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Active methods of neutron detection

Neutron metrology

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Active methods of neutron detection

  1. 1. International Atomic Energy Agency NEUTRON DOSIMETRY AND MONITORINGActive Methods of Neutron Detection
  2. 2. Active Methods of Neutron Detection - Unit ObjectivesThe objective of this unit is to present a summaryof the active detection mechanisms that can beapplied to personal and area monitoring, andcalibration instrumentation used for neutrondosimetry.At the completion of this unit, the student shouldunderstand how the detection mechanisms foractive methods employed in current neutronmonitoring problems function. The studentshould also have a general understanding of theadvantages, disadvantages, and areas ofapplication of each of these methods. International Atomic Energy Agency
  3. 3. Active Methods of Neutron Detection - Unit Outline Introduction Gas Filled Detectors Ionization chambers Proportional counters Scintillators Thermal neutron detection Fast neutron spectrometry Semiconductor Detectors Silicon Diode Based Detectors Direct Ion Storage Detectors Superheated Emulsion (Bubble) Detectors International Atomic Energy Agency
  4. 4. INTRODUCTION International Atomic Energy Agency
  5. 5. Neutron detection Neutrons can be detected only indirectly by charged particles from nuclear reactions. For spectrometry applications, the energy of these charged particles must be related to the energy of the neutron. Two kinds of reactions can be used in neutron detectors: Exothermic nuclear reactions Elastic scattering of neutrons with detector nuclei International Atomic Energy Agency
  6. 6. Exothermic reactions Result in secondary charged particles, e.g. 3He(n,p)3H + Q (Q = 764 keV). A neutron of energy En produces an electric signal at the detector output, the height of which is proportional to En+Q. International Atomic Energy Agency
  7. 7. Elastic scattering Elastic scattering of neutrons with nuclei of the filling gas, i.e. production of recoil protons in hydrogen (or a hydrogen-containing gas such as CH4) or of alpha particles in 4He filling. The maximum energy transfer (in the case of a head-on collision) from the neutron to the recoil nucleus of mass M is given by Emax = [4M/(M + 1)2] En International Atomic Energy Agency
  8. 8. GAS FILLED DETECTORS International Atomic Energy Agency
  9. 9. Gas filled detectors Voltage supplyIncident Electricradiation current or pulse Anode+ measuring device Cathode-Fill gas, e.g. air, CH4, etc. International Atomic Energy Agency
  10. 10. Gas filled detectors Region Process* I II III IV V VI Log detected charge, Q0 I Recombination II Ionization III Proportional p+ Limited IV proportionality e- V Geiger VI Breakdown* Red indicates useful for Voltage neutron applications International Atomic Energy Agency
  11. 11. Ionization Chambers International Atomic Energy Agency
  12. 12. Ionization chambers The ionization current indicates exposure rate. Very rapid response time. Dual chambers used for neutron measurement. Air equivalent walls and air fill gas for photons A-150 Tissue Equivalent plastic walls and T.E. fill gas for neutrons + photons Difference = neutrons Relatively insensitive Neutron applications - Used mainly for calibrations. International Atomic Energy Agency
  13. 13. Ionization chambers International Atomic Energy Agency
  14. 14. Proportional Counters International Atomic Energy Agency
  15. 15. Proportional counters Pulse height is proportional to the number of ions resulting from a charged particle interaction. Proton and alpha particles produce larger pulses than a beta particle or photon. Discriminator can reject photons and betas. BF3 and 3He fill gases used for thermal measurement. H2, CH4, 4He, etc. used for spectrometry. International Atomic Energy Agency
  16. 16. Pulse-height spectrum with BF3 gas International Atomic Energy Agency
  17. 17. Pulse-height spectrum with 3He gas counter International Atomic Energy Agency
  18. 18. Proportional counters Operated as recoil detectors Filled with H2 or CH4, using elastic (n,p) scattering, or 4 He gas resulting in (n,α) scattering. SP2 proportional proton recoil counter International Atomic Energy Agency
  19. 19. Proton recoil proportional countersUse the (n,p) scattering cross section: Well known and changes monotonically with energy. Isotropic in the centre-of-mass system for neutron energies less than at least 5 MeV. Described by simple scattering theory - calculations are relatively straightforward. Reasonably large absolute value so that the counters have a useable efficiency. Recoil protons have a constant mean energy loss of W = 36 eV/ ion pair produced > 3 keV. International Atomic Energy Agency
