V. Dangendorf, 10.02.03 1

V. V. DangendorfDangendorf, G. , G. LaczkoLaczko, C. , C. KerstenKersten

PhysikalischPhysikalisch--Technische Bundesanstalt Technische Bundesanstalt //BraunschweigBraunschweig, Germany, Germany

A. A. BreskinBreskin, R. , R. ChechikChechik, D. , D. VartskyVartsky

WeizmannWeizmann Institute / Institute / RehovotRehovot, Israel, Israel

Fast Neutron Resonance RadiographyFast Neutron Resonance Radiography

in a Pulsed Neutron Beamin a Pulsed Neutron Beam

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Radioscopy / RadiographyRadioscopy / Radiography

Radiationsource

X-raysneutrons

PositionSensitiveDetector

Object

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Digital Subtraction Radiography Digital Subtraction Radiography

A

B

E

σσ∆E1 ∆E2

Mat A

Mat B

Rad ∆E1

Rad ∆E2

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Properties of Fast NeutronsProperties of Fast NeutronsPROsPROs::

• high penetration powercomparable to MeV photons)

• low Z-dependence of cross sections(compared to Z - Z5 dependence of photons)

• isotope (element)-specific cross section characteristics

CONsCONs::

• complex and expensive neutron source installations

reactor, accelerator, target, shielding ...

• comparably weak sourcesneutron flux of strongest reactors is still comparable to photon flux of a candle

• difficult and little developed imaging techniquesproblems: efficiency, scattering, acquisition speed, time resolution ...

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2 4 6 8 100

1

2

3

4

C

cros

s se

ctio

n / b

arns

Neutron Energy / MeV

Cross sections of C, N, O Cross sections of C, N, Oin the in the MeV MeV regionregion

2 4 6 8 100

1

2

N-14

cro

ss s

ect

ion

/ b

arn

s

Neutron Energy / MeV

2 4 6 8 100

1

2

3

4O-16

cros

s se

ctio

n /

barn

s

Neutron Energy / MeV

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Fast-NeutronFast-Neutron Applications IApplications I Exploiting high penetration through high-Z material:

Imaging of low-Z structures behind massive high-Z shielding material

Example: (J.Hall, LLNL)

1” thick ceramic and polyethylenepolyethylene structure

behind shielding of

1” thick 238U

10 MeV neutron 8 MeV e--Bremsstrahlung

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Applications IIApplications IIExploiting cross section structure (resonance imaging):

Small objects of specific element distributions behind massive shielding

Examples:

1.) Detection of Diamonds enclosedin Mineral (Kimberlith)

Guzek et al (DeBeers / RSA)

Method: High intensity dual-energy neutron beam

utilising cross section structureof C around 8 MeV

2.) Luggage and Cargo inspection: by measuring C, N, O - distribution

• Overley et al (Ohio-University)

Method: white neutron beam with TOFtechniques for energy dependent transmission radiography

• Lanza et al (MIT):

Method: imaging with monoenergetic neutrons at several discrete energies

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Experimental

Technique

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- Transmission Image with selected, quasi-monoenergetic Neutron Energy

- Successive images at different energies

VariableVariable Monoenergetic Monoenergetic Neutron Beam Neutron Beam

deuteron beam

deuterongas target

neutron beam

Example:

deuterium projectile hitting gaseous deuterium target:

Requirements:

• high intensity deuteron beam (0,5 - 10 mA)

• high pressure windowless deuterium gas targets(e.g. 2 bar buffered towards 10-6 mbar beam tube)

• time structure of beam not relevant (DC, pulsed..)

• neutron energy selection by projectile energy or collision kinematics → variable energy accelerator→ angular variation of object & imaging system

• separate beam dump for several kW thermal powerEn ~ Ed + Q

Q(d(d,n)H3) ~ 3 MeV

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Planned at the LLNL Neutron Radiography facility

Example for Radiography System based on aExample for Radiography System based on aVariable Variable MonoenergeticMonoenergetic Neutron Beam: Neutron Beam:

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Multiple Transmission Images with Neutron Energy selected by

Neutron Time-Of-Flight (TOF)

Time-Of-FlightTime-Of-Flight MethodMethod

Requirements:

• “Medium” intensity deuteron beam (20 - 200 µA)

• solid Target (e.g Be)

• requires nanosecond beam pulsing

• neutron TOF ⇒⇒ neutron energy

• target acts also beam dump (i.e. needs cooling for about 1 kW thermal power

• need for imaging system with fast timing capability

Pulsed deuteronbeam

Be target

Pulsed neutronbeam

~ 1 µs

~ 1 ns

2TOF

2N

N 2 t

dmE

⋅=

tTOF

En(tTOF)Deuteron pulses:

• 1 ns width

• 0.5 - 1 µs distance

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1. Accelerator:

• Cyclotron (TCC-CV28)

• Ion beams: p → 2 - 24 MeV

d → 3 - 14 MeV

• Pulsing: via pulsed injection

- 1,5 ns (fwhm) wide - 500 ns pulse separation for TOF

• Beam current available:

- unpulsed: IB up to 25 (200) µA

- pulsed: IB up to 2 µµA

PTB Fast Neutron FacilityPTB Fast Neutron Facility

1

2

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2. N-production:(“high current” station with collimator)

• reaction: Be (d,n)

• Ed = 13 MeV

• thickness Be-Target: 3 mm

• beam spot: < 3 mm (but at present no online control)

Fast Neutron ProductionFast Neutron Production

2 4 6 8 10 120

200

400

600

800

1000

1200

YΩ

, E /

Q /

[1012

/(sr

C M

eV)]

energy / MeV

Forward Neutron Yield:

Y = 10 16 / (sr C)

For typical experimental setup distance source - detector: 3 m beam current: 2 µA

Differential forward neutron yield fromthick target Be(d,n) / Brede (89):

neutron flux at detector position:

ϕϕ ~ 3* 105 s-1 cm-2

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Selection of Sample MaterialSelection of Sample Material

3 4 5 6 7 8 90

1

2

3

4

SMAX

1

SMIN

1

SMAX

2

SMIN

2

cros

s se

ctio

n / b

arns

Neutron Energy / MeV

Carbon cross section and energybins for resonance imaging:

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Experimental Experimental SetupSetup of Fast Neutron of Fast NeutronRadiography ExperimentRadiography Experiment

C-samples

Be-target

collimator

neutron beamdeuteron beam

position sensitive-detectors:

FANGAS OTIFANTI

3 -3,5 m

5 - 100 cm

neutron flux per uA beam at detector position 3 m:

ϕϕ ~ 1,5 * 105 / (µµA s cm2)

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OTIFANTI:OpTIcal FAst NeuTronImaging system

Experimental Experimental SetupSetup of Fast Neutron of Fast NeutronRadiography ExperimentRadiography Experiment

pulsed neutron sourcewith collimator(13 MeV d→Be)

FANGAS: FAstNeutronGAS-filledimaging chamber

Sample: stack ofgraphite cylinders

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Detectors

Present Status

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Imaging Techniques with Imaging Techniques withTime-Of-FlightTime-Of-Flight MethodMethod

Task:Task: Simultaneous acquisition ofPosition Coodinates (X,Y) and TOF

1. Neutron Counting Imaging Techniques:

• Each Neutron is individually registered

• relevant parameters (X,Y, TOF) are measured andstored in

- 3-dimensional Histogramm- List Mode file

Advantage:

Full Information is obtained and available offline (forLM storage)

Disadvantage:- Slow (several MHz max speed)

- For LM storage: excessive diskspace required

- Dedicated Detector development necessary

2. Integrating Imaging Techniques:

• Image is captured in segmented (“pixeled”) detectors

• quantum structure is lost, only integrated “currents” intoimage cells are measured

• Requires storage structures of sufficient size and dimension(e.g. X,Y, TOF: multiple frame CCD, each frame captures image for different energy window)

Advantage:• Very high data capture rate

• Based almost entirely on industrially available techniques

Disadvantage:• requirement for proper adjustment of exposure

timing at runtime

• Fast high frequency exposure system needs some development

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FANGASFANGAS Principle of OperationPrinciple of Operation

• Neutrons interact (sometimes) in thinfoil converter (1mm PE)

• recoil protons escape from foil

• protons ionise gas along track

• electrons from gas in region close tofoil surface are amplified in ParallelPlate Avalanche Chamber (PPAC)

• wire chamber (MWPC) for finalamplification and localisation bycathode delayline readout

• TOF and position are stored inListmode or 3-d matrix

FAst NeutronGAS-filled

imaging chamber

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OTIFANTIOTIFANTIPrinciple of OperationPrinciple of Operation

• Neutrons interact in scintillatorBC400 (NE102)

• recoil protons are stopped within fewmm and produce local light spot

• optics (mirror and lens) transferimage to photon counting imageintensifier or fast framing camera(Hadland ULTRA 8)

• separate photomultiplier (PM)delivers fast trigger signal

OpTIcal FAst NeuTron Imaging system

PM

lens

Mirror

BC400(22*22 cm2

d = 10 mm )

image intensifieror fast framing

camera (ULTRA 8)

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OTIFANTI with ULTRA8OTIFANTI with ULTRA8Fast Framing CameraFast Framing Camera

• Intensified CCD camera

• segmented photocathode with 8 indepen-dently gatable frames (a 512*512 px)

• Short gating time (down to 10 ns per shot)

• Long integration time (about 1 s withreasonable noise)

• Repetitive (periodic) exposure phase-locked to beam pulse⇒ simultaneous integration of images withneutrons of up to 8 selected energy bins∆

E1

∆E

2

∆E

3

∆E

4

∆E

5

2 4 6 8 10

0

200

400

600

800

1000

1200

energy / MeV

YΩ,

E /

Q /[

1012

/(sr

C)]

