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Gamma Spectrometry

Gamma Spectrometry. Office of Nuclear Security Dep3artment of Nuclear Safety and Security. Outline. Isotopes and radiation Photon interaction and spectrum formation Measurement system (basic spectroscopy system) Detector type and structure Signal utilization

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Gamma Spectrometry

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  1. Gamma Spectrometry Office of Nuclear Security Dep3artment of Nuclear Safety and Security

  2. Outline • Isotopes and radiation • Photon interaction and spectrum formation • Measurement system (basic spectroscopy system) • Detector type and structure • Signal utilization • Gamma spectrum (photo peak, Compton, SE, DE, Ann...) • The role of gamma radiation for Nuclear Security equipment • Gamma spectrometry in the field • Features of high resolution gamma spectrometry with HPGe • Outline of spectrum analysis • Sources contributing the result’s uncertainty

  3. Isotopes and Radiation • Radiation: unstable isotope tries to get stable by releasing energy from the nucleus in form of: • Particles: Alphas, betas (positrons, electrons), neutrons, …. • Electromagnetic waves: gamma radiation • All isotopes are listed in Karlsruhe nuclide chart • X-axis: mass number (neutrons and protons) • Y-axis: atomic number (protons) • Stable isotopes: black • Positron emitters: pink • Electron emitters: blue • Alpha emitters: yellow • Nearly all particle decays releasing gamma rays too

  4. Isotopes and Radiation

  5. Gamma Radiation • Example: decay chain of Co-60 • Beta- (electron) emitter • Additional Gamma emission • Ni-60 is stable • Example: Po-210 • Alpha emitter • Daughter product out of Uranium-Radium decay chain • Additional gamma emission • Pb-206 stable • Most isotopes have a specific gamma signature (like fingerprints)!

  6. Interaction of gamma radiation in detectors • High energetic Beta particles, gamma rays and high energetic X-rays could penetrate through the detector shielding and interact with the detector material • The higher the photon energy, the deeper the radiation could penetrate into the detector  the more interactions take place • Interaction mostly ionisation • Different detector types with different processing methods for the ionisation available (semiconductor, scintillators, ..) Am-241 Cs-137 Co-60

  7. Interaction of gamma radiation in HPGe detectors • By different processes, the radiation causes free electrons and ‘holes’ which could be moved in an High Voltage electric filed • Collection of charge at the electrodes • Different detector types: planar, coaxial,…. • Noise reduction by cooling with LN2

  8. Interaction of gamma radiation in NaI detectors • By different processes, the radiation causes free electrons which are responsible for fluorescence light (scintillator = light creator) • On a photo-cathode the light creates an electron which goes through the photomultiplier and delivers an amplified signal on the detector’s signal plug

  9. Basic Gamma Spectrometry System

  10. Pulse observation and adjustment(P/Z and Pile-up adjustment) Counting rate and pulse pile up

  11. Type of Ge detector and structure (detector choice)

  12. Signal utilization • Multi Cannel Analyzer (MCA) is the part in the gamma spectrometer, which registers the incoming gamma ray to a certain energy • Linear ramp converter (Wilkinson Type) – old principle, but good for understanding

  13. Signal utilization (Spectrum formation) • Each signal is registered in definite channels

  14. Gamma spectrum

  15. Spectrometer system performance

  16. The role of gamma radiation for Nuclear Security equipment Radioactive & Nuclear materials g radiation Gamma spectroscopy systems Radionuclide & Activity (Bq) Decision Support Risk Assessment

  17. Gamma spectrometry in the field • Portable system • Easy to set up, use and maintain • Identifying radioisotopes and estimating the activity • Gamma spectrometry system with NaI(Tl) “low resolution” • Gamma spectrometry system with HPGe “high resolution”

  18. Example of portable measurement system(LN2 & electro cooling)

  19. Comparison of Detectors • NaI(Tl) detector properties: • Cheap in production • High efficiency • Bad resolution • Not temperature stable • Spectrum not linear • Hydrophilic • HPGe detector properties: • Expensive • Good efficiency • Excellent resolution • Coolant necessary (cool down time, topping) • Spectrum linear

  20. Detector response function Co-60 gamma spectra with NaI and HPGe system

  21. Detector response function

  22. Features of portable high resolution gamma spectrometry with HPGe • Easy identification of the most radionuclides of interest in the point of MEST function (NORM, medical, nuclear materials, industrial radiation sources) • Reliable results BUT • Difficult method for efficiency calibration • Maintenance with cost (LN2, electro cooling) • Expensive instruments • Expertise required

  23. Outline of spectrum analysis

  24. Sources contributing the result’s uncertainty • Background (cosmic ray, contamination, natural radioactivity existing nearby the detector) • Self absorption (container, volume, matrix, nuclide energy) • Calculation of multiple peaks • Cascade summing

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