1833-41 workshop on understanding and evaluating...
TRANSCRIPT
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1833-41
Workshop on Understanding and Evaluating RadioanalyticalMeasurement Uncertainty
Gyula KIS-BENEDEK
5 - 16 November 2007
International Atomic Energy Agency IAEAAgency's Laboratories SeibersdorfChemistry Unit, A-2444 Seibersdorf
AUSTRIA
In-situ-gamma Spectrometry
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IN SITU GAMMA RAY-SPECTROMETRY
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IN SITU GAMMA RAY-SPECTROMETRY
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Applications
• Rapid identification and determination of gamma emitting radionuclides in the environment (in the field)
- activity concentration (Bqkg-1) or- deposition (Bqm-2) - activity concentration (Bqkg-1)
• Indoor radiation studiesAnalysis of power reactor plumes
NOTE: Spatial distribution of the radionuclide(s) of interest (source geometry) has to be taken into account.
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Directly measureable natural radionuclides
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Natural radionuclides measurable through daughter products
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• Analyte: gamma emitting radionuclides • Geometry: HPGe (or other type) gamma detector usually
at 1m above the ground• Matrix: Soil and air• Measurement time: usually 20 to 40 min• MDA: about 100 Bqm-2, depending on detector efficiency,
radionuclide of interest and other radionuclides present• Accuracy: 10-50 % depending on calibration accuracy and
environmental conditions.• Prerequisite: Calibration of the gamma-ray spectrometric
system for in situ measurements
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Effective field of view – Sample size
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In situ / laboratory mesurements
• Shorter measurement time• Prompt availability of results (without sampling, sample
transport and preparation) • Averaging radionuclide activity over large area• Often large errors are observed (intercomparison
exercises)• Using laboratory results (soil profiles – depths distribution)
better results can be obtained• Based on in situ results better sampling plan can be
prepared
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Basic instrumentation
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IN SITU GAMMA RAY-SPECTROMETRY BASIC EQUIPMENT
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QA: In-situ γ-ray spectrometry Checklist
Instrumentation• Detector• Electronics (MCA, Notebook PC,
set of cables, power supply)• GPS• Dose rate meter• Distance meter • Camera
Supplies• Detector support (tripod)• Spare batteries, Spare cables,
Check sources• Liquid nitrogen (fittings for filling)• Protective clothes, plastic bags,• Basic tools
Software• MCA and Evaluation • Databases • Nuclide library, conversion
factors
Documentation• Manuals (operation, procedures)• Logbook• Nuclide library tables,
conversion factors• Map
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Shilded background
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Seminar room
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Typical environment
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Muroroa
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Theoretical model for photon flux calculation in an in situ measurement
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Commonly used depth distributions and units
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Exponential distribution of a radionuclide
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Example values for relaxation length(Soil density: 1.6 gcm-3)
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Calculation of unscattered photon flux for different radionuclide depth distributions
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Theoretical model for photon flux calculation in an in situ measurement
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Calculation of radionuclide deposition
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Detector calibration factor
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Calibration – detector characteristics
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Angular correction factor
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Examples of a detector characteristics
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Response factor – a quick reference
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Different approach – mathematical detector efficiency calculation
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Sources of errors
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Inhomogenius radionuclide distribution
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Monitoring the system performance
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Effect of count rate in well adjusted sytem
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References
Field measurements of radioactivity: introduction to in-situ γ-ray spectrometry,
Marek Makarewicz IAEA
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References• ICRU REPORT 53 Gamma –Ray Spectrometry in the Environment,
Beteheshada(1994)
• EML-557 In situ Gamma –Ray Spectrometry A tutorial for environmental radiation scientist, K.M Miller nad P. Shebell, US DOE 1993
• Generic procedures for monitoring in a nuclear or radiological emergency IAEA-TECDOC-1092, June 1999
• A theoretical comparison of methods of quantification of radioactive contamination in soil using in situ gamma spectrometry; J. MacDonald et. al; J. of Radiol. Prot. 1997 Vol. 17 No.1 3-15
• Uncertainties of in situ gamma spectrometry for environmental monitoring, W. Sowa et. al, Radiation Protection Dosimetry Vol. 27 No. 2 pp. 93-101 (1989)