Detection of Special Nuclear Materials with the Associate Particle
Technique
Cédric Carasco1, Clément Deyglun1, Bertrand Pérot1, Cyrille Eléon1, Stéphane Normand2,
Guillaume Sannié2, Karim Boudergui2, Gwenolé Corre2, Vladimir Konzdrasovs2, Philippe Pras3
CEA, DEN , Cadarache, Nuclear Measurement Laboratory, F-13108 St Paul-lez-Durance, France
CEA, DRT, LIST, Saclay, F-91191 Gif-sur-Yvette, France
CEA, DAM, DIF, F-91297 Arpajon, France
In the frame of the French trans-governmental R&D program against CBRN-E threats, CEA is
studying the detection of Special Nuclear Materials (SNM) by neutron interrogation with fast
neutrons produced by an associated particle sealed tube neutron generator. The deuterium-tritium
fusion reaction produces an alpha particle and a 14 MeV neutron almost back to back, allowing
to tag neutron emission both in time and direction with an alpha particle position-sensitive sensor
embedded in the generator. Fission prompt neutrons and gamma rays induced by tagged neutrons
are detected in coincidence with plastic scintillators. This paper presents numerical simulations
performed with the MCNP-PoliMi Monte Carlo computer code and with post processing
software developed with the ROOT data analysis package. False coincidences due to neutron and
photon scattering between adjacent detectors (cross talk) are filtered out to increase the
selectivity between nuclear and benign materials. Accidental coincidences, which are not
correlated to an alpha particle, are also taken into account in the numerical model, as well as
counting statistics, and the time-energy resolution of the data acquisition system. Such realistic
calculations show that relevant quantities of SNM (few kg) can be distinguished from cargo and
shielding materials in 10 min acquisitions. First laboratory tests of the system under development
in CEA laboratories are also presented.
Introduction
Shielded Special Nuclear Materials (SNM) is a stringent objective in the fight against terrorist
activities. In particular, Highly Enriched Uranium is difficult to detect by passive assay due
insufficient neutron emission and low-energy gamma rays easy to stop with a few cm metallic
shield. Therefore, active photon and neutron interrogation techniques are being investigated
throughout [1], the principle of which is to induce fissions in nuclear materials. High energy
photons can be used ([2], [3] and [4]) or neutrons ranging from fast to thermal energies [5] [6],
[7], [8] and [9]. Prompt or delayed fission neutrons and/or gamma rays are detected to evidence
the presence of SNM. The historical method based on the detection of photofission delayed
neutrons [10] but they are strongly attenuated in hydrogenous cargo because of their low average
energy, close to 400 keV. The detection of photofission delayed gamma rays with energy larger
than 3 MeV [2] is less sensitive to attenuation and to natural or activated gamma backgrounds.
Recent investigations focus on the detection of prompt fission particles, which are by two orders
of magnitude more numerous than the delayed ones. For instance, fission fast neutron detection
can be performed during pulses with SiC semi-conductors or plastic scintillators [11], or after
pulses using threshold activation detectors, i.e. fluorocarbon scintillators sensitive to the delayed
beta emission of 16N, the radioactive period of which being 7.1 s, created in the 19F(n,)16N fast
neutron reaction [4]. At last, nanosecond coincidence analysis can also be performed to detect
time correlated fission neutron and gamma rays, either with a ns-pulsed LINAC [12] or during a
continuous, low-dose irradiation [13].
Systems performing photon-induced fission can be implemented on existing X-ray scanners
already using a LINAC, which already allow to estimate the mean atomic number of the
inspected object [14], [15] and [4]. The presence of dense and high Z materials can be used to
trigger an alarm and the photofission system can be used as a second level inspection.
On the other hand, source photons may be strongly attenuated in dense, intermediate and high
atomic number cargos or shields, like metals, and neutron interrogating may bring valuable
information in such situations. Thermal neutron fission benefits from a large cross section but
can be impaired if cargo materials do not sufficiently moderate source fast neutrons, as for
instance metals. In addition, thermal neutrons can be absorbed in thin layers of cadmium or
boron around nuclear materials. Although having a smaller fission cross section, epithermal and
fast neutrons are less sensitive to cargo materials and shields. They also induce a higher fission
neutron multiplicity, which makes them suitable for coincidence analysis. Additionally, fast
neutron beams can perform radiographic pictures of the inspected object [16], [17], [18] and
improve the sensitivity of photon imaging systems [19]. Last but not least, they can also be used
to detect nonnuclear threats by neutron activation analysis: explosives, illicit drugs, contraband
materials, or even chemical warfare [1], [20], [21], [22].
