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| Any process which results in the removal of a bound electron (negative charge) from and electrically neutral atom or molecule by adding enough energy to the enlectron to overcome its binding energy-this leaves the atom or molecule with a net positive charge- the result is the creation of an ion pair made up of the negatively charged electron and the positively charged atom or molecule-a molecule may remain intact or break-up, depending on whether and electron that is crucial to the molecular bonds is affedted by the event |
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process that adds enough energy to an electron of an atom or molecule so that it occupies a higher energy state (smaller binding energy) than its lowest bound energy state (ground state). The electron remains bound to the atom or molecule, but depending on its role in the bonds of the molecule, mbreak-up may occur. No ions areproduced and the atom remainselectrically neutral.
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(2 protons and 2 neutrons) exciting an electron from the K shell to the L shell because of the attractive electric force (assuming there was a vacant position available in the L shell).
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| Nuclear Excitation is any process that adds energy to a nucleon in the nucleus of an atom so that it occupies a higher energy state (smaller binding energy). The nucleus continues to have the same number of nucleons and can continue in its same chemical environment |
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Bremsstrahlung is the radiative energy loss of moving charged particles as they interact with the matter through which they are movinginteraction of a high speed charged particle with the nucleus of an atom (positive charge)
In the case of a negatively charged electron, the attractive force slows down the electron, deflecting it from its original path.
The kinetic energy that the particle loses is emitted as a photon (called an x-ray because it is created outside the nucleus).
enhanced with high Z materials (larger coulomb forces) and high energy electrons (more interactions occur before all energy is lost).
After ionization, an atom with an excess of positive charge and a free electron are created. After excitation, the excited atom will eventually lose its excess energy when an electron in a higher energy shell falls into the lower energy vacancy created in the excitation process.
When this occurs, the excess energy is liberated as a photon of electromagnetic radiation (x-ray) which may escape from the material but usually undergoes other absorptive processes locally.
Following nuclear excitation analogous to atomic electron excitation above, the nucleus will eventually return to the ground state and release the excess energy in photons of electromagnetic radiation (gamma rays). |
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DIRECTLY IONIZING RADIATION
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“Coulomb force” (force from the electrical charge) will act over a distance to cause ionization and excitation in the absorber medium. Particles with charge (such as alpha and beta) that lose energy in this way are called directly ionizing radiation. The strength of this force depends on:
Energy (speed) of the particle
Charge of the particle
Density and atomic number (number of protons) of the absorber.
The “Coulomb force” for even a singly charged particle (an electron) is significant over distances greater than atomic dimensions (remember this is the same force that holds the electrons in bound energy states about the nucleus). Therefore, for all but very low physical density materials, the loss of kinetic energy for even an electron is continuous because the “Coulomb force” is constantly “pushing” on electrons of at least one atom and possibly many atoms at the same time. |
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S.I.-(average number of) ion pairs produced (by a charged particle) per unit distance traveled in an absorbing medium
Units-ion pairs/cm
As a charged particle passes through an absorber, the energy loss can be measured several ways. One method used is specific ionization. Specific ionization is the number of ion pairs formed by the particle per unit path length and is often used when the energy loss is continuous and constant such as with beta particles (electrons) or alpha particles. The number of ion pairs produced is dependent on the type of ionizing particle and the material being ionized. For example, an alpha particle traveling through air has a specific ionization of 80,000 ion pairs per cm of travel. A beta particle has a specific ionization of about 5,000 ion pairs per cm of travel in air. Specific ionization is a macroscopic quantity that accounts for all energy losses that occur before an ion pair is produced. |
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2b. linear energy transfer (LET)
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LET-(average value of) energy locally deposited (by a charged particle) in an absorbing medium per unit distance
Units-keV/cm
Another measure of energy deposited in an absorber by a charged particle is the Linear Energy Transfer (LET). The LET is the average energy locally deposited in an absorber resulting from a charged particle per unit distance of travel (keV/cm). The LET is therefore a measure of the local concentration of energy per path length resulting from ionization effects. Biological damage from radiation results from ionization. |
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S-for a given absorber, the average energy lost by a charged particle per unit distance traveled Units-keV/cm
Stopping power of an absorber is its ability to remove energy from a beam of charged particles. Stopping power is measured as the average energy lost by a charged particle per unit distanced travelled (keV/cm). Stopping power and LET may have the same units but are not equal because, although ionization may occur and removes energy from the beam, not all of that energy gets deposited locally and so does not contribute to LET. In other words, LET stopping power because some electron ions may interact via Bremsstrahlung or excitation and the resulting photons escape the local area. Materials having higher stopping power values cause the particle to lose its energy over shorter distances. |
