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

Transcription

1 Journal of Instrumentation OPEN ACCESS A single-shot nanosecond neutron pulsed technique for the detection of fissile materials To cite this article: V Gribkov et al View the article online for updates and enhancements. Related content - Detection of explosives and other illicit materials by a single nanosecond neutron pulses Monte Carlo simulation of the detection process R Miklaszewski, U Wicek, D Dworak et al. - A dense plasma focus-based neutron source for a single-shot detection of illicit materials and explosives by a nanosecond neutron pulse V A Gribkov, S V Latyshev, R A Miklaszewski et al. - Plasma dynamics in the PF-1000 device under full-scale energy storage V A Gribkov, A Banaszak, B Bienkowska et al. This content was downloaded from IP address on 06/06/2018 at 17:20

2 PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB RECEIVED: March 30, 2012 REVISED: May 20, 2012 ACCEPTED: June 7, 2012 PUBLISHED: July 12, nd INTERNATIONAL WORKSHOP ON FAST NEUTRON DETECTORS AND APPLICATIONS, NOVEMBER , EIN GEDI, ISRAEL A single-shot nanosecond neutron pulsed technique for 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. Pytel e and A. Zawadka e a Institute of Plasma Physics and Laser Microfusion, Hery 23, Warsaw, Poland b A.A. Baikov Institute of Metallurgy and Material Sciences, Russian Academy of Sciences, Leninsky prospect 49, Moscow , Russian Federation c ACS Ltd., Hery 23, Warsaw, Poland d Institute of Nuclear Physics, Polish Academy of Sciences, Krakow, Poland e Institute of Atomic Energy POLATOM, Otwock-Świerk, Poland gribkovv@yahoo.com ABSTRACT: A novel technique with the potential of detecting hidden fissile materials is presented utilizing the interaction of a single powerful and nanosecond wide neutron pulse with matter. The experimental system is based on a Dense Plasma Focus (DPF) device as a neutron source generating pulses of almost mono-energetic 2.45 MeV and/or 14.0 MeV neutrons, a few nanoseconds in width. Fissile materials, consisting of heavy nuclei, are detected utilizing two signatures: firstly by measuring those secondary fission neutrons which are faster than the elastically scattered 2.45 MeV neutrons of the D-D reaction in the DPF; secondly by measuring the pulses of the slower secondary fission neutrons following the pulse of the fast 14 MeV neutrons from the D-T reaction. In both cases it is important to compare the measured spectrum of the fission neutrons induced by the 2.45 MeV or 14 MeV neutron pulse of the DPF with theoretical spectra obtained by mathematical simulation. Therefore, results of numerical modelling of the proposed system, using the MCNP5 and the 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 1 Corresponding author. c 2012 IOP Publishing Ltd and Sissa Medialab srl doi: / /7/07/c07005

3 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 containing fissile 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 neutron radiation such as isotopes and classical neutron generators (direct-type accelerators generating 10 8 neutrons per pulse having duration 10 µs) or low-power sources like the Van de Graaff accelerator irradiating 10 3 neutrons during a pulse of duration of 2 ns [1 7]. These methods ensure the necessary solutions in some cases; yet the techniques proposed meet some awkward problems. Between them the most important one is rather low signal-to-noise ratio (SNR) at the detection part of an interrogation system. To reach a high value of SNR in those techniques it is necessary to integrate over many pulses with the above neutron sources (for generators >10 6 pulses) or to ensure a long operation time (for isotope sources > half an hour). 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 Investigation System [8, 9] a technique based on an interaction of just a single powerful nanosecond pulse of neutrons generated by a Dense Plasma Focus (DPF) device [10] with matter for the detection of hidden fissile materials. This method was developed for an interrogation of objects, in particular of fast-moving objects containing explosives. We should like to analyze an opportunity to apply this technique for interrogation of items containing fissile materials. One very preliminary result on this point with a single oscilloscope trace was presented in our work [11]. In this paper we describe all results obtained during the above-mentioned experimental session. We present their analysis and comparison with computational simulations provided specifically for our experimental conditions. Sensitivity and restriction of the method will also be discussed. A DPF device of a medium size ( 1 m 2 foot-print, 5 kj bank) [12 14] can simultaneously produce very short (τ 10 ns) and bright flashes of neutrons (up to n/s) and hard X-rays of a 1

