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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Neutron ToF (ns)

Epro

ton (

keV

)

Neutron ToF (ns)

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4000

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Neutron ToF (ns)

Epro

ton (

keV

)

Neutron ToF (ns)

0

500

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0 1 2 3 4 5

14/15Fast Neutron Imaging

TEUs

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