OBJ 1 – Radioactivity &
Radioactive Decay
1
Chart of the Nuclides
• General Layout
– Each nuclide occupies a square in a grid
where
• Atomic number (Z) is plotted vertically
• Number of neutrons (N) is plotted horizontally
– Heavily bordered square at left side of each
row gives
•
•
•
•
•
Name
Chemical symbol
Elemental Mass
Thermal neutron absorptions cross section
Resonance integral
2
Chart of the Nuclides
– Nuclides on diagonal running from upper left
to lower right have same mass numbers,
called isobars
– Colors and shading used to indicate in chart
squares used to indicate relative magnitude
of
• Half-lives
• Neutron absorption properties
– Four different colors used
•
•
•
•
Blue
Green
Yellow
Orange
3
Chart of the Nuclides
– Background color of upper half of square
represents T1/2
– Background color of lower half of square
represents greater of the thermal neutron
cross section or resonance integral
– When nuclide is stable and thermal neutron
cross section is small or unknown, entire
square is shaded grey
– Gray shading also used for unstable nuclides
having T1/2 sufficiently long (>5E8 yrs) to
have survived from the time they were
formed
4
Chart of the Nuclides
– Some squares, such as 60Co, 115In, and 116In
are divided
• Occurs when nuclide has one or more isomeric or
metastable states
• Has same A and Z, but different nuclear and
radioactive properties due to different energy
states of the same nucleus
5
Chart of the Nuclides
• Nuclide Properties Displayed on the
Chart
– Chemical Element Names and Symbols
• Same element names and symbols as used on the
Periodic Table of the Elements
– Atomic Weights and Abundances
• Isotopic masses in AMUs are given for
– Stable isotopes
– Certain long-lived, naturally occurring radioactive
isotopes
– Nuclides particle decay becomes a prominent mode
(>10%)
6
Chart of the Nuclides
– Isotopic Abundance
• Values on chart given in atom percent
• Specified for 288 nuclides (266 stable and 22
radioactive)
– Half-lives
• Half-life listed below nuclide symbol and mass
number
• Units used
–
–
–
–
–
–
–
–
–
pspicoseconds (1E-12 s)
nsnanoseconds (1E-9 s)
µsmicroseconds (1E-6 s)
ms
milliseconds (1E-3 s)
s seconds
m minutes
h hours
d days
a years
7
Chart of the Nuclides
– Background Color of Chart Square Upper
Half
•
•
•
•
1 day to 10 day  orange
>10 days to 100 days  yellow
>100 days to 10 years  green
>10 years to 5E8 years  blue
– Background Color of Chart Square Lower
Half
• Refers to thermal neutron cross section or
resonance integral
8
Chart of the Nuclides
– Major Modes of Decay and Decay Energies
•
•
•
•
•
•
•
•
•
•
•
•
•
•
α
ββ+
γ
n
p
d
t
ε
IT
eβ-βD
alpha particle
beta minus (negatron)
beta plus (positron)
gamma ray
neutron
proton
deuteron
triton
electron capture
isomeric transition
conversion electron
double beta decay
cluster decay
delayed radiation
9
Chart of the Nuclides
– To understand decay schemes
and energies, look at chart
square for 38Cl
• β- energies listed on 1st line in
order of abundance
• γ energies listed on 2nd line in
order of abundance
– Particle energies always given in
MeV
– γ energies always given in keV
10
Chart of the Nuclides
– When more than one decay
mode possible, modes listed on
chart in order of abundance or
intensity
– Different modes of beta decay
(ε, β+, β-) appear on separate
lines if intensity of one of the
decay modes is <10% absolute
intensity
– Conversely, appear on same line
if intensities of both >10%
absolute intensity with most
abundant listed first.
11
Chart of the Nuclides
– When branching decay occurs by both
β- and β+ and/or ε, and each decay is
accompanied by γ emission, format
shown in 146Pm square is used
– Metastable (or isomeric) state
frequently decays to ground by IT γ
emission, followed by one or more γ
in cascade
– Internal conversion is process
resulting from interaction between
nucleus and extra-nuclear electrons.
