Transcript Gamma Spectroscopy

HPT001.204
Rev. 1
Page 1 of 80
Gamma Spectroscopy
HPT001.204
TP-1
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Health Physics (RADCON) Initial Training Program
HPT001.204
Rev. 1
Page 2 of 80
Enabling Objectives - 1
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Purpose of Gamma-Ray Spectroscopy
Techniques for Measuring Photon Energy
Mechanisms of Interaction
Energy Range for Each Mechanism
Minimum Energy for Pair Production
Radiation Energy Measurement Unit
TP-2
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HPT001.204
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Enabling Objectives - 2
• Nuclear Decay Schemes
• Scintillation Detector Materials
• Components of Gamma Spectroscopy
System
• NaI(Tl) Detector Shapes
• Features of Gamma-Ray Spectrum
TP-3
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HPT001.204
Rev. 1
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Enabling Objectives - 3
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Differentiate Between Energy Efficiencies
Calculate Energy Resolution
Background Subtraction
Identify Nuclides in Gamma Spectrum
Materials Used in Solid State Detectors
TP-4
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Gamma Rays
• Form of Electromagnetic Radiation
• Originate in Atoms
• Fixed Energy Values
TP-5
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HPT001.204
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Cs-137 Gamma Energy
TP-6
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HPT001.204
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Purpose of
Gamma Ray Spectroscopy
• Measure:
Energy of Photons
Number of Photons
• Differentiate Photons by Energy
TP-7
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Most Common Technique
for Measuring Photon Energy
• Scintillation Material
Creates Light Pulses
Proportional to Photon Energy
TP-8
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Gamma Interactions
• Photoelectric Absorption
• Compton Scattering
• Pair Production
TP-9
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Interaction Processes
TP-10
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Photoelectric Absorption
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Photoelectric Absorption
• Most Predominant Process for X-Ray
Absorption
• Typical Energies ≤ 200 keV
• Enhanced for High Z Materials
TP-12
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Compton Scattering
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Compton Scattering
• Predominately 200-500 keV
• Decreases with Increasing Gamma
Energy
• Photon Still in Existance
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Pair Production
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Pair Production
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• Requires at Least 1.022 MeV Gamma
• Most > 5 MeV
• May Result in Annihilation Radiation
TP-16
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Electron Energy
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• Measured in Electron Volts, eV
Electron Vole - Energy Given to an
Electron by Accelerating it Through 1 Volt
of Electric Potential Difference.
• 1 eV = 1.602 * 10-19 Joules
• Usually Given in: keV (kilo eV), or
MeV (mega eV)
TP-17
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Electron Energy Equation
• E = h ν, Where,
E = Photon Energy, eV
h = Plank’s Constant, 4.135*10-15 eV-sec
ν = Frequency, sec-1
TP-18
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Wavelength of Photon
• λ = 1.24 * 10-6/E, Where
λ = Wavelength, meters
E = Photon Energy, eV
TP-19
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Decay Schemes
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• Most Decay by Electron (Beta) Emission
• Beta Creates Excited State in Daughter
• Gamma Released to Return Excited State
to Stable
TP-20
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Cs-137 Decay Scheme
TP-21
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Co-60 Decay Scheme
TP-22
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Scintillation Detectors
• Zinc Sulfide
Alpha
• Anthracene Crystals
Beta
• NaI(Tl)
Gamma
TP-23
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NaI(Tl)
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• Excellent Light Yield
• Nearly Linear Response
• Hygroscopic (Absorbs Water – Requires
Sealing)
TP-24
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Gamma Ray Spectroscopy
TP-25
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Photomultiplier (PM) Tube
TP-26
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Crystal Shapes
• Solid Right Cylinder
Excellent Light Collection
Accommodates Various Sample
Geometries
• Well Crystal
Efficiency can Approach 100 %
Uniform Counting Efficiencies
Excellent for Small Samples
TP-27
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Solid Crystal with
Marinelli Beaker
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Solid and Well Crystals