  20. 20. Spherical proton recoil counter International Atomic Energy Agency
  21. 21. Processes occur in a proportional counter The incident neutron, if scattered by a hydrogen nucleus, produces a recoil proton. Electrons produced along the proton track due to ionizations drift towards the anode wire along an electric field line. Near the wire, the electron gains enough energy for the gas atoms to be ionized. Ionization electrons can produce further electrons and thus an avalanche is produced (gas amplification). International Atomic Energy Agency
  22. 22. Processes in a proportional counter Charges are collected at the anode and a signal is produced. Signal is proportional to the amount of ionization and thus to the energy of the recoiling proton (not of the incident neutron). For each neutron energy E , proton energies E n p in the range 0 ≤ Ep ≤ En can be obtained depending on the scattering angle between the neutron and the proton. International Atomic Energy Agency
  23. 23. Spherical proton recoil counters Spherical detector response is nearly independent of the incident neutron direction. Fill gas purity and constant electric fields are important. Electric field constancy depends on: Diameter of the insulator, Diameter of the anode wire, Diameter of the wire holder, Length of the anode wire holder and Distance between the insulator and the end of the anode wire. International Atomic Energy Agency
  24. 24. Cylindrical proton recoil detectors Not as complicated in their construction. Can be manufactured with large volumes for increased sensitivity and energy range. The active volume of the detector can be exactly defined with special precautions at both ends of the anode wire – “field tubes”. Disadvantage – anisotropy. Problems in multidirectional neutron fields. International Atomic Energy Agency
  25. 25. Attributes of SP2 proportional counters * High energy resolution (ΔE/E in the order of a few per cent) for neutron spectrometry. Isotropic response. Work in high thermal and epithermal fields. Tried and tested counters - expertise in their use is available. They cover the 50–1500 keV energy range where fluence to dose equivalent coefficients vary rapidly with energy. With electronic n/γ discrimination lower energies can be measured.* Characteristic of most, well made spherical proportional counters International Atomic Energy Agency
  26. 26. Proton recoil proportional counter disadvantages May be highly microphonic. Solution - Enclosing counters in a firm metal box for acoustic noise damping with minimum neutron attenuation. May be made of aluminum or cadmium. A sheet of lead reduces gamma rays. Since 2 or 3 counters may be used in succession to cover the energy range, longer measuring times are required. Low efficiency due to the low fill gas density compared with solid scintillators. International Atomic Energy Agency
  27. 27. Ideal response function of a hydrogenrecoil counter to monoenergetic neutrons For a monoenergetic neutron fluence of energy E, the proton recoil energy distribution P(E) would ideally have the characteristic rectangular shape. International Atomic Energy Agency
  28. 28. Response of a hydrogen recoil counter Not the case because of distortion effects - the number of ions collected at the anode does not give a measure of the proton recoil energy: Not all recoil protons lose their entire energy within the counter before hitting the wall - wall distortion effects - and Gas amplification is not constant over the entire volume (electric field strength drops at the ends of the anode wire) - gas amplification distortion effects. International Atomic Energy Agency
  29. 29. SP2* counter response to 144 keV neutrons* 100 kPa International Atomic Energy Agency
  30. 30. Proportional counter response functions * Type SP2* Type SP2 International Atomic Energy Agency
  31. 31. n/γ discrimination International Atomic Energy Agency
  32. 32. Typical SP2 application energy ranges International Atomic Energy Agency
  33. 33. Proton recoil counter maximum sensitivity Maximum fluence and dose rate limits usually of the order of 5000 to 10,000 counts s-1(cps). Pile-up rejection and dead-time correction must be made correctly. Given a 1% proportional counter efficiency - the corresponding integral fast neutron fluence rate of (0.5 - 1) x 106 cm2 s-1 ⇒ dose equivalent rate of 500 - 1000 mSv h-1, respectively, is an acceptable upper limit. International Atomic Energy Agency
