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3/2003 Rev 1 I.2.6 – slide 1 of 43
Session I.2.6
Part I Review of Fundamentals
Module 2 Basic Physics and MathematicsUsed in Radiation Protection
Session 6 Modes of Radioactive Decay and Types of Radiation
IAEA Post Graduate Educational CourseRadiation Protection and Safety 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 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
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
Modes of radioactive de and types of radiation emitted will be discussed
3/2003 Rev 1 I.2.6 – slide 5 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 6 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 7 of 43
Alpha particle
charge +2
Alpha Particle Decay
3/2003 Rev 1 I.2.6 – slide 8 of 43
Alpha Particle Decay
3/2003 Rev 1 I.2.6 – slide 9 of 43
Alpha Decay Example
226Ra decays by alpha emission
When 226Ra 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
226Ra 222Rn + 4He28688
3/2003 Rev 1 I.2.6 – slide 10 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 11 of 43
Beta particlecharge -1
Beta Particle Decay
3/2003 Rev 1 I.2.6 – slide 12 of 43
Beta Particle Decay
3/2003 Rev 1 I.2.6 – slide 13 of 43
Beta Decay of 99Mo
3/2003 Rev 1 I.2.6 – slide 14 of 43
Beta Spectrum
3/2003 Rev 1 I.2.6 – slide 15 of 43
Rule of Thumb
Average energy of a beta spectrum is about one-third of its maximum energy or:
Eav = Emax13
3/2003 Rev 1 I.2.6 – slide 16 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 17 of 43
Positron (Beta+) Emission
3/2003 Rev 1 I.2.6 – slide 18 of 43
Positron Decay
3/2003 Rev 1 I.2.6 – slide 19 of 43
Positron Decay
3/2003 Rev 1 I.2.6 – slide 20 of 43
Positron Decay
3/2003 Rev 1 I.2.6 – slide 21 of 43
Positron Annihilation
3/2003 Rev 1 I.2.6 – slide 22 of 43
Orbital Electron Capture
Also called K Capture
Occurs when neutron to proton ratio is too low
Form of decay which competes 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 23 of 43
Orbital Electron Capture
3/2003 Rev 1 I.2.6 – slide 24 of 43
Orbital Electron Capture
3/2003 Rev 1 I.2.6 – slide 25 of 43
radiationpath
-1
ejectedelectron+1
ionizedatom
Ionization
3/2003 Rev 1 I.2.6 – slide 26 of 43
characteristicX-rays
X-Ray Production
electronejected
electron fillsvacancy
3/2003 Rev 1 I.2.6 – slide 27 of 43
Electromagnetic Spectrum
X- and -raysInfra-red
Ultra-violet Visible
Increase in wavelength : decrease in frequency and energy
3/2003 Rev 1 I.2.6 – slide 28 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., 99mTc. Also called “isomers”
3/2003 Rev 1 I.2.6 – slide 29 of 43
Gamma Radiation
Gamma Ray Emission
3/2003 Rev 1 I.2.6 – slide 30 of 43
Gamma Ray Emission
3/2003 Rev 1 I.2.6 – slide 31 of 43
Photon Emission
DifferenceBetween
X-Rays andGamma Rays
3/2003 Rev 1 I.2.6 – slide 32 of 43
Internal Conversion
Alternative process by which the 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 33 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 34 of 43
Internal Conversion
3/2003 Rev 1 I.2.6 – slide 35 of 43
Internal Conversion
Electron emitted
about 10%
Internal Conversion
137Cs Emits Betas
0.946 x 0.898 = 0.85
Gamma Ray emitted during 85%
of 137Cs transitions
3/2003 Rev 1 I.2.6 – slide 36 of 43
Summary of Radioactive Decay Mechanisms
DecayMode
Characteristicsof Parent
Radionuclide
Change in Atomic Number
(Z)
Change inAtomic Mass Comments
Alpha Neutron Poor -2 -4 Alphas Monoenergetic
Beta Neutron Rich +1 0 Beta Energy Spectrum
Positron Neutron Poor -1 0 Positron Energy Spectrum
ElectronCapture
Neutron Poor -1 0K-Capture; Characteristic
X-rays Emitted
GammaExcited
Energy StateNone None Gammas Monoenergetic
Internal Conversion
Excited Energy State
None NoneEjects Orbital Electrons; characteristic X-rays and Auger electrons emitted
3/2003 Rev 1 I.2.6 – slide 37 of 43
Summary
Modes of radioactive decay were discussed (including alpha, beta, gamma, positron emission, orbital electron capture, and internal conversion)
X-ray production and the differences between gamma rays and X-rays were described
3/2003 Rev 1 I.2.6 – slide 38 of 43
Where to Get More Information
Cember, H., Johnson, T. E., Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2008)
Martin, A., Harbison, S. A., Beach, K., Cole, P., An Introduction to Radiation Protection, 6th Edition, Hodder Arnold, London (2012)
Jelley, N. A., Fundamentals of Nuclear Physics, Cambridge University Press, Cambridge (1990)
Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8th Edition, 1999 update), Wiley, New York (1999)