Fast Neutron Imaging Detectors · BC400 (NE102) • recoil protons are stopped and produce local light spot • optics (mirror and lens) transfer image to photon counting image intensifier

V. Dangendorf, 25.06.04 1

Fast Neutron Imaging DetectorsFast Neutron Imaging DetectorsNew DevelopmentsNew Developments

V. Dangendorf / PTB Braunschweig

D. Vartsky / NRC Soreq

A. Breskin / Weizmann Institute, Rehovot


Task: Detectors for Fast NeutronTask: Detectors for Fast NeutronResonance RadiographyResonance Radiography

position sensitive-detectors

FANGAS OTIFANTI

samples

Be-targetneutron beam

deuteron

beam


2 4 6 8 1 00

1

2

3

4

C

cro

ss

se

cti

on

/ b

arn

s

N e u t r o n E n e r g y / M e V

2 4 6 8 1 00

1

2

N - 1 4

cro

ss

se

cti

on

/ b

arn

s


2 4 6 8 1 00

1

2

3

4O - 1 6

cro

ss

se

cti

on

/ b

arn

s


Measurement of neutron energy is a prerequisite for Resonance Imaging

Resonance ImagingResonance Imagingexploiting neutron cross-section structures


Detector Requirements forDetector Requirements forFast Neutron Resonance RadiographyFast Neutron Resonance Radiography

• Large area: > 30x30 cm2

• Detection efficiency: > 5 %

• Insensitivity to gamma-rays

• High counting rate capability: ?

• Neutron spectroscopy in 2-12 MeV range

• Energy resolution: ~ 500 keV at 8 MeV

• Position resolution: 0,5 mm


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

Task: Task: Simultaneous acquisition of position params (X,Y) and TOF

1. Neutron Counting Imaging Techniques:

• Each Neutron is individually registered

• relevant parameters (X,Y, TOF) are measured and stored in- 3-dimensional Histogramm- List Mode file

PRO:

• Full correlation of all Parameters is available offline

• Multidimensional Imaging feasible

CON:

• Slow (max several MHz speed)

• For LM storage: excessive diskspace required

• detector development necessary


Imaging Techniques with Imaging Techniques withTime-Of-Flight CapabilityTime-Of-Flight Capability

2. Integrating Imaging Techniques:

• Image is captured in segmented (“pixelized”) detectors⇒ quantum structure is lost, integrated “currents” into image cells are

measured

• Requires capture and storage structures of sufficient size and dimension(e.g. X,Y, TOF requires multiple frame CCD camera system

Task: Task: Simultaneous acquisition of position params (X,Y) and TOF

PRO:

• Very high data rate

• Based on industrially available techniques

CON:

• necessity for proper parameter selection at runtime

• fast high frequency exposure system needs some development


Status

01/03


FANGASFANGAS Principle of OperationPrinciple of Operation

• Neutrons interact in thin foilconverter (1mm PE)

• recoil protons escape from foil

• ionisation electrons are amplified inParallel Plate Avalanche Chamber(PPAC)

• wire chamber (MWPC) for finalamplification and localisation

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

FAst NeutronGAS-filled

imaging chamber


OTIFANTIOTIFANTIPrinciple of OperationPrinciple of Operation

• Neutrons interact in scintillatorBC400 (NE102)

• recoil protons are stopped andproduce local light spot

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

OpTIcal FAst NeuTron Imaging system

PM

lens

Mirror

BC400(22*22 cm2

d = 10 mm )

image intensifieror fast framing

camera (ULTRA 8)


OTIFANTI with ULTRA8OTIFANTI with ULTRA8Fast Framing TechniqueFast Framing Technique

• Intensified CCD camera

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

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

• Repetitive exposure (2MHz) triggered withbeam pulse for predefined TOF window

⇓⇓

8 images, each for a differentneutron energy

∆E

1

∆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


Summary :Summary :

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

required : > 105 s-1

OTIFANTI:

a) with 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 5 MeV neutron


New DetectorDevelopment

FANGAS


larger efficiency by usingdetector cascade⇒ 25 Dets provide 5 %

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

(> 100 kHz / module)

1 2 3 . . . . . .25

neutrons

Enhancing EfficiencyEnhancing Efficiency


GEM-FANGASGEM-FANGAS

• neutrons scatter with protons in PE/PP-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

GEMsPP-radiator

(neutron-converter)

resistive layeron insulator

R/O pads,delay lines)

neutron

proton

conversion gap

~ 12 mm


DETECTOR SETUPDETECTOR SETUP


1. Efficiency 1. Efficiency OptimsationOptimsation

Simulation Tool: GEANT 3

Efficiency vs. foil thickness of a polypropylene converter foil:

“Efficiency” is identified as charged particle escape (mainly protons)

Conclusion:

• Appropriate Foil thickness for neutrons of 2 MeV < En < 10 MeV is 1 mm

• Efficiency is 0,05 % - 0, 3 % per detector unit

0.0 0.5 1.0 1.5 2.00.0

0.1

0.2

0.3

Effi

cien

cy /

%

Foil Thickness / mm

En = 2 MeV

En = 7,5 MeV

En = 14 MeV


2. Detector Optimisation2. Detector OptimisationSimulation of Point Spread Function (PSF)Simulation of Point Spread Function (PSF)

Conclusion:

• fwhm is of the order of 0,5 - 1 mm

• Appropriate readout circuitry should have corresponding resolution

1 mm PPconverter

protonsPixel plane

50x50 micron pixels

neutrons

0.5

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e N

umbe

r of

Pro

tons

Distance from point of interaction / mm

En = 2 MeV

En = 7,5 MeV

En = 14 MeV

Simulation Tool: GEANT3


Neutron Scatter and ContrastNeutron Scatter and ContrastThe Simulation Configuration

Polypropylene sheets (1 mm thick)

. . .

