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Fundamentals of Radiation

Fundamentals of Radiation. Partial Periodic Table. The Periodic Table provides the atomic number (Z), the chemical symbol, atomic mass, and element name. It also groups the elements based on their electron structure (I.e., how they react chemically). Structure of the Atom.

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Fundamentals of Radiation

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  1. Fundamentals of Radiation

  2. Partial Periodic Table The Periodic Table provides the atomic number (Z), the chemical symbol, atomic mass, and element name. It also groups the elements based on their electron structure (I.e., how they react chemically).

  3. Structure of the Atom The nucleus contains neutrons and protons, also referred to as nucleons. The electrons orbit the nucleus. The electrons are responsible for chemical reactions (e.g., formation of molecules). The protons have a positive charge, the electrons a negative charge, and the neutrons are not charged. The nucleons are responsible for nuclear reactions (e.g., radioactive decay).

  4. Nomenclature • Z =Number of Protons(determines the chemical element) • N = Number of Neutrons(determines the isotope of the element) • A = Neutrons plus Protons(atomic mass of the isotope) A = Z + N A X X = Chemical Symbol Z N The chemical symbol and the atomic mass define the individual nuclide. (e.g., 3H has 1 proton and 2 neutrons). Isotopes of an element have the same number of protons, but a different number of neutrons in the nucleus.

  5. Forces in a Nucleus Nuclear force is an attractive force between each of the nucleons (i.e., neutrons and protons) over relatively short distances. Electrostatic force is a repulsive force between the like charged protons over a greater distance than nuclear forces. Hydrogen-3 (Tritium) Helium - 3

  6. Radioactive Decay • The nuclides, as with most things in nature, want to be at their lowest energy state which is a stable nucleus. • Radioactive decay occurs in nuclides where the nucleus is unstable. • For stable nuclides with low atomic masses, the number of neutrons is equal to, or approximately equal to the number of protons (except for 1H which only has one nucleon). • As the atomic mass of the nuclide increases, the ratio of neutrons to protons must be greater than one for it to be stable, suggesting that more neutrons are required to provide nuclear forces to offset the electrostatic repulsive force between the increased number of protons. • The nucleus may also become unstable when energy is added to it, placing it in an excited state. An example of this would be a free moving neutron inside of a reactor being captured by the nucleus of a 238U nucleus. • The nuclide reaches its stable state by undergoing radioactive decay.

  7. Types of Radiation There are four types of radiation of interest: 1) Alpha () which is a positively charged helium nucleus (2 protons and 2 neutrons). 2) Beta () which is a negatively charged electron. 3) Gamma () which is a packet of energy with zero rest mass. 4) Neutron (n) which is a released neutron. Mainly a concern during nuclear reactor operation.

  8. Alpha Particle • Helium-4 Nucleus • (2 neutrons, 2 protons) • Slow moving, but high energy • Cannot penetrate material easily • Stopped by one piece of paper • Stopped by dead layer of skin

  9. Example of Alpha Decay Alpha decay occurs when the nuclides of high atomic mass have a lower neutron to proton ratio than stable nuclides and ejects an alpha particle. Alpha decay is rare for nuclides with low or intermediate mass numbers.

  10. Beta Particle • Electron • Fast moving, Medium energy • Can penetrate material well • Stopped by 100 to 150 pieces of paper • Stopped by 0.5 -1 centimeter of water

  11. Example of Beta Decay Beta decay occurs when the nuclides have a higher neutron to proton ratio than stable nuclides. A neutron converts to a proton, electron (), and a neutrino. A neutrino is a high energy particle with zero rest mass with high penetrating capability.

  12. Gamma Decay • Electromagnetic radiation • Similar to light, x-rays, radio waves • Emitted only by certain nuclei • Speed of light; low to high energy • Highly penetrating • Stop half of the s with about 1 cm of lead • or 5 to 15 cm of water

  13. Example of Gamma Decay 238U + neutron  239U +  Gamma decay occurs as a means of removing energy from the nucleus of an excited nuclide. The gamma may be ejected alone or in conjunction with the emission of another radioactive particle (e.g., ) to reduce the nucleus energy.

