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A single-shot nanosecond neutron pulsed technique for the detection of fissile materials

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2012 JINST 7 C07005

(http://iopscience.iop.org/1748-0221/7/07/C07005)

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2012 JINST 7 C07005

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED: March 30, 2012REVISED: May 20, 2012

ACCEPTED: June 7, 2012PUBLISHED: July 12, 2012

2nd INTERNATIONAL WORKSHOP ON FAST NEUTRON DETECTORS AND APPLICATIONS,NOVEMBER 6–11 2011,EIN GEDI, ISRAEL

A single-shot nanosecond neutron pulsed techniquefor the detection of fissile materials

V. Gribkov,a,b,1 R.A. Miklaszewski,a M. Chernyshova,a M. Scholz,a R. Prokopovicz,a

K. Tomaszewski,c K. Drozdowicz,d U. Wiacek,d B. Gabanska,d D. Dworak,d K. Pytele

and A. Zawadkae

aInstitute of Plasma Physics and Laser Microfusion,Hery 23, 01-497 Warsaw, Poland

bA.A. Baikov Institute of Metallurgy and Material Sciences, Russian Academy of Sciences,Leninsky prospect 49, Moscow 119991, Russian Federation

cACS Ltd.,Hery 23, 01-497 Warsaw, Poland

dInstitute of Nuclear Physics, Polish Academy of Sciences,Krakow, Poland

eInstitute of Atomic Energy POLATOM,05-400 Otwock-Swierk, Poland

E-mail: gribkovv@yahoo.com

ABSTRACT: A novel technique with the potential of detecting hidden fissile materials is presentedutilizing the interaction of a single powerful and nanosecond wide neutron pulse with matter. Theexperimental system is based on a Dense Plasma Focus (DPF) device as a neutron source gener-ating pulses of almost mono-energetic 2.45 MeV and/or 14.0 MeV neutrons, a few nanoseconds inwidth. Fissile materials, consisting of heavy nuclei, are detected utilizing two signatures: firstly bymeasuring those secondary fission neutrons which are faster than the elastically scattered 2.45 MeVneutrons of the D-D reaction in the DPF; secondly by measuring the pulses of the slower secondaryfission neutrons following the pulse of the fast 14 MeV neutrons from the D-T reaction. In bothcases it is important to compare the measured spectrum of the fission neutrons induced by the 2.45MeV or 14 MeV neutron pulse of the DPF with theoretical spectra obtained by mathematical sim-ulation. Therefore, results of numerical modelling of the proposed system, using the MCNP5 andthe FLUKA codes are presented and compared with experimental data.

KEYWORDS: Search for radioactive and fissile materials; Interaction of radiation with matter;Neutron sources; Inspection with neutrons

1Corresponding author.

c© 2012 IOP Publishing Ltd and Sissa Medialab srl doi:10.1088/1748-0221/7/07/C07005

2012 JINST 7 C07005

Contents

1 Introduction 1

2 NINIS — a single-shot technique for disclosure of hidden objects 2

3 Monte Carlo modelling 5

4 Experimental technique 12

5 Experimental results and discussion 13

6 Conclusions 19

1 Introduction

Among promising approaches to the problem of the interrogation of hidden objects containingfissile materials, the methods which use neutrons, are of a pertinent interest at the present time.

In principle these methods can exploit a number of sources with long-pulse or continuous neu-tron radiation such as isotopes and classical neutron generators (direct-type accelerators generating6 108 neutrons per pulse having duration ∼ 10 µs) or low-power sources like the Van de Graaffaccelerator irradiating 6 103 neutrons during a pulse of duration of 2 ns [1–7].

These methods ensure the necessary solutions in some cases; yet the techniques proposedmeet some awkward problems. Between them the most important one is rather low signal-to-noiseratio (SNR) at the detection part of an interrogation system. To reach a high value of SNR inthose techniques it is necessary to integrate over many pulses with the above neutron sources (forgenerators — >106 pulses) or to ensure a long operation time (for isotope sources — > half anhour). This leads to high activation and to a long period of the interrogation of objects [1–7].

The aim of this work is to verify advantages of NINIS — Nanosecond Impulse Neutron Inves-tigation System [8, 9] — a technique based on an interaction of just a single powerful nanosecondpulse of neutrons generated by a Dense Plasma Focus (DPF) device [10] with matter for the de-tection of hidden fissile materials. This method was developed for an interrogation of objects, inparticular of fast-moving objects containing explosives. We should like to analyze an opportunityto apply this technique for interrogation of items containing fissile materials. One very preliminaryresult on this point with a single oscilloscope trace was presented in our work [11]. In this paper wedescribe all results obtained during the above-mentioned experimental session. We present theiranalysis and comparison with computational simulations provided specifically for our experimentalconditions. Sensitivity and restriction of the method will also be discussed.