Nuclear excitation energy xfr’d to
orbital electron (usually K shell) and is
indicated by e12
Chart of the Nuclides
– Delayed γ emission indicated by symbol D.
• When daughter product has too short of a half-life
to have its own spot on the chart or half-life is
much shorter than that of the parent nuclide, γ
energy is listed with the parent
13
Chart of the Nuclides
14
Chart of the Nuclides
15
Radiation Classifications
• Introduction
– All radiation possesses energy
• Inherent — electromagnetic
• Kinetic — particulate
– Interaction results in some or all of energy
being transferred to surrounding medium
• Scattering
• Absorption
16
Radiation Classifications
• Ionizing or Non-Ionizing
– Non-Ionizing
• Visible light
• Radio and TV
– Ionizing
• Particulate or Photonic
– Particulate
• α
• β
• n
– Electromagnetic
• γ
• x
17
Radiation Classifications
• Directly or Indirectly Ionizing
– Directly Ionizing
• Possesses charge
• Does not need physical contact
– Indirectly Ionizing
• Does not have charge
• Needs physical contact
18
Radiation Characteristics
• Alpha (α)
– Charge – +2
– Range – 2-4 in. (5 – 10 cm.)
– Shielding
• Paper
• Dead skin
– Hazard – Internal
– Target Organ – Anything internal (living
tissue)
19
Radiation Characteristics
• Beta (β)
– Charge
• Negatron (β-) – -1
• Positron (β+) – +1
– Range
• Average – ≈ 10 ft.
• Energy Specific – ≈ 10 – 12 ft./MeV
– Shielding
• Plastic
• Wood
• Al, Cu
Low Z
– Hazard – Internal
– Target Organ
• External – eye (lens)
• Living tissue
20
Radiation Characteristics
• Gamma (γ) and X-Ray (x)
– Charge – 0
– Range – ≈ Infinite
– Shielding High Z
• Pb
• DU
– Hazard – Internal
– Target Organ – Living tissue
21
Radiation Characteristics
• Neutron (n)
– Charge – 0
– Range – ≈ Infinite
– Shielding Hydrogenous
• H20
• Concrete
• Plastic
– Hazard – Internal
– Target Organ – Living tissue
22
Energy Transfer Mechanisms
• Ionization
– Removing bound e- from electrically neutral
atom or molecule by adding sufficient energy
to allow it to overcome its BE
– Atom has net positive charge
– Creates ion pair consisting of negatively
charged electron and positively charged
atom or molecule
23
Energy Transfer Mechanisms
Ionizing Particle
eNegative Ion
N
P+
Positive Ion
P+
N
e-
24
Energy Transfer Mechanisms
• Excitation
– Process that adds sufficient energy to e- such
that it occupies higher energy state than
lowest bound energy state
– Electron remains bound to atom
– No ions produced, atom remains neutral
– After excitation, excited atom eventually
loses excess energy when e- in higher energy
shell falls into lower energy vacancy
– Excess energy liberated as X-ray, which may
escape from the material, but usually
undergoes other absorptive processes
25
Energy Transfer Mechanisms
P+
N N
P+
e-
e+
N
N
N
+
N
e-
+
N
+
e-
e-
26
Energy Transfer Mechanisms
• Bremsstrahlung
– Radiative energy loss of moving charged particle as it interacts
with matter through which it is moving
– Results from interaction of high-speed, charged particle with
nucleus of atom via electric force field
– With negatively charged electron, attractive force slows it
down, deflecting from original path
– KE particle loses emitted as x-ray
– Production enhanced with high-Z materials (larger coulomb
forces) and high-energy e- (more interactions occur before all
energy is lost)
27
Energy Transfer Mechanisms
e-
ee-
e+
N
N
N
+
N
+
N
+
e-
e-
28
Directly Ionizing Radiation
• Charged particles don’t need physical
contact with atom to interact
– Coulombic forces act over a distance to cause
ionization and excitation
– Strength of these forces depends on:
• Particle energy (speed)
• Particle charge
• Absorber density and atomic number
• Coulombic forces significant over distances >
atomic dimensions
• For all but very low physical density
materials, KE loss for e- continuous because
of Coulombic force
29
Directly Ionizing Radiation
• Alpha Interactions
– Mass approximately 8K times > electron
– Travels approximately 1/20th speed of light
– Because of mass, charge, and speed, has high
probability of interaction
– Does not require particles touching—just
sufficiently close for Coulombic forces to interact
– Energy gradually dissipated until α captures two
