MCNP Simulations of Fast Neutron Scattering by Various Elements and their Compounds in View of Elaboration of a Single Shot Inspection System

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1 MCNP Simulations of Fast Neutron Scattering by Various Elements and their Compounds in View of Elaboration of a Single Shot Inspection System Urszula Wiacek a, Krzysztof Drozdowicz a, Vladimir Gribkov b, Ryszard Miklaszewski c a The Henryk Niewodniczaski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland b Institute for Theoretical and Experimental Physics, Moscow, Russia c Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland Plasma-focus device is intended to be used in a single shot inspection system for a detection of hidden explosives. Fast neutrons emitted during a short, intense pulse are scattered by the investigated object. Unfolding of the elemental content of unknown bulk sample should be possible from the recorded energy spectrum of the scattered neutrons. The time-of-flight detection technique can be involved in the identification procedure. Numerical Monte Carlo simulations of fast (2.45 and 14 MeV) neutron scattering by various elements and their combinations have been performed. Examples of results obtained using the MCNP code are presented in the paper in the form of the scattered neutron energy and TOF spectra registered by a hypothetic detector at 1 m distance. 1. Introduction Fight against terrorism is a complicated, multidisciplinary task involving political, economical, psychological, organizational, scientific, technical, and other issues. Efficient method of detection of explosives and other illicit materials is of principal importance. However, despite the long lasting efforts of scientists and engineers such a method still does not exist. Several non-destructive nuclear methods have been proposed and investigated up to now to detect explosives and other illicit materials in luggage or containers. Neutron-based diagnostics are quite widely developed and used. Generally, they make use of the neutron transport characteristics through matter (transmission, absorption, elastic and inelastic scattering) and various neutron sources are being exploited like radioisotopes, accelerators and portable neutron generators producing continuous or pulsed neutron beams. The following methods can be distinguished among others: Thermal neutron analysis, TNA, with detection of prompt γ-rays [1] Fast neutron analysis, FNA, with detection of γ-rays from activated nuclei [2, 3] Pulsed fast neutron analysis, PFNA, with detection of γ-rays from inelastic scattering [4] Pulsed fast-thermal neutron analysis, PFTNA, using a microsecond pulse neutron generator [5] Pulsed fast neutron transmission spectroscopy, PFNTS, using a nanosecond pulse accelerator [6] Fast neutron scattering analysis, FNSA, with detection of forward and backward neutrons from elastic and inelastic scattering [7] Associated particle imaging, API, with detection of the α particle associated with generated neutron and of the γ-rays from inelastic scattering of neutrons [8] and other combinations of these techniques. Isotopic sources are often inconvenient, and the neutron bursts from the pulsed sources applied up till now must be repeated many times because of a relatively low neutron yield. We propose here a new approach to the Fast Neutron Scattering Analysis (FNSA) using a 1

2 2 U. Wiacek Figure 1: Principle of the detection method. Figure 2: Geometry of the MCNP simulations. pulsed neutron source of the plasma-focus type. The device produces high intensity neutron pulses in the nanosecond range. 2. Principle of a Single Shot Inspection System New-generation plasma-focus devices can produce very intense and very short neutron pulses [9]. The PF-6 facility [10], operating in the Institute of Plasma Physics and Laser Microfusion (Warsaw, Poland), generates neutron pulses of about 10 ns. The neutron yield during one pulse is up to 10 9 from the reaction in pure deuterium plasma (2.5 MeV neutrons) and up to from the deuterium-tritium mixture (14 MeV neutrons). These features make possible 1. to irradiate the material under interrogation with a neutron pulse sufficiently intense in order to 2. detect elastically and inelastically scattered neutrons and 3. to use the time-of-flight (TOF) method for spectrometric analysis of these neutrons. Nuclide-specific information is present in the scattered neutron field and deconvolution of the registered energetic spectra allows detection of the type of irradiated material. A schematic idea of the outlined single shot inspection system is presented in Fig. 1. A database of the neutron scattering signatures has to be prepared in order to get information from the stored data on proportions of contents of elements which are characteristic for explo- sives (as H, C, N, O). The unfolding method has to distinguish signal from everyday-use materials, which contain the same elements in other proportions. We present here preliminary results of the Monte Carlo simulations of the scattered neutron fields from several elements. 3. Monte Carlo Simulations of Fast Neutron Scattering by Various Elements The MCNP code (Monte Carlo N-Particle) has been used for simulation of the neutron transport in matter. A small cube has been modelled as the scattering volume. Artificial samples of the scattering material have been assumed, like pure C, pure N, pure O, pure S (the later to introduce an element which is usually absent in the materials under question), and mixtures of the elements, (C + N), (C + O), (N + O), or (C + S), etc. The geometry of the simulated experiment is shown in Fig. 2. The monoenergetic neutron source emits a parallel neutron beam that falls on the whole surface of the cube wall. Two incident neutron energies are used: 2.45 MeV (corresponding to the d d reaction), and 14 MeV (corresponding to the d t reaction). Neutron scattering is observed in five directions with respect to the incident beam. The angles are defined from the centre of the cube: 30, 45 (Forward), 90, 120, 150 (Backward). For the MCNP calculation purposes, a ring detector of 1 m radius is used owing to the system symmetry. This procedure accelerates the computer data collection, non-influencing the physical idea of the phenomena. The neutron scattering has been investigated on two volumes of the samples in order to de-