A DPF device of a medium size (∼ 1 m2 foot-print, ∼ 5 kJ bank) [12–14] can simultaneouslyproduce very short (τ ∼ 10 ns) and bright flashes of neutrons (up to 1020 n/s) and hard X-rays of a

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Figure 1. Schematic of the process used in the NINIS technique.

few J. Such a DPF device is a neutron source that is able to generate pulses of almost monochro-matic (∆En/En ≈ 3–5%) neutrons of the energy of En = 2.45 MeV and/or En = 14.0 MeV. Theneutron yield (per one shot) of these DPF devices is on average 109 neutrons when operating withpure deuterium as a working gas, and 1011 neutrons when a deuterium-tritium mixture is used. Theabove figures are quantitatively stable in sense of order of magnitude. In fact, the neutron yield in-creases/decreases from shot to shot, and the changes are on average about a factor of 2. These typesof variations in the total neutron yields were observed during longer shot series (from 30 to 200consecutive shots) for the both operating modes. The repetition rate of such a device obtained up tonow is 16 Hz if water-cooled electrodes are used (potentially up to 50 Hz for this DPF’s size). To in-crease an average neutron yield per shot in a repetitive mode it is necessary to maintain the optimumpressure inside the DPF chamber by purging the deuterium continuously at an optimal flow rate.The most sensitive element is the anode of the discharge chamber, which has a life-time of about103÷104 shots depending on its construction. With a proper design the chamber can be changed ina few minutes. All other elements can survive during about 106÷107 shots. These features providean opportunity to use time-of-flight (TOF) technique with relatively short flight base (a few me-tres) for the potential NINIS application. Important elements of the NINIS are also detectors withnanosecond (ns) and sub-ns time resolution (temporal resolution for neutron pulses with full widthat half maximum ∆τ = 3.0–0.276 ns in our case) and mobile cabinets for them with good shielding.

2 NINIS — a single-shot technique for disclosure of hidden objects

In the NINIS technique we use elastic and inelastic scattering of our mono-energetic neutronsproduced in D-D or D-T nuclear fusion reactions upon nuclei of unknown elements (figure 1).

After this collision the elastically scattered neutron will change its energy En (and speed v)depending on the mass of the nucleus-scatterer, the angle of scattering, and the kinematics of theelastic scattering of neutrons in dependence of these parameters can easily be calculated, as repre-sented in figure 2 [1].

However contrary to many techniques where individual flashes are collected during many irra-diation shots (so detectors are working in the regimes of counting of pulses with terms of “loads”)we are working with our detectors in their current mode when an amplitude of a current producedby merged pulses are measured during a nanosecond time interval. It means that inside the scintil-

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Figure 2. Dependence of energy loss of a neutron on nuclei masses A and on angles of elastic scattering α .

lator 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 impacts by neutrons.But individual flashes with their dissimilar amplitudes start to merge when intensity of the regis-tered neutrons is increased (see figure 3).

For each group of ∼ 10 mono-energetic neutrons a linear dependence is proved for the aver-aged (merged) nanosecond signal (i.e. for the total integral under the pulse trail) versus the numberof neutrons. This dependence will be violated due to saturation effects in the photon detector (amicro-channel plate (MCP) and a regular photo-multiplier tube (PMT)). They also suffer satura-tion problems and secondary pulses (afterpulses) when exposed to short, intense light flashes aswell. But if one can use a high-current PMT (like the SNFTT M type with linear current up to 7 –10 A) the saturation effect may appear when micro-spheres of the action of recoil protons start tooverlap each other resulting in change of excitation mechanism. However for a scintillator blockwith a diameter Ø = 10 cm and a length l= 10 cm it will be at its registration of about 1015 neutronsper the block of this size. This value is much higher than the total neutron yield of our device andvery far from those amplitudes of the scattered neutron pulses registered in our experiments.

Thus, the restrictions are implied: from the lower limit — by the averaging-out of individualpulses, and from the upper one — by the saturation. So the technique based on NINIS is operationalfor the number of neutrons N registered in a single pulse of several nanoseconds duration with thescintillation block of the above-mentioned dimensions and with the high-current PMT lying withinthe limits: 102 < N < 1014.

