fast neutron imaging detectors · bc400 (ne102) • recoil protons are stopped and produce local...
TRANSCRIPT
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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
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Task: Detectors for Fast NeutronTask: Detectors for Fast NeutronResonance RadiographyResonance Radiography
position sensitive-detectors
FANGAS OTIFANTI
samples
Be-targetneutron beam
deuteron
beam
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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
N e u t r o n E n e r g y / M e V
2 4 6 8 1 00
1
2
3
4O - 1 6
cro
ss
se
cti
on
/ b
arn
s
N e u t r o n E n e r g y / M e V
Measurement of neutron energy is a prerequisite for Resonance Imaging
Resonance ImagingResonance Imagingexploiting neutron cross-section structures
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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
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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
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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
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Status
01/03
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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
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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)
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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
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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
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New DetectorDevelopment
FANGAS
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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
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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
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DETECTOR SETUPDETECTOR SETUP
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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
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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
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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
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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
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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
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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)
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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
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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)
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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
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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
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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)
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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)
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FANGASFANGAS
ExperimentalExperimentalResultsResults
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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
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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)
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Position Resolution and ContrastPosition Resolution and Contrast
open field imagefor flat fieldcorrection
Raw image
Corrected image
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Position Resolution and ContrastPosition Resolution and Contrast
20 40 60 80 100200
400
600
800
1000
dN/d
x
x / mm
60mm 40mm 20mm 10mm 7mm
ToDo:
• MTF plot
• Abltg fwhm
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Resonance ImagingResonance Imaging
Measurements with carbon cylinders and steel wrench
∅ 30 L20 ∅ 30 L60
∅ 30 L40
∅ 20 L20 ∅ 20 L60 ∅ 20 L40
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Resonance ImagingResonance Imaging
neutron energy : broad spectrum (2 - 10 MeV)acquisition time: 5.5 h1200 c/pixel (matrix 300 x 300 pixel)
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Resonance ImagingResonance Imaging
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
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New DetectorDevelopment
OTIFANTI
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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 →
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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
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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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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
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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)
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Properties and ResultsProperties and Results
TOF (ns)
• Camera(∆t ~ 10 ns)
• PM∆t ~ 2.5 ns)
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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
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Resonant imaging with OTIFANTIResonant imaging with OTIFANTI
Ratio ofimages
ON-image(7.7 MeV)
OFF-image(6.8 MeV)
processed image