Chapter 2. Radiation

1
Chapter 2. Radiation

• Radioactivity
• 2.Radiation interaction with Matter
• 3.Radiation Doses and hazard Assessment
•  

2
2.1 Radioactivity

• Overview
• Types of Radioactive Decay
• Energetics of Radioactive Decay
• Characteristics of Radioactive Decay
• Decay Dynamics
• Naturally Occurring Radionuclides

3
2.2 Radiation interaction with Matter

1. overview
2. Photon Interactions
3. Neutron Interactions
4. Interaction of Heavy Charged Particles with
   Matter
5. Scattering of Electrons in a Medium

4
Radiation is everywhere
1) overview
Cosmic
Inhaled Radon
Bodies
Plants
Radioactive Elements
We live in a sea of radiation
Rocks
5
NCRP National Council on Radiation Protection and
Measurements
6
Discovery of Ionization by Radiation
X-rays and radioactivity discharged a charged
electroscope. Curie and Rutherford attributed the
discharge to the ionization of air by these rays.
7
1) overview
8
directly ionizing radiation
indirectly ionizing radiation
9
Interaction of Photons with Matter
Photon Energies Visible red light 1.5
eVvisible blue light 3.0 eV UV few
eV-hundreds eV X-rays 1 to 60 keV Gamma
rays keV - some MeV
Interactions of gamma rays with
matter photoelectric effect Compton effect Pair
productions
10
Photoelectric process
KEh?-EB
11
Compton Effect of Gamma Rays
When a photon transfers part of its energy to an
electron, and the photon becomes less energetic
is called Compton effect.
12
Pair Production of Gamma Rays
Gamma photons with energy greater than 1.02 MeV
produce a electron-positron pair is called pair
production.
13
Gamma-ray Three Modes of Interaction with Matter
Photoelectric effect Compton scattering pair
production
14
(No Transcript)
15
Attenuation of Gamma Rays by Matter
Gamma-ray intensity decreases exponentially as
the thickness of the absorber increases. I Io
eµx I Intensity at distance xµ absorption
constantx thickness
16
Average Travel Distance Before An Interaction
the interaction probability P(x) that a particle
interacts somewhere along a path of length x is
The probability th that a particle does not
interact while traveling a distance x
p(x)dx be the probability that a particle
interacts for the first time between x and x dx.
17
the average distance the average distance such a
particle travels before it interacts.
mean-free-path length
Half-Thickness the thickness of a medium
required for half of the incident radiation to
undergo an interaction
the thickness of a medium required for half of
the incident radiation to undergo an interaction
18
Absorption of neutrons
Elastic scattering
neutron collides with proton (e.g. hydrogen
nucleus) and shares its kinetic energy dominant
process with fast neutrons of energy lt 6 MeV in
tissue
19
Absorption of neutrons
Inelastic scattering
fast neutron ( 6 MeV and above) interacts with
nucleus and causes disintegration
with the atomic nuclei
20
Neutrons lose very little energy per collision
when they collide with heavy nuclei. Nuclei of
hydrogen and neutrons have approximately the same
mass. In collisions with hydrogen nuclei,
neutrons can transfer almost all their kinetic
energy to the hydrogen nuclei. Thus,
hydrogen-containing compounds such as H2O,
paraffin wax, and hydrocarbons (oil and grease)
slow down neutrons rapidly.
21
Thermal Neutrons Cross Sections
Uranium for Fission Fuel in Nuclear Reactor
113Cd 233U 235U 238U ? c /b 19,820 46
98 2.7 ?f /b 530 580 2.710-6 t1/2/y 1
.6105 7108 4.5109
22
Thermal Neutrons Cross Sections
Cross section (?) a measure of reaction
probabilityThermal neutron cross sections
(?c)Thermal neutron cross section for fission
(?f)
1H 2H 12C 14N 16O 113Cd ? c
/b 0.33 0.00052 0.0034 1.82 0.0002 19,820
Moderators H2O vs. D2O vs. C
23
Thermal Neutrons Cross Sections
The extremely large thermal neutron cross section
of 113Cd makes cadmium a good neutron absorber or
eliminator.
Neutrons Capture Cross Sections of Cadmium
Isotopes 106Cd 108Cd 110Cd 111Cd 112Cd 113Cd 11
4Cd ? c / b 1 1 0.1 24 2.2 19,820 0.3
Abundance/ 1.25 0.89 12.45 12.80 24.13 12.22 28.
37
the neutron-capture reaction 113Cd (n, ?) 114Cd
leads to a stable isotope. These properties made
cadmium a very desirable material for the nuclear
technology industry.
24
ConclusionSlow neutrons (0.03 to 0.001 eV) are
more effective for inducing fission of 235U Fast
neutrons (10 MeV to 10 KeV) favours neutron
capture reaction of 238U Light atoms are
effective moderators
25
4) Interaction of Heavy Charged Particles with
Matter
Fast moving protons, 4He, and other nuclei are
heavy charged particles. Coulomb force dominates
charge interaction. They ionize and excite (give
energy to) molecules on their path.
The Born-Bethe Formula for Energy Loss of
Charged Particles.
26
Range of Heavy Charged Particles in a Medium
Particles lose all their energy at a distance
called range.
27
Scattering of Electrons in a Medium
Fast moving electrons are light charged
particles. They travel at higher speed., but
scattered easily by electrons.
28
Range of Light Charged Particles in a Medium
Range of b particles is not as well defined as
heavy charged particles, but measured range is
still a useful piece of information.
29
Braking Radiation of b particles Influenced by
Atom
Bremsstrahlung (braking) radiation refers to
photons emitted by moving electrons when they are
influence by atoms.
30
Interaction of Beta particles with Matter
Beta particles interact with matter mainly via
three modes Ionization (scattering by
electrons) Bremsstrahlung (braking) radiation
Annihilation with positrons
31
Example At what energy does an electron moving
through gold lose as much energy by
bremsstrahlung as it does by ionizing and
exciting gold atoms?
For gold Z 79 and for equal energy loss by both
mechanisms, we have find for electrons M me
that E 700/79 8.9 MeV.
32
Stopping power (dE/ds)/p in mass units (MeV
cm2/g) for protons and electrons.
33
Range or path length pR, in mass units (g/cm2),
in the continuous slowing down approximation.
34
aß?ioization radiation
a ß ? ionizing
process D D
I track
Straight Defle Straight ionization
Large medium
Small Penetration weak
medium long
2 MeV range(m) ion pairs/mm a 0.01
6000 ß 2-3 60 ? 10
1
air
35
????
36