  34. 34. Proton recoil counter limit of detection Determined by the statistical uncertainty and acceptable measuring time. Assume a required number of counts is at least a few tens of thousands of events. Necessary count rate for a 2 h measurement is about 5 cps: Fast neutron fluence rate ≈ 5x102 cm-2 s-1, or Dose equivalent rate of 500 µSv h-1. Definite values cannot be given. International Atomic Energy Agency
  35. 35. SCINTILLATION DETECTORS International Atomic Energy Agency
  36. 36. Scintillation detectors Radiation enters detector material. Interaction causes light flash (scintillation). Scintillation detected by photomultiplier. Signal processed by electronics Pulse height proportional to energy deposited. International Atomic Energy Agency
  37. 37. Scintillation detection Photocathode Window Reflector P.M. Photoelectron tube from photocathode Scintillator γ β+Source α β- Neutron p U.V. photons produced Dynode (secondary Anode from local excited electron emission) states following ionization International Atomic Energy Agency
  38. 38. Thermal Neutron Detection International Atomic Energy Agency
  39. 39. Thermal neutron detector – 6LiI(Eu) Used for moderated detector (multisphere) measurements. Reaction energy - 4.780 MeV - shared by the resulting alpha particle and triton. Appears in a pulse-height spectrum as a broad quasi-Gaussian full-energy peak. LiI crystal usually connected to a 6 photomultiplier with a light pipe. Crystal sizes typically: 4 mm x by 8 mm, 8 mm x 8 mm, or 12.7 mm x 12.7 mm. International Atomic Energy Agency
  40. 40. 6 LiI pulse-height spectrum4mm x 4mm detectora-b. Range of fitted datac. Gaussian peakd. Photon backgrounde. Sum of fitted componentsf. Lower discriminator limit International Atomic Energy Agency
  41. 41. Fast Neutron Detection International Atomic Energy Agency
  42. 42. Organic scintillation detectors Organic scintillators are best neutron spectrometry at higher energies (>1 MeV). Scintillation materials include: Plastic Anthracene Stilbene Liquid scintillators International Atomic Energy Agency
  43. 43. Organic scintillation detectors High detection efficiency due to: High n-p scattering cross section, and Higher density of scintillation detectors compared with gas counters. Neutron response well calculated from cross sections, up to 20 MeV. All organic scintillation detectors are equally sensitive to photons and neutrons. Photon energies of up to 10 MeV must be taken into consideration in mixed fields. International Atomic Energy Agency
  44. 44. Organic scintillator characteristics Plastic scintillators not suitable for mixed fields – no n/γ discrimination. Good for neutron time-of-flight spectrometry - excellent sub-nanosecond time resolution. Stilbene crystals have excellent n/γ discrimination. Light production by the secondary charged particles depends on the ion direction. ∴ neutron response functions depend on angle of incidence - must be determined for the actual neutron directional distribution. International Atomic Energy Agency
  45. 45. Liquid scintillator characteristics NE213 or BC501A liquid scintillators do not have directionality drawback. n/γ discrimination properties are also good. Xylene is the basic liquid, so container must be carefully prepared. Concerns: Chemical properties Rather large xylene expansion coefficient. International Atomic Energy Agency
  46. 46. Liquid scintillator characteristics Aluminum capsules in polyethylene expansion tubes - can be used from 5°C to 35°C. Any size and shape can be constructed for coupling to one or two phototubes as appropriate for optimal response. Liquid scintillators encapsulated in aluminium are well suited for most applications. NE213 scintillators no longer available BC501A (model MAB-1F) are identical to NE213 in design and chemical composition. International Atomic Energy Agency
  47. 47. Charged particle ranges organic scintillators International Atomic Energy Agency
  48. 48. NE213 organic scintillator assembly NE213 Plexiglas scintillator lightguiden/γ field Light emitting diode (LED) Photomultiplier International Atomic Energy Agency
  49. 49. Calculated NE213 response function 10 MeV neutrons International Atomic Energy Agency
  50. 50. Calculated NE213 response functions International Atomic Energy Agency
  51. 51. SEMICONDUCTOR DETECTORS International Atomic Energy Agency
  52. 52. Silicon Diode Based Detectors International Atomic Energy Agency
  53. 53. Detection mechanisms for SSD Use same principles as passive dosimeters. Detect charged particles in detector, or Use converter layers (e.g. polyethylene). Silicon diodes. Direction ion storage. International Atomic Energy Agency