For light elements of the first part of the periodic table of elements the energy change ina scattering act (i.e. lower speed v of scattered neutrons) will result in a later time of arrival ofthe scattered neutrons to a detector — in our case it is a photomultiplier with plastic scintillator(PMT+S) — compared with the arrival time of “direct” neutrons coming to the detector from thesource without scattering [1, 8]. Taking into consideration these time lags, amplitudes of pulsesand corresponding cross-sections of scattering it is possible to reconstruct an elemental content ofthe material under interrogation from an oscilloscope trace [1, 8, 9, 11].

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Figure 3. Oscilloscope traces of pulses of X-rays and neutrons generated by DPF and registered by a detectorwith a scintillator having time resolution equal to 0.275 ns (a): groups of individual flashes produced insidea scintillator by each hard X-ray photon (i.e. by photoelectron) (1) and 14 MeV neutron (i.e. by recoilproton) (2) during the time intervals when the radiations have low intensities; their merged parts (3) and(4) correspondingly, which were registered during the period of their high intensities; red and yellow tracesrefer to two detectors placed at slightly different distances from the source (6 and 7 meters accordingly); thesame traces (b) for the distance of about 2 m but for temporal resolution of PMT+S of 1.3 ns; both traces (a)and (b) were registered in a relatively good environment without many scatterers.

However, as one may see from figure 2, this difference between energy of direct neutronscoming from the source to the detector and elastically scattered by the nuclei becomes negligiblefor heavy nuclei comprising fissile materials (elements like Th, U, etc. from the end part of theperiodic table of elements). Fortunately neutrons produced due to fission reactions at the bom-bardment of nuclei of fissile materials by 14 MeV neutrons have a wide energy spectrum extendedapproximately from about 0.1 MeV to ∼ 5 MeV with a peak at about 0.7÷1.0 MeV as it is in theclassical Watt distribution. However one has to take into account that for different commercialfuel assemblies these energy spectra (and maxima) depend strongly on their configurations (ele-mental contents and geometries). To understand whether it would be possible to distinguish themfrom elastically scattered neutrons we have provided numerical modelling of the interaction of ourmono-energetic neutrons with specific fuel elements (assemblies) taking into consideration realexperimental conditions.

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3 Monte Carlo modelling

We simulated scattering of 2.45 MeV and 14 MeV neutrons by various objects using FLUKA [15–17] and MCNP [18, 19] codes. For quick, time-independent calculations we have used the FLUKAcode. More sophisticated problems, where a time-dependent source had to be applied, were solvedby means of MCNP5. These our computational works were aimed first at elaboration of the theo-retical basis for the fissile materials detection concept, and to verify expected experimental resultsin this study.

The first part of simulations applies the FLUKA code to investigate detailed interaction ofneutrons of energy En = 2.45 MeV with localized objects. The fuel assembly MR-6/80, whosefuel section includes uranium enriched to 80% of U-235 isotope, was used as a target. The MRassembly consists of a few consecutive cylindrical layers of aluminium and of uranium fuel. Thistarget was irradiated by the parallel neutron beam of the diameter Ø = 7 cm and neutron energyequal to 2.45 MeV as is outlined in figure 4a. In figure 4b one can see an example of energydistributions of neutrons outgoing from the MR assembly (in units “neutrons per cm2 per lethargyunit, and per one incident beam neutrons”). The spectrum was taken under a certain angle (45 ˚ ),but we have checked that the shapes of these spectra are almost independent on the angle.

The solid line in red in figure 4b shows a result for the original MR assembly. The greendashed line presents a result obtained for an extra case, where the fuel layers of the MR assemblywere substituted by equivalent aluminium layers, in order to show clearly an effect of the “purealuminium background”. All neutrons with energy above the visible “green wall” — the boundaryenergy equal to the energy of the irradiating neutron beam — can clearly descend only from fissionof uranium in the fuel layers. Due to this feature fission neutrons are clearly seen in this energydistribution.

In principle this group of “outrunning” neutrons, having spectrum extended to higher energies,can easily be distinguished from the almost monochromatic primary neutrons and all other lowerenergy neutrons scattered by the MR assembly materials. This group of fission neutrons will beseen as a “precursor” at the front of the pulse of scattered neutrons (pulse overshoot).

However, we have found that at the distance r = 2 m from the target we can expect no morethan (2–3)×10−9 neutrons of fission origin (with energy above 2.45 MeV) per cm2 and per beamneutron hitting the MR-6/80 target.