1Detector

2 3Det. 25

Samples

NeutronSource

0 300 623 624.8 644.7644.6624 cm

Carbon

Fe

Poly-propylene

Al

Simulation Tool: MCNP4

(by I. Mor)

En = 7.8 MeV


Neutron Scatter and ContrastNeutron Scatter and ContrastTransmitted vs. Scattered RadiationTransmitted vs. Scattered Radiation

• Number of scattered neutrons increases initially with detector number until it reaches maximum at around detector 13

• The T/S ratio drops sharply until detector 13

• After det. 13 the ratio is fairly stable


Detector OptimisationDetector Optimisation 3. 3. Position ReadoutPosition Readout

Requirements:• few electronic channels per detector element• dead time < 500 ns• 100 kHz rate capability per element

Solution:•delayline readout (5 channels / element)• resistive anode technique to obtain

– sufficient charge spread of signal onR/O pads

– galvanic decoupling between detectorand readout

– limiting discharge energy ( to protectpreamps)

GEM

charge cloudin

induction gap

resistive anode

insulator

R/O board

2 mm


Resistive Anode TransparencyResistive Anode Transparency

Remarks:

• C-lacquer is simple, cheapest and best suited for large surfaces but requires R-tuning to achieve better transparency

• Stability of Ge-layer 5 weeks:

1st 5 weeks: R increases by 10 - 20 %

1 year (meas.: Apr. 2004): R increased by factor 2

R Transp. X/SUM Y/Sum

(Mohm) (%) (%) (%)Ge-160nm/G10(1) 30 94 59,2 40,8 1,45 1,14Ge-30nm on G10 370 95 58,7 41,3 1,42 1,14

C-lacquer(1) 3,1 71 59,6 40,4 1,48 1,14C-lacquer(2) 3 65 59,2 40,8 1,45 1,14

Ge-160nm/G10(2) 30 101 59,6 40,4 1,48 1,14

Electrode X/Y X/Y(fast)


Readout Electrode and Readout Electrode and DelaylineDelayline

• Position Encoding via Delay Line Readout

• Signal induction to pads (“diamonds”) ofpickupelectrode

• Pads are interconnected in lines (backside) and rows (frontside)

• non-overlapping pads on front and backside to minimize capacitive cross talk

• π-delayline with SMD-parts integrated toelectrode

C2C1

L

Z = 100 Ω, τd = 2,7 nstotal length: 135 ns


Optimisation of Pad StructureOptimisation of Pad Structurehorizontal charge distributionhorizontal charge distribution

Measurement method:

• irradiating of double GEM detector with 5, 9 keV X-ray beam from 55Fe source

• width of X-ray beam in conversion gap: 0,47 mm

• 160 nm Ge-anode on glass → 63 MΩ•

PA 1

PA 2

PA 3

Ch 2

Ch 1

TDS3052

ExtTrig

R/O electrode

R/O electrodeGe on glassdouble GEM

d

55Fe5,9keV

Ar / CO2p=1 bar

• recording ratio of charge on central strip to total vs source position

• Variation of d to match lateral width of induced charge with pitch of strips(2 mm)


Optimisation of Pad StructureOptimisation of Pad Structurehorizontal charge distributionhorizontal charge distribution

16 18 20 220

20

40

60

80

100

120

140

rela

tive

char

ge /

%

position / mm

Experiment 4 (d=1.0 mm)

Gauss Fit X

c=18.9 mm

σ=0.86 mm

14 16 18 20 22 24 260

10

20

30

40

50

60

rela

tive

char

ge /

%

position / mm

Experiment 2 (d=2.6 mm)

Gauss Fit X

c=20.1 mm / σ=2,19 mm

fwhm:5,1 mm

fwhm:2,0 mm

Result:

2 mm pitch of readout pads selected

⇒⇒Optimum gap between anode and

RO pads is d = 1 mm

previous “knowledge” from wire chamberexperiments: w ~ d is not valid !

d = 2,6 mm

d = 1,0 mm


Optimisation of Pad StructureOptimisation of Pad Structurevertical charge distributionvertical charge distribution

Measurement method:• irradiating double-GEM detector

with 5, 9 keV X-ray beam from 55Fe source

• recording ratio of charge from front- to back side of R/O pads

• Variation of pad size (area covered by front and back side structure)

•

PA 1

PA 2

Ch 1

TDS3052

Ch2

R/O electrode

from backside

front side back side

Goal:• equal charge on front and back side of R/O pad


Optimisation of Pad StructureOptimisation of Pad Structurevertical charge distributionvertical charge distribution