3 MCNP Simulations of Fast Neutron Scattering 3 8 cm. The mass density ρ = 2.3 g/cm 3 has been assumed (following e.g., real density of carbon). The same value has been used in all cases in order not to change the signal by varying material density. For gaseous samples, it should be then understood as if there would be a material with the given density in which we observe neutron scattering on nuclei of N or O. The scattered neutron energy distribution has been registered in the five directions specified above. The most often used energy mesh has been as follows: (i) in the case of the 2.45 MeV source neutrons: 0.1, 1, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, and 2.45 [MeV], (ii) in the case of the 14 MeV source neutrons: 0.1, 1, 2, 4, 6, 8, 10, 12, 13, and 14 [MeV], (sometimes an additional mesh used between 13 and 14 MeV). The scattered neutron flux Φ/E (fluence) is obtained as a function of energy [neutrons per cm 2 per MeV per one starting source neutron] at 1 m distance. The angle-energy distributions of the neutrons scattered by particular media are the result. The corresponding TOF distributions of neutrons have been obtained on 1 m flight path. Three examples of the obtained energy distributions are shown in Fig. 3. The spectra are clearly different. The contributions in the interesting parts are below 10 6 of the initial neutron flux. At the neutron yield on the order of 10 9 (per one shot) this corresponds to about 10 3 counts (neglecting the detector efficiency). Examples of TOF spectra coming from the initial 14 MeV neutrons are presented in Fig. 4. Figure 3: Comparison of scattered neutron energy spectra from the cube a = 3 cm of a) Carbon, b) Sulphur, c) C+N+O+S mixture. Source energy E = 2.45 MeV. tect an influence of the size of the material on the effect of multiple scattering resulting in the neutron emission in various final directions. The two following cube edges have been used: 3 and 4. Conclusions The energy spectra of neutrons scattered from different elements (or their combinations) are sufficiently different to create a base of scattering signatures. Detection of neutrons scattered (elastically and inelastically) at different angles brings information helpful for determination of the amounts and positions of the scattering nuclei, especially when two incident neutron energies (2.45 and 14 MeV) are used. A short-pulse

4 4 U. Wiacek Monte Carlo simulation method verified with a number of relevant experiments. REFERENCES Figure 4: Comparison of TOF spectra (base = 1 m) of neutrons scattered by the 3 cm cube of Carbon and Sulphur. Source energy E = 14 MeV. source (ns) of plasma-focus type, makes it possible to use the TOF method for the spectroscopy of the scattered fast neutrons. The measurement on the 1-meter flight path in the case of the 14 MeV neutrons (time ranging ns) can be perturbed by the initial pulse width ( 10 ns). This inconvenience can be partly compensed by a higher neutron yield ( per pulse) than in the case of 2.45 MeV neutrons ( 10 9 ). A longer base, of course, can be used to improve the measurement accuracy. The work should be continued to create an extensive database of signatures by means of the 1. W.C. Lee, D.B. Mahood, P. Ryge, P. Shea, T. Gozani, Thermal neutron analysis (TNA) explosive detection based on electronic neutron generators. Nucl. Instrum. Methods B 99, Z.P. Sawa, PFN GASCA technique for detection of explosives and drugs. Nucl. Instrum Methods B 79, Smith R.C., Hurwitz M.J., Tran K.-C., System to detect contraband in cargo containers using fast and slow neutron irradiation and collimated gamma detectors. Nucl. Instrum. Methods B 99, D.R. Brown, T. Gozani, R. Loveman, J. Bendahan, P. Ryge, J. Stevenson, F. Liu, M. Sivakumar, Application of pulsed fast neutrons analysis to cargo inspection. Nucl. Instrum. Methods A 353, P.C. Womble, F.J. Schultz, G. Vourvopoulus, Nondestructive characterization using pulsed fast-thermal neutrons. Nucl. Instrum. Methods B 199, J.C. Overlay, Element-sensitive computed tomography with fast neutrons. Nucl. Instrum. Methods B 24/25, A. Buffler, F.D. Brooks, M.S. Allie, K. Bharuth-Ram, M.R. Nchodu, Material classification by fast neutron scattering analysis. Nucl. Instrum. Methods B 173/4, E. Rhodes, C.E. Dickerman, A. DeVolpi, C.W. Peters, APSTNG: radiation interrogation for verification of chemical and nuclear weapons. IEEE Trans. Nucl. Sci. 39(4), S. Lee, G. Zhang, X. Feng, V.A. Gribkov, M. Liu, A. Serban, T.K.S. Wong, High rep rate high performance plasma focus as a powerful radiation source, IEEE Trans. on Plasma Science 26(4), V.A. Gribkov, L. Karpiński, P. Strzyzewski, M. Scholz, A. Dubrovsky, New efficient low-energy dense plasma focus in IP-

5 MCNP Simulations of Fast Neutron Scattering 5 PLM. Czechoslovak J. of Physics 54, Suppl. C, C191-C197.