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Accurate gamma-ray spectrometry of environmental samples: a challenge

Accurate gamma-ray spectrometry of environmental samples: a challenge. O. Sima - Bucharest University D. Arnold - PTB Braunschweig C. Dovlete - ERL Bucharest. Accurate gamma-ray spectrometry of environmental samples: a challenge. Introduction

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Accurate gamma-ray spectrometry of environmental samples: a challenge

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  1. Accurate gamma-ray spectrometry of environmental samples: a challenge O. Sima - Bucharest University D. Arnold - PTB Braunschweig C. Dovlete - ERL Bucharest

  2. Accurate gamma-ray spectrometry of environmental samples: a challenge • Introduction • Problems in efficiency calibration of the spectrometer • Coincidence summing effects • Matrix effects • Geometry effects • GESPECOR • Summary and conclusions

  3. Introduction • Assessment of radioactivity of environmental samples: • carefully designed sampling procedures • appropriate sample preparation • accurate sample measurement • rigorous analysis of the results • Modern requirements and conditions: • low detection limits • accurate evaluation of uncertainty • high number of samples, various types, matrices, available quantities • high efficiency detectors available

  4. Problems in efficiency calibration • Low level activity + low detection limit: • high efficiency measurement conditions • volume sources • Detection efficiency for high efficiency measurements: • nuclide specific coincidence summing effects • Detection efficiency for volumic samples: • dependent upon sample matrix and density => Direct experimental calibration: - limited number of matrices - specific nuclides - expensive, problems with the management of radioactive material => Additional procedures for a complete calibration required

  5. Coincidence summing effects • Are encountered in the case of measurement of nuclides which decay through the emission of coincident radiation (cascading photons, X-rays, annihilation photons etc) • Depend on the details of the decay scheme: • Nuclide and peak specific effects • Are much enhanced in high efficiency measurement conditions • Effects: • summing out (coincidence losses from the peak) => decrease of the apparent efficiency • summing in (additional counts in the sum peak) => increase of the apparent efficiency

  6. SPECTRUM OF 22Na (WELL-TYPE DETECTOR)

  7. Coincidence summing: - sample analysis - efficiency calibration: ex: 1 l Marinelli beaker Co-60 => 1173 keV 0.926, 1332 keV 0.924 Y-88 => 898 keV 0.932, 1836 keV 0.920 ex: Well-type detector: Co-60 => 1173 keV 0.445, 1332 keV 0.424 Y-88 => 898 keV 0.472, 1836 keV 0.390 => Accurate procedures for the evaluation of the effects required

  8. Matrix effects • Matrix effects are encountered when the calibration source has a different composition and density than the sample of interest • Depend on: • sample geometry • linear attenuation coefficient • photon energy • detector parameters • Linear attenuation coefficient obtained from: • sample composition and density; • transmission experiments

  9. - Transmission factor approximated by exp(-d) ?

  10. Transmission factors (log scale). Sample: R=3.5, H=2 cm 1 0.1 exp(-d) 0.01 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Linear attenuation coefficient (1/cm)

  11. Relative error  (%). Soil sample, R=3.5 cm, H=2 cm. 30 20 10 0 - 10 - 20 10 100 1000 Energy (keV)

  12. Geometry effects • For samples measured close to the end cap of a closed end coaxial detector efficiency very sensitive to geometry details • For some samples (e.g. powder) it is difficult to assure exactly the standard geometry • Detectors parameters may vary in time (e.g. the entrance window of the end cap)

  13. Relative error of activity (A) (%). Soil sample, R=3.5 cm Relative error of activity (A) (%). Soil sample, R=3.5 cm 0 - 2 - 4 - 6 - 8 - 10 10 100 1000 Energy (keV)

  14. GESPECOR • Monte Carlo based software dedicated to solve problems in gamma spectrometry: • - computation of coincidence summing corrections • - computation of self-attenuation effects (matrix effects) • - computation of the efficiency • Typical applications: environmental spectrometry • - detectors: HPGe (closed end or well-type), Ge(Li) • - sources: cylinder, Marinelli, point, parallelepiped, ring • - matrix: any (known composition) or known  • - nuclides: ~ 100 (for coincidence summing effects) • Extension for very large samples: variance reduction techniques (focused photon emission, weighted emission point)

  15. Direct efficiency calibration: - typical sources (cylinder, Marinelli, point sources) - special geometries: Parallelepiped (Al-26 in meteorite samples) Spherical source (Rn-222 sources) In situ measurements Drum waste containersEfficiency transfer – less sensitive to detector details

  16. Summary and conclusions • Accurate assessment of the radioactivity of environmental samples – a challenge • Coincidence summing effects • Matrix effects • Geometry effects • The GESPECOR software can solve typical problems required by an accurate assessment of the radioactivity of environmental samples

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