But our DPF device (PF-6 [12]) can produce neutron yield in full solid angle on the level of6 109 neutrons per pulse only. This constitutes an insuperable obstacle for our source of neutronswith E0 ≈ 2.45 MeV, i.e. for the PF-6 device operating with pure deuterium as the working gas. Itis easy to calculate that at a distance to the target (MR) from the point isotropic neutron source ofR ≈ 10 cm; at the irradiation of the whole MR fuel element (l= 100 cm and d =7 cm); and at adistance of the scintillator from the target equal to r = 2 m the number of high-energy (i.e. fission)neutrons coming to the detector having the area S = 80 cm2 in the above geometry will be ∼ 10−2

neutrons only in total. It is not enough of course to be detected in a single pulse. So this methodcould be successfully applied only with the DPF neutron yield ∼ 1011 and for a distance betweenthe fuel element and the detector r 6 1 metre.

Consequently we decided to use in our simulation 14 MeV neutrons from DPF operating withD-T mixture as a working gas. In this arrangement the total neutron yield of PF-6 in a single shotis two orders of magnitude higher (∼ 1011 n/pulse). However, in this case in final spectrum the

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a)

b)

Figure 4. Schematic of irradiation (a); the number of neutrons produced due to fission processes in thefuel element MR-6/80 and registered at 2 m from it (red) and neutrons scattered on the aluminium layerssubstituted instead of the fuel rods (green) (b) as it was obtained from FLUKA simulations.

fission neutrons will have energies much less compared with the almost mono-energetic primaryneutrons peaked near the energy E0 = 14 MeV. So, the main idea is now just opposite to the pre-

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Table 1. Composition of the fuel assembly EK-10.

Element Atomic number Atomic weight [u] Proportion by Proportion byN[%] weight [g/cm3]

U−235 92 235.044 2.19 0.4184U−238 92 238.051 19.71 3.8113O 8 15.999 43.80 0.5693Mg 12 24.305 34.30 0.6772

Fuel density: 5.4762 g/cm3

vious one concerning neutrons with the energy of 2.45 MeV. Indeed in the case of the source ofneutrons with energy E0 = 14 MeV we shall distinguish fission neutrons having now much lowerenergy compared to the energy of primary neutrons.

As a consequence the next step in our simulations was done in the geometry more close to ourreal experiments performed with the PF-6 device (yet still idealized). We put the DPF chamberas a point source at the distance of 10 cm to the centre of the Fuel Element EK-10 (this assemblyincludes 10% enriched uranium, what is also more realistic) and we use a distance r = 6 m from theFuel Element to the detector. The device was operated in the experiments with the DPF chamberfilled with D-T mixture as it was mentioned above.

The EK-10 fuel assembly was modelled as 16 cylindrical fuel pipes. Each pipe is 50 cm highand has an external diameter equal to 10 mm (7 mm of fuel + 2×1.5 mm of aluminium wall). Thepipes are parallel to each other and they form sort of a ring bundle. Looking at the bundle crosssections (horizontal plane) the centres of these 16 pipes are on the ring with diameter 6 cm, andgo around, thus the pipes are almost close to each other. In this way we can say that the “externaldiameter” of this ring of the pipes (whole assembly) is equal to 7 cm, and its “internal diameter” isequal to 5 cm.

In this modelling the source is very simple, a mono-energetic (E0 = 14 MeV), point, instant,and fully isotropic source. For this reason one cannot observe in the spectra a peak at the energyE1 = 2.45 MeV originated from the D-D reaction taking place inside the real DPF chamber andseen in the actual oscilloscope traces (see below). With this geometry we again provided numericalmodelling by use the FLUKA code.

For the fuel assembly EK-10, the composition is mixed UO2 and Mg (73.33 g of U-238, 8.05g of U-235, 13.03 g of Mg). An individual fuel element is a cylinder: 50 cm high and 7 mm indiameter, enclosed in an aluminium envelope. Fuel density was 5.4762 g/cm3. Total amount ofU-238 was 1173.28 g, whereas of U-235 — 128.8 g. It is presented in the table 1.

Figure 5 and 6 show results of these calculations for the neutrons of energy E0 = 14 MeVscattered by all materials of the EK-10 fuel assembly. For these figures the energy spectra wererecalculated into the shapes of the oscilloscope trace derivatives expected at a distance equal to 6 m.Time scale starts (i.e. t = 0) at the moment of the neutron pulse beginning inside the DPF chamber.

