Neutron Detection

� Example of n detection: Well logging

Reservoir/Formation Evaluation

� Brief introduction to neutron generation

– Continuous sources

– Large accelerators

– Pulsed neutron generators– Pulsed neutron generators

� n interactions with matter

� n detection

� High/low energy n detectors

Measurement Environment

CementCement

Formation RegionFormation Region

Casing & BH FluidCasing & BH Fluid

In most logging applications, pulsedneutron tools should be run decentralizedin the wellbore.

Borehole RegionBorehole Region

The borehole region encompassesanything that’s before the formation.This includes tubulars, gravel packs, etc.

in the wellbore.

High Energy Neutron Reactions

γ γ γ γ γ γ γ γ of Captureof CaptureInelastic Inelastic γγγγγγγγ

High Energy NeutronHigh Energy Neutron Inelastic Inelastic γ γ γ γ γ γ γ γ

elastic

Inelastic Porosity Region -

Compton Scattering effect

similar to Gamma-Gamma

logging.

Gamma transport is a

function of Density.

Capture Porosity Region -.

Gamma transport is a

function of Hydrogen

Index.

Neutron Energy Losses

Element Avg. # ofCollisions

Max.EnergyLoss perCollision

AtomicWeight

AtomicNumber

Calcium 371 8% 40.1 20

Chlorine 316 10% 35.5 17Chlorine 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 10 µµss

1000 1000 µµss

Gamma Rays From Gamma Rays From Inelastic CollisionsInelastic Collisions

Gamma Rays From Gamma Rays From

NN

1000 1000 µµss

Seconds,Minutes, Hours, DaysSeconds,Minutes, Hours, Days

Gamma Rays From Gamma Rays From Thermal Neutron CaptureThermal Neutron Capture

Gamma Rays From Neutron Gamma Rays From Neutron Activation ProductsActivation Products

Gamma Ray Detection Methods

γγγγ γγγγDetectorDetector

Photomultiplier Photomultiplier TubeTube

P

P P

γγγγγγγγ’s Sorted by ’s Sorted by Time and Time and grouped in Gatesgrouped in Gates

Number of

counts

Gates

(gross counts)

γγγγ

Number of

counts

Gamma Ray Energy

γγγγγγγγ’s Sorted by energy ’s Sorted by energy levels (256)levels (256)(Not Unlike the Colors (Not Unlike the Colors of the Rain Bow)of the Rain Bow)

Time

256 channels

Typical Capture Cross Sections for Formation Minerals

Mineral ΣΣΣΣ @20°°°°C,c.u. Typically used ΣΣΣΣ valuesSandstone 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 20Oil 16 to 22 20Oil 16 to 22 20

Gas 2 to 15ƒƒƒƒ(Temperature, Pressure &

Specific Gravity)

Fresh Water 22.20 20

Salt Water (100 Kppm) 59 59

Salt Water (240 Kppm) 119 119

Oil 16 to 22 20

Fresh Water 22.20 25

Gas 2 to 15 ƒƒƒƒ(Temperature, Pressure & Specific Gravity)

Response

for

Reservoir 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 108 neutrons/sec.

– no measurable radioactivity when off

� 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

Neutron Detection

interactions with matter are detected instead

� Elastic, inelastic and n capture: basic interactions

– Scintillation detectors

– Gas Proportional counters - ionization chambers

– Semiconductor detectors

Cross section of n interaction with He, B, Li

Neutron Detection

3 3 1

6 4 3

10 7 * 3 7 4

7 4

0.764

4.79

0.48

n He H H MeV

n Li He H MeV

n B Li He Li He MeV

Li He

γ

+ → + +

+ → + +

+ → + → + +

→ +

� 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-

Neutron Detection

Lithium scintillation detectors (thermal neutrons)

� Gamma ray strikes photocathode and creates an e-

� e- charge multiplied in PMT � output pulse

6 4 3

4.79n Li He H MeV+ → + +

Li glass (Ce) 1.75×1022 0.45 % 395 nm ~7,000

Material

Density of6Li atoms(cm-3)

Scintillation

efficiency

Photon

wavelength (nm)

Photons per neutron

Li scintillators exhibits low efficiency � add Eu, Zn, others

Neutron Detection

Li glass (Ce) 1.75×1022 0.45 % 395 nm ~7,000

LiI (Eu) 1.83×1022 2.8 % 470 ~51,000

ZnS (Ag) - LiF 1.18×1022 9.2 % 450 ~160,000

� 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

H scintillation detectors (fast neutrons)

� e- charge multiplied in PMT � output pulse

Scintillation detectors

Neutron Detection

� 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

Neutron DetectionGas filled (proportional) n detectors

electrical pulse3 3 1

10 7 * 3 7 4

7 4

0.764

0.48

n He H H MeV

n B Li He Li He MeV

Li He

γ

+ → + +

+ → + → + +

→ +

� n reaction with B, LiF converts n � charged particles

� T or alpha particle create e- hole pairs

� Electric pulse produced at contacts

Neutron DetectionSemiconductors n detectors

6 4 3

4.79n Li He H MeV+ → + +

τeC

jP e

⋅=

3 3 1

6 4 3

10 7 * 3 7 4

7 4

0.764

4.79

0.48

n He H H MeV

n Li He H MeV

n B Li He Li He MeV

Li He

γ

+ → + +

+ → + +

+ → + → + +

→ +