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Direct Monitoring Methods

ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKE S OF RADIONUCLIDES. Direct Monitoring Methods. Direct Measurements – Unit Objectives.

siobhan-crofton
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Direct Monitoring Methods

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  1. ASSESSMENT OF OCCUPATIONAL EXPOSURE DUE TO INTAKES OF RADIONUCLIDES Direct Monitoring Methods

  2. Direct Measurements – Unit Objectives The objective of this unit is to provide a review of the principles and techniques that are used for the direct measurement of radioactive material deposited in the human body. It reviews detection methods, facility requirements, background control, calibration and data analysis. At the completion of the unit, the student should understand direct measurement principles and limitations, how select the appropriate detection techniques, and appropriate criteria for acceptable performance.

  3. Direct Measurements - Unit Outline • Introduction • Detectors • Measurement Geometry • Background Control • Calibration • Data Analysis • Operational Considerations

  4. Introduction

  5. Measurements for internal dose assessment • Direct measurement - the use of detectors placed external to the body to detect ionizing radiation emitted by radioactive material contained in the body. • Indirect measurement - the analysis of excreta, or other biological materials, or physical samples to estimate the intake of radioactive material.

  6. Interpretation of monitoring measurements Direct Measurements (In vivo) Indirect Measurements m(t) Excretion rate, M Air concentration m(t) Body/organ content, M Estimated intake DAC-hr e(g)j Dose rate Committed effective dose

  7. Direct measurement applications • Radiation must be able to leave the body • Gamma rays • X-rays • Bremsstrahlung • Energetic betas • Fission and activation products • Uranium and transuranium radionuclides • Naturally occurring radioactive materials • Radiopharmaceuticals

  8. Public exposure assessment • Accident monitoring • Dose assessment and reconstruction • Public exposure to natural radiation • Long term follow up of doses from injection of thorotrast

  9. Direct measurement advantages • Accurate for most photon emitters • Measure radioactivity in specific organs • Prompt data acquisition and analysis • Metabolic models not needed to estimate organ or body radioactivity content • Simultaneous detection of many nuclides • Avoid handling of biological materials

  10. Direct measurement limitations • Lower detection sensitivity than excretion analyses for many radionuclides • Equipment usually elaborate and expensive • Value limited for low energy emitters •  and  radiations not normally detected • Need for calibration sources and phantoms • Potential confusion from body contamination

  11. Relative transmission through tissue for a) 13.6, b) 17.1, c) 20.3 and d) 59.5 keV photons 1 1 10-1 Adipose 10-2 10-1 10-3 10-4 10-2 Fraction of photons transmitted Skeletal Muscle 10-5 10-6 10-3 Cortical Bone 10-7 10-8 10-4 10-9 10-10 10-5 Tissue thickness - mm 1 1 10-1 10-1 10-2 10-3 10-2 0 10 20 30 40 50 0 0 0 10 10 10 20 20 20 30 30 30 40 40 40 50 50 50 (b) Adipose 13.6 keV 17.1 keV Fraction of photons transmitted Skeletal Muscle Cortical Bone Tissue thickness - mm (c) (d) Adipose Adipose Skeletal Muscle 20.3 keV Cortical Bone Fraction of photons transmitted Fraction of photons transmitted Skeletal Muscle 59.5 keV Cortical Bone Tissue thickness - mm Tissue thickness - mm

  12. Photon transmission depends on adipose content 1 10-1 50% adipose, 50% muscle 10-2 20% adipose, 80% muscle 100% muscle 20 30 40 50 60 70 80 90 100 Fraction of photons transmitted Photon energy - keV

  13. Photon transmission through muscle 1 Thickness 10 mm 10-1 20 mm 10-2 30 mm 40 mm 10-3 50 mm 10-4 10 15 20 30 40 50 60 70 80 90 100 Fraction of photons transmitted Photon energy - keV

  14. Detectors

  15. Resolution and efficiency • Resolution Ability to distinguish photons with different energies • Efficiency Ability to detect low levels of activity

  16. Detector resolution Resolution : R = FWHM / H0 Y  Counts FWHM HO H Energy

  17. Detector efficiency • Detection efficiency, E = Td/Te where, Td are the transformations detected and Te are the transformations emitted • Photons must reach the sensitive volume of the detector to be detected • The probability that it will do so is the geometric efficiency, Eg • Governed by the solid angle • Depends on distance and detector area

  18. Detector efficiency • Intrinsic efficiency, Ei,is the ratio of photons interacting in the detector to the number incident. • Photon absorption is determined by the μ/ρ, ρ and material thickness. • If photon deposits all its energy in the detector, it contributes to the photopeak. • Photopeak efficiency, Ep, is the ratio of photons depositing all their energy to those incident.