e- and becomes a He atom
– α from given nuclide emitted with same energy,
consequently will have approximately same
range in a given material
30
Directly Ionizing Radiation
• Beta Interactions
– Interaction between β- or β+ and an orbital e- is
interaction between 2 charged particles of similar mass
– βs of either charge lose energy in large number of
ionization and/or excitation events, similar to α
– Due to smaller size/charge, lower probability of
interaction in given medium; consequently, range is >> α
of comparable energy
– Because β’s mass is small compared with that of nucleus
• Large deflections can occur, particularly when low-energy βs
scattered by high-Z elements (high positive charge on the
nucleus)
• Consequently, β usually travels tortuous, winding path in an
absorbing medium
– β may have Bremsstrahlung interaction resulting in X-rays
31
•
•
•
•
Indirectly Ionizing Radiation
No charge
γ and n
No Coulomb force field
Must come sufficiently close for physical
dimensions to contact particles to
interact
32
Indirectly Ionizing Radiation
• Small probability of interacting with matter –
Why?
– Doesn’t continuously lose energy by constantly
interacting with absorber
– May move “through” many atoms or molecules
before contacting electron or nucleus
– Probability of interaction depends on its energy
and absorber’s density and atomic number
– When interactions occur, produces directly
ionizing particles that cause secondary
ionizations
33
Indirectly Ionizing Radiation
• Gamma absorption
– γ and x-rays differ only in origin
– Name used to indicate different source
• γs originate in nucleus
• X-rays are extra-nuclear (electron cloud)
– Both have 0 rest mass, 0 net electrical
charge, and travel at speed of light
– Both lose energy by interacting with matter
via one of three major mechanisms
34
Indirectly Ionizing Radiation
• Photoelectric Effect
– All energy is lost – happens or doesn’t
– Photon imparts all its energy to orbital e– Because pure energy, photon vanishes
– Probable only for photon energies < 1 MeV
– Energy imparted to orbital e- in form of KE,
overcoming attractive force of nucleus,
usually causing e- to leave orbit with great
velocity
– Most photoelectrons are inner-shell e35
Indirectly Ionizing Radiation
– High-velocity e-, called photoelectron
• Directly ionizing particle
• Typically has sufficient energy to cause secondary
ionizations
– Most photoelectrons are inner-shell
electrons
36
Indirectly Ionizing Radiation
eGamma Photon
(< 1 MeV)
Photoelectron
e-
e+
N
N
N
+
N
+
N
+
e-
e-
37
Indirectly Ionizing Radiation
• Compton Scattering
– Partial energy loss for incoming photon
– Dominant interaction for most materials for photon
energies 200 keV – 5 MeV
– Photon continues with less energy in different
direction to conserve momentum
– Probability of Compton interaction  with distance
from nucleus — most Compton electrons are
valence electrons
– Beam of photons may be randomized in direction
and energy, so that scattered radiation may appear
around corners and behind shields where there is no
direct line of sight to source
– Probability of Compton interaction  with distance
from nucleus — most Compton electrons are
valence electrons
38
Indirectly Ionizing Radiation
• Pair Production
– Occurs when all photon energy is converted to
mass (occurs only in presence of strong electric
field, which can be viewed as catalyst)
– Strong electric fields found near nucleus and are
stronger for high-Z materials
– γ disappears in vicinity of nucleus and β-- β+ pair
appears
– Will not occur unless γ > 1.022 MeV
– Any energy > 1.022 MeV shared between the β-β+ pair as KE
– Probability < photoelectric and Compton
interactions because photon must be close to
the nucleus
39
Indirectly Ionizing Radiation
Electron
eGamma Photon
(E > 1.022 MeV)
ee+
e+
N
N
N
+
N
e-
+
Positron
N
+
e-
e0.511 MeV Photons
e-
e-
40
Indirectly Ionizing Radiation
• Neutron Interactions
– Free, unbound n unstable and disintegrates
by β- emission with half-life of ≈ 10.6
minutes
– Resultant decay product is p+, which
Category
Energy
eventually
combines with
freeRange
e- to become
Thermal
~ 0.025 eV (< 0.5 eV)
H atom
Intermediate energy0.5