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R-average distance traveled by a radiation in an absorbing medium Units-cm
Inversely related to the stopping power of the absorber is the range of the charged particle. The concept of range only has meaning for charged particles whose energy is kinetic energy which is lost continuously along their path. The range of a charged particle in an absorber is the average depth of penetration of the charged particle into the absorber before it loses all its kinetic energy and stops. If a particle has a high range, the absorber has a low stopping power. If the particle has a short range, the absorber has a high stopping power. |
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W-the average amount of energy needed to produce an ion pair in a given medium Units-eV/ion pair
Specific Ionization, Linear Energy Transfer, Stopping Power, and Range can all be related to each other if one knows the average amount of energy needed to ionize a material. The average amount of energy needed to create an ion pair in a given medium is called the W-Value for the medium. Table 1 below summarizes the terms used in describing the energy losses from radiations in matter. |
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strongly bound together by nuclear forces. If such a particle approaches an electron (negatively charged), it experiences a strong electrostatic attraction, whereas if it approaches an atomic nucleus (also positively charged) it will tend to be repelled. Alpha particles have a mass about 8,000 times that of an electron. They are ejected from the nuclei of radioactive atoms with velocities of the order of 1/20 the speed of light. All of these properties--its large mass, its charge, and its high velocity tend to make the alpha particle an efficient projectile when it encounters atoms of an absorbing material. In other words, it would have a high probability of interacting, or colliding, with orbital electrons, and also atomic nuclei.
When speaking of “collisions” between subatomic particles, it should be understood that the particles (for example an alpha and an electron) need approach each other only sufficiently close for Coulomb forces to interact. Such an interaction may then be referred to as a collision.
Figure 3 below schematically shows such a collision, resulting in ionization. In this case, the kinetic energy of the alpha particle is decreased and shows up as a free electron with kinetic energy. The free electron’s kinetic energy is less than the alpha energy loss by the amount of energy necessary to free the electron (its binding energy). Because the alpha particle is so much more massive than the electron, the alpha particle typically only loses a small fraction of its energy in any collision and travels in a relatively straight path through the material |
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two major mechanisms of energy transfer for alpha particulate radiation.
3 ionization andor Excitation
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Alpha collisions may result in energy transfer by 1) ionization and/or 2) excitation. And since a finite amount of energy is required to ionize or excite an atom, the kinetic energy of the alpha particle is gradually dissipated by such interactions until it captures two electrons and settles down to a quiet existence as a helium atom. Since the average amount of energy to ionize most materials is much less than the initial energy of most alpha particles, many ionizations will occur before the alpha particle is stopped.
Due to the high probability of interaction between an alpha particle and the orbital electrons of the absorbing medium and because of the +2 charge, a large number of ion pairs are formed per unit path length. Therefore, this type of radiation loses its energy over a relatively short distance. For these reasons, the range of alpha particles is much less than the range of other forms of radiation. It is, in summary, a highly ionizing, weakly penetrating radiation.
Alpha particles from a given radionuclide are all emitted with the same energy, consequently those emitted from a given source will have approximately the same range in a given material. Alpha particle range is usually expressed in centimeters of air. The relationship between range and energy has been expressed empirically as follows:
Ra = 0.56E for E < 4 MeV
Ra = 1.24E-2.62 for 4 < E < 8 MeV
where:
Ra = Range in cm of air at 1 atmosphere and 0C
E = Energy in MeV.
As stated above, the number of ion pairs formed per centimeter of path in any given medium is called the specific ionisatio for the a particular ionizing radiation.
on 34 eV of energy is los for each primary ion pair formed in air. Only 2/3 of the energy is actually required to remove the orbital electron the balanc being los tin electron exitation processes. energy of alpha particle 5-80 k formed per centimeter of path in air. |
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4
three major mechanisms of energy transfer for beta particle radiation
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| Like an alpha particle, a beta particle may transfer energy through ionization and excitation. In addition, a beta may have a Bremsstrahlung interaction with an atom which results in the production of X-rays. Figure 4, below, schematically shows a Bremsstrahlung interactionIn this case, a high energy beta penetrates the electron cloud surrounding the nucleus of theatom, and experiences the strong electrostatic attractive force of the positively charged nucleus. This results in a change in velocity/kinetic energy of the particle and the emission of a Bremsstrahlung X-ray. Figure 4. Bremsstrahlung Radiation The energy of the X-ray emitted depends on how much deflection of the beta particle occurred, which in turn, depends on how close the electron cam to the nucleus. |
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