4 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 monochromatic ( E n /E n 3 5%) neutrons of the energy of E n = 2.45 MeV and/or E n = 14.0 MeV. The neutron yield (per one shot) of these DPF devices is on average 10 9 neutrons when operating with pure deuterium as a working gas, and neutrons when a deuterium-tritium mixture is used. The above figures are quantitatively stable in sense of order of magnitude. In fact, the neutron yield increases/decreases from shot to shot, and the changes are on average about a factor of 2. These types of variations in the total neutron yields were observed during longer shot series (from 30 to 200 consecutive shots) for the both operating modes. The repetition rate of such a device obtained up to now is 16 Hz if water-cooled electrodes are used (potentially up to 50 Hz for this DPF s size). To increase an average neutron yield per shot in a repetitive mode it is necessary to maintain the optimum pressure 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 about shots depending on its construction. With a proper design the chamber can be changed in a few minutes. All other elements can survive during about shots. These features provide an opportunity to use time-of-flight (TOF) technique with relatively short flight base (a few metres) for the potential NINIS application. Important elements of the NINIS are also detectors with nanosecond (ns) and sub-ns time resolution (temporal resolution for neutron pulses with full width at half maximum τ = 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 neutrons produced 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 E n (and speed v) depending on the mass of the nucleus-scatterer, the angle of scattering, and the kinematics of the elastic scattering of neutrons in dependence of these parameters can easily be calculated, as represented in figure 2 [1]. However contrary to many techniques where individual flashes are collected during many irradiation 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 produced by merged pulses are measured during a nanosecond time interval. It means that inside the scintil- 2

5 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 registered neutrons is increased (see figure 3). For each group of 10 mono-energetic neutrons a linear dependence is proved for the averaged (merged) nanosecond signal (i.e. for the total integral under the pulse trail) versus the number of neutrons. This dependence will be violated due to saturation effects in the photon detector (a micro-channel plate (MCP) and a regular photo-multiplier tube (PMT)). They also suffer saturation problems and secondary pulses (afterpulses) when exposed to short, intense light flashes as well. But if one can use a high-current PMT (like the SNFT T 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 to overlap each other resulting in change of excitation mechanism. However for a scintillator block with a diameter Ø = 10 cm and a length l= 10 cm it will be at its registration of about neutrons per the block of this size. This value is much higher than the total neutron yield of our device and very 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 individual pulses, and from the upper one by the saturation. So the technique based on NINIS is operational for the number of neutrons N registered in a single pulse of several nanoseconds duration with the scintillation block of the above-mentioned dimensions and with the high-current PMT lying within the limits: 10 2 < N < For light elements of the first part of the periodic table of elements the energy change in a scattering act (i.e. lower speed v of scattered neutrons) will result in a later time of arrival of the 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 the source without scattering [1, 8]. Taking into consideration these time lags, amplitudes of pulses and corresponding cross-sections of scattering it is possible to reconstruct an elemental content of the material under interrogation from an oscilloscope trace [1, 8, 9, 11]. 3

6 Figure 3. Oscilloscope traces of pulses of X-rays and neutrons generated by DPF and registered by a detector with a scintillator having time resolution equal to ns (a): groups of individual flashes produced inside a scintillator by each hard X-ray photon (i.e. by photoelectron) (1) and 14 MeV neutron (i.e. by recoil proton) (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 traces refer to two detectors placed at slightly different distances from the source (6 and 7 meters accordingly); the same 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 neutrons coming from the source to the detector and elastically scattered by the nuclei becomes negligible for heavy nuclei comprising fissile materials (elements like Th, U, etc. from the end part of the periodic table of elements). Fortunately neutrons produced due to fission reactions at the bombardment of nuclei of fissile materials by 14 MeV neutrons have a wide energy spectrum extended approximately from about 0.1 MeV to 5 MeV with a peak at about MeV as it is in the classical Watt distribution. However one has to take into account that for different commercial fuel assemblies these energy spectra (and maxima) depend strongly on their configurations (elemental contents and geometries). To understand whether it would be possible to distinguish them from elastically scattered neutrons we have provided numerical modelling of the interaction of our mono-energetic neutrons with specific fuel elements (assemblies) taking into consideration real experimental conditions. 4

