DUNBLAD, the Delft University Neutron Backscatter
LAnd-mine Detector, a status report
Victor R. Bom∗ , Cor P. Datema∗† and Carel W.E. van Eijk∗
∗
Delft University of Technology, IRI, Mekelweg 15, 2629 JB Delft, the Netherlands
†
Currently at Philips, Best, The Netherlands
Abstract. The neutron backscattering technique may be applied to search for non-metallic land mines in relatively dry sandy
soils. A novel, ergonomic detector system has been constructed. Tests with real land mines in a realistic environment show
that anti-tank mines can reliably be found, but that anti-personnel mines may escape detection.
The performance could improve when an image of the mine signal could be obtained. One approach is to use an array of
position sensitive 3 He detectors placed close to the soil. If a pulsed neutron generator is used further improvement could be
reached by applying a time window on the neutron transit time.
The possibilities of neutron backscattering imaging systems are investigated using Monte Carlo simulations with GEANT4. A neutron backscattering imaging device with a 2D sensitive detection plane is currently under development.
INTRODUCTION
The detection of non-metallic land mines has been a
problem for many years and until now no adequate
method to track these ’hidden killers’ is available . One
of the techniques currently under investigation is neutron
backscattering [1, 2, 3, 4, 5, 6]. Here use is made of the
circumstance that non-metallic land mines contain much
more hydrogen atoms than the average dry sandy soil.
Hydrogen is an extremely effective thermalizing agent
for fast neutrons. Thus when the soil is irradiated with
high-energy neutrons the number of low-energy neutrons
which is scattered back from the soil forms a good indicator of a hydrogen-rich anomaly, c.q. a mine.
Many of the existing systems use a radio isotopic
neutron source and a single pixel detector for low-energy
neutrons. Unfortunately, such devices have significant
problems in finding the relatively small anti-personnel
mines. Typically, these have a diameter of ≈6 cm and
contain less than 100 gram of explosive material.
A prototype handheld detector was developed at the
Inter-faculty Reactor Institute (IRI) at Delft based on
preliminary tests [8] and on Monte Carlo simulations.
The simulations served to examine the performance of
the system as a function of parameters such as: the mine
depth, the distance between the detector and the soil, the
soil moisture content, and the neutron reflector material.
From test with this demonstration model [3, 9] several new ideas emerged which have recently been implemented in the DUNBLAD system. In addition to the neutron system this latter detector also features a metal de-
tector which allows for the detection of both metallic and
hydrogen-rich (plastic) land mines. Other improvements
are an ergonomic carrying harness [10], new electronics
and a more robust detector head.
DUNBLAD DESCRIPTION
The geometry of the detector-head is shown in Figure 1.
It consists of two times four helium detectors with a
length of 50 cm and a diameter of 2.54 cm, placed 18
cm apart. A 252 Cf source is situated between the two arrays with the reflector, a glass container filled with heavy
water, positioned above it. The container serves to reflect
neutrons which are emitted from the source in an upward
direction down into the soil. It increases the flux of high
energy neutrons into the soil. Consequentely the detector
count rate increases by approximately 60% without reducing the mine/no-mine signal ratio. The helium tubes
and the source are both made of metal. Therefore they
influence the sensitivity of the metal detector somewhat.
However, as long as these metal part do not move in relation to the coils, the metal detector works properly. The
main material that is used for the construction is polychlorotrifluorethylene (PCTFE), a plastic that does not
contain any hydrogen.
The detector-head is carried in the way shown in Figure 2. Two carbon fiber poles hold the detector head at
approximately 1.5 meter away from the person carrying the detector. The counter balance weight contains
the batteries for the electronics. The center of gravity of
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FIGURE 3. Scans of the AT-mine buried at various depths.
Measuring time: 20 seconds per scan position.
FIGURE 1. Photograph of the detector head. The radioactive
source, which is on the end of a 20 cm long stick, is placed at
the center. The reflector is positioned directly above it. To the
left one can see the two coils of the metal detector. The neutron
detector arrays are also shown.
the user. The current source strength is 1.1 MBq, which
results in a count rate of about 400 c/s. With the current
system the dose rate for the user is less than 0.5 µ Sv/h,
which is well below the international standard of 15 mSv
per year.
EXPERIMENTAL RESULTS
The system has been tested in sandboxes, indoors and
outdoors. Indoors the sand is very dry (<1% weight). The
maximum depth to which our dummy AT-mine could be
found was about 15 cm when a threshold to the mine/nomine signal ratio of 10% was applied.
FIGURE 2. Photograph of the DUNBLAD system. It is carried by the deminer using a comfortable harness. The audio
signals from both detectors are clearly distinguishable.
the complete unit is significantly lower than the suspension point. This makes the construction very stable and
sudden uncontrolled movements do not result in touching the soil with the detector head. The detector head is
scanned very much like a metal detector with a comparable scanning speed.
The success of the present type of detector is mainly
determined by the mine/no-mine signal ratio and by the
count rate. On one hand the count rate should be high
enough to get good statistics, but on the other hand it
should be as low as possible to limit the dose rate for
At the IRI outdoor facilities test were done with two
real defused land mines kindly supplied by the Dutch
Ministry of Defense. The AT-mine contained 9 kilogram
of TNT, dimensions: diameter 30 cm and height 15 cm.
