8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Imaging of Neutron Fields with

Submicron Resolution

R. Gregory Downing

Inorganic Chemical Metrology GroupAnalytical Chemistry Division

Chemical Science and Technology LaboratoryNational Institute of Standards and Technology

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

The Challenge:

develop a new concept for neutron radiography detectors

An imaging technique that would …

break the 10 micrometer spatial resolution barrier

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

basically limited by

the size of the electron charge cloud

produced following a neutron-atom reaction…

Existing Technologies:

Phosphors light cones

Pixels Capacitance well size

Gaseous amplifiers electron clouds

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

a paradigm shift in how we capture the informationon where the neutron reaction occur!

What really is required is…

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

n

E = 2054 keV

6Li

E = 2727 keV

4He

3H3He 5333b 3He(n,p)3H

6Li 940b 6Li(n,3H)4He

10B 3837b 10B(n,4He)7Li

xxxGd >>10000b xxxGd(n,e)xxyGd

Particle emission is diametrical

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Neutron Converter

Encoder

Encoder

Time-of-Flight (ToF) Coincidence

Neutron Beam

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

The reaction gives a unique coordinate solutionKnown:• Mass of each particle• Initial energy of each particle• Stopping power of converter• Stopping rate for each particle is different

Measure:• The unique time of flight (ToF) for each particle pair • Two PSD encoders establish the x-y coordinates for each pair

Calculate:• TOF Residual energy for each particle pair unique depth (x) of each reaction• Position sensitive encoder establishes a unique (y,z) position for the reaction• Variation in time/energy/stopping power/x-y position give spatial uncertainty• List mode output

Impose conditions:• Min./Max. delta time window for the coincidence pair• Line segment must pass through detector volume• Particle pair must yield a unique depth• A Jacobian Transformation defines unique angular emission & confirms measured angle

t1

t2

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Charged particle ranges in the converter material for the 6Li(n,3H)4He reaction defines the optimal thickness

Particle Energy (keV)3H 2727.8774He 2055.515

3H 4He

LiF 33.45 6.07Aluminum 37.94 6.73Silicon 42.95 7.51Lithium 132.85 23.29

Max. Range (micrometers)Substrate

Gold 1002 Å 1060Å for 45/22.5 keV

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Plot of Thermal Neutron Attenuationin Various Converter Targets

Neutron Transmission (ratio)

0.0001 0.001 0.01 0.1 1

Dep

th (

mm

)0.0

0.5

1.0

1.5

2.0

natLi Metal

6LiF

6Li Metal

natB Metal

10B Metal

Converter Range1 µm Range2 µm

Li metal 132.85 23.29LiNbO4 51.11 8.77

LiF 33.45 6.07B metal 3.54 1.83

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

High-speed encoder electronics

n

3H

4He 3H

4He

Li - richconverter

6Li(n,3H)4He

Assume a 5 m thickness of LiF,

Encoders are 5 cm away from LiF

Then the particle TOF differences are

either 118 ns (T/ or 10 ns (/T)

Thus for 50 nm position resolutionin the converter thickness dimension:

The timing resolution required is varies from 5.6 ns to 0.22 ns

Resolution along length of a plate converter is < 40 m

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Encoder response time to an MCP electron pulse

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Example of an encoder - > 45 mm x 45 mm active area

Therefore the converter must be thin (few m), but can be several cm in length,giving good solid angle coverage to collect charged particles emerging from the converter.

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Encoder shown mounted MCP electron amplifier

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

… but a progressive loss of resolution due to L/D beam broadening

Off-set stacking to gain increased imaging area …

Top view

Side view

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Potential Error Sources:

BeamDivergence

SurfaceRoughness

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Sample for Study

Intense Neutron Beam

DetectorSystem

Resulting Real-TimeHigh-Resolution Radiograph

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

into screen is the direction of neutron travel

An alternative converter geometry leading to more precise X-Y definition?

Encoder

Encoder

Cylindrical Converter

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Top View Top View

Alternative Converter Concepts with Increased Area

Side View Side View

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Weaknesses:

• Accuracy of stopping power tabulations

• Getting sufficient neutrons on the detector to take full advantage of resolution!

Challenges:

• Alignment of detector with beam direction

• Alignment of sample with detector

• Good quality material and precise production of the converter shape

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Strengths:

• Resolution in one dimension: 0.1 to << 0.01 m

• Resolution in 2nd dimension: > 1) 40 m or 2) << 0.01 m

• Speed – Real-time image building

• Reasonable coverage area 3 orders of magnitude

• Continuous coverage – virtually nonpixilated detector

• Position uncertainty determined with each data point

• Encoders are out of the neutron/gamma beam.

• Scanning of sample by moving it over the converter and time tagging each event with the known sample location.

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Extra Slides

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

• Sum along the length of the converter to increase statistics while at high resolution for 1D studies. Also gives realistic possibility for real-time analysis

• Single or few cell studies in biological specimens

• Grain boundaries in geological, metallurgical or biological specimens

Potential applications

8th World Conference on Neutron Radiography Gaithersburg, MD USA October 19 2006

Channel blocking

Random

Channeling

Charged-Particle Channeling

Possible measured energy variations due to converter crystal structure