  54. 54. Semiconductor based detectors Semiconductor detectors detect charged particles generated in neutron-induced nuclear reactions: In the detector itself, or Charged particles generated in converter layers mounted close to the detector. Conventional semiconductors will not detect neutrons below 1 MeV since they do not contain hydrogenous material. International Atomic Energy Agency
  55. 55. Detection of low energy neutrons Radiators such as 6LiF or 10B can be used. Albedo neutrons can be detected with this type of converter. Converters are layers upon or incorporated into charged particle detectors. Secondary charged particle energy deposition allows discrimination against intrinsic noise and photons. International Atomic Energy Agency
  56. 56. Detection of higher energy neutrons Above several tens of keV recoil protons from elastic scattering in hydrogen play the important role in generating dose equivalent in tissue. Recoil protons from hydrogenous converters can be detected in this energy range. A 20mm (CH2)n converter will provide an acceptable dose equivalent response. International Atomic Energy Agency
  57. 57. Illustration of a Si diode neutron dosimeter International Atomic Energy Agency
  58. 58. Cross section of a silicon detector n n γ n γ ≥2 mm Radiator ~2 mm Air gap Dead layer ~4 μm SiO2 Positive traps Holes Electrons~300 μm Compton electrons Protons Silicon substrate International Atomic Energy Agency
  59. 59. Energy dependence Range of low-energy recoil protons is short. Detector “dead layer" between converter and sensitive layer reduces the proton response. Dead layers (~ 50 nm to 300 nm) result from: Construction of junction devices requiring a surface electrode, or Basic physics of devices which may result in a surface undepleted layer. Typical noise levels correspond to energy depositions of 10 keV to 20 keV in silicon. International Atomic Energy Agency
  60. 60. Energy dependence At low proton energies, pulse energy is similar to that deposited by photon interactions. Typical sensitive layer thicknesses are of a few tens to a few hundreds of µm. Silicon ranges of 50 keV and 100 keV electrons are, respectively, 24 mm and 78 mm ⇒ LETs are 1.2 keV mm-1 and 0.76 keV mm-1. Result many non-neutron induced pulses of a few tens of keV. International Atomic Energy Agency
  61. 61. Neutron-photon discrimination Two-diode devices can be used to subtract the photon component with paired detectors. With S.S. detectors, the fast neutron detection threshold via recoil protons can be reduced to ~200 keV using just an electronic threshold. Pulse shape analysis can be used for photon discrimination, but needs special electronics. International Atomic Energy Agency
  62. 62. Alternate approach to n/γ discrimination Use of very small volumes ⇒ neutron events confined to smaller volumes than γ events. Small volumes:  Arrange strip or pixel structures ~ few μm dimensions as arrays on one silicon chip.  Anti-coincidence between neighboring elements suppresses photons. Threshold should be reduced to ~ 100 keV, with acceptable noise level. Charge-coupled devices (CCDs) would have smallest volumes and low noise, but dead layers may cause problems. International Atomic Energy Agency
  63. 63. Example of a silicon based detector International Atomic Energy Agency
  64. 64. Direct Ion Storage Detectors International Atomic Energy Agency
  65. 65. DIS detectors are small ion chambers Information stored as charge trapped on the floating gate of a MOSFET transistor. Charge in each memory cell can be made fully variable. Result memory cell used to store analog information. Control gate Silicon oxide Oxide Electron tunneling Floating gate Analog-EEPROM paths memory cell Source Drain Si International Atomic Energy Agency
  66. 66. DIS detectors are small ion chambers Charge on floating gate set by tunneling electrons through the oxide layer. Charge is permanently stored on the gate. Stored information is read without disturbing the charge stored, by measuring the channel conductivity of the transistor. Radiation incident on the oxide layer produces electron-ion pairs but most of the free charge is neutralised before it has a chance to cross the metal-oxide interface. International Atomic Energy Agency
  67. 67. Cross section of DIS Fill gas Opening Floating gateOxide Electron tunneling path Source Drain Si Modified transistor with ion chamber International Atomic Energy Agency
