1. Gamma Spectroscopy HPT001.204 TVAN Technical Training
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2. Enabling Objectives - 1 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
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3. Enabling Objectives - 2 Nuclear Decay Schemes
Scintillation Detector Materials
Components of Gamma Spectroscopy System
NaI(Tl) Detector Shapes
Features of Gamma-Ray Spectrum
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4. Enabling Objectives - 3 Differentiate Between Energy Efficiencies
Calculate Energy Resolution
Background Subtraction
Identify Nuclides in Gamma Spectrum
Materials Used in Solid State Detectors
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5. Gamma Rays Form of Electromagnetic Radiation Originate in Atoms
Fixed Energy Values
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6. Cs-137 Gamma Energy TVAN Technical Training
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7. Purpose of Gamma Ray Spectroscopy
Measure: Energy of Photons Number of Photons
Differentiate Photons by Energy
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8. Most Common Technique for Measuring Photon Energy
Scintillation Material Creates Light Pulses Proportional to Photon Energy
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9. Gamma Interactions Photoelectric Absorption Compton Scattering
Pair Production
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10. Interaction Processes
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11. Photoelectric Absorption
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12. Photoelectric Absorption
Most Predominant Process for X-Ray Absorption
Typical Energies ≤ 200 keV
Enhanced for High Z Materials
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13. Compton Scattering TVAN Technical Training
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14. Compton Scattering Predominately 200-500 keV
Decreases with Increasing Gamma Energy
Photon Still in Existance
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15. Pair Production TVAN Technical Training
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16. Pair Production Requires at Least 1.022 MeV Gamma Most > 5 MeV
May Result in Annihilation Radiation
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17. Electron Energy
Measured in Electron Volts, eV Electron Vole - Energy Given to an Electron by Accelerating it Through 1 Volt of Electric Potential Difference.
1 eV = * Joules
Usually Given in: keV (kilo eV), or MeV (mega eV)
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18. 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
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19. Wavelength of Photon
λ = * 10-6/E, Where λ = Wavelength, meters E = Photon Energy, eV
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20. Decay Schemes Most Decay by Electron (Beta) Emission
Beta Creates Excited State in Daughter
Gamma Released to Return Excited State to Stable
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21. Cs-137 Decay Scheme TVAN Technical Training
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22. Co-60 Decay Scheme TVAN Technical Training
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23. Scintillation Detectors
Zinc Sulfide Alpha
Anthracene Crystals Beta
NaI(Tl) Gamma
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24. NaI(Tl) Excellent Light Yield Nearly Linear Response
Hygroscopic (Absorbs Water – Requires Sealing)
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25. Gamma Ray Spectroscopy
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26. Photomultiplier (PM) Tube
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27. 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
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28. Solid Crystal with Marinelli Beaker
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29. Solid and Well Crystals
Solid Crystal
Well Crystals
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30. 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
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31. MCA Output 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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32. Gaussian Distribution38
Symmetrical about the mean
One  includes 68.3% of area under curve
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33. Ideal Gamma Ray Spectrum
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34. Spectrum TVAN Technical Training
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35. Energy Resolution - Diagram
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36. 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)
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37. Energy Resolution Assume FWHM = 50 keV Ho = 662 keV, then,
For NaI(Tl), R Typically 5-10 %
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38. The Gamma Spectrum Compton Scattering
Compton edge
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39. The Gamma Spectrum X-Ray Escape Peaks
Photoelectric Absorption near Surface
X-Rays Escape Detector
Results in Loss of Energy
Most Prominent at Low Incidence Gamma-Ray Energies
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40. The Gamma Spectrum Annihilation Radiation
Occurs When Positron Emitted
Positron Expends Energy & Combines with an Electron
Two MeV Photons Emitted from the Annihilation of the Positron
Adds a Peak at the MeV Line
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41. The Gamma Spectrum Bremsstrahlung
Many Gamma Interactions Release Beta Particles
Beta Particles Create Secondary Bremsstrahlung
These Radiations Added to the Continuum
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42. The Gamma Spectrum Backscatter
Results from Compton Scattering in Shielding Material
Some Gamma Rays Reflected Back into the Detector
Usually ≤ 0.25 MeV
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43. The Gamma Spectrum Characteristic X-Rays
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
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44. The Gamma Spectrum Summation Effects
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)
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45. Sum Peak, Sc-46 TVAN Technical Training
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46. The Gamma Spectrum Background Radiation
Terrestrial
Cosmic
Laboratory
Example: K-40
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47. Potassium-40 Spectrum TVAN Technical Training
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48. Single Channel Analyzer Spectrum
LLD
ULD
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49. Components of a Gamma Spectrum
Photopeak
Backscatter Peaks
Compton Continuum
Compton Edge
Annihilation Peak
Escape Peaks
Sum Peaks
Bremsstrahlung
Background Peaks
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50. Spectrum with Components
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51. Detector Efficiency
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
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52. Detector Efficiency – Solid Right Cylinder
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53. Detector Efficiency – Well Crystal
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54. Detector Efficiency Critical Data
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
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55. Peak Area DeterminationB
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56. Peak Area Determination- EquationB
Peak Area = ∑ Ci , i=A Where, Ci = Number of Counts in ‘i’ Channel A & B = Channels Marking Either Side of the Peak Area
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57. Peak Area Determination – Background Subtraction
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’
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58. Background Subtraction - Spectrum Stripping
Accumulate a Background Spectrum
Subtract Background Channel Counts from Respective Sample Channel Counts
Analyze Resulting Spectrum Photopeak
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59. I-131 Spectrum, With Background
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60. Background Spectrum TVAN Technical Training
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61. I-131 Spectrum, Background Subtracted
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62. Identifying Gamma Emitters Unknown Spectrum
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63. Identifying Gamma Emitters Solution Spectrum
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64. Comparison of NaI(Tl) and GeLi Spectra
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65. Other Detectors – Advantages
Solid State GeLi HPGe
More Efficient
Compact Size
Fast Timing Characteristics
Effective Thickness can be Varied to Match Requirements of Applications
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66. Other Detectors – Disadvantages
Must be Cooled to Liquid Nitrogen Temperatures GeLi – Continuously Cooled HPGe – Cooled Only While in Use
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67. Practical Exercises Problem # 1
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68. Practical Exercises Problem # 1 - Solution
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69. Practical Exercises Problem # 2
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70. Practical Exercises Problem # 2 - Solution
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71. Practical Exercises Problem # 3
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72. Practical Exercises Problem # 3 -Solution
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73. Practical Exercises Problem # 4
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74. Practical Exercises Problem # 4 - Solution
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75. Practical Exercises Problem # 5
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76. Practical Exercises Problem # 5 - Solution
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77. Summary - 1
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
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78. Summary - 2
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
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79. Summary - 3
Gamma Ray Detection Efficiency: 1. Absolute – Based on Radiation Released from the Source 2. Intrinsic – Based on Radiation Incident on the Detector
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80. Summary - 4 Other Gamma Ray Detector Systems 1. GeLi 2. HPGe
Advantages 1. Compact 2. More Efficient 3. Faster
Disadvantage – Operate at Liquid Nitrogen Temperatures
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