Fast Neutron Imaging for SNM Detection

Transcription

1 Fast Neutron Imaging for SNM Detection Victor Bom Delft University of Technology, The Netherlands Delft University of Technology, Faculty of Applied Physics

2 Special Nuclear Materials Terrorist threat Detection by fast neutron emissions passive active intensities in (kg.s) -1 2/21

3 Flux from 1 kg plutonium (WGP) Plutonium n-emission (n.kg -1.s -1 ) 236 Pu Pu Pu Pu Pu kg WGP 6% 240 Pu + 94% 239 Pu n/s at 7 m distance π = 0.01 n cm -2 s -1 3/21

4 Neutron back ground Neutron back ground cosmic sun earth crust ship effect 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) 4/21

5 Imaging Back ground reduction angular resolution, say 10 o reduction factor π ( o r tan10 ) 4πr 2 2 = 2 tan 10 4 o = now 1 kg WGP detectable up to 70 m distance above back ground Need direction sensitive detector for fast neutrons 5/21

6 Back ground from cargo Standard detection portals? not direction sensitive Activity present in normal cargo p.e. Tiles filling fraction 10% fraction K 1% 40 K fraction 0.012% half life 10 9 yr 6x10 6 Bq decay by β-emission (80%) 6/21

7 Detection principle Two successive n-p elastic scattering Determine: interaction positions energy scattered neutron E n direction scattered neutron energy of the first recoil proton p 1 Determine the incident neutron energy p 1 Θ n n' p 2 E = E + n E p1 n' Calculate scatter angle Θ Construct cone Θ = arcsin E p 1 Common direction on several cones points to the source E n 7/21

8 Existing systems P.E. Vanier et al 8/21

9 Existing systems Bravar et al. 9/21

10 Detector schematic Interaction positions light distribution on PMTs Time difference t p2 -t p1 scintillation light flash timing Energy first proton light intensity Positions and time difference gives E n and direction scattered neutron time differences ~ ns track lengths ~ cm Fast scintillator necessary neutrons plastic fast scintillator PMTs on all sides 10/21

11 Scintillation light pulses NE111 decay: 1.4 ns photons/mev LaBr like decay 16 ns photons/mev 11/21

12 Position determination Light intensity pos ~ intensity difference intensity sum 0.8 mm suppose linear relation σ of 3 mm n Time difference of light t right t left = ( L+ x) ( L x) c n c = 2x = 0.1[ns/cm] c n n x x L=10 cm Anger principle accuracy? 12/21

13 Experimental test 252 Cf Lead shielding PM NE111 PM 8GHz sampler 13/21

14 Energy response Back ground no lead shield 252 Cf source no lead shield 252 Cf source with lead shield 14/21

15 Scintillation light pulses BTP3_14 3-bromo-p-terphenyl rise: 0.18 ns, decay: 0.57 ns 3300 photons/mev NE111 decay: 1.4 ns photons/mev 15/21

16 Sample pulse shapes

17 Direction determination Assume time resolution 0.4 ns position resolution 5 mm energy resolution 16% Calculate (fully drawn lines) scatter angle 1 σ error ~ 12 o Disregard events (dashed lines) E p1 < 200 kev track length < 5 mm time difference < 0.4 ns offset 17/21

18 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? 18/21

19 Efficiency Simulation of 2.5 MeV neutrons in10 cm 3 cube scintillator E p1 versus time difference t p2 -t p1 theory: flat distribution of E p 2500 Some events below detection limits 2250 t p2 -t p1 below time 2000 resolution E p1 below 200 kev 1750 limit assuming 200 kev lower energy 1000 limit 0.4 ns time resolution 70% detected Eproton (kev) Neutron ToF (ns) Overall efficiency 0.7x0.27 = 19% Eproton (kev) Neutron ToF (ns) Neutron ToF (ns) Neu 19/21

20 TEUs 20/21

21 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 cm 3 cube detector at 25 m, rate: cm 2 = n/s 2 4π 2500 back ground rate: in 10 minutes: π ( ) o 2 ( r tan12 ) cm 2 = n/s 4πr 90 Pu counts on a background of 14 counts 2 50 m 50 m 21/21

22 Thank you 22/21

23 23/21