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Gamma Spectroscopy

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Gamma Spectroscopy

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  1. Gamma Spectroscopy HPT001.204 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  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 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  3. Enabling Objectives - 2 • Nuclear Decay Schemes • Scintillation Detector Materials • Components of Gamma Spectroscopy System • NaI(Tl) Detector Shapes • Features of Gamma-Ray Spectrum TVAN Technical Training Health Physics (RADCON) Initial Training Program

  4. Enabling Objectives - 3 • Differentiate Between Energy Efficiencies • Calculate Energy Resolution • Background Subtraction • Identify Nuclides in Gamma Spectrum • Materials Used in Solid State Detectors TVAN Technical Training Health Physics (RADCON) Initial Training Program

  5. Gamma Rays • Form of Electromagnetic Radiation • Originate in Atoms • Fixed Energy Values TVAN Technical Training Health Physics (RADCON) Initial Training Program

  6. Cs-137 Gamma Energy TVAN Technical Training Health Physics (RADCON) Initial Training Program

  7. Purpose of Gamma Ray Spectroscopy • Measure:Energy of PhotonsNumber of Photons • Differentiate Photons by Energy TVAN Technical Training Health Physics (RADCON) Initial Training Program

  8. Most Common Techniquefor Measuring Photon Energy • Scintillation MaterialCreates Light PulsesProportional to Photon Energy TVAN Technical Training Health Physics (RADCON) Initial Training Program

  9. Gamma Interactions • Photoelectric Absorption • Compton Scattering • Pair Production TVAN Technical Training Health Physics (RADCON) Initial Training Program

  10. Interaction Processes TVAN Technical Training Health Physics (RADCON) Initial Training Program

  11. Photoelectric Absorption TVAN Technical Training Health Physics (RADCON) Initial Training Program

  12. Photoelectric Absorption • Most Predominant Process for X-Ray Absorption • Typical Energies ≤ 200 keV • Enhanced for High Z Materials TVAN Technical Training Health Physics (RADCON) Initial Training Program

  13. Compton Scattering TVAN Technical Training Health Physics (RADCON) Initial Training Program

  14. Compton Scattering • Predominately 200-500 keV • Decreases with Increasing Gamma Energy • Photon Still in Existance TVAN Technical Training Health Physics (RADCON) Initial Training Program

  15. Pair Production TVAN Technical Training Health Physics (RADCON) Initial Training Program

  16. Pair Production • Requires at Least 1.022 MeV Gamma • Most > 5 MeV • May Result in Annihilation Radiation TVAN Technical Training Health Physics (RADCON) Initial Training Program

  17. Electron Energy • Measured in Electron Volts, eVElectron 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) TVAN Technical Training Health Physics (RADCON) Initial Training Program

  18. Electron Energy Equation • E = h ν, Where,E = Photon Energy, eVh = Plank’s Constant, 4.135*10-15 eV-secν = Frequency, sec-1 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  19. Wavelength of Photon • λ = 1.24 * 10-6/E, Whereλ = Wavelength, metersE = Photon Energy, eV TVAN Technical Training Health Physics (RADCON) Initial Training Program

  20. Decay Schemes • Most Decay by Electron (Beta) Emission • Beta Creates Excited State in Daughter • Gamma Released to Return Excited State to Stable TVAN Technical Training Health Physics (RADCON) Initial Training Program

  21. Cs-137 Decay Scheme TVAN Technical Training Health Physics (RADCON) Initial Training Program

  22. Co-60 Decay Scheme TVAN Technical Training Health Physics (RADCON) Initial Training Program

  23. Scintillation Detectors • Zinc Sulfide Alpha • Anthracene Crystals Beta • NaI(Tl) Gamma TVAN Technical Training Health Physics (RADCON) Initial Training Program

  24. NaI(Tl) • Excellent Light Yield • Nearly Linear Response • Hygroscopic (Absorbs Water – Requires Sealing) TVAN Technical Training Health Physics (RADCON) Initial Training Program

