Neutron Detection. n interactions with matter n detection High/low energy n detectors

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John Flowers
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1 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

2 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.

3 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

4 Neutron Energy Losses Element Avg. # of Collisions Max. Energy Loss per Collision Atomic Weight Atomic Number Calcium 371 8% Chlorine % Silicon % Oxygen % Carbon % Hydrogen % Hydrogen Avg. Loss due to Angular Collisions is 63%

5 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

6 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

7 Typical Capture Cross Sections for Formation Minerals Mineral C,c.u. Typically used Σ values Sandstone 7 to Limestone 7 to Dolomite 8 to 12 9 Shales 20 to 50 Varies for Formation Oil 16 to ƒ(temperature, Pressure & Gas Gas 2 to 15 Specific Gravity) 2 to 15 ƒ(temperature, Pressure & Specific Gravity) Fresh Water Fresh Water Salt Water (100 Kppm) Salt Water (240 Kppm)

8 Response for Reservoir Monitoring (soft rock formations)

9 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

10 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

11 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

12 Neutron Detection Cross section of n interaction with He, B, Li n + He H + H MeV n + Li He + H MeV 10 7 * n B Li He Li He MeV Li He γ

13 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 + Li He + H MeV

14 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) % 395 nm ~7,000 LiI (Eu) % 470 ~51,000 ZnS (Ag) - LiF % 450 ~160,000

15 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

16 Neutron Detection Scintillation detectors

17 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 + He H + H + MeV 10 7 * n + B Li + He Li + He + MeV Li He 0.48 γ

18 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 n + Li He + H MeV

19 je P = τ C e n + He H + H + MeV n + Li He + H + MeV 10 7 * n B Li He Li He 0.48MeV Li He γ

EEE4106Z Radiation Interactions & Detection

EEE4106Z Radiation Interactions & Detection 2. Radiation Detection Dr. Steve Peterson 5.14 RW James Department of Physics University of Cape Town steve.peterson@uct.ac.za May 06, 2015 EEE4106Z :: Radiation