  68. 68. DIS detectors for neutron dosimetry Ion chambers can be made sensitive to neutrons and photons. DIS for neutron dosimetry requires two chamber system. One chamber with high neutron sensitivity. One chamber with low neutron sensitivity. Signals must be differentiated. Photon energy dependence of the chambers must be almost equal. International Atomic Energy Agency
  69. 69. Dual ion chamber DIS system Neutron sensitive detector Neutron insensitive detector thermal fast neutrons neutrons photons electrons photonsα particles protons electrons A-150 plastic with BN Graphite or Teflon Ion chamber with A−150/PE Ion chamber with Teflon or containing BN/LiNO3 graphite International Atomic Energy Agency
  70. 70. SUPERHEATED EMULSION (BUBBLE) DETECTORS International Atomic Energy Agency
  71. 71. Bubble Damage Polymer Detector Superheated droplets are suspended in a firm elastic polymer. Neutrons trigger droplets giving rise to formation sites. Number of bubbles is a measure of the neutron dose. International Atomic Energy Agency
  72. 72. Bubble formation steps in superheated emulsions International Atomic Energy Agency
  73. 73. Halocarbons used in superheated emulsions Boiling Critical Empirical point point Chemical name formula T (°C)* Tc (°C) b 1,2-dichlorotetrafluoroethane C2Cl2F4 3.65 145.7 Octafluorocyclobutane C4F8 -6.99 115.22 Dichlorofluoromethane CCl2F2 -29.76 111.8 1,1,1,2-tetrafluoroethane C2H2F4 -26.07 101.2 Hexafluoropropylene (HFP) C3F6 -29.40 85.0 Monochloropentafluoroethane C2ClF5 -39.17 79.9 Octafluoropropane C3F8 -36.65 71.95* At atmospheric pressure (101 kPa) International Atomic Energy Agency
  74. 74. Superheated Drop detectors use different detection mechanisms Bubble Technology TM APFEL Liquid Matrix Bubble Dosimeter Superheated Drop Detector Cap Glass or Plastic Event Event Tube Acoustical Acoustical Elastic Polymer Transducer Transducer (Gel) Trapped Bubbles ~1 mm diam. Superheated Anti-Coincidence Counting and Liquid Drops Circuitry Display Circuitry ~0.025 mm diam. Noise Acoustical Transducer International Atomic Energy Agency
  75. 75. Bubble damage polymer detector Passive readout – optical bubble detection. Active readout – acoustical detection of bubble formation. Extremely sensitive to neutrons (in µSv range). Completely insensitive to gamma rays. Can be made with neutron energy thresholds from <20 keV to several MeV. International Atomic Energy Agency
  76. 76. ReferencesBURGESS, P.H., MARSHALL, T.O., PIESCH, E.K.A., The design of ionisation chamber instruments for themonitoring of weakly penetrating radiation, Radiat. Prot. Dosim. 39, No. 3 157-160 (1991) .D’ERRICO, F. AND MATZKE, M., Neutron Spectrometry in Mixed Fields: Superheated Drop (Bubble)Detectors, Radiat. Prot. Dosim. 107, Nos 1–3, pp. 111–124 (2003).INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment of Occupational Exposure Due to ExternalSources of Radiation, Safety Guide RS-G-1.3 (1999).INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR OFFICE, Occupational RadiationProtection, Safety Standards Series No. RS-G-1.1, IAEA, Vienna (1999).INTERNATIONAL ATOMIC ENERGY AGENCY, Calibration of Radiation Protection Monitoring Instruments,Safety Series No. 16 (2000).INTERNATIONAL ATOMIC ENERGY AGENCY, Neutron Monitoring for Radiological Protection, TechnicalReports Series No. 252, IAEA, Vienna (1985).INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Measurement of DoseEquivalents Resulting from External Photon and Electron Radiations, Report No. 47, ICRU, Bethesda, MD(1992).INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Quantities and Units inRadiation Protection Dosimetry, Report No. 51, ICRU, Bethesda, MD (1993).INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, General Principles for the RadiationProtection of Workers, Publication No. 75, Pergamon Press, Oxford and New York (1997). International Atomic Energy Agency
  77. 77. ReferencesKLEIN, H., Neutron Spectrometry in Mixed Fields: Ne213/BC501A Liquid Scintillation Spectrometers, Radiat.Prot. Dosim. 107, Nos 1–3, pp. 73–93 (2003).KLEIN, H. AND NEUMANN, S. Neutron and photon spectrometry with liquid scintillation detectors in mixedfields. Nucl. Instrum. Methods A476, 132–142 (2002)KNOLL, G. F. Radiation Detection and Measurement, 3rd edition (New York: John Wiley) (2000).Nakamura, T., Nunomiya, T. and Sasaki, M., Development of active environmental and personal neutrondosemeters, Radiat. Prot. Dosim. 110, Nos 1-4, pp. 169-181 (2004).TAGZIRIA, H. AND HANSEN, W., Neutron Spectrometry in Mixed Fields: Proportional Counter Spectrometers,Radiat. Prot. Dosim. 107, Nos 1–3, pp. 95–109 (2003). International Atomic Energy Agency