MCNP Simulations of Fast Neutron Scattering by Various Elements andtheir Compounds in View of Elaboration of a Single Shot InspectionSystem

Urszula Wiaceka, Krzysztof Drozdowicza, Vladimir Gribkovb, Ryszard Miklaszewskic

aThe Henryk Niewodniczaski Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland

bInstitute for Theoretical and Experimental Physics, Moscow, Russia

cInstitute 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 theelemental content of unknown bulk sample should be possible from the recorded energy spectrum of the scatteredneutrons. The time-of-flight detection technique can be involved in the identification procedure. Numerical MonteCarlo simulations of fast (2.45 and 14 MeV) neutron scattering by various elements and their combinations havebeen performed. Examples of results obtained using the MCNP code are presented in the paper in the form ofthe scattered neutron energy and TOF spectra registered by a hypothetic detector at 1 m distance.

1. Introduction

Fight against terrorism is a complicated, mul-tidisciplinary task involving political, economi-cal, psychological, organizational, scientific, tech-nical, and other issues. Efficient method of detec-tion of explosives and other illicit materials is ofprincipal importance. However, despite the longlasting efforts of scientists and engineers such amethod still does not exist.

Several non-destructive nuclear methods havebeen proposed and investigated up to now to de-tect explosives and other illicit materials in lug-gage or containers. Neutron-based diagnostics arequite widely developed and used. Generally, theymake use of the neutron transport characteristicsthrough matter (transmission, absorption, elas-tic and inelastic scattering) and various neutronsources are being exploited like radioisotopes, ac-celerators and portable neutron generators pro-ducing continuous or pulsed neutron beams. Thefollowing methods can be distinguished amongothers:

– Thermal neutron analysis, TNA, with de-tection 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 scatter-ing [4]

– Pulsed fast-thermal neutron analysis,PFTNA, using a microsecond pulse neu-tron generator [5]

– Pulsed fast neutron transmission spec-troscopy, PFNTS, using a nanosecond pulseaccelerator [6]

– Fast neutron scattering analysis, FNSA,with detection of forward and backwardneutrons from elastic and inelastic scatter-ing [7]

– Associated particle imaging, API, with de-tection of the α particle associated withgenerated neutron and of the γ-rays frominelastic scattering of neutrons [8]

– and other combinations of these techniques.

Isotopic sources are often inconvenient, and theneutron bursts from the pulsed sources applied uptill now must be repeated many times because ofa relatively low neutron yield.

We propose here a new approach to the FastNeutron Scattering Analysis (FNSA) using a

1

2 U. Wiacek

Figure 1: Principle of the detection method.

pulsed neutron source of the plasma-focus type.The device produces high intensity neutron pulsesin the nanosecond range.

2. Principle of a Single Shot InspectionSystem

New-generation plasma-focus devices can pro-duce very intense and very short neutronpulses [9]. The PF-6 facility [10], operating inthe Institute of Plasma Physics and Laser Mi-crofusion (Warsaw, Poland), generates neutronpulses of about 10 ns. The neutron yield dur-ing one pulse is up to 109 from the reaction inpure deuterium plasma (2.5 MeV neutrons) andup to 1011 from the deuterium-tritium mixture(14 MeV neutrons). These features make possi-ble

1. to irradiate the material under interroga-tion with a neutron pulse sufficiently intensein order to

2. detect elastically and inelastically scatteredneutrons and

3. to use the time-of-flight (TOF) method forspectrometric analysis of these neutrons.

Nuclide-specific information is present in the scat-tered neutron field and deconvolution of the reg-istered energetic spectra allows detection of thetype of irradiated material. A schematic idea ofthe outlined single shot inspection system is pre-sented in Fig. 1.

A database of the neutron scattering signatureshas to be prepared in order to get informationfrom the stored data on proportions of contentsof elements which are characteristic for explo-

Figure 2: Geometry of the MCNP simulations.

sives (as H, C, N, O). The unfolding method hasto distinguish signal from everyday-use materials,which contain the same elements in other propor-tions. We present here preliminary results of theMonte Carlo simulations of the scattered neutronfields from several elements.

3. Monte Carlo Simulations of Fast Neu-tron Scattering by Various Elements

The MCNP code (Monte Carlo N-Particle) hasbeen used for simulation of the neutron transportin matter. A small cube has been modelled as thescattering volume. Artificial samples of the scat-tering material have been assumed, like pure C,pure N, pure O, pure S (the later to introduce anelement which is usually absent in the materialsunder question), and mixtures of the elements, (C+ N), (C + O), (N + O), or (C + S), etc.

The geometry of the simulated experiment isshown in Fig. 2. The monoenergetic neutronsource emits a parallel neutron beam that fallson the whole surface of the cube wall. Two inci-dent neutron energies are used: 2.45 MeV (corre-sponding to the d−d reaction), and 14 MeV (cor-responding to the d − t reaction). Neutron scat-tering is observed in five directions with respectto the incident beam. The angles are defined fromthe centre of the cube: 30◦, 45◦ (Forward), 90◦,120◦, 150◦ (Backward). For the MCNP calcula-tion purposes, a ring detector of 1 m radius isused owing to the system symmetry. This pro-cedure accelerates the computer data collection,non-influencing the physical idea of the phenom-ena.

