Transcript Session I206 Disintegration - International Atomic Energy Agency

Session I.2.6
Part I Review of Fundamentals
Module 2 Basic Physics and Mathematics
Used in Radiation Protection
Session 6 Modes of Radioactive Decay and
Types of Radiation
3/2003 Rev 1
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources
I.2.6 – slide 1 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
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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
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Overview
 Fundamental atomic structure will be
described
 Modes of radioactive disintegration
and types of radiation emitted will be
discussed
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Atomic Structure
proton
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neutron
electron
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Atomic Number (Z)
Hydrogen
Carbon
Cobalt
Selenium
Iridium
Uranium
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1
6
27
34
77
92
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Isotopes
An isotope of an element has:
 the same number of protons
 a different number of neutrons
1H
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2H
3H
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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.
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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
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The Line of Stability
N>Z
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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
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Alpha Particle Decay
Alpha particle
charge +2
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Alpha Particle Decay
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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
88
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
222Rn
86
+ 42He
I.2.6 – slide 14 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)
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Beta Particle Decay
Beta particle
charge -1
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Beta Particle Decay
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Beta Decay of 99Mo
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Beta Spectrum
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Rule of Thumb
Average energy of a beta spectrum is about
one-third of its maximum energy or:
Eav =
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1
3
Emax
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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)
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Positron (Beta+) Emission
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Positron Decay
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Positron Decay
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Positron Decay
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Positron Annihilation
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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
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Orbital Electron Capture
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Orbital Electron Capture
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Ionization
ionized
atom
+1
ejected
electron
-1
radiation
path
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X-Ray Production
electron fills
vacancy
electron
ejected
characteristic
x-rays
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Electromagnetic Spectrum
x- and -rays
Ultraviolet
Visible
Infrared
Increase in wavelength : decrease in frequency and energy
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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”
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Gamma Ray Emission
Gamma Radiation
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Gamma Ray Emission
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Photon Emission
Difference
Between
X-Rays and
Gamma Rays
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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
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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
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Internal Conversion
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Internal Conversion
137Cs
Emits
Betas
Gamma
Ray emitted
during 85%
of 137Cs
transitions
0.946 x 0.898 = 0.85
Internal
Conversion
Electron
emitted
about 10%
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Summary of Radioactive Decay Mechanisms
Decay
Mode
Characteristics
of Parent
Radionuclide
Change in
Atomic Number
(Z)
Change in
Atomic
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
Electron
Capture
Neutron Poor
-1
0
K-Capture; Characteristic
X-rays Emitted
Gamma
Excited
Energy State
None
None
Gammas Monoenergetic
Internal
Conversion
Excited
Energy State
None
Ejects Orbital Electrons;
characteristic x-rays and
Auger electrons emitted
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None
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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
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Where to Get More Information
 Cember, H., Introduction to Health Physics, 3rd
Edition, McGraw-Hill, New York (2000)
 Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds.,
Table of Isotopes (8th 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)
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