Transcript Ionizing Radiation radioactivity measurements

Ionizing Radiation
radioactivity measurements
High energy particles and photons that ionise atoms and molecules
along their tracks in a medium are called ionizing radiation. For
example, a, b, g, cosmic rays and X-rays are ionizing radiation.
Most radioactive measurement are based on their ionizing effect.
Ionizing radiation causes illness such as cancer and death.
Radiation effect is a health and safety concern.
Ionizing radiation can also be used in industry for various purposes.
Light and microwaves that do not ionize atoms and molecules are
called non-ionizing radiation.
Ionizing Radiation
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Discovery of Ionization by Radiation
Electroscopes
X-rays and radioactivity
discharged a charged
electroscope. Curie and
Rutherford attributed the
discharge to the ionization
of air by these rays.
Charged
Ionizing Radiation
Discharged
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Ionization Energy of Gases
The minimum energy required
to remove an outer electron
from atoms or molecules is
called ionization potential.
Ionizing radiation also remove
electrons in atomic inner shell,
and the average energy per
ion pair is considered
ionization energy
He + 25 eV  He+ + eHe+ + 54 eV  He2+ + e-
Ionization energy (IE eV) per ion pair of some substances
Material
Air
Xe
He
NH3
Ge-crystal
Average IE
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22
43
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2.9
Ionizing Radiation
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Primary and Secondary Ion Pairs
Primary ion
pairs are caused
directly by radiation.
Secondary ion
pairs are generated
by high-energy primary
electrons.
Molecular density
(molecules/mL)
air = 2.7e19
water = 3.3e22
Primary and secondary ion Pairs
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Ionizing Radiation
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Interaction of Heavy Charged Particles with
Matter
Sketch of Alpha Particle Paths in a Medium
Fast moving protons, 4He,
and other nuclei are
heavy charged particles.
Coulomb force dominates
charge interaction.
a source
Shield
They ionize and excite
(give energy to) molecules
on their path.
Ionizing Radiation
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Energy Loss of Heavy Charged Particles in Matter
Stopping power is the rate of energy loss per unit length along the path.
Stopping power is proportional to the mass number A, and to the square
of atomic number, Z2, of a medium, but inversely proportional to the energy
of the particle E.
Ion Pair Density Along the Path of Heavy
Charged Particles in a Medium
The surges of
ion density
before they
stop give the
Bragg peaks.
Ion pairs
density
a particle
proton
Distance along the path
Ionizing Radiation
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Range of Heavy Charged Particles in a Medium
Variation of a Intensity as a Function of Thickness
Detector
Intensity
Range
Absorber
straggling
Particles lose
all their energy
at a distance
called range.
a source
a source
thickness
Ionizing Radiation
Shield
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Range of Heavy Charged Particles in a Medium
Ranges as Functions of Energy
Variation of a Intensity as a Function of Thickness
Detector
Intensity
100 mg/cm2
Range
Absorber
straggling
10
thickness
Range
of
protons
0.1
The range can be used to
determine the energy of the
particles and the radiation
source.
Range
of a
1
0.1
a source
1
10/MeV
Ionizing Radiation
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Speed of Particles
Speed of 1 MeV (= 1.6e-13 J) electron
Speed of 1 MeV a particle
1.6e-13 J = (1/2) m v2
= (½) 9.1e-31 kg v2
Solving for v,
v2 = 3.52e17 (m/s)2
or
v = 5.9e8 m/s.
1.6e-13 J = (½) m v2
= (½)(41.66e-27 kg) v2
Solving for v
v2 = 4.82e13 (m/s)2
v = 6.9e6 m/s
exceeds c (=3e8 m/s), the speed limit
Proper evaluation method shown next
Ionizing Radiation
Still reasonable
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Proper Evaluation of Particle Speed
The relativity mass m of a particle of kinetic energy Ek is the
sum of the rest mass and its kinetic energy
m = Ek MeV + 0.51 MeV (rest mass of electron)
For an electron with Ek = 1.0 MeV, m = 1.51 MeV
The speed of an electron with a kinetic energy 1.0 MeV is
evaluated by applying the Einstein’s equation:
m = mo / (1-(v/c)2)
me
0.51
v  (1  ) c  (1 
)3 108  2.4 108 m/s
m
1.51
This speed is a 80% of c, the speed of light.
Ionizing Radiation
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Scattering of Electrons in a Medium
An Imaginary Path of a b particle in
a Medium
Fast moving electrons are
light charged particles.
They travel at higher speed.,
but scattered easily by
electrons.
a source
Shield
Ionizing Radiation
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Range of Light Charged Particles in a Medium
Intensity (I ) of Electrons with the Same Kinetic Energy
as a Function of Thickness (x) of Absorber.
detector
I
I0
Extrapolated
range
I
x absorber
Range
straggling
I0
Variation of a Intensity as a Function of Thickness
x
Range of b particles is not as well defined
as heavy charged particles, but measured
range is still a useful piece of information.
Ionizing Radiation
Detector
Intensity
Range
Absorber
a source
straggling
thickness
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Braking Radiation of b particles Influenced by
Atom
Bremsstrahlung Radiation and its
Feynmann Diagram
E=hv
.h v
e–
Feynmann
diagram
Bremsstrahlung (braking) radiation refers to photons emitted by moving
electrons when they are influence by atoms.
Ionizing Radiation
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Interaction of Beta particles with Matter
Ionization
Annihilation
Beta particles interact with
matter mainly via three
modes:
Ionization (scattering by electrons)
Bremsstrahlung (braking) radiation
Annihilation with positrons
Braking radiation
Ionizing Radiation
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Interaction of Photons with Matter
Photon Energies
Visible red light
visible blue light
UV
X-rays
1.5 eV
3.0 eV
few eV-hundreds eV
1 to 60 keV
Gamma rays keV - some MeV
Interactions of gamma
rays with matter:
photoelectric effect
Compton effect
Pair productions
Ionizing Radiation
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Compton Effect of Gamma Rays
Spectra of an Original and Scattered X-rays
at a Particular Fixed Angle.
Intensity
arbitrary
scale
Original
spectrum
When a photon transfers
part of its energy to an
electron, and the photon
becomes less energetic is
called Compton effect.
Feynman Diagram for
the Compton Effect