eV–10 keV –classified
– n interactions
dependent
Faston KE
10 keV–20 MeV
based
Relativistic
> 20 MeV
41
Indirectly Ionizing Radiation
• Classifying according to KE important from
two standpoints:
– Interaction with the nucleus differs with n
energy
– Method of detecting and shielding against
various classes are different
• n detection relatively difficult due to:
– Lack of ionization along their paths
– Negligible response to externally applied
electric, magnetic, or gravitational fields
– Interact primarily with atomic nuclei, which are
extremely small
42
Indirectly Ionizing Radiation
• Slow Neutron Interactions
– Radiative Capture
• Radiative capture with γ emission most common
for
A slow1n
A1 *
A1
 ZD 
Z P 0 n
Z D   nuclei
• Reaction
often results
in radioactive
• Process is called neutron activation
43
Indirectly Ionizing Radiation
– Charged Particle Emission
• Target atom absorbs a slow n, which  its mass
and internal energy
• Charged particle then emitted to release excess
mass and energy
• Typical examples include (n,p), (n,d), and (n,α).
For example
A
Z
X  n
1
0
A1
Z
D 
*
A 4
Z 2
D 
4
2
44
Indirectly Ionizing Radiation
– Fission
• Typically occurs following slow n absorption by
several of the very heavy elements
• Nucleus splits into two smaller nuclei, called
primary fission products or fission fragments
• Fission fragments usually undergo radioactive
decay to form secondary fission product nuclei
• There are some 30 different ways fission may
take place with the production of about 60
primary fission fragments
45
Indirectly Ionizing Radiation
• Fast Neutron Interactions
– Scattering
• Free n continues to be free n following interaction
• Dominant process for fast n
– Elastic Scattering
• Occurs when n strikes nucleus of approx. same mass
• Neutron can xfer much of its KE to that, which recoils
off with energy lost by n
• No γ emitted by nucleus
• Recoil nucleus can be knocked away from its e- and,
being (+) charged, can cause ionization and excitation
46
Indirectly Ionizing Radiation
e-
N
PN+
47
Indirectly Ionizing Radiation
– Inelastic Scattering
• Occurs when n strikes large nucleus
– n penetrates nucleus for short period of time
– Xfers energy to nucleon in nucleus
– Exits with small decrease in energy
• Nucleus left in excited state, emitting γ radiation,
which can cause ionization and/or excitation
48
Indirectly Ionizing Radiation
e-
e-
γ
P+
N
N
N
N
P+
P+
N
ee-
49
Indirectly Ionizing Radiation
• Reactions in Biological Systems
– Fast n lose energy in soft tissue largely by
repeated scattering interactions with H
nuclei
– Slow 0n1 captured in soft tissue and release
energy in one of two principal mechanisms:
and
1
0
1
0
n H  H  H  
1
1
2
1
*
2
1
n N  N  C  p  
14
7
15
7
*
14
6
1
1
(2.2 MeV)
(0.66 MeV)
50
Radioactivity and Radioactive
Decay nucleus is usually more
– Following a transformation,
–
–
–
–
stable than it was, but not necessarily stable
Another transformation will take place by nucleus
emitting radiation
Amount of energy given off and emission type
depends on nucleus’ configuration immediately
before transformation
As nucleus’ energy , nucleus disintegrates or
decays
Called radioactive decay
• Atom before decay—parent
• Atom after decay—daughter
– Steps from parent to daughter traced to stability
called decay chain
51
Radioactivity and Radioactive
Decay
• Parent-Daughter
Relationships and
Equilibrium
– Produces daughter product and radiation is
emitted
– Daughter also produces radioactivity when it
decays, as does each successive daughter until
stability is reached
– Activity contributed by the parent vs. daughters
varies based on half-life of both parent and
daughters
– When activity production rate is same as
product decay rate, equilibrium is said to exist
52
Radioactivity and Radioactive
Decay
– Secular Equilibrium
– Τ1/2,P >> Τ1/2,D (Parent half-life infinitely >
daughter)
– As parent activity , daughter  proportionately
– During 10 half-lives of the daughter, essentially no
parent decay takes place during secular
equilibrium
– Two conditions necessary
Rule
Thumb
– Parent must have
Τ ofmuch
longer than any other nuclide
1/2
in the series
Secular–equilibrium
is reached
in ≈ must
6 daughter
half-lives.
