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Radiation interacting with matter can be either scattered or absorbed. The mechanisms of the absorption of radiation are of interest because:
Ionization: The process resulting in the removal of an electron from an atom, leaving the atom with a net positive charge.
Excitation: Addition of energy to an atomic system, transferring it from the ground state to an excited state.
Radiation can be classified into two groups:
All atoms are normally electrically neutral. When a charged particle strikes an orbital electron, it ejects it from the atom resulting in the formation of an ion pair. Since the removal of the electron from the atom decreases the total number of negative charges by one, it leaves the atom with a net positive charge. The ion pair consists of:
The term used to compare and relate the ionizing powers of different types of charged particles is called the "specific ionization" Specific ionization is defined as the number of ion pairs per unit path length formed by ionizing radiation in a medium.
The specific ionization is dependent on the velocity of the charged particle (and therefore its energy), and the density of the absorbing material (the number of atoms available for ionization).
An alpha particle is a helium nucleus stripped of its orbital electrons. It is emitted from a radioactive atom with a velocity of about 1/20 that of the speed of light and with energies ranging from 4 to 9 MeV. Alpha particles cause ionizations in matter when they are deflected by the positive charge of a nucleus and pull the orbital electrons (attracted by the alpha's positive charge) along with them. Alpha particles also cause excitation along their path by pulling inner orbital electrons to outer orbits. No ion pair is formed, but energy is lost from the alpha particle and added to the atom. The added energy is then given off by the atom as fluorescent radiation or low energy X-Rays when the electrons drop back down to the inner orbital vacancies.
Because of its relatively large mass (2 neutrons and 2 protons), high electrical charge (+2) and low velocity, the specific ionization of an alpha particle is very high. That is, it creates many ion pairs in a very short path length. Because of this, it loses all of its energy in a very short distance. The range in air is only several centimeters even for the most energetic alpha particles.
Since the alpha particle has a very limited range in matter, it presents no external radiation hazard to man. Many alpha particles cannot penetrate the protective layer of skin. However, once inside the body, surrounded by living tissue, damage will be to the local area in which the alpha emitter is deposited. Thus, alpha emitters are an internal hazard and intake to the body must be prevented. (See Chapter 7 "Internal Radiation Protection Techniques", page 51).
Beta particles are emitted from the nucleus of a radioactive atom with a wide range of energies up to some maximum value. When a beta is emitted that is below the maximum value, the neutrino carries away the rest of the energy. Beta particles, like alpha particles, lose their energy by ionization and excitation, but because of their small mass (1/7300 of an alpha) and lower charge (1/2 of that of an alpha) the interactions take place at less frequent intervals. Therefore, the beta particles do not produce as many ion pairs per centimeter of path as alpha particles, and thus, have a greater range in matter. The beta particle's range in matter depends on the energy and the composition of the material.
Beta particles can interact with a nucleus of an element and give rise to X-rays by a method called Bremsstrahlung. Bremsstrahlung (German for "Breaking Radiation") occurs when high speed beta particles approaches the nucleus of an atom. The electrical interaction between the negative beta particle and the positively charged nucleus causes the beta particle to be deflected from its original path or stopped all together. Their stoppage or deflection results in a change in velocity of the beta particle with the emission of X-rays of various energies. The likelihood of Bremsstrahlung production increases with increasing atomic number of the absorber. For this reason, beta shields are made from low atomic numbered material, like aluminum or plastics.
Beta particles require an energy of greater than 70 keV to penetrate the protective layer of the skin, and thus, are somewhat of an external hazard. The beta can also constitute an internal hazard. A beta particle has a greater range in tissue compared to an alpha particle due to its low specific ionization. The beta particle gives up less energy per unit volume of tissue and, therefore, is not as effective in causing damage as an alpha particle.
From a practical radiation protection point of view, X-rays and gamma rays are identical, differing only in their place of origin. Gamma rays are emitted from excited nuclei with a discrete energy. X-rays are emitted when the extra-nuclear atomic structure undergoes a transition; i.e., an outer shell electron replaces a missing lower shell electron and an X-ray is produced. The energy of the X-ray is approximately equal to the difference in the electron energy levels.
Since X and γ-rays are chargeless, they do not interact by electrostatic forces as in the case of charged particles, which cause ionization of matter directly along their path of travel. However, X and γ-rays do have sufficient energy to release high energy secondary charged particles (electrons) from matter through one of three basic interactions:
The Photoelectric Effect is the interaction of X or γ-ray photons**** as well as other photons (such as light), whereby all of the energy of the photon is transferred to an inner shell electron (usually the K shell), ejecting it from the atom and leaving the atom with an inner shell vacancy. This shell vacancy creates an excitation energy which corresponds to the Binding Energy (BE) of the ejected photoelectron.
Figure 5 Photoelectric effect: An X-ray or a γ -photon ejects an electron from the inner shell. KEphotoelectron=Ex or Eγ - BE of the inner electron.
The Kinetic Energy (KE) of the photoelectron is equal to the energy of the X or γ-ray photo minus the BE of the electron ejected. If the X or γ -ray photon does not have sufficient energy to knock the inner shell electron loose, the reaction will not occur. The resultant atom is now in an excited state and will decay to the ground state by emission of X-rays and fluorescent radiation with the total energy equal to the BE of the photoelectron. The energies of the secondary radiations are usually much lower than the primary X or γ-ray energies.
Gamma rays emitted from excited nuclei, and X-rays emitted from excited atoms, have discrete energy characteristics of the specific nuclides and elements, respectively. Thus, the energy of these γ or X-ray photons can be used as "finger prints" to identify unknown nuclides and elements.
Photons with energies much greater than the BE of the electrons in an atom may interact through essentially elastic scattering interactions in which the total KE of the system is conserved. In this interaction, the electron appears to the photon as a free electron.
Figure 6 Compton effect: A photon interacts with an electron and transfer some of its energy.
The primary γ loses part of its energy to the Compton electron which gets scattered at an angle from the original direction of the incident , while the compton scattered γ (γ ') is scattered as an angle. In this process, the scattered photon and Compton electron share the energy of the incident γ.
The KE carried off by the Compton electron may be deposited locally (i.e., absorbed immediately by the surroundings). However, the energy carried off by the Compton scattered photon is not deposited locally. Therefore, this scattered photon can significantly contribute to the dose outside a shielding apparatus.
High energy gamma photons transfer their energy primarily by pair production. A high energy X or γ-ray passing close to a nucleus suddenly disappears and an electron and a positron appear in its place. This interaction must take place in the neighborhood of a nucleus to conserve momentum. Since both particles are created from energy supplied by the incident photon, the process is energetically possible only if Eγ or EX is greater than 1.02 MeV.
Figure 7 Pair production: A photon passing close to the nucleus disappears and a positron and an electron appear in its place.
When the positron slows down (i.e., loses its KE), it will annihilate itself by combining with an electron. This produces two photons with an energy of 0.51 MeV each. This "annihilation radiation" represents the energy equivalent of the rest mass of two electrons which is converted to pure energy according to the principles of Einstein's theories, in particular, E = mc2; where E = energy of two 0.51 MeV photons m = the rest mass of two electrons (1/1840 amu) c = the velocity of light (3×108 m/sec)
Figure 8 Annihilation: When a positron slows down it will annihilate itself by combining with an electron.
Applications of Pair Production Due to characteristic peaks observed for various known nuclides, Pair Production is an aid in the identification of unknowns.