RADIATION INTERACTION WITH MATTER

RADIATION INTERACTION WITH MATTER

When X or g radiation passes through a medium, it interacts with anatom and produces moving electrons. These electrons travel in themedium, interact with other atoms and produce ionization and excitation.As a result, energy is deposited on the cells, which are either damagedpartially or completely. In addition, sufficient amount of heat is alsoproduced. In summary, the x or g photon transfer energy to the electrons,which in turn transfer the energy to the cell system and produce thebiological effect. That is why they are called as indirectly ionizingradiations.The above interaction is said to have wavelike and particle likeproperties. X and gamma rays interacts with structures that are similar in size to their wavelength. Low energy photons tend to interact with

atoms, medium energy to that of electrons and high energy photons with that of nuclei. The above structural level interactions may be performed by five mechanisms, namely (i) coherent scattering, (ii) photoelectric absorption, (ii) compton scattering, (iii) pair production,and (v) photodisintegration. The compton scattering and photoelectric absorption are the two most important interactions in diagnostic radiology.

COHERENT OR RAYLEIGH SCATTERING

The photon interacts with electron of an atom and sets the atom in the excited state. The excited atom releases excess energy as scattered X-ray with wavelength equal to the incident photons. The emitted radiation will have the same energy of the incident photon. But the direction of the scattered photon is different to that of the incident photon. Thus, in coherent scattering, the photon undergoes a change in direction without change in wavelength (Fig. 5.3). In this process, no energy is transferred and no ionization occurs and most of the photons are scattered in the forward direction. The scattering angle increases asthe X-ray energy decreases. This interaction occurs mainly in low energy photons, may be in mammography (15–30 keV).

COMPTON SCATTERING

In Compton scattering, a photon interacts with a free electron (valence)of an atom and gets scattered with partial energy . The other part of energy is given to the valence electron, which is ejected from the atom. The ejected electron loses energy by ionization and excitation of atoms in the tissue, there by contributing to patient dose. The scattered photon may travel in the medium with or without interaction in the medium by Compton scattering or photoelectric absorption. The scattered photon will have longer wavelength compared to the incident photon. The energy of the incident photon (E0) is equal to the sum of the energy of the scattered photon (Esc) and the kinetic energy of the ejected electron.

  

E0

Esc = ———————————————

1 + [E0 (1-cos q) ÷ 511 keV]

where, q is the angle of scattered photon. As the incident photon energy increases, both photons and electrons are scattered in forward direction. The fraction of energy transferred to the scattered photon decreases with increase of incident photon energy, for a given angle

of scatter. If the photon makes a direct hit on the electron, the electron will travel straight forward (j = 0) and the scattered photon will be scattered back with q = 180o. In this collision, the electron will gets its maximum energy, while the scattered photon goes with minimum energy. If the photon makes a grazing hit with the electron, the electron will be emitted at right angles (j = 90o) and the scattered photon will go in the forward direction (q = 0). In this collision, the electron receives minimal energy and the scattered photon goes with maximal energy. The Compton scattering involves

an interaction between a photon and a free electron, resulting ionization of atom. The incident photon energy is shared between the scattered photon and ejected electron. The

probability of Compton scattering depends upon the electron density (number of electrons per gram × density) in the medium. Except hydrogen, the electron density is constant for tissue and it is independent of atomic number (Z). Probability of Compton interaction decreases with increase

of X-ray energy and it is 1/E. The probability per unit volume isNproportional to the density of the material. Hydrogenous material has higher probability for Compton scattering.

Compton scattering occurs in all energies in tissue, important in X-ray imaging. It is a predominant interaction in the diagnostic energy range with soft tissue (100 keV-10 MeV). Scattered X-rays provide no useful information, reduces image contrast, create radiation hazards in radiography and fluoroscopy. In fluoroscopy, large amount of radiation is scattered from the patient and contributes to occupational radiationexposure.

PHOTOELECTRIC EFFECT

In the photoelectric effect (PE), a photon of energy E collides with an atom and ejects one of the bound electrons from the K or L shells. The ejected electron is called photoelectron and it has kinetic energy equal to E - orbital binding energy. In this process, all the incident photon energy is transferred to the electron. The incident photon must have energy equal or greater than the orbital binding energy of the electron, to perform photoelectric effect. After the photoelectric effect, the atom is said to be ionized and there is a vacancy in the shell. This vacancy is filled by a electron of lower binding energy from higher orbit. This will create a cascadeof electron transition event from outer orbit to inner orbit. The difference

in binding energy is released as characteristic X-rays or Auger electrons. The photoelectric effectinvolves tightly bound electrons. The tightly bound electrons aremostly available in the K shell andhence, most photoelectric interactions occur at the K shell.The probability of photoelectric cross section per unit mass isproportional to Z3/E3, where Z is the atomic number and E is the incident

photon energy. As the X-ray photon energy increases, the subject contrast decreases. As the atomic number increases, the subject contrast increases, that is why barium (Z = 56) and iodine (Z = 53) are used as contrast agents. Even though photoelectric effect decreases with increase of energy,there are exceptions. The absorption of photon increases markedly

as the incident photon energy is increased from below to above the binding energy of the K-shell. This is known as K-edge absorption. The relation between probability of photoelectric absorption with photon energy may be plotted. The elements exhibit sharp discontinuities called absorption edges. For example, the biding energy of the K-shell in iodine is 33.2 keV, which will have 6 times higher absorption at the K-edge.

The photon energy corresponding to a absorption edge increases with atomic number. The absorption edges of elements H,C,N, and O present in the soft tissue are well below < 1 keV. The iodine, barium and lead absorption edges are 33.2, 37.4 and 88keV,respectively.

The photoelectric effect is very important in soft tissue imaging for photon energy < 50 keV. It will differentiate attenuation between two tissues with slightly varying atomic number.

PAIR PRODUCTION

When a photon having energy > 1.02 MeV, passes near the nucleusof an atom, will be subjected to strong nuclear field . The photon may suddenly disappear and become a positron and electron pair. For each particle 0.511 MeV energy is required and the excess energy > 1.02 MeV, would be shared between the positron and electron as kinetic energy. Actually, the interaction is between a photon and the nuclear field. This process is an example for the conversion of energy into mass as predicted by Einstein. The threshold energy for the pair

production process is 1.02 MeV. The probability of pair production increases with energy for a given material and also increases with atomic number (Z2). It is very important for photons having energy > 5 MeV .The electron loses energy by excitation and ionization and filling vacancy in the orbital shells. The positron travels in the medium and loses its energy by ionization, excitation and bremsstrahlung process.

Finally, the positron combines with a free electron andproduces two photons of each energy of 0.511 MeV that are ejected in opposite directions, to conserve momentum. Pair production is a true absorption because all the energy of the original photon is transformed.

Positron Annihilation

After the pair production process, the electron comes to rest, by joining with an atom. The positron comes to rest by combining with an electron and the two particles annihilates each other. The combined mass of the two particles is converted into energy in the form of two photons. The combined mass of two particles is 1.02 MeV and this energy is shared by the two photons. Hence, each photon will have energy of 0.511 MeV. The above process is called the positron annihilation. This is an example of conversion of mass into energy and forms the basis for positron emission tomography.