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Gamma-Ray Spectroscopy

This comprehensive overview presents the principles of gamma-ray spectroscopy, focusing on various types of semiconductor detectors such as germanium and silicon-based systems. Authored by Dr. Ir. Peter Bode, Associate Professor in Nuclear Science & Engineering, the material explores solid-state ionization detectors, the role of impurities in semiconductor performance, energy resolution, and calibration techniques. The advancements in detector technology, including the use of LaBr3(Ce) scintillation detectors and modern amplification systems, are also discussed, providing insights into achieving high-resolution spectroscopy.

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Gamma-Ray Spectroscopy

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  1. Gamma-Ray Spectroscopy Dr.Ir. Peter Bode Associate Professor Nuclear Science & Engineering

  2. INAA:Semiconductor detectors RNAA:Semiconductor detectors Scintillation detectors

  3. Solid-state ionisation detectors Principle of a semiconductor detector

  4. Solid-state ionisation detectors N-type Ge: Impurities such as P and As as electron donors P-type Ge: Impurities as B, Al, Ga as positive charge donors

  5. Solid-state ionisation detectors Semiconductor detector: Junction diode with P and N type impurities on either side Applying a reverse bias: A P-I-N structure is formed

  6. Solid-state ionisation detectors n+ contact dq = (qedV1 + qhdV2 )/V e dV1 qe h i = dq/dt qh dV2 n-type silicon • silicon diode • germanium detector p+n junction -V 0 reverse bias, fully depleted

  7. Solid-state ionisation detectors Some properties of semiconductor materials

  8. Solid-state ionisation detector Schematic representation of a Ge-semiconductor detector,

  9. Solid-state ionisation detector

  10. Solid-state ionisation detectors Contacts: n+: diffusion of Li-atoms 700 – 1000 m (dead layer) p+: implantation of B-atoms 0.3 m

  11. Solid-state ionisation detector • Different types of Ge semiconductor detectors

  12. Solid-state ionisation detector No.of pulses (* 1000) Channel number pulse height

  13. Solid-state ionisation detector

  14. Solid-state ionisation detector Pulse height spectra obtained with Si(Li) detectors.Left: X-ray spectrum of 241AmRight: - spectrum of 241Am

  15. Solid-state ionisation detector Different types of cryostats for use with Ge-semiconductor detectors

  16. Energy resolution Usually: Full Width on Half Maximum @ 1332 keV of 60Co @ 122 keV of 57Co @ 6 keV of 55Fe

  17. Energy resolution State-of-the-art: 1332 keV: 1.58 – 2.0 keV, depending on crystal size 122 keV: 0.6 – 1 keV 5.9 keV: 0.2 – 0.5 keV

  18. Peak Shape Ratio of : FWHM/Full Width 0.1 M FWHM/Full Width (1/50) M

  19. Gamma-ray peak shape and tail parameters

  20. Peak Shape Ratio of : FWHM/Full Width 0.1 Mtheoretically: 1.83 FWHM/Full Width (1/50) Mtheoretically: 2.38 Importance: symmetry !!!

  21. High energy (top) and low energy tail parameters

  22. High energy tail of pulser peak

  23. High energy tail of pulser peak

  24. Calibration source activity

  25. Peak-to-Compton ratio Defined as: Ratio of peak height at 1332 keV and average peak height in energy range between 1040 and 1096 keV

  26. Peak-to-Compton ratio State-of-the-art: p/C ~ 50-100, depending on size of crystal: pC = 34.75 + 1.068 (εCo-60) - 4.96.10-3 (εCo-60)2

  27. Efficiency Absolute efficiency defined as: Relative to the efficiency of a 3” x 3 ” NaI(Tl) detector, defined as 1.2.10-3 counts/1332 keV photon,measured at a source-detector distance of 25 cm

  28. Determination of photopeak efficiency curve Absolute: Using calibrated sources with known gamma-ray emission rates and activity values, traceable to Bq Single gamma-ray emitting radionuclides Point sources Extended sources Problem:Many sources contain 60Co and 88Y; corrections for coincidence effects require also the p/T curve

  29. Determination of photopeak efficiency curve Relative: Using mix of sources with well-established gamma-ray intensity ratios 1 source for entire energy range, e.g. 152Eu 2-5 sources, e.g. 182Ta + 133Ba + 75Se + 24Na + … Problem:Intensity ratios not always well established

  30. Determination of photopeak efficiency curve Relative: 1 source:advantage: simpledisadvantage: do not always fully cover entire energy range; inter/extra-polation disputable in 80-150 keV range 3-5 sources:advantage: better coverage all energy rangesdisadvantage: more cumbersome, problems with non-matching parts

  31. Determination of efficiency curves Relative: Using mix of sources with well-established gamma-ray intensity ratios 1 source for entire energy range, e.g. 152Eu 2-5 sources, e.g. 182Ta + 133Ba + 75Se + 24Na + …

  32. New Tools for Nuclear Spectroscopy Better and bigger Ge detectors High count rate electronics High-resolution scintillation detectors (LaBr3(Ce)) Position-sensitive (strip) detectors Monte Carlo modeling Image processing

  33. Bigger Ge- Detectors Absolute photopeak efficiency 3 %90 % 560 cm3 well 0.3 %20 % 75 cm3 (17 %) 4 cm Photon energy, keV

  34. 0,01 0,1 1 0,01 0,1 0,25-0,3 1 0,01 0,1 0,15-0,25 1 0,01 0,1 1 Bigger Ge-Detectors Typical improvement in detection limits Arbitrary units 20 % 100 % well125 cm3 well560 cm3 0,07 - 0,1

  35. New Tools for Nuclear Spectroscopy LaBr3(Ce) scintillation spectra P.Doorenbos et.al., IEEE Transactions 51 (2004) 1289Developed and Patented by T.U.Delft: produced by Saint Gobain, France

  36. Preamplifiers • Resistor feedback- Pulse optical feedback high resolutions (planar detectors) • Transistor feedback high count rates

  37. NIM bin and power supply • Adequate capacity • standard: +/- 24 V +/- 12 V +/- 6 V

  38. High Voltage supply Typically (+/-) 3-5 kV Different power supplies for Ge and NaI(Tl) detectors dV/dt networks LN2 switchoff option

  39. Spectroscopy Amplifiers Analogue systems Digital systems - Gaussian shaping- Triangular shaping- Gated-integrated shaping

  40. Baseline retoration and Pole-zero setting

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