In the frame of the French trans-governmental R&D program against CBRN-E threats, CEA is
studying the detection of Special Nuclear Materials (SNM) by neutron interrogation with 14
MeV neutrons produced by the 3H(2H,n) fusion reaction in an associated particle sealed tube. A
position-sensitive alpha detector allows determining the direction of the nearly opposite neutron
and its time of flight (TOF) [23]. Coincidence analysis between alpha and fission particles
decreases dramatically the random background and false alarm rates [13], [12]. In particular, the
detection of at least three fission neutrons or gamma rays, in coincidence with the alpha particle,
signs the presence of SNM with a much larger selectivity than two-fold coincidences, which are
sensitive to (n,2n) or (n,n’) reactions in nonnuclear materials [24], [25] and [26]. The system
developed by CEA is based on this principle. Fission particles are detected by a matrix of plastic
scintillators within a time window triggered by the alpha detector. TOF information enables
neutron-gamma discrimination and it is not necessary to use expensive and toxic liquid
scintillators. High efficiency LaBr3(Ce) or NaI(Tl) gamma-ray detectors will also be
implemented to detect non-nuclear threats by associated particle imaging. The plastic and
inorganic scintillators can also be used for passive neutron-gamma detection. At last, the use of a
small size associated particle neutron generator from EADS SODERN allows designing systems
ranging from portable devices1 to large cargo container inspection portals with several neutron
generators. As reported in references [27] and [13], coupling photon and associated particle
imaging provides extended threat detection capability: X-ray radiography allows fast detection of
a suspicious area, on which tagged neutron inspection can be focused for further identification.
The performances of the system under development in CEA laboratories have been studied with
the MCNP PoliMi Monte Carlo code [28] coupled to a post processing software based on the
ROOT package [29]. Accidental coincidences not correlated with an alpha particle are taken into
account in post processing algorithms, because they may reduce the selectivity between nuclear
and other materials. Counting statistics, as well as time and energy resolutions of the data
acquisition system, are also introduced in the simulation. This paper describes the simulation
See for instance http://www.sodern.com/sites/en/ref/ULIS_80.html, EADS SODERN, ULIS “Unattended Luggage
Inspection System”, or http://www.apstec.ru/pr31.php?kuna=en, APSTEC, Smart SENNA, Portable Device for
Detection of Hazardous Materials.
1
approach, calculated performances for typical inspection geometries, and first passive laboratory
tests.
Numerical simulation and data post processing
The model of an ideal geometry for cargo container inspection is shown in Fig. 1. The container
is surrounded by four large segmented plastic scintillation plates. Detection pixels are not shown
in the MCNP model because segmentation is only introduced in post processing algorithms. On
the other hand, some of the large detection plates can be disabled in the calculation depending on
practical implementation constraints. For instance, if the cargo is transported by a truck, the
bottom plate can be suppressed.
In a real system, a few hundred nanosecond window is opened after the alpha particle detection,
during which fission particles induced by the tagged neutron are expected. The process is similar
in simulation, but time origin is the source neutron emission. Ideally, a true coincidence is due to
the detection of two, three, or more fission neutrons or gamma rays, by different segmented
detectors during the inspection time window. However, neutron and photon scattering between
adjacent detectors leads to cross talk coincidences. To diminish the number of such events,
adjacent pixels triggered within the inspection time window are grouped into clusters, each
cluster being supposed to have detected only one particle. One-fold, two-fold, three-fold…
coincidences refer to the number of clusters [30] [Brevet], which is called coincidence “order” or
coincidence “level” in this paper.
Figure 1. Model of the cargo container inspection system showing in blue
the cells used as detectors in MCNP Polimi simulations.
Simulations are performed with the MCNP PoliMi computed code [29] [Polimi] which handles
coincidences between several detectors. MCNP PoliMi provides an inventory of histories, each
connected to one 14 MeV source neutron (corresponding to a tagged neutron), in which
collisions occurred in at least N detectors, N being set by the user. For each collision, MCNP
Polimi provides rich information: energy deposited in the detector, time and position of the
interaction, projectile type (neutron or photon), collision type (elastic or inelastic scattering,
capture, fission…), target nucleus, reaction originating the particle (for instance a fission) in case
of secondary particle, etc. As only 47 detectors can be described with MCNP PoliMi, only the 7
cells shown in Fig. 1 have been declared as detectors and N has been set to unity. Post processing
software based on CERN’s ROOT package [29] allows segmentation of the detectors into an
arbitrary number of pixels. Collisions coordinates provided by MCNP PoliMi are used to assign
each collision to a detection pixel. The detection clusters mentioned above are built to create
TOF spectra with different coincidence levels, TOF being here defined as the time between the
source neutron emission and the detection of the first particle in coincidence. The number of
detection pixels, energy threshold, inspection time window boundaries, coincidence level, as well
as the time and energy resolutions of the plastic scintillators and data acquisition system, are set
in the form of text entries of a Graphical User Interface (GUI).
