3/2003 Rev 1 I.2.6 – slide 1 of 43 Session I.2.6 Part I Review of Fundamentals Module 2Basic Physics and Mathematics Used in Radiation Protection Modes of Radioactive Decay and Types of Radiation Session 6 Modes of Radioactive Decay and Types of Radiation IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

3/2003 Rev 1 I.2.6 – slide 2 of 43 Introduction  Modes of radioactive decay and types of radiation emitted will be discussed  Students will learn about basic atomic structure; alpha, beta, and gamma decay; positron emission; differences between gamma rays and x-rays; orbital electron capture; and internal conversion

3/2003 Rev 1 I.2.6 – slide 3 of 43 Content  Basic atomic structure and isotopes  Alpha, beta, and gamma decay  Decay spectra  Differences between gamma rays and x-rays  Positron emission  Orbital electron capture  Internal conversion

3/2003 Rev 1 I.2.6 – slide 4 of 43 Overview  Fundamental atomic structure will be described  Modes of radioactive disintegration and types of radiation emitted will be discussed

3/2003 Rev 1 I.2.6 – slide 5 of 43 protonneutronelectron Atomic Structure

3/2003 Rev 1 I.2.6 – slide 6 of 43 Atomic Number (Z) Hydrogen1 Carbon6 Cobalt27 Selenium34 Iridium77 Uranium92

3/2003 Rev 1 I.2.6 – slide 7 of 43 Isotopes An isotope of an element has:  the same number of protons  a different number of neutrons 1H1H 2H2H 3H3H

3/2003 Rev 1 I.2.6 – slide 8 of 43 Isotopes The number of protons determines the element. Elements with the same number of protons but different numbers of neutrons are called isotopes. Some isotopes are radioactive.

3/2003 Rev 1 I.2.6 – slide 9 of 43 Radioactive Decay  Spontaneous changes in the nucleus of an unstable atom  Results in formation of new elements  Accompanied by a release of energy, either particulate or electromagnetic or both  Nuclear instability is related to whether the neutron to proton ratio is too high or too low

3/2003 Rev 1 I.2.6 – slide 10 of 43 The Line of Stability N > Z

3/2003 Rev 1 I.2.6 – slide 11 of 43 Alpha Emission  Emission of a highly energetic helium nucleus from the nucleus of a radioactive atom  Occurs when neutron to proton ratio is too low  Results in a decay product whose atomic number is 2 less than the parent and whose atomic mass is 4 less than the parent  Alpha particles are monoenergetic

3/2003 Rev 1 I.2.6 – slide 12 of 43 Alpha particle charge +2 Alpha Particle Decay

3/2003 Rev 1 I.2.6 – slide 13 of 43 Alpha Particle Decay

3/2003 Rev 1 I.2.6 – slide 14 of 43 Alpha Decay Example 226 Ra decays by alpha emission When 226 Ra decays, the atomic mass decreases by 4 and the atomic number decreases by 2 The atomic number defines the element, so the element changes from radium to radon 226 Ra  222 Rn + 4 He 28688

3/2003 Rev 1 I.2.6 – slide 15 of 43 Beta Emission  Emission of an electron from the nucleus of a radioactive atom ( n  p + + e -1 )  Occurs when neutron to proton ratio is too high (i.e., a surplus of neutrons)  Beta particles are emitted with a whole spectrum of energies (unlike alpha particles)

3/2003 Rev 1 I.2.6 – slide 16 of 43 Beta particle charge -1 Beta Particle Decay

3/2003 Rev 1 I.2.6 – slide 17 of 43 Beta Particle Decay

3/2003 Rev 1 I.2.6 – slide 18 of 43 Beta Decay of 99 Mo

3/2003 Rev 1 I.2.6 – slide 19 of 43 Beta Spectrum

3/2003 Rev 1 I.2.6 – slide 20 of 43 Rule of Thumb Average energy of a beta spectrum is about one-third of its maximum energy or: E av = E max 1 3

3/2003 Rev 1 I.2.6 – slide 21 of 43 Positron (Beta + ) Emission  Occurs when neutron to proton ratio is too low ( p +  n + e + )  Emits a positron (beta particle whose charge is positive)  Results in emission of 2 gamma rays (more on this later)