Result:

strongly asymmetric size of

readout pads required

Optimized values for 0,5 mm boards and

2 mm pitch:

front side pads: 0,5 mm

back side pads: 1,5 mm

0.0 0.5 1.0 1.5 2.00

1

2

3

4

5

char

ge r

atio

ratio of areas (Af / A

b )

Qfront

/Qback

linear fit (without 2 last points)


Interface and DAQInterface and DAQ

DAQ:DAQ:

CAMDA

• WIN based ⇒ unreliable,• ca 35 kevt/s

ATMD/LEA

• Linux based

• Frontend: ATMD F. Kaufmann, PTB

• Backend: SATAN M. Krämer, GSI

• Rate capability: ca 90 kevt / s

TDC:TDC: ATMD-board from ACAM ATMD-board from ACAM• F1/ATMD PC-hosted 8 channel TDC

with 125 ps resolution, 7us full range

• Q-T converter LeCroy MTQ100

• FIFO-buffered output ⇒ dt < 1 µs • data-throughput about 1 MWord/s

(about 100 kEvts/s)


FANGASFANGAS

ExperimentalExperimentalResultsResults


Energy SpectrumEnergy Spectrum

2 4 6 8 10 120

200

400

600

800

1000

1200

YΩ

, E /

Q /[

1012

/(sr

C M

eV)

Neutron Energy / MeV

Compare:Neutron yield in forward direction for13 MeV deuterons on thick Be target[Brede et al]

Measured energy spectrum with FANGASwith and without 8 cm C absorber(not efficiency corrected)

2 4 6 8 10

20

40

60

1

2

3

σσ N /

bndN/d

E

EN / MeV

no object 8 cm carbon carbon n x-section


Position Resolution and ContrastPosition Resolution and Contrast

7 mm 10 mm 20 mm

40 mm

60 mmn beam

1 2 3 5 10 mm

Measurements with step wedge

made of polyvinyltoluene leaves (NE102)



open field imagefor flat fieldcorrection

Raw image

Corrected image



20 40 60 80 100200

400

600

800

1000

dN/d

x

x / mm

60mm 40mm 20mm 10mm 7mm

ToDo:

• MTF plot

• Abltg fwhm


Resonance ImagingResonance Imaging

Measurements with carbon cylinders and steel wrench

∅ 30 L20 ∅ 30 L60

∅ 30 L40

∅ 20 L20 ∅ 20 L60 ∅ 20 L40



neutron energy : broad spectrum (2 - 10 MeV)acquisition time: 5.5 h1200 c/pixel (matrix 300 x 300 pixel)



Processed (median filter) ratio

ON

OFF

RATIO

1700 1750 1800 1850 1900 1950 2000

4000

6000

8000

dN/N

TOF / ns

Full Behind 60 mm C

OFF ON


New DetectorDevelopment

OTIFANTI


OTIFANTIOTIFANTI

lens

mirror

separate ICCDcameras

Scintillatingfiber screen

Improvements• Thicker scintillating screen (20 mm)• Better lens (F# = 1.0)• Larger diameter intensifier (40 mm)

⇒⇒Factor 17 increase in overall detection

efficiency

Multiple-Energy Imaging• Large diameter ungated

optical preamp with fastphosphor (E36)

• Multiple II CCD cameras,each gated on a differentenergy window

ιd < 2 ns

t →


OTIFANTIOTIFANTI

Presently available:• 1 camera which can be individually

triggered with 2 MHz repetition rate

PM

lens

BC 400scintillator

screen

Couplinglenses

gatedintensifier

mirror

Cooled CCDcamera


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

!


New Optical DetectorNew Optical DetectorFast Gated IntensifierFast Gated Intensifier

∅ 40 mmhν

hν’

e-

+50 - 250 V

0 V

2 kV

8 kV

photocathode

MCPselectron amplifier

phosphor

gating electrode


Intensifier Exposure ControlIntensifier Exposure Control High Voltage Gating UnitHigh Voltage Gating Unit

Requirements:Gating control:• Computer Control

(GDG via RS232 from Weierganz/Mugai)

• Phase locked to Cyclotron HF

High Voltage Pulser:• < 10 ns- pulse width

• 2 MHz repetition rate

• 250 Vpp( +50 bis - 200 V)


Properties and ResultsProperties and Results

TOF (ns)

• Camera(∆t ~ 10 ns)

• PM∆t ~ 2.5 ns)


7.7 MeV image10 min 100 c/pixel

6.8 MeV10 min ~ 100c/pix

ResultsResultsResonant imaging with OTIFANTI

Gamma- image Background image All-energies1 min 160 c/pixel

Carbon phantom

TOF

γ n


Resonant imaging with OTIFANTIResonant imaging with OTIFANTI

Ratio ofimages

ON-image(7.7 MeV)

OFF-image(6.8 MeV)

processed image

Documents

Fast Neutron Imaging Detectors · BC400 (NE102) • recoil protons are stopped and produce local light spot • optics (mirror and lens) transfer image to photon counting image intensifier