Neutron TOF spectroscopy in a single-shot nanosecond neutron pulsed technique for a disclosure of hidden explosives and fissile materials GRIBKOV V.A. Institute of Theoretical and Experimental Physics, Moscow, R.F.
COLLABORATION Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland ACS Ltd., Warsaw, Poland Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland Institute of Atomic Energy, Otwock-Swierk, Poland A.A. Baikov Institute of Metallurgy and Material Sciences, Russian Academy of Sciences, Moscow, Russian Federation
CONTENTS Introduction NINIS a single-shot technique for disclosure of hidden objects MCNP modeling Experimental technique Results and discussions Conclusions
INTRODUCTION Spectroscopy of neutrons elastically and/or inelastically scattered upon an object with a use of the Time-Of-Flight (TOF) technique demands long enough flight bases to convert the temporal neutron pulse shape into the energy spectral distribution Use of classical neutron sources like generators with direct electrostatic acceleration of deuterons having neutron pulse duration of 1 µs has a necessity of the base of the order of hundred meters At this distance neutron signals recoded by a PMT appears to be of a very low intensity
Classical sources with short-pulse (ns) neutron radiation are usually have low intensity by themselves Among promising approaches to the problem of interrogation of hidden objects the methods, which use neutrons, are of a pertinent interest at present time. Usually these methods exploit sources with long-pulse or continuous neutron radiation such as isotopes and classical neutron generators (direct-type accelerators 108-10 n/shot for 1-10-μs pulse s duration) or low-power sources like the Van der Graff accelerator: 103 n/shot during a 2-ns flash These methods ensure the necessary solutions; yet the techniques meet some awkward problems
Between them the most important one is rather low signal-to-noise ratio at the detection part of a system The problem results in a necessity to produce many shots with the above neutron sources (for generators > 106 pulses) or in a long operation time (for isotope sources > half an hour) and thus in high activation and in a long period of an interrogation of objects Dense Plasma Foci devices (DPF) of small sizes (having a 1-m2 footprint) might occupy a niche within the contemporary neutron-based methods of unveiling of hidden objects; they can produce short (τ 10 ns) and bright flashes of neutrons (up to 1020 n/s ster.) and hard (and soft) X-rays of a few J at once
The aim of the work is to demonstrate advantages of NINIS: Nanosecond Impulse Neutron Investigation System - a technique based on an interaction of a single powerful nanosecond pulse of neutrons with matter developed for a disclosure of hidden objects (explosives and fissile materials) Our experimental system consists of a Dense Plasma Focus (DPF) device as a neutron source generating pulses of almost monochromatic (ΔE/E 1-5%) 2.45-MeV and/or 14.0-MeV neutrons in the nanosecond ( 10 ns) range plus detectors with subns time resolution
NINIS a single-shot technique for disclosure of hidden objects We use elastic for explosives and inelastic for fissile materials - scattering of our almost monoenergetic neutrons (ΔE n /E 0 3-5%) produced in D-D or D-T nuclear fusion reaction upon nuclei of unknown elements Neutron source Neutron Nucleus of an element Target of atomic mass M Scintillator Flash Recoil proton
After this collision the elastically scattered neutron will change its energy (and speed v) depending: 1) On mass of the nucleus-scatterer 2) On angle of scattering
Energy change (lower speed v of scattered neutrons) will result in a later time of arrival of the scattered neutrons to PMT+S compared with the arrival time of direct neutrons Inside the scintillator all neutrons will produce a distribution of brightness of flashes depending on impact parameters, i.e. on energies of recoil protons appeared inside the scintillator block after collisions with neutrons But for 10 1 2 neutrons a linear dependence is proved for the amplitude of an averaged nanosecond signal versus number of neutrons This dependence will be violated only when microspheres of the action of recoil protons start to overlap each other, i.e. at overloading. However for our scintillator block it will be at 10 15 neutrons per this block (<< our load)
MCNP modeling We undertake attempts to simulate scattering of 2.45-MeV and 14-MeV neutrons from various objects by means of full MCNP calculations using standard MCNP-5, version 5, developed at Los Alamos National Laboratory A special so-called input programme to the MCNP code that allows following neutrons time histories has been prepared for our purposes Here as an example we present its usage in particular to simulate scattering of neutrons by a long object (a 1-meter high-pressure aluminum cylinder filled with deuterium at 70 atm); its length l was: l > v x τ (v speed, τ pulse duration of neutrons)
1 b) 3 2 z 1 2 3 a) Scheme of numerical experiment (a), energetic distribution (b) and temporal behaviour (c) of neutrons coming to the detector: 1 direct neutrons (2.45 MeV), 2 neutrons scattered by aluminium, 3 neutrons scattered by deuterium c)
Y Detector n Target r=5 cm: U or Th or Al theta_max theta_centre theta_min neutrons 2.45 or 14 MeV 15 Detector 1 Z 15 r=100cm Detector s surface Detector n Geometry of the fissile materials computer modeling
Neutron fluence [n/cm2/ns] Beam: 14 MeV & Gauss time profile with FWHM=10 ns, Target: Uranium 10% enriched 15_deg 30_deg 45_deg 60_deg 75_deg 90_deg 105_deg 120_deg 135_deg 150_deg 165_deg 5,0E-07 4,5E-07 4,0E-07 3,5E-07 3,0E-07 2,5E-07 2,0E-07 1,5E-07 1,0E-07 5,0E-08 0,0E+00 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 TOF [ns] The expected oscilloscope trace at a 1-meter distance of a PMT