7 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 FLUKA code. More sophisticated problems, where a time-dependent source had to be applied, were solved by means of MCNP5. These our computational works were aimed first at elaboration of the theoretical basis for the fissile materials detection concept, and to verify expected experimental results in this study. The first part of simulations applies the FLUKA code to investigate detailed interaction of neutrons of energy E n = 2.45 MeV with localized objects. The fuel assembly MR-6/80, whose fuel section includes uranium enriched to 80% of U-235 isotope, was used as a target. The MR assembly consists of a few consecutive cylindrical layers of aluminium and of uranium fuel. This target was irradiated by the parallel neutron beam of the diameter Ø = 7 cm and neutron energy equal to 2.45 MeV as is outlined in figure 4a. In figure 4b one can see an example of energy distributions of neutrons outgoing from the MR assembly (in units neutrons per cm 2 per lethargy unit, 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 green dashed line presents a result obtained for an extra case, where the fuel layers of the MR assembly were substituted by equivalent aluminium layers, in order to show clearly an effect of the pure aluminium background. All neutrons with energy above the visible green wall the boundary energy equal to the energy of the irradiating neutron beam can clearly descend only from fission of uranium in the fuel layers. Due to this feature fission neutrons are clearly seen in this energy distribution. 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 lower energy neutrons scattered by the MR assembly materials. This group of fission neutrons will be seen 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 more than (2 3) 10 9 neutrons of fission origin (with energy above 2.45 MeV) per cm 2 and per beam neutron hitting the MR-6/80 target. But our DPF device (PF-6 [12]) can produce neutron yield in full solid angle on the level of 10 9 neutrons per pulse only. This constitutes an insuperable obstacle for our source of neutrons with E MeV, i.e. for the PF-6 device operating with pure deuterium as the working gas. It is easy to calculate that at a distance to the target (MR) from the point isotropic neutron source of R 10 cm; at the irradiation of the whole MR fuel element (l= 100 cm and d =7 cm); and at a distance 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 cm 2 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 method could be successfully applied only with the DPF neutron yield and for a distance between the fuel element and the detector r 1 metre. Consequently we decided to use in our simulation 14 MeV neutrons from DPF operating with D-T mixture as a working gas. In this arrangement the total neutron yield of PF-6 in a single shot is two orders of magnitude higher ( n/pulse). However, in this case in final spectrum the 5

8 a) b) Figure 4. Schematic of irradiation (a); the number of neutrons produced due to fission processes in the fuel element MR-6/80 and registered at 2 m from it (red) and neutrons scattered on the aluminium layers substituted 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 primary neutrons peaked near the energy E 0 = 14 MeV. So, the main idea is now just opposite to the pre- 6

9 Table 1. Composition of the fuel assembly EK-10. Element Atomic number Atomic weight [u] Proportion by Proportion by N[%] weight [g/cm 3 ] U U O Mg Fuel density: g/cm 3 vious one concerning neutrons with the energy of 2.45 MeV. Indeed in the case of the source of neutrons with energy E 0 = 14 MeV we shall distinguish fission neutrons having now much lower energy compared to the energy of primary neutrons. As a consequence the next step in our simulations was done in the geometry more close to our real experiments performed with the PF-6 device (yet still idealized). We put the DPF chamber as a point source at the distance of 10 cm to the centre of the Fuel Element EK-10 (this assembly includes 10% enriched uranium, what is also more realistic) and we use a distance r = 6 m from the Fuel Element to the detector. The device was operated in the experiments with the DPF chamber filled 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 high and has an external diameter equal to 10 mm (7 mm of fuel mm of aluminium wall). The pipes are parallel to each other and they form sort of a ring bundle. Looking at the bundle cross sections (horizontal plane) the centres of these 16 pipes are on the ring with diameter 6 cm, and go around, thus the pipes are almost close to each other. In this way we can say that the external diameter of this ring of the pipes (whole assembly) is equal to 7 cm, and its internal diameter is equal to 5 cm. In this modelling the source is very simple, a mono-energetic (E 0 = 14 MeV), point, instant, and fully isotropic source. For this reason one cannot observe in the spectra a peak at the energy E 1 = 2.45 MeV originated from the D-D reaction taking place inside the real DPF chamber and seen in the actual oscilloscope traces (see below). With this geometry we again provided numerical modelling by use the FLUKA code. For the fuel assembly EK-10, the composition is mixed UO 2 and Mg (73.33 g of U-238, 8.05 g of U-235, g of Mg). An individual fuel element is a cylinder: 50 cm high and 7 mm in diameter, enclosed in an aluminium envelope. Fuel density was g/cm 3. Total amount of U-238 was g, whereas of U g. It is presented in the table 1. Figure 5 and 6 show results of these calculations for the neutrons of energy E 0 = 14 MeV scattered by all materials of the EK-10 fuel assembly. For these figures the energy spectra were recalculated 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 aluminium tubes, for fuel rods and for the whole EK-10 assembly. One may see two peaks covered