In figure 5 one may see the results of calculations provided separately for empty aluminiumtubes, for fuel rods and for the whole EK-10 assembly. One may see two peaks –covered 280–300

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Figure 5. The oscilloscope trace derivative for neutrons with E0 = 14 MeV elastically and inelasticallyscattered by aluminium and by fuel rods of the EK-10 with fission neutrons for a PMT with plastic scintillatorat a distance r = 6 m calculated by FLUKA code; the arrows show two peaks at 4.0 MeV (TOF = 217 ns)and 2.4 MeV (TOF = 280 ns) associated with the fuel rods.

ns (En = 2.4 MeV) and covered 221 ns (En = 4.0 MeV) that are related to the fuel elements andwhich are absent in the aluminium curve. Figure 6 presents results of calculations for the same fueltype made for all elements of the fuel assembly. In the next step of calculations we have used the“real geometry”, which is much closer to actual experimental hall where our measurements weremade. This room has walls, a floor, a ceiling and a number of other objects made of concrete. This“real case” demands more sophisticated modelling of the neutron source (taking into account thetime distributions of the emitted neutrons), as well as the environment. So we have provided theMCNP simulation for geometries of installation-specific positioning of the DPF chamber PF9 ofour PF-6 device and the fuel element EK-10 (figure 7), a paraffin screen and different objects thatscatter neutrons during our experiments (figure 8a and b).

In previous tests we have found that in “clean” conditions, i.e. in a big empty hall, our neutronpulse has a bell-like shape with its rise-time almost equal to its pulse decay time (∼ 10÷20 ns) (seefigure 3). However, in figure 8 one may see that in present experimental conditions, which are veryfar from the ideal ones, we can expect a long tail of our scattered neutrons as result of multiplescattering from numerous objects of the hall.

A view from the Faraday cage (movable cabinet) with a photomultiplier plus scintillator block(PMT+S) to the fuel element taken along the PMT axis is shown in figure 9.

During different shots of the DPF we put our 1000×200 mm2 paraffin screen perpendicular tothe direct neutron beam (as in figure 9) or along its direction (as in figure 8) sometimes adding a

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Figure 6. FLUKA modelling calculations for all elements of the EK-10 fuel element; note at the left-handside several narrow peaks produced due to scattering on nuclei of various elements and two peaks shown byarrows which are of the same origin as in figure 5; expected peak at 0.7÷1.0 MeV of “pure fission neutrons”produced by 14 MeV neutrons must be in the range 440÷520 ns with maximum at about 500 ns (shown bythe oval).

a) b)

Figure 7. Arrangement of the DPF chamber PF9 of the device PF-6 and the fuel element EK-10 (a) and thesame with enlargement (b); diameters of the PF9 chamber of the PF-6 device (DPF) and of the fuel assembly(FA) are 120 mm and 70 mm correspondingly.

block of several paraffin plates (as it is shown in figure 9). Our calculations have shown that in thisgeometry we have a very asymmetric pulse of scattered neutrons indeed with a very long decaytime (extended till 800 ns).

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a)

b)

Figure 8. Schematic (a) and side-view (b) of the experiment with sizes/distances in mm.

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Figure 9. Photograph of the PMT+S Faraday cage.

Figure 10. Calculated oscilloscope trace (blue) derivative with two peaks expected at r =6 m from the EK-10 bundle (the DPF source of neutrons with E0 = 14 MeV shielded with the paraffin block). Comparison of“All” and “Fuel Flagged” neutrons expected at the same PMT+S placed at a distance r = 6 m.

In figure 10 we present our calculations by MCNP code for a comparison of “All” and “FuelFlagged” neutrons, counted by the same scintillator. Here “All” neutrons are the overall neutronselastically and inelastically scattered inside the experimental hall and registered by the photomulti-plier tube plus scintillator. The “Fuel Flagged” neutrons are the neutrons, which crossed (escaped)one of the fuel filed parts of the EK-10 assembly, believing that neutrons produced by fission reac-tions in the fuel element are presented in these pulses.

The same calculated trace derivative related to “fuel flagged neutrons” only and flipped ver-tically is presented in figure 11. One may see a specific peak observed at about 180 ns after thebeginning of the neutron pulse or about 300–340 ns in relation to the start of neutron generationinside the DPF chamber, as shown in figure 6 and figure 5. This peak corresponds to the energy ofneutrons in the range 1.6–2.1 MeV that are in the middle of the spectrum of neutrons generated inuranium by external neutrons of the energy E0 = 14 MeV.

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Figure 11. The same trace derivative as it is shown in figure 11 but which is calculated for the “fuelflagged neutrons” only. It is enlarged and turned upside-down. Note again as in figure 5 and 6 two peaks(peculiarities) shown by arrows.