scattered
spectrum

Ionizing Radiation
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The animated Compton effect
Ionizing Radiation
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Pair Production of Gamma Rays
Feynman Diagram for Pair Production
A nucleus or field.
A negative charge in reverse is
equivalent to a plus charge.
Gamma photons with energy greater than 1.02 MeV produce a
electron-positron pair is called pair production.
Ionizing Radiation
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Gamma-ray Three Modes of Interaction with
Matter
Interaction of Photons with Matter
Photoelectric
Pair
production
Compton scattering
1
Photoelectric effect
5/MeV
Compton scattering
Ionizing Radiation
pair production
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Attenuation of Gamma Rays by Matter
Intensity of Parallel Gamma Rays as a
Function of Absorber Thickness.
Gamma-ray intensity
decreases exponentially
as the thickness of the
absorber increases.
Intensity, I
I = Io e–c x
I: Intensity at distance x
c: absorption constant
x: thickness
Thickness x
Ionizing Radiation
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Ionizing Radiation Measurements
Radiation Detectors - overall view
electroscopes
ionization chambers
proportional counters
Geiger-Muller counters
solid-state detectors
photographic films and photographic emulsion plates
bubble chambers and cloud chambers
scintillation counters and fluorescence screen
Ionizing Radiation
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Ionization Chambers
Key Components in a Simple Ionization Chamber
Ionizing
radiation
Battery
+
Load
resister
–+–+–
+–+–+
–
Detector
chamber
Amperemeter
Current (A) is proportional to charges collected on electrode in
ionization chambers.
Ionizing Radiation
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Proportional Counters
Gas Multiplication
–+