Sufficiently long
period of time
have elapsed
to
allow for in-growth of the decay products
53
Radioactivity and Radioactive
Decay
54
Radioactivity and Radioactive
Decay
Τ1/2 = 53 m
Τ1/2 = 15.3 m
Τ1/2 = 6.57 h
55
Radioactivity and Radioactive
Decay
– Transient Equilibrium
– Τ1/2,P > Τ1/2,D (Parent half-life > daughter, but not
infinitely)
– Daughter activity decays at same approx. rate as
parent
– Different way of saying – daughter atom formation
rate = daughter atom decay rate
– Same fractional decrease in parent and daughter
Rule of Thumb
activities
Transient equilibrium is reached in ≈ 4 daughter half-lives.
56
Radioactivity and Radioactive
Decay
57
Radioactivity and Radioactive
– Transient—Τ Decay
>Τ
, but not very long
1/2,P
1/2,D
Τ1/2 = 1.68 d
Τ1/2 = 12.75 d
58
Radioactivity and Radioactive
Decay
– No Equilibrium
– Τ1/2,P < Τ1/2,D
– Parent activity decays at faster rate than daughter
– Equilibrium is never reached
59
Radioactivity and Radioactive
Decay
60
Radioactivity and Radioactive
Decay
Τ1/2 = 3.1 m
Τ1/2 = 27 m
Τ1/2 = 19.9 m
Τ1/2 = 23.3 y
61
Decay Modes and Emissions
• Alpha Decay (α)
– With few exceptions, only relatively heavy
nuclides decay by α emission
– Essentially a helium nucleus (2 p+, 2 n)
– Charge of +2
62
Decay Modes and Emissions
p
n
63
Decay Modes and Emissions
• Beta (Negatron) Decay (β-)
– High n:p ratio usually β- decays
– n changed into p
–  n:p ratio, results in β- emission
– Have same mass as e– Because n has been replaced by p, Z  1, but
A remains unchanged
64
Decay Modes and Emissions
– Because n has been replaced by p, Z  1, but
A remains unchanged
- decay is:
– Standard
notation
for
β
A
A

Z
X Z 1 X   
210Bi as
– For 210
example, 210Pb210
β- decays to produce

follows:
82
83
Pb Bi   
65
Decay Modes and Emissions
p
n
66
Decay Modes and Emissions
– Neutrinos and anti-neutrinos—neutral
particles with negligible rest mass
– Travel at speed of light and are noninteracting
– Account for energy distribution among β+
(positrons) and β- (negatrons)
67
Decay Modes and Emissions
– Nuclide having low n:p ratio) tends to decay
by positron emission
– Positron often mistakenly thought of as
positive electron
– In reality, positron is anti-particle of electron
(has charge of +1)
– β+ used to designate positrons
– With positron emitters, parent nucleus
changes p+ into n and emits a β+
– Because p+ replaced by n, Z  1 and A
remains unchanged
– Neutrino also emitted during β+ emission
68
Decay Modes and Emissions
– Standard notation for β+ decay is:
A
Z
X
A
Z 1
X   

– For example, 57Ni β+ decays to produce 57Co as
follows: 57
57

28
Ni27 Co   
69
Decay Modes and Emissions
p
n
70
Decay Modes and Emissions
• Electron Capture (EC)
– For radionuclides with low n:p ratio, another
decay mode, known as EC, can occur
– Nucleus captures e- (usually from K shell)
– Could capture L-shell electron, but K-electron
capture much more probable
– Decay frequently referred to as K-capture
– Can result in formation of Auger e• In lieu of characteristic X-ray being emitted
• Atom ejects bound e• Auger e- are monoenergetic
71
Decay Modes and Emissions
– Transmutation resembles positron emission
A
Z
X
A
Z 1
X 
– Electron combines with p+ to form a n,
followed by neutrino emission
– Electrons from higher energy levels fill
vacancies left in inner, lower-energy shells
– Excess energy emitted causes cascade of
characteristic X-rays
72
Decay Modes and Emissions
• Gamma Emission (γ)
– Decay resulting in transmutation generally
leaves nucleus in excited state