  14. Examples of Neutron Emission There are Neutron (n) emissions associated with the following reactions. 2H +  1H + neutron 9Be +  2 (4He) + neutron 9Be +  12C + neutron Neutron emissions of interest in a nuclear reactor occur when the excited nucleus of a specific high atomic mass nuclide splits into two or more smaller nuclides during the fission process. 235U + n  135I + 97Y + 3n Neutrons with high kinetic energy are released in the process.

  15. Half-life • Each radioactive nucleus has a certain probability of decay per time • Some decay quickly (fractions of a second), some later (thousands of years) • Rate of decay depends on the number of nuclei available • As number decreases, rate of decay decreases

  16. Half-life • In theory, all the radioactive material will never totally decay • Define Half-life • Time for half of the sample to decay

  17. Half-life

  18. Example Half-lives - Natural • Uranium-238 (In soil) • 4.5 Billion years • Potassium-40 (in soil and body) • 1.3 Billion years • Carbon-14 (In all living tissue) • 5730 years • Hydrogen-3 (in all water) • 12 years

  19. Example Half-lives - Natural • Radium-226 (In soil - produces radon) • 1600 years • Radon-222 (in soil and air) • 3.8 days • Polonium-214 (radon progeny that decays in lungs) • 164 microseconds (0.000164 s)

  20. Example Half-lives - Medical Uses • Iodine - 131 (Thyroid treatment) • 8 days • Technetium-99m (Nuclear medicine) • 6 hours • Gold-198 (Tumor therapy) • 2.7 days

  21. Activity • Activity = Decays per time • Units: • 1 Becquerel = 1 decay per second (dps) • 1 Curie = 37 Billion dps • 1 microCurie (mCi) = 37,000 dps • 1 picoCurie (pCi) = 0.037 dps

  22. Example Activities - Regulations • Typical maximum alpha emitting radionuclide allowed without a license (some exceptions) • 0.1 mCi • Typical maximum beta emitting radionuclide allowed without a license (some exceptions) • 10 mCi

  23. Example Activities - Natural Radioactivity • Uranium-238 in cup of soil (typical) • 0.003 mCi = 3000 pCi • Radon-222 in air • 0.5 pCi per liter (outdoor air) • 1 to hundreds of pCi per liter (houses) • Potassium-40 in human body • 0.1 mCi

  24. Radiation Dose • Dose = Energy absorbed per mass • Units: • Rad • Gray (Gy) [1 Gy = 100 rad)

  25. Radiation Dose Equivalent • Different radiations do different amounts of biological damage • Dose Equivalent = Dose X QF • QF = Quality factor • Betas, Gamma: QF = 1; Alpha: QF = 20 • Units • Rem (1 mrem = 0.001 rem) • Sievert (Sv) [1 Sv = 100 rem)]

  26. Radiation Exposure • Old unit of exposure • Amount of radiation present in air • Only applicable for x-rays and gamma radiation • Units: • Roentgen (R) • 1 R exposure in air will produce about 1 rad dose in human tissue

  27. Example Doses • Natural annual background radiation • cosmic: 27 mrem (0.27 mSv) • Terrestrial: 28 mrem (0.28 mSv) • Internal: 39 mrem (.039 mSv) • [total natural (excl. Radon): ~100 mrem] • Radon-lungs: 2400 mrem (24 mSv) [effective whole body: 200 mrem] • Source: NCRP Report #93

  28. Example Doses • Medical Radiation (Effective Whole Body Dose Equivalent) • Chest X-ray: 8 mrem (0.08 mSv) • Head CT scan: 111 mrem (1.11 mSv) • Barium Enema: 406 mrem (4.06 mSv) • Extremity X-ray: 1 mrem (0.01 mSv) • Source: NCRP Report 100

  29. Radiation Safety • Radioactive materials produce a dose rate per time • To reduce total dose: • Minimum time • Half the time - half the dose • Shielding • Maximum distance • Twice the distance - one-fourth the dose

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