∆E

6

∆E1 ∆E2

∆E3 ∆E4

∆E1

∆E5 ∆E6 ∆E6

∆E4

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OTIFANTI with ULTRA8OTIFANTI with ULTRA8Fast Framing CameraFast Framing Camera

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Imaging properties (FANGAS)Imaging properties (FANGAS)

Radiographic images of Carbonsamples obtained with FANGASintegrated over all neutron energies

Images obtained after correcting for

flatness of field andefficiency and linearity inhomogeneities of detector

d= 5 mm

l=20 mm3 cm

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Imaging properties (FANGAS)Imaging properties (FANGAS)

Differential Imaging withFANGAS

a) “off” resonance image

b) “on” resonance image

c) differential image (smoothed)

Comment:

(taken from earlier paper of Watterson et al):

“...because of the difficulties withsources and the low efficiencies ofdetectors, images are often limited byPoisson statistics”

_

=

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Imaging properties (OTIFANTI)Imaging properties (OTIFANTI)

Images with OTIFANTI and intensified UV-CCD camera

d-beam current: 20 µA

no energy windows selected, exposure time: 30 s (300 imagesat 100 ms / frame)

3 cm

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Summary of present statusSummary of present status

FANGAS: . - Detector worked well but has low detection efficiency: εFA ~ 0,2 % - Data Acquististion slow : ~ 104 s-1 at present

required : 106 s-1

OTIFANTI:

a) with Ultra8 framing camera: - small optical efficiency due to problem with image splitter - limited pulsing possibility (present frame exposure rate: ~ 2500 s-1, required: 2*106 s-1 )

b) with present standard intensified camera: - due to integrating system →→ high acquisition speed

- only single frame possible, i.e. 1 energy range per exposure cycle- optical efficiency needs improvement (at present ~ 60 % QE per absorbed neutron

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NewDetector

Development

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Detector Development: FANGASDetector Development: FANGAS

Efficiency problem:

larger efficiency by stackingof detectors ⇒ 25 Dets provide 5 %

Requirements:- simple and industrial production- robust and easy to operate- cheap high rate readout system

(several 100 kHz / module)

1 2 3 . . . . . .25

neutrons

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Detector Development IDetector Development IFANGASFANGAS

• neutrons scatter with protons in PE-radiator

• protons produce electrons in conversion gap

• electrons are amplified in multistage GEM structures

• final electron avalanche is collected on resistive layer

• moving electrons induce signal on pickup electrode

• integrated delayline structures encode position information

GEMsPE-radiator

(neutron-converter)

resistive layeron insulator

multilayer PCB(pickup, delay lines)

neutron

proton

conversion gap

~ 12 mm

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conversionand drift

amplification

transfer andsignal induction

Cross section view of 1 hole

All photos taken from F. Saulis webpage: http://gdd.web.cern.ch/GDD/

∅ hole ~80 µm, ∅ hole center ~70 µm,

hole spacing: 140 µm

~70 µm

80 µm

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Detector Development IIDetector Development IIOTIFANTIOTIFANTI

Modifications of Otifanti:

• Optical Preamplifier to increase light detection efficiency

• Modifying ULTRA8 to-enable repetitive triggering for about 1s with > 1 MHz rate- increase sensitivity in near UV

• Replace ULTRA8 by separate cameras which can be individually triggered with 2 MHz repetition rate

mirror

PM

lens

scintillatorscreen

fast framingcamera (ULTRA 8)

optical preamp(image intensifier)

lens

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OPTICAL PREAMPLIFIEROPTICAL PREAMPLIFIER

photocathode

MCPselectron amplifier

phosphor

hν

hν’

e-

∅ 75 mm

ιd < 2 ns

t →

Fast light decay in phosphorto preserve time resolution

I

-250 V 0 V

2 kV

8 kV

!

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ULTRA 8 - REQIREMENTSULTRA 8 - REQIREMENTS

Further use of ULTRA8 depends on

• Achieving sensitivity in UV to avoid wavelenght shifter(namely in the λ= 370 - 470 nm region)

- new beam splitter (proposed by manufactuer)

• Implementation of fast (> 1 MHz) repetitive photocathode-pulsing

- upgrade of pulser electronics by manufacturer

• Implemetation of more flexible trigger schemes

- external access to HV-pulsers

- modification of camera firmware by manufacturer

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Individual ICCD CamerasIndividual ICCD Cameras

PM

lens

mirrorBC400 screen

(22*22 cm2

d = 10 mm )

separate ICCDcameras

optical preamp(image intensifier)

lens

Individual Intensified CCD cameras (ICCD) view preampintensifier at slight angles to optical axis

each ICCD is independently triggered and read out

modular from 1 up to 9 cameras (first step: 1 camera for testing concept)

Estimate of optical efficiency: (assuming standard ICCD)90 pe- / n (fiberplate outp. window)720 pe- / n (fused silica outp.window)

Problems to be solved:Funding (~ 25 k$ per camera)PC-Pulser with 2 MHz repetition rate (e.g. Photek GM 150-20 (modified) DEI HV Modules