Delft University of Technology, Faculty of Applied Physics Fast Neutron Imaging for SNM Detection Victor Bom Delft University of Technology
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
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2500
0 1 2 3 4 50
20
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100
120
Neutron ToF (ns)
Epro
ton (
keV
)
Neutron ToF (ns)
0
500
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2500
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3500
4000
0 1
0
250
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0 1 2 3 4 50
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120
Neutron ToF (ns)
Epro
ton (
keV
)
Neutron ToF (ns)
0
500
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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