– Nucleus can reach unexcited, or ground,
state by emitting γ
– Gammas are type of electromagnetic
radiation—behave as small bundles or
packets of energy, called photons, and travel
at speed of light
73
Decay Modes and Emissions
– γ essentially the same as X-ray
• γ usually higher energy (MeV); whereas, X-rays
usually in keV range
• Basic difference between γ and X-ray is origin—γ
originate in nucleus, X-rays originate in electron
A
*
A
shells
Z X Z X  
• General decay equation slightly different from
others
– Most decay reactions have γ emissions
associated with them
– Some decay by particulate emission with no
74
Decay Modes and Emissions
• Isomeric Transition (IT)
– Commonly occurs immediately after particle
emission
– Nucleus may remain in excited state for
measurable period of time before dropping
to ground state
– Nucleus that remains excited known as
isomer because it is in a metastable state
– Differs in energy and behavior from other
nuclei with the same Z and A
– Generally achieves ground state by emitting
delayed γ (usually > 10-9 s)
75
Decay Modes and Emissions
• Internal Conversion
– An alternative isomeric mechanism to
radiative transition
– Excited nucleus of γ-emitting atom gets rid of
excitation energy
– Tightly bound e- (K or L) interacts with
nucleus by absorbing Eexcitation and is ejected
– Electron known as conversion electron
– Distinguished from β- by energy
• Conversion e- − monoenergetic
• β- − spectrum of energies
76
Decay Modes and Emissions
• Each radionuclide, artificial or natural, has
characteristic decay pattern
• Several aspects associated with pattern:
– Decay modes
– Emission types
– Emission energies
– Decay rate
77
Decay Modes and Emissions
• All nuclei of given radionuclide seeking
stability decay in specific manner
– 226Ra decays by α emission, accompanied by
γ—only decay mode open to 226Ra
– Some nuclides may decay with branching,
where a choice of decay modes exists
– Some nuclides may decay with branching,
where a choice of decay modes exists
•
57Ni,
mentioned previously, decays 50% by EC (K
capture) and 50% by β+ emission
– Nuclides decay in constant manner by
emission types, and emissions from each
nuclide exhibit distinct energy picture
78
Decay Modes and Emissions
• Single Ra nucleus may disintegrate at
once or wait 1000s of years before
emitting an α
– All that can be predicted with certainty is 1/2
of all 226Ra nuclei present will disintegrate in
1,622 years
– Called the half-life
– Half-lives vary greatly for naturally occurring
radioisotopes
79
Natural Decay Series
• Natural Decay Series
– Uranium, radium, and thorium occur in three natural
decay series, headed by uranium-238, thorium-232,
and uranium 235, respectively
– In nature, in secular equilibrium
80
Natural Decay Series
• Uranium-238 (Radon-222) (Radon)
81
Natural Decay Series
• Uranium-235 (Radon-219) (Actinon)
82
Natural Decay Series
• Thorium-232 (Radon-220) (Thoron)
83
Radioactive Decay Law
• Radioactive Decay Law
– Decays at a fixed rate and is not a function of
temperature, pressure, etc.
– Half-life defined as amount of time it takes for
activity to be reduced to 1/2 the original value
– Occurs at an exponential rate
84
Radioactive Decay Law
85
Radioactive Decay Law
– Can be expressed as (½)n or e-λt.
– When calculating half-life, units of time (t) must be
the same time units as the half-life (Τ½)
• Radioactive Decay Formula
At  A0 (1 / 2)
n
or
At  A0 e
 t
• Decay Constant (λ)
– Equivalent to natural log of 2 (ln 2) divided by halflife (Τ1/2)
86