Radiation Source Use and Replacement Abbreviated Version (2008) / Chapter Skim
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APPENDIX B: BACKGROUND ON THE ATOM, RADIOACTIVE DECAY, RADIATION, AND RADIATION DOSE DEPOSITION
APPENDIX B: BACKGROUND ON THE ATOM, RADIOACTIVE DECAY, RADIATION, AND RADIATION DOSE DEPOSITION
Pages 193-208
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From page 193... ...
The proton and neutron have nearly identical rest masses; the rest mass of the electron is about 2,000 times smaller than that of the proton or neutron. FIGURE B-1 Schematic diagram of the Rutherford atomic model.
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From page 194... ...
These are more stable than the parent nucleus but may still be unstable and will decay further through a chain of radioactive decays until a stable nuclear configuration is reached. Radioactive decay is usually accompanied by emission of energetic particles and/or gamma rays which together form a class of radiation that is referred to as ionizing radiation.
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From page 195... ...
• The mode of radioactive decay depends upon the particular nuclide involved. • Radioactive decay processes are governed by general formalism that is based on the definition of the activity A (t)
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From page 196... ...
Modes of Radioactive Decay A closer look at radioactive decay processes shows that they are divided into seven main categories: 1. alpha decay producing alpha particles, 2.
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From page 197... ...
• Beta minus decay is characterized by a nuclear transformation in which a neutron transforms into a proton, and an electron and anti-neutrino, sharing the available energy, are ejected from the nucleus. Energetic electrons emitted in beta minus decay have a relatively small mass and can penetrate human tissue to a depth of a few centimeters, and so they pose a hazard to humans both as external and internal radiation sources.
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From page 198... ...
Note: the two gamma photons are called cobalt-60 gamma rays, yet they actually originate in nickel-60. The average energy of the two is 1.25 MeV.
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From page 199... ...
Both platinum-192 and osmium192 are produced in various excited states and they both instantaneously attain their ground states through emission of gamma photons. The spectrum of iridium-192 gamma photons thus consists of many gamma photons, and the effective energy of the iridium-192 gamma photons is of the order of 0.34 MeV.
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From page 200... ...
All excited states instantaneously decay through various excited states into the ground state of neptunium-237 through emission of gamma photons. SOURCE: Image provided by committee.
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From page 201... ...
In the case of x-rays and gamma rays, the energetic charged particles released in the absorbing medium are electrons or positrons produced through photoelectric effect, Compton effect or pair production; in the case of neutrons, these energetic charged particles are protons or heavier nuclei released in the absorbing medium through nuclear reactions. It originates from: • radioactive decay producing gamma rays in gamma decay, neutrons in spontaneous fission; • an electron accelerator producing x-rays in x-ray machine, linear accelerator, betatron, or microtron; • a neutron generator producing neutrons in a charged particle electrostatic accelerator or through bombarding a beryllium target with alpha particles produced through alpha decay (e.g., Am-Be)
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From page 202... ...
Hot cathodes emit electrons through thermionic emission and are still in use today in modern x-ray tubes, now called Coolidge tubes, and in electron guns of linear accelerators. The maximum x-ray energy produced in the x-ray target (anode)
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From page 203... ...
In the use of ionizing radiation in industrial imaging, it is the interactions of the radiation with the imaged objects and the dose deposition in the image receptors that govern the image quality as well as the radiation safety requirements. Medical imaging with ionizing radiation is limited to the use of x-ray beams in diagnostic radiology and gamma rays in nuclear medicine (see Figure B-9a)
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From page 204... ...
Although all four radiation categories are used in radiotherapy, routine radiotherapy is generally done with x-rays, gamma rays, or electrons, the beam choice depending on the location of the treated tumor and availability of equipment. Dose Distributions for Photon Beams in Water A photon beam propagating through air or vacuum is governed by the inverse-square law which results in the diminution of the beam's intensity as the inverse square of the distance from the radiation source.
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From page 205... ...
beams; 0.5 cm for cobalt-60 gamma rays; 1.5 cm for 6-MV beams; 2.5 cm for 10-MV beams; and 4 cm for 22-MV beams. The relatively low surface dose for high-energy photon beams (referred to as the skin sparing effect)
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From page 206... ...
Dose Distributions for Neutron Beams in Water Neutron beams belong to the group of indirectly ionizing radiation, but rather than releasing electrons like photons do, they release protons or heavier nuclei which then deposit their energy in absorbing media through Coulomb interactions with the electrons and nuclei of the absorber. As shown in Figure B-10b, the dose deposition characteristics of neutrons in water are similar to those of photon beams.
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From page 207... ...
Because of their large mass compared to the electron mass, heavy charged particles lose their kinetic energy, only interacting with orbital electrons of the absorber. Since they do not lose any appreciable amount of energy through bremsstrahlung interactions with absorber nuclei, their depth dose curves do not exhibit a bremsstrahlung contamination tail.
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