–+–+–+

–+–+–+–+–+–+–+–+–+

–+–+–+–+–+–+–+–+–+–
+–+–+–+–+–+–+–+–+–+–
+–+–+–+–+–+–+–+–+–+–
+–+–+–+–+–+–+
Key Components in a Simple Ionization Chamber
Ionizing
radiation
Battery
+
Load
–+–+–
+–+–+
X00 V
resister
–
Detector
chamber
Amperemeter
Proportional counters
Gas multiplication due to
secondary ion pairs when the
ionization chambers operate at
higher voltage.
Ionizing Radiation
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Key Components in a Simple Ionization Chamber
Geiger-Muller Counters
Working Components of a Geiger Muller Counter
Ionizing
radiation
Battery
+
1X00 V
Load
resister
–+–+–
+–+–+
–
Geiger-Muller Counter:
Pulse counting electronics
Detector
chamber
Amperemeter
Dead Time in Pulse Counting
–
Dead time
+
1500 V
supplier
Detector
Source
Every ionizing particle causes a discharge in the detector of G-M counters.
Ionizing Radiation
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Solid-state Detectors
A P-N junction of semiconductors placed under reverse bias has no current
flows. Ionizing radiation enters the depleted zone excites electrons causing
a temporary conduction. The electronic counter register a pulse
corresponding to the energy entering the solid-state detector.
+ + depleted
Negative P +
+ +
-- N Positive
zone
--
electronic
counter
Ionizing Radiation
See: bo.iasf.cnr.it/ldavinci/programme/Presentazioni/Harrison_cryo.pdf
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A simple view of solid-state detectors
Solid-state detectors are usually made from germanium or cadmiumzinc-telluride (CdZnTe, or CZT) semiconducting material. An incoming
gamma ray causes photoelectric ionization of the material, so an
electric current will be formed if a voltage is applied to the material.
Digirad has developed and made commercially available the world's
first solid-state, digital gamma camera for the nuclear medicine
imaging market. Our proprietary, solid-state imaging technology is
based on a patented, silicon photodiode technology that replaces the
vacuum photomultipier tubes (PMTs) used in all other gamma
cameras. These photodiodes are coupled to individual scintillation
crystals to create a unique detection element for each addressable
spatial location of the camera's head. We call this Digital Position
Sensing™ technology. It provides images with excellent contrast and
spatial resolution.
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Ionizing Radiation
Scintillation Counters
The Key Components of a Typical Scintillation Counter
Na(Tl)I
crystal
X- or
g rays
Thin Al
window
Photocathode
High voltage
supplier and
multi-channel
analyzer /
computer
system
Photomultiply tube
Photons cause the emission of a
short flash in the Na(Tl)I crystal.
The flashes cause the photo-cathode
Radiation
toIonizing
emit electrons.
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Scintillation
Detector
and
Photomultiplier
tube
Ionizing Radiation
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Gamma ray spectrum of 207mPb (half-life 0.806 sec)
207mPb
13/
- Intensity (log scale)
Decay Scheme
2+____________1633.4
keV
g-ray
spectrum
of 207mPb
1063
-1e4
569
5/
2-____________569.7
keV
1063
-1e3
569
-1e2
-10
-1
1/ -____________0.0
2
stable
569 + 1063
Ionizing Radiation
Energy
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Fluorescence Screens
Fluorescence materials absorb invisible energy and emit
visible light.
J.J. Thomson used fluorescence screens to see electron tracks in
cathode ray tubes. Electrons strike fluorescence screens on computer
monitors and TV sets give dots of visible light.
Röntgen saw the shadow of his skeleton on fluorescence screens.
Rutherford observed alpha particle on scintillation material zinc sulfide.
Fluorescence screens are used to photograph X-ray images using
films sensitive visible light.
Ionizing Radiation
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Cloud and Bubble Chambers
The ion pairs on the tracks
of ionizing radiation form
seeds of gas bubbles and
droplets. Formations of
droplets and bubbles
provide visual appearance
of their tracks, 3-D
detectors.
C.T.R. Wilson shared the
Nobel prize with Compton
for his perfection of cloud
chambers.
Photographing the Particle Tracks
radia
tion
Cloud or bubble chamber
Ionizing Radiation
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Image Recorded in Bubble Chambers
Charge exchange of
antiproton produced neutronantineutron pair.
A Sketch of the Tracks of Charge Exchange
and Antineutron-Proton Annihilation.
antiproton
p + p  n + n (no tracks)
–
Charge
exchange
Annihilation of neutronantineutron pair produced 5
pions.
n +n  3+ + 2- + ?
Only these tracks are
sketched.
+
Ionizing Radiation
Antineutronneutron
annihilation
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Bubble Chambers
The Brookhaven 7-foot bubble chamber
and
the 80-inch bubble chamber 
Ionizing Radiation
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Image from
bubble
chamber
This image shows a historical
event: one of the eight beam
particles (K- at 4.2 GeV/c)
which are seen entering the
chamber, interacts with a
proton, giving rise to the
reactions
K– p  – K+ K0
K0  + –
–  0 K–
K+  + 0
Ionizing Radiation
 0  p –
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Photographic Emulsions and Films
Sensitized silver bromide grains of emulsion develope
into blackened grains. Plates and films are 2-D
detectors.
Roentegen used photographic plates to record X-ray image.
Photographic plates helped Beckerel to discover radioactivity.
Films are routinely used to record X-ray images in medicine but
lately digital images are replacing films.
Stacks of films record 3-dimensional tracks of particles.
Photographic plates and films are routinely used to record
images made by electrons.
Ionizing Radiation
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Overall View
Ionizing radiation interacts with matter in various ways: ionization
(photoelectric effect), excitation, braking radiation, Compton effect,
pair production, annihilation etc.
Mechanisms of interaction are utilized for the detection of ionizing
radiation.
Function and principles of electroscope, ionization chambers,
proportional chambers, Geiger-Muller counters, solid-state
detectors, and scintillation counters, bubble chambers, and cloud
chambers have been describe.
Ionizing Radiation
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The Sudbury Neutrino
Observatory
SNO will contain 1000 tonnes of heavy
water, held in a 12-m diameter
spherical acrylic vessel. It has the
ability to detect all three types of
neutrinos.
Ionizing Radiation
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