10 Figure 5. The oscilloscope trace derivative for neutrons with E 0 = 14 MeV elastically and inelastically scattered by aluminium and by fuel rods of the EK-10 with fission neutrons for a PMT with plastic scintillator at 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 (E n = 2.4 MeV) and covered 221 ns (E n = 4.0 MeV) that are related to the fuel elements and which are absent in the aluminium curve. Figure 6 presents results of calculations for the same fuel type 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 were made. 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 the time distributions of the emitted neutrons), as well as the environment. So we have provided the MCNP simulation for geometries of installation-specific positioning of the DPF chamber PF9 of our PF-6 device and the fuel element EK-10 (figure 7), a paraffin screen and different objects that scatter 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 neutron pulse has a bell-like shape with its rise-time almost equal to its pulse decay time ( ns) (see figure 3). However, in figure 8 one may see that in present experimental conditions, which are very far from the ideal ones, we can expect a long tail of our scattered neutrons as result of multiple scattering 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 mm 2 paraffin screen perpendicular to the direct neutron beam (as in figure 9) or along its direction (as in figure 8) sometimes adding a 8

11 Figure 6. FLUKA modelling calculations for all elements of the EK-10 fuel element; note at the left-hand side several narrow peaks produced due to scattering on nuclei of various elements and two peaks shown by arrows which are of the same origin as in figure 5; expected peak at MeV of pure fission neutrons produced by 14 MeV neutrons must be in the range ns with maximum at about 500 ns (shown by the oval). a) b) Figure 7. Arrangement of the DPF chamber PF9 of the device PF-6 and the fuel element EK-10 (a) and the same 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 this geometry we have a very asymmetric pulse of scattered neutrons indeed with a very long decay time (extended till 800 ns). 9

12 a) b) Figure 8. Schematic (a) and side-view (b) of the experiment with sizes/distances in mm. 10

13 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 E 0 = 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 Fuel Flagged neutrons, counted by the same scintillator. Here All neutrons are the overall neutrons elastically and inelastically scattered inside the experimental hall and registered by the photomultiplier 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 reactions in the fuel element are presented in these pulses. The same calculated trace derivative related to fuel flagged neutrons only and flipped vertically is presented in figure 11. One may see a specific peak observed at about 180 ns after the beginning of the neutron pulse or about ns in relation to the start of neutron generation inside the DPF chamber, as shown in figure 6 and figure 5. This peak corresponds to the energy of neutrons in the range MeV that are in the middle of the spectrum of neutrons generated in uranium by external neutrons of the energy E 0 = 14 MeV. 11

14 Figure 11. The same trace derivative as it is shown in figure 11 but which is calculated for the fuel flagged 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 or deuterium-tritium mixture as working gas. This transportable device (IPPLM) has the following parameters: bank energy 7 kj, amplitude of the discharge current 760 ka, neutron yield per shot 10 9 for pure D 2 or for a D T mixture, total weight 400 kg, footprint 1 m 2. The chambers used for the device, PF7 and PF9 (see figure 7b), were manufactured at the VNIIA, RF [20]), which are designed for the neutron yield and MeV neutrons per pulse correspondingly. Detection of hard X-rays and neutrons (both directly coming from the source and scattered by the object under interrogation and by surrounding items as well) was provided by photomultiplier tubes plus scintillators (PMT+S) detectors and chevron micro-channel plates plus PMT plus scintillator (MCP-PMT+S) with time resolution 3 and ns correspondingly. The second one 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 of the 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 we should like to have a temporal resolution of about 0.3 ns we cannot use a scintillator thickness more than 1 cm because TOF of 14 MeV neutrons of 1 cm is about this value. But in this case we lose sensitivity in a very high degree. From the other side a scintillator with its thickness equal to 10 cm 12

