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Ionizing Radiation radioactivity measurements

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Ionizing Radiation radioactivity measurements High energy particles and photons that ionise atoms and molecules along their tracks in a medium are called ionizing ... – PowerPoint PPT presentation

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Title: Ionizing Radiation radioactivity measurements


1
Ionizing Radiationradioactivity 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.
2
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.
3
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 e- He 54 eV ? He2 e-
Ionization energy (IE eV) per ion pair of some
substancesMaterial Air Xe He NH3 Ge-crystalAver
age IE 35 22 43 39 2.9
4
Primary and Secondary Ion Pairs
Primary and secondary ion Pairs
ooooooooooooooooo oooooooooo?oooooo
oooooooo-ooooooo oooooo-ooooooooo
oooo-ooooooooooo oo-ooo-oooooooo
-ooooooooooooooo ?ooooooooooo-oooo
ooooooooooooooooo
Primary ion pairs are caused directly by
radiation. Secondary ion pairs are generated by
high-energy primary electrons. Molecular density
(molecules/mL)air 2.7e19water 3.3e22
5
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.
6
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.
The surges of ion density before they stop give
the Bragg peaks.
7
Range of Heavy Charged Particles in a Medium
Particles lose all their energy at a distance
called range.
8
Range of Heavy Charged Particles in a Medium
The range can be used to determine the energy of
the particles and the radiation source.
9
Speed of Particles
Speed of 1 MeV a particle 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
Speed of 1 MeV ( 1.6e-13 J) electron 1.6e-13 J
(1/2) m v2 (½) 9.1e-31 kg v2Solving for v,
v2 3.52e17 (m/s)2or v 5.9e8 m/s.
exceeds c (3e8 m/s), the speed limitProper
evaluation method shown next
Still reasonable
10
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 Einsteins equation
m mo / ?(1-(v/c)2)
This speed is a 80 of c, the speed of light.
11
Scattering of Electrons in a Medium
Fast moving electrons are light charged
particles. They travel at higher speed., but
scattered easily by electrons.
12
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.
13
Braking Radiation of b particles Influenced by
Atom
Bremsstrahlung (braking) radiation refers to
photons emitted by moving electrons when they are
influence by atoms.
14
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
15
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
16
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.
17
The animated Compton effect
18
Pair Production of Gamma Rays
Gamma photons with energy greater than 1.02 MeV
produce a electron-positron pair is called pair
production.
19
Gamma-ray Three Modes of Interaction with Matter
Photoelectric effect Compton scattering pair
production
20
Attenuation of Gamma Rays by Matter
Gamma-ray intensity decreases exponentially as
the thickness of the absorber increases. I Io
ec x I Intensity at distance xc absorption
constantx thickness
21
Ionizing Radiation Measurements
Radiation Detectors - overall view electroscopesi
onization chambersproportional
countersGeiger-Muller counters solid-state
detectors photographic films and photographic
emulsion plates bubble chambers and cloud
chambers scintillation counters and fluorescence
screen
22
Ionization Chambers
Current (A) is proportional to charges collected
on electrode in ionization chambers.
23
Proportional Counters
Gas Multiplication ? ?
?

X00 V
Proportional counters Gas multiplication due to
secondary ion pairs when the ionization chambers
operate at higher voltage.
24
Geiger-Muller Counters
1X00 V
Every ionizing particle causes a discharge in the
detector of G-M counters.
25
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 - - P - N
zone - -
Negative
Positive
electronic counter
See bo.iasf.cnr.it/ldavinci/programme/Presentazio
ni/Harrison_cryo.pdf
26
A simple view of solid-state detectors
Solid-state detectors are usually made from
germanium or cadmium-zinc-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.
27
Scintillation Counters

Photons cause the emission of a short flash in
the Na(Tl)I crystal.The flashes cause the
photo-cathode to emit electrons.
28
Scintillation Detector and Photomultiplier tube
29
Gamma ray spectrum of 207mPb (half-life 0.806
sec)

207mPb Decay Scheme
13/2____________1633.4 keV -
Intensity (log scale)
1063 -1e4 569
5/2-____________569.
7 keV 1063 -1e3
569
1/2-____________0.0 stable -1e2
-10 569
1063 -1
Energy
?-rayspectrum of 207mPb
30
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.
31
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.
32
Image Recorded in Bubble Chambers
Charge exchange of antiproton produced
neutron-antineutron pair. p p ? n n (no
tracks) Annihilation of neutron-antineutron pair
produced 5 pions. n n ? 3p 2p- ? Only
these tracks are sketched.
33
Bubble Chambers
?The Brookhaven 7-foot bubble chamberand the
80-inch bubble chamber ?
34
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 ?0 ? p ?
35
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.
36
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.
37
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.
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