Accurate gamma-ray spectrometry of environmental samples: a challenge

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Accurate gamma-ray spectrometry of environmental
samples a challenge

• O. Sima - Bucharest University
• D. Arnold - PTB Braunschweig
• C. Dovlete - ERL Bucharest

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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

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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

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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

gt Direct experimental calibration - limited
number of matrices - specific nuclides -
expensive, problems with the management of
radioactive material
gt Additional procedures for a complete
calibration required
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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) gt
  decrease of the apparent efficiency
• summing in (additional counts in the sum peak) gt
  increase of the apparent efficiency

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  SPECTRUM OF 22Na (WELL-TYPE DETECTOR)          
           
 
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Coincidence summing - sample analysis -
efficiency calibration ex 1 l Marinelli
beaker Co-60 gt 1173 keV 0.926, 1332 keV
0.924 Y-88 gt 898 keV 0.932, 1836 keV
0.920 ex Well-type detector Co-60 gt 1173
keV 0.445, 1332 keV 0.424 Y-88 gt 898 keV
0.472, 1836 keV 0.390 gt Accurate procedures for
the evaluation of the effects required
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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

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- Transmission factor approximated by exp(-?d) ?
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Transmission factors (log scale). Sample R3.5,
H2 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)
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Relative error ? (). Soil sample, R3.5 cm, H2
cm.
30
20
10
0
- 10
- 20
10 100
1000
Energy (keV)
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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)

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Relative error of activity ?(A) (). Soil sample,
R3.5 cm
Relative error of activity ?(A) (). Soil sample,
R3.5 cm
0
- 2
- 4
- 6
- 8
- 10
10 100
1000
Energy (keV)
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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)

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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 containersEfficien
cy transfer less sensitive to detector details
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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