15 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 neutrons number in the scintillator. Thus we used a scintillator of the thickness of 1 cm with MCP-PMT for characterization of the neutron source itself whereas for our scattering experiments we used 2 sizes 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 neutrons with E 0 = 14 MeV generated by the PF-6 and directly coming from the source to the scintillator probes MCP-PMT (without shielding) are shown in figure 12 for their dissimilar intensities and for different sensitivities of the detector channels. In the second case (b) our probe is in a saturation mode. In these measurements made with two scintillation probes and for different distances from the source to the probe we found that the delay time of the temporal neutron peak intensity as appeared inside the DPF chamber in relation to the front of the hard X-ray pulse (and very often to 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 with fuel element EK-10 is presented in figure 7 whereas its surrounding, paraffin screens and concrete scatterers, 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 assembly EK-10). We have done it in the geometry as it is presented in figure 8. Our preliminary estimations and examination of the above figure 5, 6, 10, 11 and some others gave us the following positions of energy peaks expected in spectral distributions and, correspondingly, time neutron peaks t n in our real TOF oscilloscope traces in respect to the start of the X-ray pulse t X taking into account the neutron delay time t d and TOF of r= 6 m by neutrons t n TOF and by hard X-rays t X TOF : t n = t n TOF t X TOF +t d 1) The main neutron energy peak from the DPF source originating from D-T nuclear reactions according to our previous measurements is observed at the energy E n 14.0 MeV; its TOF of the distance from the DPF chamber to the detector (6 m) is equal to t n TOF 120 ns. 13

16 2) Peaks expected because of scattering on nuclei of 8 O 16 should be at the energies equal to the values E n 12.5; 11.0; ; 2.6; 2.2 MeV; corresponding TOFs are at t n TOF 126; 134; 170; 180, 225; 310 ns. 3) 13 Al 27 : E n 7.7; 6.7; 5.5; 4.0; 2.7 MeV; t n TOF 160; 170; 185; 220; 267 ns. 4) 12 Mg 24 : E n 12.5; 7.7; 5.76; 4.0; 1.1 MeV; t n TOF 126; 160; 180; 220; 430 ns. 5) Neutron energy peak originating from D-D nuclear reactions in the PF-6 (having more than 2 orders of magnitude lower amplitude compared with the main peak at 14 MeV) according to our previous measurements has the energy equal to E n 2.7 MeV; t n TOF = 267 ns. 6) Two relatively weak peaks (better to name them peculiarities ), which were observed in the above modelling of the real EK-10 assembly must be situated in the positions around values t n TOF 200 ns and 290 ns. 7) Fission neutrons 92 U 238 and 92 U 235 : E n 5... ( ) MeV with a peak at energy in the range E n MeV (Watt spectrum) may appear in the time gap t n TOF 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 of its amplitude in these our experiments is the same as it was measured for this device (15 ns) in good premises without concrete scatterers. Yet the tail of the neutron pulse appeared to be very long here (as it was predicted by MCNP calculations of figure 10 and 11). It reflects scattering of neutrons by many concrete blocks filling the experimental hall. Fortunately as one may see in figure 13 taken without fuel element (the reference pulse) this tail is very smooth having no visible peaks. In this control experiment we had just one screen made by paraffin and installed perpendicular to the direct neutron beam as it is shown in figure 9 (so with week shielding). Below we have presented several oscilloscope traces obtained during this session what continued during the time period of the availability of the fuel element. We provided 13 shots without fuel element to fit sensitivity and trigger level of the oscilloscope and to adapt the geometry of shields (partially). Then we provided 10 shots with fuel element. Among them we have registered good signals in 9 shots by one or two channels of the oscilloscope (working with dissimilar sensitivities and with different neutron yields of the PF-6). Throughout this period we changed positions and orientation of our shield(s), used different pressures of working gas and moved our PMT+S trying to find the highest signal at the lowest noise. Neutron yields measured in these experiments by activation of copper and by silver activation counter were within the range neutrons of energy E 0 = 14 MeV per shot. Analysis of the peaks appeared in the oscilloscope traces of figure 14 through 20 has shown that they are coincided with the above estimations for scattering (elastic and inelastic) on nuclei of elements contained in the fuel element as well as with our modelling simulations presented in figure 5, 6, 10 and 11. We have to mention here that one may see several additional peaks on the oscilloscope s traces having nothing with our object under interrogation. They result from several small abruptions of 14