Understanding the impact of the random background in such a system is essential. Indeed,
random background can transform a n-fold coincidence into a false (n+1)-fold coincidence. For
instance, if a source neutron undergoes a (n,2n) reaction on an iron nucleus and the two outgoing
neutrons are detected by different clusters, while a particle not correlated with the original tagged
neutron is accidentally detected in another cluster during the same inspection window, it will
appear as a three-fold coincidence. Therefore, the total uncorrelated count rate of the system is
calculated separately and introduced in the flow of collisions for all MCNP PoliMi histories, thus
simulating the random background. The rate, spatial distribution and energy distribution of
background counts are calculated with MCNP PoliMi [28] and set as entries in the GUI text. In
this way, several background configurations, corresponding for example to different neutron
source shielding options, can be tested in a straightforward way.
Numerical performance assessment
The expected performances of a container inspection system were calculated in case of 0.2 g/cm3
iron and wood cargos including, in their center, a 5 cm edge cube of highly enriched uranium (~
2.3 kg of metallic uranium with a 93% 235U isotopic fraction), shielded with 2 mm cadmium and
3 cm lead shields. Containers transporting metallic goods represent about 15-20% of the
maritime traffic [31], [32], for which fast neutron is favorable due to limited neutron attenuation
and slowing-down effects [20]. However, organic goods (clothes, wood products, food…) span
the majority of cargo containers [31], [32], for which neutron scattering on hydrogen nuclei is
known to be a strong limitation [33], [34]. Therefore, we studied the performances for these two
cargos, with the average density of 0.2 g.cm-3 also observed on a large sample of containers [32].
Calculations were performed with the model shown in Fig. 2. The detection plate below the
container was not implemented because inspection is planed to be performed on trucks. The
plates are segmented in 37.5 cm  40 cm pixels with a thickness of 10 cm. The pyramidal shield
is made of polyethylene in its first part, to slow down generator fast neutrons, completed by lead
in its second part to stop gamma rays produced in polyethylene by inelastic scattering and
capture reactions.
Figure 3 shows two-fold and three-fold coincidence TOF spectra, corresponding to 10 min
acquisitions with a total neutron flux of 3.107 n/s, in case of bare and shielded HEU in the center
of the metallic container. A 283 kc/s total count rate, corresponding to 171 kc/s for neutrons and
112 kc/s for gamma rays, was calculated for the whole detection plates (in fact, it is predominant
in the plate close to the generator). This background count rate does not vary a lot with cargo
materials. A third calculation is reported with the shielding materials only (i.e. 15 kg of lead and
280 g of cadmium) in the center of the metallic cargo, as well as a fourth calculation with the
bare cargo to show that the system can distinguish SNM from nonnuclear materials.
The [70 ns; 180 ns] window corresponding to coincidences involving only neutrons allow the
best discrimination between SNM and nonnuclear materials, and as explained above three-fold
coincidences show a better contrast than two-fold coincidences. The peak at 40 ns corresponds to
coincidences involving at least one gamma particle. In case of shielded HEU, it does not allow
evidencing SNM because fission gamma rays have a relatively low average energy, around 1
MeV, and are stopped by the 3 cm lead shield.
Table 1 reports the total number of three-fold coincidences in the [70 ns; 180 ns] window, and
the net signal after subtraction of the counts recorded with the bare matrix. In the experimental
system under development, the cargo materials background will be acquired thanks to a fraction
of the tagged neutron beam not aiming at the region suspected to include SNM, in view to be
subtracted to the signal of this region of interest. In the simulation, the number of coincidences of
the calculation without SNM is simply subtracted to the one with SNM.
In the case of a high random background level, accidental detection of an uncorrelated particle
before 70 ns may lead to lose a true fission neutron coincidence in the [70 ns; 180 ns] window.
Therefore, coincidence numbers reported in Table 1 were calculated only with particles having a
TOF in the [70 ns, 180 ns] window, which explains the little difference between the signal-tonoise ratio observed in Figure 3 and in Table 1.
Figure 2 : Inspection system configuration
Fission, capture or
inelastic gamma rays
Shielded HEU
Bare HEU
Lead and cadmium shield
Bare matrix
Transmitted neutrons
Fission neutrons
Figure 3. Two-fold coincidences (above) and three-fold coincidences (below) after 10 min inspections
of a container filled with a 0.2 g.cm-3 homogenous metallic cargo.
Table 1. Number of two- and three-fold coincidences in the [70 ns, 180 ns] time window
of the TOF spectra reported in Fig. 3.
