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 Problems in efficiency calibration of the spectrometer Coincidence summing effects Matrix effects Geometry effects GESPECOR Summary and conclusions

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

Problems in efficiency calibration Low level activity + low detection limit: –high efficiency measurement conditions –volume sources 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 Detection efficiency for high efficiency measurements: –nuclide specific coincidence summing effects

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

SPECTRUM OF 22 Na (WELL-TYPE DETECTOR)

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

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

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

Transmission factors (log scale). Sample: R=3.5, H=2 cm Linear attenuation coefficient (1/cm) exp(-  d)

Relative error  (%). Soil sample, R=3.5 cm, H=2 cm. Energy (keV)

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)

Relative error of activity  (A) (%). Soil sample, R=3.5 cm Energy (keV)

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)

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 containers Efficiency transfer – less sensitive to detector details

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