  25. Gamma Ray Spectroscopy TVAN Technical Training Health Physics (RADCON) Initial Training Program

  26. Photomultiplier (PM) Tube TVAN Technical Training Health Physics (RADCON) Initial Training Program

  27. Crystal Shapes • Solid Right CylinderExcellent Light CollectionAccommodates Various Sample Geometries • Well CrystalEfficiency can Approach 100 %Uniform Counting EfficienciesExcellent for Small Samples TVAN Technical Training Health Physics (RADCON) Initial Training Program

  28. Solid Crystal withMarinelli Beaker TVAN Technical Training Health Physics (RADCON) Initial Training Program

  29. Solid and Well Crystals Solid Crystal Well Crystals TVAN Technical Training Health Physics (RADCON) Initial Training Program

  30. Components of a GammaSpectroscopy 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 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  31. MCA Output • Numbers Correlate with the Activity • Amplitudes Correlate with Gamma Energy • Expectation: Single Vertical Line at the Energy of the Gamma Ray TVAN Technical Training Health Physics (RADCON) Initial Training Program

  32. Gaussian Distribution 38 • Symmetrical about the mean • One  includes 68.3% of area under curve TVAN Technical Training Health Physics (RADCON) Initial Training Program

  33. Ideal Gamma Ray Spectrum TVAN Technical Training Health Physics (RADCON) Initial Training Program

  34. Spectrum TVAN Technical Training Health Physics (RADCON) Initial Training Program

  35. Energy Resolution - Diagram TVAN Technical Training Health Physics (RADCON) Initial Training Program

  36. The Gamma Spectrum • Distribution of the Data (Resolution) • Energy Resolution, “R”R = (FWHM * 100)/Ho, whereFWHM = Full Width, Half Max, (eV) Ho = Location of Peak Centroid (eV) TVAN Technical Training Health Physics (RADCON) Initial Training Program

  37. Energy Resolution • Assume FWHM = 50 keV • Ho = 662 keV, then, • R = (50 * 100)/622 = 7.55 % • For NaI(Tl), R Typically 5-10 % TVAN Technical Training Health Physics (RADCON) Initial Training Program

  38. The Gamma SpectrumCompton Scattering Compton edge TVAN Technical Training Health Physics (RADCON) Initial Training Program

  39. The Gamma SpectrumX-Ray Escape Peaks • Photoelectric Absorption near Surface • X-Rays Escape Detector • Results in Loss of Energy • Most Prominent at Low Incidence Gamma-Ray Energies TVAN Technical Training Health Physics (RADCON) Initial Training Program

  40. The Gamma SpectrumAnnihilation Radiation • 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 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  41. The Gamma SpectrumBremsstrahlung • Many Gamma Interactions Release Beta Particles • Beta Particles Create Secondary Bremsstrahlung • These Radiations Added to the Continuum TVAN Technical Training Health Physics (RADCON) Initial Training Program

  42. The Gamma SpectrumBackscatter • Results from Compton Scattering in Shielding Material • Some Gamma Rays Reflected Back into the Detector • Usually ≤ 0.25 MeV TVAN Technical Training Health Physics (RADCON) Initial Training Program

  43. The Gamma SpectrumCharacteristic 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 ShieldingLow Z Material for Lining of Shielding TVAN Technical Training Health Physics (RADCON) Initial Training Program

  44. The Gamma SpectrumSummation 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) TVAN Technical Training Health Physics (RADCON) Initial Training Program

  45. Sum Peak, Sc-46 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  46. The Gamma SpectrumBackground Radiation • Terrestrial • Cosmic • Laboratory • Example: K-40 TVAN Technical Training Health Physics (RADCON) Initial Training Program

  47. Potassium-40 Spectrum TVAN Technical Training Health Physics (RADCON) Initial Training Program

  48. Single ChannelAnalyzer Spectrum LLD ULD TVAN Technical Training Health Physics (RADCON) Initial Training Program

  49. Components of a Gamma Spectrum • Photopeak • Backscatter Peaks • Compton Continuum • Compton Edge • Annihilation Peak • Escape Peaks • Sum Peaks • Bremsstrahlung • Background Peaks TVAN Technical Training Health Physics (RADCON) Initial Training Program

  50. Spectrum with Components TVAN Technical Training Health Physics (RADCON) Initial Training Program