Solid Crystal
TP-29
Well Crystals
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Components of a Gamma
Spectroscopy System
• Detector – Detects the Radiation Energy
• Photocathode–Converts Light to Electrons
• Photomultiplier Tube – Multiplies the
Number of Pulses
• Amplifier – Amplifies & Shapes the Pulses
• Analog-to-Digital Convert (ADC) Converts
the Pulses to Numbers
• Multi-Channel Analyzer (MCA) – Sorts and
Stores the Pulses & Their Amplitudes
TP-30
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MCA Output
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• Numbers Correlate with the Activity
• Amplitudes Correlate with Gamma
Energy
• Expectation: Single Vertical Line at the
Energy of the Gamma Ray
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Gaussian Distribution
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• Symmetrical about the mean
• One  includes 68.3% of area under
curve
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Ideal Gamma Ray Spectrum
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Spectrum
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Energy Resolution - Diagram
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The Gamma Spectrum
• Distribution of the Data (Resolution)
• Energy Resolution, “R”
R = (FWHM * 100)/Ho, where
FWHM = Full Width, Half Max, (eV)
Ho = Location of Peak Centroid (eV)
TP-36
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Energy Resolution
• Assume FWHM = 50 keV
• Ho = 662 keV,
then,
• R = (50 * 100)/622 = 7.55 %
• For NaI(Tl), R Typically 5-10 %
TP-37
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The Gamma Spectrum
Compton Scattering
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100000
photopeak
10000
counts/channel
backscatter
peak
Compton continuum
1000
Compton
edge
100
10
1
0
64
128
192
256
320
384
MCA channel number
TP-38
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448
512
The Gamma Spectrum
X-Ray Escape Peaks
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Photoelectric Absorption near Surface
X-Rays Escape Detector
Results in Loss of Energy
Most Prominent at Low Incidence
Gamma-Ray Energies
TP-39
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The Gamma Spectrum
Annihilation Radiation
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• Occurs When Positron Emitted
• Positron Expends Energy & Combines
with an Electron
• Two 0.511 MeV Photons Emitted from
the Annihilation of the Positron
• Adds a Peak at the 0.511 MeV Line
TP-40
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The Gamma Spectrum
Bremsstrahlung
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• Many Gamma Interactions Release Beta
Particles
• Beta Particles Create Secondary
Bremsstrahlung
• These Radiations Added to the
Continuum
TP-41
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The Gamma Spectrum
Backscatter
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• Results from Compton Scattering in
Shielding Material
• Some Gamma Rays Reflected Back into
the Detector
• Usually ≤ 0.25 MeV
TP-42
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The Gamma Spectrum
Characteristic X-Rays
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• Photoelectric Absorption in Shield Material
• Generates Characteristic X-Ray From the
Shield Material
• Especially Prominent with High Z Material
• Compensate by Graded Shielding:
High Z Material for Primary Shielding
Low Z Material for Lining of Shielding
TP-43
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The Gamma Spectrum
Summation Effects
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• Occurs When Two Photons are Counted
at the Same Time
• Detector Sees them as One Event
• Energy is the Sum of the Two Photon
Energies
• Results in Sum Peak
(Example, Co-60 & Sc-46)
TP-44
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Sum Peak, Sc-46
TP-45
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The Gamma Spectrum
Background Radiation
• Terrestrial
• Cosmic
• Laboratory
• Example: K-40
TP-46
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HPT001.204
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Potassium-40 Spectrum
TP-47
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Single Channel
Analyzer Spectrum
LLD
TP-48
ULD
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Components of a
Gamma Spectrum
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Photopeak
Backscatter Peaks
Compton Continuum
Compton Edge
Annihilation Peak
Escape Peaks
Sum Peaks
Bremsstrahlung
Background Peaks
TP-49
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Spectrum with Components
TP-50
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Detector Efficiency
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• Measured Efficiency from Known Activity
1. Same or Similar Geometry as Unknown
2. Separate Efficiency for Each Geometry
• Calculated Efficiency Based on