  19. Detector efficiency • Photofraction, Fp = Ep/Ei Where μ/ is the mass attenuation coefficient  is the material density t is the material thickness • μ/ depends highly on energy

  20. Scintillation detectors • Thick crystals for energetic photons (E > 50 keV) • Phoswich detectors for lower energy photons for transuranic nuclides • Plastic detectors for screening • BGO (Bi4Ge3O12) for wound counting, etc. • Liquid scintillator

  21. Scintillation detectors

  22. Cross section of a phoswich detector Photomultiplier tube CsI(Tl) - 5.0 cm NaI(Tl) - 0.1 cm Be entrance window (0.25 mm)

  23. Solid state detectors • Germanium • HPGe • Planar • Broad energy germanium (BEGe) • Silicon • CdTe • HgI2

  24. Illustration of superior GeLi resolution 106 NaI 105 104 GeLi 103 102 10 110mAg Counts per keV Energy - keV

  25. Germanium detectors offer better resolution

  26. Gas filled detectors • Ionization chambers • Proportional • Large volumes • Used for measuring x rays • Resolution of 13-15 per cent for the 13.6- and 17.2-keV L x rays from 239Pu • Backgrounds from < 2 to 400 cpm in the (10-24 keV), methods to suppress bkgd. • GM • Used for post-accident screening

  27. Detector applications - Photons

  28. Detector applications - Betas

  29. Direct measurement electronics • Photomultiplier • Pre-amplifier • Main amplifier • Analogue - to - digital converter (ADC) • High voltage power supply (HPS) • Multi channel analyzer • Special • pulse shape analyzer • multiplexer • etc.

  30. Electronic components of a direct measurement system Detector HV / Bias Supply Multi-channel Analyzer Amplifier ADC* preamplifier * Analogue to Digital Converter

  31. Phoswich system electronic components Linear Gate Delay Multiplexer Router A CsI(Tl) A Amp. Shape Gate B A Shape Gate B Pre. Amp. B 16 K Computer Based Analyzer High gain FET Input Preamplifier Spectroscopy Amplifier NaI(Tl) Delay Delay Amplifier and Pulse Shape Analyzer

  32. Measurement Geometry

  33. Direct measurement in various geometries • Whole and partial body measurement • Individual organ measurement • Methods to determine radionuclide distribution • Special methods

  34. Whole body measurement • Chair geometry • Arc geometry • Bed geometry

  35. Chair geometry Detector

  36. Arc geometry Detector r typically = 2 meter r

  37. Bed geometry Detector Detector a) Detector Detector Stretcher Scanning bed b)

  38. Beds with moving detectors

  39. Shadow shielded bed

  40. Comparison of commonly used geometries Geometry Mechanical arrangement Uniformity of response Distribution information Sensitivity Arc fixed very good no low Chair fixed poor no high fixed good possible high Static array Scan moving detector good yes high Shadow shield moving bed good yes medium

  41. Examples of whole-body counter MDAs Time of measurement MDA(Bq) Detector No Size Geometry 137Cs 88Y NaI(Tl) 1 29.2 x 10.1 chair 3600 s 7 11 NaI(Tl) 1.5 x 10 scanning bed 2700 s 36 5 32 NaI(Tl) 1 10 x 10 chair 900 s 100 85 NaI(Tl) 20 x 10 scanning bed 1200 s 21 4 27 HPGe 3 50% scanning bed 1200 s 89 93 chair HPGe 55% 1200 s 60 1 90 HPGe 4 83% scanning bed 600 s 26 8

  42. Individual organ measurement Lungs Thyroid Liver Skeleton

  43. Measurement of radioiodine in the thyroid

  44. Measurement of radioactivity in the lung

  45. Measurement of skeletal activity A) Phoswich detectors B) Germanium detectors

  46. Special direct measurement methods • Wound monitoring • Beta and bremsstrahlung measurement • Quick body monitoring • Low energy measurements with silicon detectors • Mobile direct measurement systems • Measurement of neutron induced radioactivity

  47. Portable wound counter

  48. Beta and bremsstrahlung measurement

  49. Quick body monitoring

  50. Jacket design for low energy radionuclides Silicon detector arrays

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