4 Experimental technique

As a neutron and X-ray source we used the PF-6 device (figure 7) working both with deuterium ordeuterium-tritium mixture as working gas. This transportable device (IPPLM) has the followingparameters: bank energy — 7 kJ, amplitude of the discharge current — 760 kA, neutron yield pershot ∼ 109 for pure D2 or ∼ 1011 for a D−T mixture, total weight — 400 kg, footprint — ∼ 1m2. The chambers used for the device, PF7 and PF9 (see figure 7b), were manufactured at theVNIIA, RF [20]), which are designed for the neutron yield 1010 and 1011 14 MeV neutrons perpulse correspondingly.

Detection of hard X-rays and neutrons (both directly coming from the source and scatteredby the object under interrogation and by surrounding items as well) was provided by photo-multiplier tubes plus scintillators (PMT+S) detectors and chevron micro-channel plates plus PMTplus scintillator (MCP-PMT+S) with time resolution 3 and 0.276 ns correspondingly. The secondone is equipped with BC-422Q(0.5) ultra-fast scintillator (Bicron) with a pulse width (FWHM)0.29 ns, 6:1 FO Taper (Incom, U.S.A.) and Microchannel Plate - Photomultiplier Tube (MCP-PMT) R3809U-52 type by Hamamatsu Photonics Deutschland GmbH, having declared pulse width(FWHM) 0.35 ns. They were positioned inside a movable Faraday-cage stand. The construction ofthe cabinet used for the detectors ensures 90 dB shielding at 500 MHz.

Choosing one of the above techniques we have to find a compromise. From one side if weshould like to have a temporal resolution of about 0.3 ns we cannot use a scintillator thickness morethan 1 cm because TOF of 14 MeV neutrons of 1 cm is about this value. But in this case we losesensitivity in a very high degree. From the other side a scintillator with its thickness equal to 10 cm

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Figure 12. Pulses of hard X-rays (1) and neutrons (2) at different intensities of the DPF source.

ensures the 2-ns time resolution, which is enough for our case with appropriate collection of neu-trons number in the scintillator. Thus we used a scintillator of the thickness of 1 cm with MCP-PMTfor characterization of the neutron source itself whereas for our scattering experiments we used 2sizes of scintillators with 5 or with 10 cm (for both their diameters and lengths correspondingly).

Low (a) and high (b) intensities of hard X-rays (photon energy > 60 keV) and neutronswith E0 = 14 MeV generated by the PF-6 and directly coming from the source to the scintillatorprobes MCP-PMT (without shielding) are shown in figure 12 for their dissimilar intensities and fordifferent sensitivities of the detector channels. In the second case (b) our probe is in a saturationmode. In these measurements made with two scintillation probes and for different distances fromthe source to the probe we found that the delay time of the temporal neutron peak intensity asappeared inside the DPF chamber in relation to the front of the hard X-ray pulse (and very oftento its peak) in these experiments varies in the range 4÷6 ns.

A photograph of the experiment with the PF-6 chamber PF9 filled with D-T mixture and withfuel element EK-10 is presented in figure 7 whereas its surrounding, paraffin screens and concretescatterers, are shown in figure 8 and 9.

5 Experimental results and discussion

We used 14 MeV neutrons in initial experiments on irradiation of fissile materials (Fuel assemblyEK-10). We have done it in the geometry as it is presented in figure 8. Our preliminary estimationsand examination of the above figure 5, 6, 10, 11 and some others gave us the following positionsof energy peaks expected in spectral distributions and, correspondingly, time neutron peaks tn inour real TOF oscilloscope traces in respect to the start of the X-ray pulse tX taking into account theneutron delay time td and TOF of r= 6 m by neutrons tn−TOF and by hard X-rays tX−TOF :

tn = tn−TOF − tX−TOF + td

1) The main neutron energy peak from the DPF source originating from D-T nuclear reactionsaccording to our previous measurements is observed at the energy En ≈ 14.0 MeV; its TOFof the distance from the DPF chamber to the detector (6 m) is equal to tn−TOF ≈ 120 ns.

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2) Peaks expected because of scattering on nuclei of 8O16 should be at the energies equal to thevalues En ≈ 12.5; 11.0; 8.0. . . 5.0; 2.6; 2.2 MeV; corresponding TOFs are at tn−TOF ≈ 126;134; 170; 180, 225; 310 ns.

3) 13Al27: En ≈ 7.7; 6.7; 5.5; 4.0; 2.7 MeV; tn−TOF ≈ 160; 170; 185; 220; 267 ns.

4) 12Mg24: En ≈ 12.5; 7.7; 5.76; 4.0; 1.1 MeV; tn−TOF ≈ 126; 160; 180; 220; 430 ns.