Knowledge of Gamma Ray Interactions
TP-51
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Detector Efficiency –
Solid Right Cylinder
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Detector Efficiency –
Well Crystal
TP-53
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Detector Efficiency
Critical Data
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• Category of Efficiency
1. Absolute – Based on Radiation Emitted
2. Intrinsic – Based on Radiation Incident
on Detector
• Size & Shape of Crystal
Example – Solid or Well
• Geometry of Sample
• Absorption Between Source & Detector
TP-54
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Peak Area Determination
A
B
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Peak Area DeterminationEquation
B
• Peak Area = ∑ Ci ,
i=A
Where,
Ci = Number of Counts in ‘i’ Channel
A & B = Channels Marking Either Side of
the Peak Area
TP-56
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Peak Area Determination –
Background Subtraction
B
• Peak Area = ∑ Ci – (B - A)[CA + CB)/2]
i=A
Where,
A & B = Channels Marking Either Side of
the Peak Area
Ci = Number of Counts in ‘i’ Channel
CA = Counts in Channel ‘A’
CB = Counts in Channel ‘B’
TP-57
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Background Subtraction Spectrum Stripping
• Accumulate a Background Spectrum
• Subtract Background Channel Counts
from Respective Sample Channel Counts
• Analyze Resulting Spectrum Photopeak
TP-58
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I-131 Spectrum,
With Background
TP-59
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Background Spectrum
TP-60
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HPT001.204
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I-131 Spectrum,
Background Subtracted
TP-61
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Identifying Gamma Emitters
Unknown Spectrum
TP-62
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Identifying Gamma Emitters
Solution Spectrum
TP-63
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Comparison of NaI(Tl)
and GeLi Spectra
TP-64
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HPT001.204
Rev. 1
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Other Detectors –
Advantages
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• Solid State
GeLi
HPGe
• More Efficient
• Compact Size
• Fast Timing Characteristics
• Effective Thickness can be Varied to
Match Requirements of Applications
TP-65
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Other Detectors –
Disadvantages
• Must be Cooled to Liquid Nitrogen
Temperatures
GeLi – Continuously Cooled
HPGe – Cooled Only While in Use
TP-66
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Rev. 1
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Practical Exercises
Problem # 1
TP-67
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Practical Exercises
Problem # 1 - Solution
TP-68
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Practical Exercises
Problem # 2
TP-69
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Practical Exercises
Problem # 2 - Solution
TP-70
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Practical Exercises
Problem # 3
TP-71
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Practical Exercises
Problem # 3 -Solution
TP-72
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Practical Exercises
Problem # 4
TP-73
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Practical Exercises
Problem # 4 - Solution
TP-74
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Practical Exercises
Problem # 5
TP-75
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Practical Exercises
Problem # 5 - Solution
TP-76
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Summary - 1
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• Gamma Rays:
1. Form of Electromagnetic Radiation
2. Energy Levels from 0.01 to 10 MeV
3. Measured by Scintillation Detectors
4. Interactions with Matter:
a. Photoelectric Absorption
b. Compton Scattering
c. Pair Production
TP-77
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Summary - 2
HPT001.204
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Page 78 of 80
• Gamma Ray Spectrum: Components
1. Photopeak
2. Backscatter Peak
3. Compton Continuum
4. Annihilation Peaks
5. Escape Peaks
6. Sum Peaks
7. Bremsstrahlung
8. Characteristic X-Rays
9. Background Radiation
TP-78
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Summary - 3
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• Gamma Ray Detection Efficiency:
1. Absolute – Based on Radiation
Released from the Source
2. Intrinsic – Based on Radiation Incident
on the Detector
TP-79
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Summary - 4
HPT001.204
Rev. 1
Page 80 of 80
• Other Gamma Ray Detector Systems
1. GeLi
2. HPGe
• Advantages
1. Compact
2. More Efficient
3. Faster
• Disadvantage – Operate at Liquid Nitrogen
Temperatures
TP-80
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