Hidden material in
metallic matrix
Bare metallic matrix
Shielded HEU
Bare HEU
Lead and cadmium
shield only (without
HEU inside)
Two-fold coincidences
Gross counts Net counts
9592
34200
24608
41083
31491
14832
5240
Three-fold coincidences
Gross counts Net counts
561
6589
6028
9508
8947
734
173
In case of the wood matrix, see Figure 4 and Table 2, three-fold coincidences involving gamma
rays ([40 ns, 46 ns] window) is a strong indication of the presence of unshielded nuclear material.
However, fission gamma rays are stopped by the 3 cm lead shield, and due to neutron scattering
and slowing down on hydrogen nuclei, fission neutron three-fold coincidences ([70 ns, 180 ns]
window) are very few.
Shielded HEU
Bare HEU
Lead and cadmium shield
Bare matrix
Figure 4. Two-fold coincidences (on the left) and three-fold coincidence (on the right) after 10 min inspection
of a container filled with a 0.2 g.cm-3 homogenous wood cargo.
Table 2. Number of two- and three-fold coincidences in the [70 ns, 180 ns] time window of the TOF spectra
reported in Fig. 3. Nota: negative net signals are due to insufficient counting statistics.
Hidden
material in
wood
matrix
Bare wood
matrix
Shielded
HEU
Bare HEU
Lead and
cadmium
shield
(without
HEU
inside)
Two-fold coincidences
[40 ns, 46 ns]
[70 ns, 180 ns]
Gross Net
Gross Net
counts counts counts counts
270
1718
-
Three-fold coincidences
[40 ns, 46 ns]
[70 ns, 180 ns]
Gross
Net counts Gross
Net counts
counts
counts
5
32
-
324
54
1600
-118
3
-2
46
14
1148
312
878
42
1733
1501
15
-217
94
9
89
4
57
25
25
-7
The tagged neutron flux could be increased, for instance by implementing several neutron
generators, and acquisition time could be extended to improve performances for wood cargo
inspections. We are also studying the detection of the suspicious item in other positions inside
the container, and the detection limits for the different situations.
Development of a laboratory test bench
The data acquisition electronics (DAQ) is under development at CEA LIST Saclay. It consists of
compact and independent FPGA cards being each associated to a unique pixel of the alpha
position sensitive detector, or to a unique plastic scintillator. Detection time and energy of the
pulses are determined in each board by sampling the signal with four 200 MHz ADC (Analog to
Digital Converters) shifted by a quarter of the sampling period, to increase sampling frequency to
an equivalent ~ 800 MHz.
Figure 6: Electronic board.
Figure 5: 10 cm  10 cm 
10 cm plastics scintillation
detectors.
The relative time between detector cards is determined by synchronization modules. Time and
energy information are transferred to a data acquisition PC which can be embedded with the
DAQ in case of a portable system.
The DAQ was first tested with 252Cf and AmBe sources placed in the center of three 10 cm  10
cm  10 cm plastic scintillation detectors. Figure 7 shows the time difference between the counts
recorded in two of the three detectors with the 252Cf source. A classical symmetric pattern [35]
can be observed, with a narrow peak between - 2 ns and + 2 ns due to - detection, and broader
structures above 2 ns and below -2 ns corresponding to n-n, -n, and n- coincidences.
Delta t (ns)
Delta t (ns)
Figure 7. Correlation function between two plastic scintillators recorded with the 252Cf source: MCNP PoliMi
calculation above; experimental data below.
In Figure 8, the time difference between the 4.439 MeV gamma-ray and neutron detections is
consistent with a typical AmBe neutron energy spectrum [36].
Figure 8. Experimental correlation function between two plastic scintillators recorded with the AmBe source
with a 0.15 MeVee (MeV equivalent electron) threshold in both detectors.
Conclusion
CEA laboratories are developing a fast neutron interrogation system based on the associate
particle technique to detect special nuclear materials in various scenarios. The system will consist
of a sealed tube neutron generator with an embedded alpha detector from EADS SODERN, a few
dozen plastic scintillators from ELJEN, and a dedicated data acquisition electronics under
development in CEA. In order to discriminate SNM from non-nuclear materials, three-fold
coincidences are detected between fission particles induced by tagged fast neutrons, i.e. in an
inspection time window following the detection of an alpha particle. The performances of the
system are studied with MCNP PoliMi and a dedicated post processing software based on ROOT
data analysis package. Calculated performances for a ~ 2.3 kg shielded HEU (Highly Enriched
Uranium) block hidden in the middle of a metallic cargo are very encouraging, HEU being
unambiguously discriminated from nonnuclear materials in 10 min. However, only unshielded
SNM can be detected in the organic wood cargo with the current setup and we are studying
different ways to improve performances, such as using several neutron generators, as well as
detection limits in various inspection situations. On the other hand, the data acquisition
electronics was successfully tested in passive mode with radioactive sources in view to prepare
the tests with the associated particle neutron generator.
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