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Delft University of Technology, Faculty of Applied Physics
Fast Neutron Imaging for SNM Detection
Victor Bom
Delft University of Technology
2/15Fast Neutron Imaging
Special Nuclear Materials
• Terrorist threat
• Detection by fast neutron emissions
• passive
• active
3/15Fast Neutron Imaging
• Plutonium n-emission (n.kg-1.s-1)
• 1 kg WGP
• 6% 240Pu + 94% 239Pu
• 6.104 n/s
• at 7 m distance
236Pu 3560238Pu 2660240Pu 920242Pu 1790244Pu 1870
01.07004
10.62
4
=π
n cm-2 s-1
Flux from 1 kg plutonium (WGP)
4/15Fast Neutron Imaging
Neutron back ground
• Neutron back ground
• cosmic
• sun
• earth crust
• Flux
• varies
• in time -> solar activity
• with height / location
• 10-3 n cm-2 s-1 MeV-1
• for 1-10 MeV 0.01 n cm-2 s-1
• equal to Pu rate at 7 m! M.S. Gordon et al., IEEE
TNS 51, no:6 (2004)
5/15Fast Neutron Imaging
Imaging
• Back ground reduction
• angular resolution, say 10o
• reduction factor
• now 1 kg WGP detectable up to 70 m distance
above back ground
• Need direction sensitive detector for fast neutrons
( )128
1
4
10tan
4
10tan 2
2
2
==oo
r
r
π
π
6/15Fast Neutron Imaging
Back ground from cargo
• Standard detection portals?
• not direction sensitive
• Activity present in normal cargo
• p.e. Tiles
• filling fraction 10%
• fraction K 1%
• 40K fraction 0.012%
• half life 109 yr
• decay by β-emission (80%)
6x106 Bq
7/15Fast Neutron Imaging
Detection principle
• One large organic scintillator
• Two successive n-p elastic scattering
• Determine:
• interaction positions
• energy scattered neutron En’
• direction scattered neutron
• energy of the first recoil proton p1
• Determine the incident neutron energy
• Calculate scatter angle Θ
• Construct cone
Common direction on several cones points to the source
p2
p1
nΘΘΘΘ
n'
'1 npn EEE +=
n
p
E
E 1arcsin=Θ
8/15Fast Neutron Imaging
Detector schematic
• Interaction positions
• light distribution on PMTs
• Time difference tp2-tp1
• scintillation light flash timing
• Energy first proton
• light intensity
• Positions and time differencegives En’ and direction scattered
neutron
• time differences ~ ns
• track lengths ~ cm
• Fast scintillator necessary
neutrons
plastic fast scintillator
PMTs onall sides
9/15Fast Neutron Imaging
Scintillation light pulses
NE111
decay: 1.4 ns
10200 photons/MeV
LaBr like
decay 16 ns
80000 photons/MeV
Perovskite
decay: 0.4 ns
4000 photons/MeV
10/15Fast Neutron Imaging
Position determination
• Light intensity
• pos ~
• suppose linear relation
• σ of 3 mm
• Time difference of light
• Anger principle
• accuracy ?
0.8 mmsumintensity
differenceintensity
L=10 cm
x
n
xnc
x
ncxL
ncxL
tt leftright
[ns/cm] 1.02
)()(
==
−−+=−
11/15Fast Neutron Imaging
Direction determination
• Assume
• time resolution 0.4 ns
• position resolution 5 mm
• energy resolution 16%
• Calculate (fully drawn lines)
• scatter angle
• 1 σ error ~ 12o
• Disregard events (dashed lines)
• Ep1 < 200 keV
• track length < 5 mm
• time difference < 0.4 ns
⇒ offset
12/15Fast Neutron Imaging
Efficiency
• n-p and n-C interactions
• n-C interactions
• small light yield ⇒ go undetected
• but change n-direction
• only n-p interactions useable
• for hydro-carbon scintillator (10 cm cube) ⇒ 27% of all events
• other scintillators?
33% for perovskite
(n-C6H13NH3)2PbI4
13/15Fast Neutron Imaging
Efficiency
• Simulation of 2.5 MeV neutrons in10 cm3
cube scintillator
• Ep1 versus time difference tp2-tp1
• theory: flat distribution of Ep
• Some events below detection limits
• tp2-tp1 below time resolution
• Ep1 below 200 keV limit
• assuming
• 200 keV lower energy limit
• 0.4 ns time resolution
• ⇒ 70% detected
• Overall efficiency 0.7x0.27 = 19%
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
0 1 2 3 4 50
20
40
60
80
100
120
Neutron ToF (ns)
Epro
ton (
keV
)
Neutron ToF (ns)
0
500
1000
1500
2000
2500
3000
3500
4000
0 1
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
0 1 2 3 4 50
20
40
60
80
100
120
Neutron ToF (ns)
Epro
ton (
keV
)
Neutron ToF (ns)
0
500
1000
1500
2000
2500
3000
3500
4000
0 1 2 3 4 5
15/15Fast Neutron Imaging
Application
• Port of Rotterdam
• Container stack
• 50 x 50 m2 ~ 2500/(2.5x12) ~ 80 TEUs
• stacked 4 layers ⇒ over 300 containers
• 1 kg Pu, 10 cm3 cube detector at 25 m, rate:
• back ground rate:
• in 10 minutes:
⇒ 90 Pu counts on a background of 14 counts
( )n/s cm 2 076.0100
25004
10.62
4
=π
( )n/s cm 2 011.010001.0
4
12tan2
2
=×r
r
π
π o
50 m
50 m