• 2.1 Two-body collisions
• Formula
• Tacit assumptions
• Well defined Z1
• Independent two body collisions
• Stochastic process, average E.L.
• 2.2 Collisions with atoms
• Elastic and inelastic energy loss
• 2.3 Adiabatic cutoff
• Momentum approximation free
• Harmonic model free bounded
• 2.4 Under which circumstances is
• classical mechanics applicable

37
????
INCIDENT ION BEAM




38
?1-1 ??-??????
39
(No Transcript)
40
????????
b collision diameter Closest distance in
repulsive potential
41
????




42
2.2 Collisions with atoms Elastic and
inelastic energy loss

• Elastic
• moving the center of the mass of the
  atom-- nuclei
• Inelastic
• leading to excitation of internal degrees
  of
• freedom--electrons

43
(No Transcript)
44
????

electrons feels a constant force during
collision time
45
?????
y the distance of the electron away from the
equilibrium position
??? y0
???? mÿ-m?2yK 0t t
mÿ1 - m?2y1
?
46
??????
?tltlt1
free
?tgtgt1
?t2
47
(No Transcript)
48
2.4 Under which circumstances is classical
mechanics applicable
49
(No Transcript)
50
(No Transcript)
51
(No Transcript)
52

• ? ? ?? ,
  ???
• ?Lindhard?

53
TUNNELING (WKB??)
b?????, ???E????????
54
2.2 Radiation interaction with Matter

1. overview
2. Photon Interactions
3. Neutron Interactions
4. Attenuation of Charged Particles