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Methods and Instruments for Dosimetric Measurements in Mixed Neutron-Gamma Radiation Fields

Methods and Instruments for Dosimetric Measurements in Mixed Neutron-Gamma Radiation Fields. NATALIA GOLNIK NATIONAL CENTRE FOR NUCLEAR RESEARCH. NCBJ in OTWOCK-ŚWIERK. MARIA reactor Laboratory for Mixed Radiation Dosimetry. Neutron dosimetry.

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Methods and Instruments for Dosimetric Measurements in Mixed Neutron-Gamma Radiation Fields

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  1. Methods and Instruments for Dosimetric Measurements in Mixed Neutron-Gamma Radiation Fields NATALIA GOLNIK NATIONAL CENTRE FOR NUCLEAR RESEARCH

  2. NCBJ in OTWOCK-ŚWIERK MARIA reactor Laboratory for Mixed RadiationDosimetry

  3. Neutron dosimetry The basic protection quantity for ionising radiation is the absorbed dose in an organ or tissue. Because different types of radiations have different degrees of biological effectiveness account must be taken for radiation type. The concept of weighted dose seems at present the best available. The present system, recommended by ICRP involves a complexcombination of effectivedose, the socalled „protectionquantity” whichisused in legislation and the operationalquantitieswhichinstruments and dosemetersshouldrealise.

  4. Neutron dosimetry Aongside the problems of the compatibility of the protection and operationalquantitiesthereremains the problem of providing devices whichmeasure the operationalquantitiesaccurately, in a routine and simpleway. Thereremains much to do ifsuch devices are to becomewidelyavailable.

  5. U C NU NC U DLT

  6. 1. CalibrationlaboratoryP. Tulik, Sz. Domański Neutron sources 241Am Be, 252Cf, traceable to National Physical Laboratory, UK, 239Pu Be Sourcescan be exposed free in air or in modifying filters.Doseequivalent rate: 2 ÷1400 μSv/h. 137Cs gamma sources give fields traceable to Polish Committee for Normalisation and Measurements; dose equivalent rate of 2 μSv/h÷ 20 mSv/h

  7. 1. Calibrationlaboratory

  8. 1. Calibrationlaboratory

  9. 2. Instrument for low-levelgamma and neutron monitoringS. Pszona, A. Bantsar, K. Wincel. B. Zaremba Photons in common/availableneutron monitors are usually discriminated newdesigneddevicegivespossibility of registration gamma and neutron components in separate „windows”

  10. 2. Instrument for low-level gamma and neutron monitoring for thermal and epithermalneutronsthereis a significantcontributionfrom secondary gammagenerated by neutrons in (n,p) reactions (montecarlocalculations) valuenormalized to h*(10)

  11. 2. Instrument for low-level gamma and neutron monitoring Almostequalresponse to gamma and neutrons Pb filtergivespossibility to correct gamma energydependence Channels 125-160 givespossibility to correctresponse >10MeV

  12. Recombination Chamber High-pressure, tissue-equivalent ionization chamber, operated under conditions of initial recombination of ions • REM-2 recombination chamber • Large chamber (6 kg, 1800 cm3), 25 TEelectrodes with the diameter of 12cm; • distance between electrodes - 7 mm. • Usually filled with a mixture of methane andnitrogen up to about 1 MPa.

  13. saturation current i f = • ionis. current i 0 0 0 voltage voltage 0 0 Ioncollectioneffciency

  14. Dependence on radiationquality

  15. Recombinationindex of radiationquality

  16. AMBIENT DOSE EQUIVALENT*(10) H*(10) = D*(10) x Q Approximation within recombination methods: H*(10) ≈ D*(10) x Q4

  17. 3. Monitoring system for workplaces Keithley 6517B Interface KUSB 448B REM-2 Software

  18. RESULTS

  19. OVERNIGHT MEASUREMENTS

  20. 3. Monitoring system for workplaces T

  21. 3. Monitoring system for workplaces The idea of the triple-mode detector (summation mode raises sensitivity, differential mode gives possibility of direct measurements of the ambient dose equivalent rate, in case of fluctuations of dose rate. Two switches are needed for proper configuration of polarizing electrodes and for measurements of ionization current.

  22. 4. Ionizationchambercontaining10BM.A. Gryziński, K. Tymińska, M. Zielczyński Development of a method for determination of ambientdoseequivalent, H*(10), usinganionizationchamberfilled with BF3 TARGET • Considered solutions • Classic recombination chamber with addition of BF3 to the filling gas; • High-pressure chamber filled with BF3 and placed in external moderator; • Relatively small chamber (BOR-2) in spherical moderator • Air filled ionization chamber with 10B in material of electrodes

  23. Ionization chamber containing 10Bclassic REM-2 with addition of BF3 to the filling gas Doseresponsecalculated with MCNPX code (per 1 neutron) Addition of BF3 (about 0,1% ) improves the chamber sensitivity for low energy neutrons (<10keV) and only slightly influences the energy dependence for fast neutrons.

  24. Ionization chamber containing 10B - high pressurechamber with moderator Thickness of the moderator, gas pressure and polarising voltage should be optimised in order to obtain similar sensitivity to H*(10) for neutrons and gamma radiation in certain energy range

  25. Ionization chamber containing 10B - high pressurechamber with moderator Moderatorsmade with polyethylene Angulardependence of the chamberresponse

  26. Ionization chamber containing 10B - high pressurechamber with moderator Saturationcurves of the REM-2 chamberfilled with BF3havebeendetermined in neutron and gamma radiationfields. Relativeioncollectionefficiency for 137Cs and 252Cf

  27. Ionization chamber containing 10B high pressurechamber with moderator • Optimum values • Thickness of the external moderator – ca. 3 cm • Gas pressure – 130kPa • Polarising voltage – 350 V for 4 cm moderator.

  28. 5. Model of recombinationchamber with spectrometricpropertiesM. Zielczyński, N. Golnik, M.A. Gryziński, M. Maciak, K. Tymińska Patent application • Considered solutions • Number of electrodes - 21; • Made with PP-EL • Thickness of electrodes – 10 mm; • Electrodescovered with B4C Model. MCNPX Visual Editor

  29. 5. Model of recombinationchamber with spectrometricproperties Neutron flux on the chamber lid (0) andchosen electrodes (1‐21) at 25 mg/cm3 B4Clayer thickness for both Monte Carlo codes.

  30. 5. Model of recombinationchamber with multilayer moderator Neutron flux on the chamber lid (0) andchosen electrodes (1‐21) at 600 mg/cm3 B4C layer thickness for both Monte Carlo codes.

  31. 5. Model of recombinationchamber with multilayer moderator Neutron flux on the lowest electrode (21) forvarious values of the B4C layer thickness forboth Monte Carlo codes.

  32. 6. Passivedetector with multilayer moderatorN. Golnik, K. Dworecki, M.A. Gryziński, P.Tulik, K. Tymińska Radiation

  33. 6. Passivedetector with multilayer moderator

  34. 6. Passive detector with multilayer moderator

  35. 7. Accident dosimetry

  36. 7. Accidentdosimetry Capacitors as a detector

  37. THANK YOU FOR YOUR ATTENTION WE WILL SHOW MUCH MORE IN NEXT YEAR

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