5) Neutron energy peak originating from D-D nuclear reactions in the PF-6 (having more than2 orders of magnitude lower amplitude compared with the main peak at 14 MeV) accordingto our previous measurements has the energy equal to En ≈ 2.7 MeV; tn−TOF = 267 ns.

6) Two relatively weak peaks (better to name them “peculiarities”), which were observed in theabove modelling of the real EK-10 assembly must be situated in the positions around valuestn−TOF ≈ 200 ns and 290 ns.

7) Fission neutrons92U238 and 92U235: En ≈ 5 . . . (2.1–1.6) . . . 0.1 MeV with a peak at energyin the range En ≈ 0.7÷1.0 MeV (Watt spectrum) may appear in the time gap tn−TOF ≈440÷520 ns.

One may see that some of the above-mentioned peaks are overlap.We found that the rise-time of the neutron peak on the levels 0.1. . . 0.9 of its amplitude in

these our experiments is the same as it was measured for this device (15 ns) in good premiseswithout concrete scatterers. Yet the tail of the neutron pulse appeared to be very long here (as itwas predicted by MCNP calculations of figure 10 and 11). It reflects scattering of neutrons bymany concrete blocks filling the experimental hall. Fortunately as one may see in figure 13 takenwithout fuel element (the “reference” pulse) this tail is very smooth having no visible peaks. Inthis control experiment we had just one screen made by paraffin and installed perpendicular to thedirect neutron beam as it is shown in figure 9 (so with week shielding).

Below we have presented several oscilloscope traces obtained during this session whatcontinued during the time period of the availability of the fuel element. We provided 13 shotswithout fuel element to fit sensitivity and trigger level of the oscilloscope and to adapt thegeometry of shields (partially). Then we provided 10 shots with fuel element. Among them wehave registered good signals in 9 shots by one or two channels of the oscilloscope (working withdissimilar sensitivities and with different neutron yields of the PF-6). Throughout this period wechanged positions and orientation of our shield(s), used different pressures of working gas andmoved our PMT+S trying to find the highest signal at the lowest noise. Neutron yields measuredin these experiments by activation of copper and by silver activation counter were within the range4×1010. . . 2×1011 neutrons of energy E0 = 14 MeV per shot.

Analysis of the peaks appeared in the oscilloscope traces of figure 14 through 20 has shownthat they are coincided with the above estimations for scattering (elastic and inelastic) on nucleiof elements contained in the fuel element as well as with our modelling simulations presented infigure 5, 6, 10 and 11.

We have to mention here that one may see several additional peaks on the oscilloscope’s traceshaving nothing with our object under interrogation. They result from several small abruptions of

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Figure 13. Oscilloscope trace of hard X-ray (first) and neutron (second) pulses taken at a distance r= 6 mfrom the PF-6 chamber filled with DT mixture; the trace was obtained without fuel element and with justone screen made by paraffin that was installed perpendicular to the direct neutron beam (“reference pulse”).

Figure 14. Oscilloscope traces taken in the same shot of DPF with fuel element (as in figure 7) with twodissimilar sensitivities of channels; the shield was placed perpendicular to the direct neutron beam as it is infigure 9 and 15; one may see the expected peak at 300 ns better pronounced at the right-hand side trace (b)where we use higher sensitivity of the oscilloscope channel; in the same trace 14 (b) several additional peaksthat corresponds to different materials contained in the fuel element may be seen; note a small peak in thetrace (b) with a position related to neutrons with energy 2.7 MeV; if it is connected with D-D neutrons gen-erated by the DPF and elastically scattered by the environment, the area of that pulse must be lower than thearea of the scattered pulse of neutrons of energy E0 = 14 MeV by more than 200 times: this demand is met.

current after the main one in the DPF (see their “signatures” also correspondingly in the tail ofthe X-ray pulses) that were produced unfortunately in this shot. However we know where we haveto expect the peaks of our interest in the oscilloscope traces beforehand, and we have found them

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Figure 15. Two shots produced in the same conditions as in figure 14.

namely in these time positions. Happily in this case we have no overlapping of these peaks withpeaks from other small abruptions as it was also proved by the comparative amplitude analysis andcontrol experiments.

A very similar result was obtained in the next two shots made in the same conditions (figure 15a and b). The oscilloscope trace was taken here with the higher sensitivity compared to that in thefigure 14.

Then we have reoriented our main paraffin screen installing it along the neutron beam as it isseen in Fig 8. We suppressed direct beam in a higher degree and our expected pulses became moreprofound (figure 16).