17 Figure 13. Oscilloscope trace of hard X-ray (first) and neutron (second) pulses taken at a distance r= 6 m from the PF-6 chamber filled with DT mixture; the trace was obtained without fuel element and with just one 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 two dissimilar sensitivities of channels; the shield was placed perpendicular to the direct neutron beam as it is in figure 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 peaks that corresponds to different materials contained in the fuel element may be seen; note a small peak in the trace (b) with a position related to neutrons with energy 2.7 MeV; if it is connected with D-D neutrons generated by the DPF and elastically scattered by the environment, the area of that pulse must be lower than the area of the scattered pulse of neutrons of energy E 0 = 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 of the X-ray pulses) that were produced unfortunately in this shot. However we know where we have to expect the peaks of our interest in the oscilloscope traces beforehand, and we have found them 15

18 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 with peaks from other small abruptions as it was also proved by the comparative amplitude analysis and control experiments. A very similar result was obtained in the next two shots made in the same conditions (figure 15 a and b). The oscilloscope trace was taken here with the higher sensitivity compared to that in the figure 14. Then we have reoriented our main paraffin screen installing it along the neutron beam as it is seen in Fig 8. We suppressed direct beam in a higher degree and our expected pulses became more profound (figure 16). Then we install an additional paraffin block with dimensions 50 by 50 by 50 cm 3 along the direct beam of neutrons thus suppressing the main neutron pulse in a higher extent. The result is shown in figure 17. Comparison of one of these experimental oscilloscope traces versus our preliminary MCNP calculations is presented in figure 18. One may see the coincidence of the two peaks in both the simulation curve and the oscilloscope trace. Two vertical lines show the moments of starts of pulses of neutrons scattered by the fuel 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 interest we can try to confront them with the calculated (and estimated) peaks related to other materials composing our fuel element. For this purpose we have subtracted one oscilloscope trace taken without fuel element from the other which was taken with the EK-10 in its position near the DPF chamber (see figure 19). This oscilloscope traces subtraction with attempts of attribution of different peaks and comparison with results of MCNP modelling calculations are presented in figure 20. All other peaks without any indications on the picture are presumably connected with additional small current abruptions, 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 for the main hard X-ray and neutron pulses. 16

19 Figure 16. The shot with the main paraffin screen placed along the axis of the direct neutron beam from DPF 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 associated with the fuel assembly. According to MCNP calculations their intensity at the distance 6 m from 17

20 Figure 18. The oscilloscope trace (a) versus the preliminary MCNP calculation curve (b) of figure 11. Three vertical lines show start of the main neutron pulse and two peaks associated with a fuel assembly. the fuel element is I = n/cm 2 ns. Taking into consideration volume of our scintillators V (5 or 10 cm in lengths l and diameters d), efficiency of neutrons registration (k 1/2) and pulse durations registered (τ 40 ns) one can find the figure for the worst case (d = l = 5 cm): N =I V k τ =( n/cm 2 ns) {( )/(4 2)cm 2 } (40ns)=10 8 neutrons per pulse taken from the number of neutrons irradiating the fuel element. Geometry of irradiation decreases this figure in relation to the overall neutron yield of the device Y n by another one order of magnitude. It gives for each pulse from 40 up to 200 neutrons registered depending of our neutron yield of the device (Y n = neutrons per pulse in full solid angle). This statistics is above the border of acceptability presented in the section 2 (minimal number of registered mono-energetic neutrons must be higher than 10). 18

21 Figure 19. Two oscilloscope traces overlapping one another: one is taken without fuel element (the black one, smooth) and another one is taken with fuel element (the blue one with multiple peaks); the red one is the 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 TOF spectrum of a DPF can be observed by a single-shot nanosecond neutron pulsed technique. It is quite evident that in future experiments with our PF-6 device we have to find better premises to eliminate parasitic scatterers (to diminish the long tail produced by them), to create better shielding configuration, to decrease distance between an object and the detector till 1 meter (what is acceptable for the same DPF neutron source, detector and fuel assembly according to our present experience and MCNP modelling), and to use a scintillator with d = 100 and l = 100 mm. In this case 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 widerange MCNP calculations, we are of the opinion that the NINIS technique can probably be used for a disclosure of hidden objects containing fissile materials. In the case of success of these future works the main perspective of this method seems to be in unveiling of fissile materials at the express interrogation 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 these studies in the frame of CRP IAEA grants Nos and The work was partly sponsored from the research grant of the Polish Ministry of Science and Higher Education (MNiSZW) No. O N

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