3/2003 Rev 1 I.2.6 – slide 22 of 43 Positron (Beta + ) Emission

3/2003 Rev 1 I.2.6 – slide 23 of 43 Positron Decay

3/2003 Rev 1 I.2.6 – slide 24 of 43 Positron Decay

3/2003 Rev 1 I.2.6 – slide 25 of 43 Positron Decay

3/2003 Rev 1 I.2.6 – slide 26 of 43 Positron Annihilation

3/2003 Rev 1 I.2.6 – slide 27 of 43 Orbital Electron Capture  Also called K Capture  Occurs when neutron to proton ratio is too low  Form of decay competing with positron emission  One of the orbital electrons is captured by the nucleus: e -1 + p +1  n  Results in emission of characteristic x-rays

3/2003 Rev 1 I.2.6 – slide 28 of 43 Orbital Electron Capture

3/2003 Rev 1 I.2.6 – slide 29 of 43 Orbital Electron Capture

3/2003 Rev 1 I.2.6 – slide 30 of 43 radiation path ejected electron +1 ionized atomIonization

3/2003 Rev 1 I.2.6 – slide 31 of 43 characteristic x-rays X-Ray Production electronejected electron fills vacancy

3/2003 Rev 1 I.2.6 – slide 32 of 43 Electromagnetic Spectrum x- and  -rays Infra-redUltra-violet Visible Increase in wavelength : decrease in frequency and energy

3/2003 Rev 1 I.2.6 – slide 33 of 43 Gamma Ray Emission  Monoenergetic radiations emitted from nucleus of an excited atom following radioactive decay  Rid nucleus of excess energy  Have characteristic energies which can be used to identify the radionuclide  Excited forms of radionuclides often referred to as “metastable”, e.g., 99m Tc. Also called “isomers”

3/2003 Rev 1 I.2.6 – slide 34 of 43 Gamma Radiation Gamma Ray Emission

3/2003 Rev 1 I.2.6 – slide 35 of 43 Gamma Ray Emission

3/2003 Rev 1 I.2.6 – slide 36 of 43 Photon Emission DifferenceBetween X-Rays and Gamma Rays

3/2003 Rev 1 I.2.6 – slide 37 of 43 Internal Conversion  Alternative process by which excited nucleus of a gamma emitting isotope rids itself of excitation energy  The nucleus emits a gamma ray which interacts with an orbital electron, ejecting the electron from the atom  Characteristic x-rays are emitted as outer orbital electrons fill the vacancies left by the conversion electrons

3/2003 Rev 1 I.2.6 – slide 38 of 43 Internal Conversion  These characteristic x-rays can themselves be absorbed by orbital electrons, ejecting them.  These ejected electrons are called Auger electrons and have very little kinetic energy

3/2003 Rev 1 I.2.6 – slide 39 of 43 Internal Conversion

3/2003 Rev 1 I.2.6 – slide 40 of 43 Internal Conversion Electron emitted about 10% Internal Conversion 137 Cs Emits Betas x = 0.85 Gamma Ray emitted during 85% of 137 Cs transitions

3/2003 Rev 1 I.2.6 – slide 41 of 43 Summary of Radioactive Decay Mechanisms DecayModeCharacteristics of Parent Radionuclide Change in Atomic Number (Z) Change in Atomic Mass Comments Alpha Neutron Poor -2-4 Alphas Monoenergetic Beta Neutron Rich +10 Beta Energy Spectrum Positron Neutron Poor 0 Positron Energy Spectrum ElectronCapture Neutron Poor 0 K-Capture; Characteristic X-rays Emitted GammaExcited Energy State NoneNone Gammas Monoenergetic Internal Conversion Excited Energy State NoneNone Ejects Orbital Electrons; characteristic x-rays and Auger electrons emitted

3/2003 Rev 1 I.2.6 – slide 42 of 43 Summary  Basic atomic structure was described  Isotopes were defined  Modes of radioactive disintegration were discussed (including alpha, beta, gamma, positron emission, orbital electron capture, and internal conversion)  Ionization was defined  X-ray production and the differences between gamma rays and x-rays were described

3/2003 Rev 1 I.2.6 – slide 43 of 43 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)