Experimental technique PF-6 Transportable device PF-6: 7 кдж, 750 ka, 10 9 D 2 or 10 11 DT n/pulse, weight - 400 кг (IPPLM)
PF9 chamber PF7 chamber Neutron-producing chambers of PF-6 device
Fast detectors Fast MCP-based detectors at the Bora facility (ICTP)
Pulse Amplitude [a.u.] 0,10 DIFFERENT PMTs TIME RESPONSE COMPARISON 0,00-0,10-0,20-0,30-0,40-0,50-0,60-0,70-0,80 H1949-51 Module (S/N WA5952) - Clasic PMT with 10 ldynodes arranged in inear structure; HVPS -1.4 kv; FWHM = 3.12 ns H6780 Metal Package PMT; HVPS - 0.8 kv; FWHM = 1.3 ns -0,90 MCP-PMT Chevron R3809U-52 type; HVPS - 3 kv; FWHM = 0.276 ns -1,00-5,0-4,0-3,0-2,0-1,0 0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0 9,0 10,0 Time [ns] Comparison of the response time (registration of cosmic neutrons) for three types of neutron probes used in experiments on the PF-6 device
a) b) 2 5 3 4 1 c) Low (a), medium (b) and high (c) intensity of 14-MeV neutrons generated by PF-6: 1- intrinsic and 2- prompt-gamma X-Rays, 3, 4 and 5 direct neutrons and neutrons scattered by Al and D 2
PF-6 with the chamber filled with DT mixture, with fuel element EK-10 and with shields
General view of the experimental hall
Work-place
Results and discussions Typical scheme of the experiment
For calculations of positions and amplitudes of the expected peaks of direct and scattered neutrons on the oscilloscope traces we took into consideration: - average energy of neutrons (in this set of experiments E 0 =2.55 MeV) irradiating a target - energy of neutrons directly arrived to the scintillator from the DPF chamber E 0 =2.70 MeV - angle, at which we produce irradiation of the target ( ), as well as angles of direct beams propagation to the S+PMT ( and ) and scattering beams ( )
- distances from the source to the axis of the bottle L and from the target center to the scintillator of PMT l - effective irradiation angle of the target for our geometry ( ) - time delay of neutron maximum in relation to the front of hard X-Rays inside the DPF chamber (6 and 13.5 ns respectively for the above presented shots) - respective cross-sections of elastic scattering of neutrons on the particular nuclei
a) Scattering by a 1-litre bottle of H 3 PO 4 acid (3x10 8 n/shot of PF-6, a single pulse) The neutron detector signals contain in this our case of low dose of neutrons an essential noise component Trying to de-noise this signal, we used wavelet method suggested by MATLAB Wavelet Toolbox We used De-noising 1-D; we considered the noise to be un-scaled white Top: Original signal Bottom: Signal with denoising by the dmey wavelet
b) Scattering by lengthy objects (L > v n τ n ) 4 MCNP We used our 1-m high-pressure aluminium cylinder filled with deuterium having pressure of 70 atmospheres The geometry of the experiment was the same as shown in our MCNP simulations 3 2
Oscilloscope traces for the case without a paraffin screen (a) and with the screen (b) 1 X-rays, 2 direct neutrons, 3 neutrons scattered by Al, 4 neutrons scattered by D 2 Small difference in time lags between real and numerical data appeared because of difference in neutron energies - 2.45- MeV neutrons in simulations vs 2.7 MeV in the experiments
One may see than the whole process of collecting information by an oscilloscope during a single DPF shot lasts about 2 5x10-7 s It is clear that for the express information gaining during the same period of time we have to elaborate: - in-line data acquisition - fast digital pulse/signal processing with the denoising procedure - fast data analysis based on pattern recognition (e.g. comparison of experimental oscilloscope traces with the triple-pulse structures related to scattering on C, N, O nuclei) In this case we shall be able to work with fast moving objects like cars, trains, etc.
1 116 231 346 461 576 691 806 921 1036 1151 1266 1381 1496 1611 1726 1841 1956 2071 2186 2301 2416 2531 2646 2761 2876 2991 3106 3221 3336 3451 3566 3681 3796 3911 4026 4141 4256 4371 4486 4601 b) Scattering by fissile materials DPF 2.00E-01 0.00E+00-2.00E-01-4.00E-01-6.00E-01-8.00E-01-1.00E+00-1.20E+00-1.40E+00-1.60E+00-1.80E+00 An oscilloscope trace without fuel element
DPF WITH FUEL ELEMENT -1.80E-01-1.60E-01-1.40E-01-1.20E-01-1.00E-01-8.00E-02-6.00E-02-4.00E-02-2.00E-02 0.00E+00 2.00E-02 1 119 237 355 473 591 709 827 945 1063 1181 1299 1417 1535 1653 1771 1889 2007 2125 2243 2361 2479 2597 2715 2833 2951 3069 3187 3305 3423 3541 3659 3777 3895 4013 4131 4249 4367 4485 4603
Overlapping of with and without Amplitude (a. u.) 0 500 Time(ns) Time (ns)
Images subtraction
Attribution A result of the subtraction of oscilloscope traces with attribution of different peaks and comparison with results of MCNP modelling calculations
CONCLUSION Due to these experiments that were supported by a wide-range MCNP calculations we are of the opinion now that the NINIS technique can find its niche among neutron-based methods of disclosure of hidden illegal objects We see the mains perspectives of this method in interrogation of fast-moving vehicles (cars, trains ) and in unveiling of fissile materials The method demands an in line fast digitizing technique supported by denoising and pattern recognition procedures
Acknowledgements The author is greatly indebted to the coworkers participating in different sections of these investigations as follows: MIKLASZEWSKI R.A., CHERNYSHOVA M., SCHOLZ M., PROKOPOWICZ, TOMASZEWSKI K., DROZDOWICZ K., WIACEK U., GABANSKA B., DWORAK D., PYTEL K., ZAWADKA A. These works were partly supported by the International Atomic Energy Agency