likely to occur when the photon energy and electron binding energy are nearly the same.
• The tighter an electron is bound in its orbit the more likely it is to be involved in a photo-electronic reaction

Application of photoelectron to Diagnostic Radiology

Positive effect

. it produces radiographic images of excellent quality. The quality is good for two reasons

• the photoelectric effect does not produce scatter radiation

-it enhances natural tissue contrast(x-ray image contrast depends on the tissue absorbing more x-rays than other tissue, contrast is greatest when the difference in absorption between adjacent tissues is large) Negative effect

From the point of view of patient exposure, the patient receives more radiation from photoelectric reactions than from any other type of interaction.

All the energy of the incident photon is absorbed by the patient in the photoelectric reaction (The importance of the photoelectric effect can be minimized by using high energy KVp technique).

COMPTON SCATTERING

Almost all the scatter radiation that we encounter in diagnostic radiology comes from Compton scattering.

An incident photon with relatively high energy strikes a free outer shell electron, ejecting it from its orbit. The photon is deflected by the electron so that it travels In a new direction as scatter radiation The reaction produces an ion pair, a positive atom, and a negative electron which is called a coil electron The probability of a Compton reaction depends on the total number of electrons in an absorber which in turn depends on its density and the number of electron per gram.

PAIR PRODUCTION AND PHOTODISINTEGRATION

The last two basic interactions, pair production and photodisintegration do not occur in the diagnostic energy range.

They have no importance in diagnostic radiology.

In Pair production a high-energy photon interacts with the nucleus of an atom, the photon disappears and its energy is converted into matter in the form of two particles

• ordinary electron
• A positron with the same mass as an electron but with a positive charge

PHOTODISINTEGRATION

In the photodisintegration part of the nucleus of an atom is ejected by high-energy photon The ejected portion may be a neutron, a proton, and an alpha particle or cluster of particles.

The photon must have sufficient energy to overcome nuclear binding energies of the order of 7-15 MeV.