Neutron Detection Example of n detection: Well logging Reservoir/Formation Evaluation Brief introduction to neutron generation – Continuous sources – Large accelerators – Pulsed neutron generators n interactions with matter n detection High/low energy n detectors Measurement Environment Casing & BH Fluid Cement Formation Region In most logging applications, pulsed neutron tools should be run decentralized in the wellbore. The borehole region encompasses anything that’s before the formation. This includes tubulars, gravel packs, etc. Borehole Region High Energy Neutron Reactions Inelastic γγγ γ of Capture Inelastic Porosity Region - Compton Scattering effect Capture Porosity Region -. similar to Gamma-Gamma logging. Gamma transport is a function of Hydrogen Gamma transport is a Index. function of Density. elastic High Energy Neutron Inelastic γ Neutron Energy Losses Element Avg. # of Max. Atomic Atomic Collisions Energy Weight Number Loss per Collision Calcium 371 8% 40.1 20 Chlorine 316 10% 35.5 17 Silicon 261 12% 28.1 14 Oxygen 150 21% 16.0 8 Carbon 115 28% 12.0 6 Hydrogen 18 100% 1.0 1 Hydrogen – Avg. Loss due to Angular Collisions is 63% Gamma Rays From Neutron Decay 10 µµss Gamma Rays From Inelastic Collisions 1000 µµss Gamma Rays From Thermal Neutron Capture Seconds,Minutes, Hours, Days NN Gamma Rays From Neutron Activation Products Gamma Ray Detection Methods Gates γγγ’s Sorted by Number γγγ γγγTime and of (gross counts) Detector grouped in Gates counts P Photomultiplier P P Tube γγγ’s Sorted by energy Time γγγ levels (256) (Not Unlike the Colors of the Rain Bow) Number of counts 256 channels Gamma Ray Energy Typical Capture Cross Sections for Formation Minerals Mineral ΣΣΣ @20 °°°C,c.u. Typically used ΣΣΣ values Sandstone 7 to 14 10 Limestone 7 to 15 12 Dolomite 8 to 12 9 Shales 20 to 50 Varies for Formation Oil 1616 to to 22 2020 ƒƒƒ(Temperature, Pressure & Gas 2 2to to 15 15 ƒƒƒ(Temperature,Specific Pressure Gravity) & Specific Gravity) Fresh Water Water 22.20 22.20 25 20 Salt Water (100 Kppm) 59 59 Salt Water (240 Kppm) 119 119 Response for Reservoir Monitoring (soft rock formations) Neutron Sources Continuous (ex. AmBe) – DOE/DHS efforts to eliminate them Large Accelerators (ex. SNS) – for large projects Pulsed Neutron Generators – Compact, convenient replacement of chemical sources n generator tube Pulsed Neutron Sources • Pulsed Accelerator Neutron Source – deuterium ( 2H) and tritium ( 3H) collided at 100keV D + T →→→ n + 4He – produces bursts of neutrons with 14MeV energy – ~1 x 10 8 neutrons/sec. – no measurable radioactivity when off Neutron Detection n don’t interact directly with e in matter Indirect methods of detection needed Charged particles and gammas created during n interactions with matter are detected instead Elastic, inelastic and n capture: basic interactions – Scintillation detectors – Gas Proportional counters - ionization chambers – Semiconductor detectors Neutron Detection Cross section of n interaction with He, B, Li n+3 He → 3 H + 1 H + 0.764 MeV n+6 Li → 4 He + 3 H + 4.79 MeV n+→10 B 7*3 Li + He →+ 7 Li 4 He + 0.48 MeV γ →7Li + 4 He Neutron Detection Lithium scintillation detectors (thermal neutrons) Lithium capture a thermal n Lithium transforms into He and tritium + ~4.8Mev Kinetic energy of particles deposited into crystal Crystal emit a gamma ray Gamma ray strikes photocathode and creates an e - e- charge multiplied in PMT output pulse n+6 Li → 4 He + 3 H + 4.79 MeV Neutron Detection Li scintillators exhibits low efficiency add Eu, Zn, others Density of Scintillation Photon Photons Material 6Li atoms efficiency wavelength per neutron (cm -3) (nm) Li glass (Ce) 1.75 ×10 22 0.45 % 395 nm ~7,000 LiI (Eu) 1.83 ×10 22 2.8 % 470 ~51,000 ZnS (Ag) - LiF 1.18 ×10 22 9.2 % 450 ~160,000 Neutron Detection H scintillation detectors (fast neutrons) Scintillation with hydrogenous material Elastic interaction of n with H n loss energy Thermal n is captured by H H emits 2.1 MeV gamma Gamma ray strikes photocathode and creates an e- e- charge multiplied in PMT output pulse Neutron Detection Scintillation detectors Neutron Detection Gas filled (proportional) n detectors Based on n interaction with B, He Low energy (thermal) neutrons interact with gas Charge particles (alpha) and H recoil ionize gas Avalanche dischrge between cathode and anode creates electrical pulse n+3 He → 3 H + 1 H + 0.764 MeV n+→10 B 7*3 Li + He →+ 7 Li 4 He + 0.48 MeV γ →7Li + 4 He Neutron Detection Semiconductors n detectors n reaction with B, LiF converts n charged particles T or alpha particle create e- hole pairs Electric pulse produced at contacts n+6 Li → 4 He + 3 H + 4.79 MeV j P = e τ C ⋅ e n+3 He → 3 H + 1 H + 0.764 MeV n+6 Li → 4 He + 3 H + 4.79 MeV n+→10 B 7*3 Li + He →+ 7 Li 4 He + 0.48 MeV γ →7Li + 4 He.