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 Formation Region Cement In most logging applications, pulsed neutron tools should be run decentralized in the wellbore. Borehole Region The borehole region encompasses anything that s before the formation. This includes tubulars, gravel packs, etc.
High Energy Neutron Reactions Inelastic γ γ of Capture Inelastic Porosity Region - Compton Scattering effect similar to Gamma-Gamma logging. Gamma transport is a function of Density. High Energy Neutron Inelastic γ Capture Porosity Region -. Gamma transport is a function of Hydrogen Index. elastic
Neutron Energy Losses Element Avg. # of Collisions Max. Energy Loss per Collision Atomic Weight Atomic Number 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 µs Gamma Rays From Inelastic Collisions 1000 µs Gamma Rays From Thermal Neutron Capture N Seconds,Minutes, Hours, Days Gamma Rays From Neutron Activation Products
Gamma Ray Detection Methods γ Detector γ γ s Sorted by Time and grouped in Gates Number of counts Gates (gross counts) P P P Photomultiplier Tube γ γ s Sorted by energy levels (256) (Not Unlike the Colors of the Rain Bow) Time Number of counts 256 channels Gamma Ray Energy
Typical Capture Cross Sections for Formation Minerals Mineral Σ @20 C,c.u. Typically used Σ values Sandstone 7 to 14 10 Limestone 7 to 15 12 Dolomite 8 to 12 9 Shales 20 to 50 Varies for Formation Oil 16 to 22 20 ƒ(temperature, Pressure & Gas Gas 2 to 15 Specific Gravity) 2 to 15 ƒ(temperature, Pressure & Specific Gravity) Fresh Water 22.20 25 Fresh Water 22.20 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 ( 2 H) and tritium ( 3 H) collided at 100keV D + T n + 4 He 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 3 3 1 n + He H + H + 0.764MeV 6 4 3 n + Li He + H + 4.79MeV 10 7 * 3 7 4 n B Li He Li He MeV + + + + 0.48 Li + 7 4 He γ
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 6 4 3 n + Li He + H + 4.79MeV
Neutron Detection Li scintillators exhibits low efficiency add Eu, Zn, others Material Density of 6 Li atoms (cm -3) Scintillation efficiency Photon wavelength (nm) Photons per neutron 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 0.764 3 3 1 n + He H + H + MeV 10 7 * 3 7 4 n + B Li + He Li + He + MeV Li + 7 4 He 0.48 γ
Semiconductors n detectors Neutron Detection n reaction with B, LiF converts n charged particles T or alpha particle create e- hole pairs Electric pulse produced at contacts 6 4 3 n + Li He + H + 4.79MeV
je P = τ C e 0.764 3 3 1 n + He H + H + MeV 4.79 6 4 3 n + Li He + H + MeV 10 7 * 3 7 4 n B Li He Li He 0.48MeV + + + + Li + 7 4 He γ