The neutron scattering has been investigatedon two volumes of the samples in order to de-

MCNP Simulations of Fast Neutron Scattering 3

Figure 3: Comparison of scattered neutron energyspectra 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 onthe effect of multiple scattering resulting in theneutron emission in various final directions. Thetwo following cube edges have been used: 3 and

8 cm. The mass density ρ = 2.3 g/cm3 has beenassumed (following e.g., real density of carbon).The same value has been used in all cases in or-der not to change the signal by varying materialdensity. For gaseous samples, it should be thenunderstood as if there would be a material withthe given density in which we observe neutronscattering on nuclei of N or O.

The scattered neutron energy distribution hasbeen registered in the five directions specifiedabove. The most often used energy mesh has beenas 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 be-tween 13 and 14 MeV).

The scattered neutron flux Φ/E (fluence) is ob-tained as a function of energy [neutrons per cm2

per MeV per one starting source neutron] at 1 mdistance. The angle-energy distributions of theneutrons scattered by particular media are theresult. The corresponding TOF distributions ofneutrons have been obtained on 1 m flight path.

Three examples of the obtained energy distri-butions are shown in Fig. 3. The spectra areclearly different. The contributions in the inter-esting parts are below 10−6 of the initial neutronflux. At the neutron yield on the order of 109 (perone shot) this corresponds to about 103 counts(neglecting the detector efficiency).

Examples of TOF spectra coming from the ini-tial 14 MeV neutrons are presented in Fig. 4.

4. Conclusions

The energy spectra of neutrons scattered fromdifferent elements (or their combinations) are suf-ficiently different to create a base of scatter-ing signatures. Detection of neutrons scattered(elastically and inelastically) at different anglesbrings information helpful for determination ofthe amounts and positions of the scattering nu-clei, especially when two incident neutron ener-gies (2.45 and 14 MeV) are used. A short-pulse

4 U. Wiacek

Figure 4: Comparison of TOF spectra (base = 1 m)of neutrons scattered by the 3 cm cube of Carbon andSulphur. Source energy E = 14 MeV.

source (ns) of plasma-focus type, makes it possi-ble to use the TOF method for the spectroscopyof the scattered fast neutrons. The measure-ment on the 1-meter flight path in the case ofthe 14 MeV neutrons (time ranging 20 ∼ 75 ns)can be perturbed by the initial pulse width ( 10ns). This inconvenience can be partly compensedby a higher neutron yield (∼ 1011 per pulse) thanin the case of 2.45 MeV neutrons (∼ 109). Alonger base, of course, can be used to improvethe measurement accuracy.

The work should be continued to create an ex-tensive database of signatures by means of the

Monte Carlo simulation method verified with anumber of relevant experiments.

REFERENCES

1. W.C. Lee, D.B. Mahood, P. Ryge, P. Shea,T. Gozani, 1995. Thermal neutron analy-sis (TNA) explosive detection based on elec-tronic neutron generators. Nucl. Instrum.Methods B 99, 739- 742.

2. Z.P. Sawa, 1993. PFN GASCA technique fordetection of explosives and drugs. Nucl. In-strum Methods B 79, 593- 596.

3. Smith R.C., Hurwitz M.J., Tran K.-C., 1995.System to detect contraband in cargo con-tainers using fast and slow neutron irradia-tion and collimated gamma detectors. Nucl.Instrum. Methods B 99, 733 -735.

4. D.R. Brown, T. Gozani, R. Loveman, J. Ben-dahan, P. Ryge, J. Stevenson, F. Liu,M. Sivakumar, 1994. Application of pulsedfast neutrons analysis to cargo inspection.Nucl. Instrum. Methods A 353, 684- 688.

5. P.C. Womble, F.J. Schultz, G. Vourvopoulus,1995. Nondestructive characterization usingpulsed fast-thermal neutrons. Nucl. Instrum.Methods B 199, 757- 760.

6. J.C. Overlay, 1987. Element-sensitive com-puted tomography with fast neutrons. Nucl.Instrum. Methods B 24/25, 1058- 1062.

7. A. Buffler, F.D. Brooks, M.S. Allie,K. Bharuth-Ram, M.R. Nchodu, 2001.Material classification by fast neutron scat-tering analysis. Nucl. Instrum. Methods B173/4, 483–502.

8. E. Rhodes, C.E. Dickerman, A. DeVolpi,C.W. Peters, 1992. APSTNG: radiation inter-rogation for verification of chemical and nu-clear weapons. IEEE Trans. Nucl. Sci. 39(4),1041 -1045.

9. S. Lee, G. Zhang, X. Feng, V.A. Gribkov,M. Liu, A. Serban, T.K.S. Wong, 1998. Highrep rate high performance plasma focus asa powerful radiation source, IEEE Trans. onPlasma Science 26(4), 1119- 1126.

10. V.A. Gribkov , L. Karpinski, P. Strzyzewski,M. Scholz, A. Dubrovsky, 2004. New effi-cient low-energy dense plasma focus in IP-

MCNP Simulations of Fast Neutron Scattering 5

PLM. Czechoslovak J. of Physics 54, Suppl.C, C191-C197.