Then we install an additional paraffin block with dimensions 50 by 50 by 50 cm3 along thedirect beam of neutrons thus suppressing the main neutron pulse in a higher extent. The result isshown in figure 17.

Comparison of one of these experimental oscilloscope traces versus our preliminary MCNPcalculations is presented in figure 18.

One may see the coincidence of the two peaks in both the simulation curve and the oscillo-scope trace. Two vertical lines show the moments of starts of pulses of neutrons scattered by thefuel element and peaks at 200 ns and 300 ns discussed above.

Because one may see in our oscilloscope traces several other peaks inside the zone of interestwe can try to confront them with the calculated (and estimated) peaks related to other materialscomposing our fuel element. For this purpose we have subtracted one oscilloscope trace takenwithout fuel element from the other which was taken with the EK-10 in its position near the DPFchamber (see figure 19).

This oscilloscope traces subtraction with attempts of attribution of different peaks and com-parison with results of MCNP modelling calculations are presented in figure 20. All other peakswithout any indications on the picture are presumably connected with additional small currentabruptions, which produce low-amplitude flashes of hard X-rays and neutrons. These neutron“sub-pulses” can easily be correlated with hard X-ray sub-pulses using the same time-delay as forthe main hard X-ray and neutron pulses.

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Figure 16. The shot with the main paraffin screen placed along the axis of the direct neutron beam fromDPF chamber to the PMT+Scintillator; higher neutron yield and better shielding.

Figure 17. Signals for the case with two shields.

It is easy to estimate how many neutrons Nwe have registered in the above pulses associatedwith the fuel assembly. According to MCNP calculations their intensity at the distance 6 m from

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Figure 18. The oscilloscope trace (a) versus the preliminary MCNP calculation curve (b) of figure 11. Threevertical lines show start of the main neutron pulse and two peaks associated with a fuel assembly.

the fuel element is I = 2.5×10−11 n/cm2ns. Taking into consideration volume of our scintillatorsV (5 or 10 cm in lengths l and diameters d), efficiency of neutrons registration (k ≈ 1/2) and pulsedurations registered (τ ≈ 40 ns) one can find the figure for the worst case (d = l = 5 cm):

N = I×V × k× τ =(2.5×10−11 n/cm2ns)×{(3.14×25)/(4×2)cm2}× (40ns)=10−8 neutrons

per pulse taken from the number of neutrons irradiating the fuel element. Geometry of irradiationdecreases this figure in relation to the overall neutron yield of the device Yn by another one order ofmagnitude. It gives for each pulse from 40 up to 200 neutrons registered depending of our neutronyield of the device (Yn = 0.4. . . 2.0×1011 neutrons per pulse in full solid angle). This statisticsis above the border of acceptability presented in the section 2 (minimal number of registeredmono-energetic neutrons must be higher than 10).

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Figure 19. Two oscilloscope traces overlapping one another: one is taken without fuel element (the blackone, smooth) and another one is taken with fuel element (the blue one with multiple peaks); the red one isthe same as the blue one taken with much lower sensitivity of the oscilloscope channel.

6 Conclusions

In the previous sections we showed that the signature of fissile material in the neutron TOFspectrum of a DPF can be observed by a single-shot nanosecond neutron pulsed technique. Itis quite evident that in future experiments with our PF-6 device we have to find better premisesto eliminate parasitic scatterers (to diminish the long tail produced by them), to create bettershielding configuration, to decrease distance between an object and the detector till 1 meter (whatis acceptable for the same DPF neutron source, detector and fuel assembly according to our presentexperience and MCNP modelling), and to use a scintillator with d = 100 and l = 100 mm. In thiscase we may expect to increase our signal by two orders of magnitude.

Due to these particularly “proof-of-principle” experiments, which were supported by the wide-range MCNP calculations, we are of the opinion that the NINIS technique can probably be usedfor a disclosure of hidden objects containing fissile materials. In the case of success of these futureworks the main perspective of this method seems to be in unveiling of fissile materials at the expressinterrogation of them, e.g. in fast-moving vehicles (cars, wagons of a train, etc.).

Acknowledgments

The authors wish to thank the International Atomic Energy Agency for a partial support of thesestudies in the frame of CRP IAEA grants Nos. 16954 and 16956. The work was partly sponsoredfrom the research grant of the Polish Ministry of Science and Higher Education (MNiSZW) No.O N202 049735.

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Figure 20. A result of the subtraction of oscilloscope traces with attribution of different peaks and compar-ison with results of MCNP modelling calculations (see figure 5 and 6).

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