This mine comes with no casing and does not contain any
metal parts. The AP-mine contained 17 grams of tetryl
and 57 grams of trotyl. The plastic casing was only a
few millimeters thick. The water content of the sand was
determined through several gravimetric measurements.
On average, the sand at a depth of 5 to 10 cm contained
3% (weight) water.
The tests consisted of scans of both mines buried at
various depth. In addition several blind tests were done,
in which the anti-tank mine was buried at various places
in the 5x2 meter sandpit and the operator had to find the
mine with the detector.
In the scans the detector head has been moved on a
rail system across the buried land min at a height of 5
cm above the sand. The results for the AT-mine buried at
depths of 2, 7 and 12 cm are shown in Figure 3.
Under these conditions, at a depth of 12 cm, this mine
only gave a marginal signal. The results for the scans
with the AP-mine were inconclusive: the signal with the
mine did not exceed the signal without the mine. The
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blind test was performed twice and both times the ATmine was found without any problems. It was buried at a
depth of 3-5 cm at which mines are commonly found.
We can conclude from these tests that the current
system is able to detect non-metallic AT-mines in fairly
dry sandy soils, but that it has severe problems with the
small AP-mines.
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FUTURE DEVELOPMENTS
To increase the sensitivity of the current system the
mine/no-mine signal ratio should be to be improved. Two
ways to do this are:
By forming a 2D image additional information is obtained of the hydrogen-rich anomaly in the soil.
Over the image the mine/no-mine signal ratio will
vary. At the mine location this ratio can be better
than for the image as a whole.
By applying a time window on the neutron transit time
the no-mine signal is suppressed. One of the main
conclusions from earlier Monte Carlo simulations
[7] was that timing information could improve the
mine/no-mine signal ratio by more than a factor
of two. This can be done by ignoring detector hits
that occur within the first 5 microseconds after the
neutron leaves the source. Such hits are mainly due
to neutrons that came straight from the source and
thus carry no information on the soil hydrogen.
Tagged neutron sources, where the direction of the emitted neutron is known, can also provide a significant reduction of the signal without a mine. However the maximum count rate with such sources is very limited because
the neutron transit time may be up to several hundred microseconds, and only one neutron can be allowed in the
setup at any time.
Monte Carlo simulations
The two issues mentioned above have been investigated with Monte Carlo simulations. We used the
GEANT4 package. At the position of the old detector
head a position sensitive detection plane is simulated
with a spatial resolution of 2.5x2.5 cm and with a dimension of 40x50 cm. This rather crude resolution is
more than sufficient to ‘see’ all sizes of land mines. The
neutron source is placed directly above the center of the
imaging plane; a reflector is used as before. The standoff
distance used was 5 cm above the soil. The soil used in
the simulations was sand (SiO2 ) with a moisture content
of 2% weight.
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FIGURE 4. Simulated images of an AT-mine at 3 cm depth
made with a 252 Cf source. The vertical Z-axis denotes number
of counts which are recorded as function of position in the
detector plane. This detector is the ground plane of the figures
measuring 400x500 mm. The left image: signal without a mine,
the right image: result after subtraction of the no-mine signal
from the signal obtained with the AT-mine.
There are two concepts for the next generation Neutron Backscattering Imaging Detector (DUNBID).
Imaging, without timing Without using timing information the 252 Cf source can still be used, although
a neutron generator can also be applied. Such a system can be carried very much like the DUNBLAD
system, The images may be shown in real-time on a
small screen.
Imaging, with timing When discrimination on the neutron transit time is applied, a pulsed neutron generator is required. Considering the weight, this system
should then be placed on a vehicle.
Figure 4 shows the results for the first concept after
simulation of 106 neutrons. An intense central peak is
present regardless whether there is a mine or not. This
peak originates to a large extend from neutrons which
came into the detector directly from the source. For the
high energy of such neutrons the efficiency of the 3 He
detector is small, but the number of neutrons is large due
to the close proximity of the source. With an AT-mine
at 3 cm depth the total intensity increases by 50% to
≈20000 counts over the detector area. After subtraction
of the signal without a mine, a circular image with a
diameter of about 20 cm becomes visible containing
≈7500 counts.
Figure 5 shows the results for the second concept,
using a pulsed 106 n/s DD neutron generator (2.5 MeV)
and an AT-mine at 3 cm depth. The count rate without a
mine is in the order of 4000 c/s. With a mine the count
rate increases to more than 11000 c/s. This produces
good images to identify the anomaly. However, the count
rate of the same mine at the edge of the detection plane is
somewhat lower: 6000 c/s, still sufficient to identify the
anomaly.
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DUNBID, the Delft University Neutron
Backscattering Imaging Detector
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FIGURE 5. Simulated images made with a DD neutron generator applying a 50 µ s time threshold. The vertical Z-axis denotes the number of counts recorded as function of position in
the detector. This detector is the ground plane of the figures
measuring 400x500 mm. The left image refers to the situation
without a mine; the right image shows the signal of an AT-mine
at 3 cm depth.
The maximum stand-off distance is about 10 cm. This
distance has a strong influence on the image quality and it
is important that the detector is placed as close to the soil
as possible. The time it takes to achieve an image with
≈5000 counts with a 106 n/s source is about 1 second.
This means that the time to investigate 1 m2 is about 5
seconds with the current system.
One of the outcomes of the simulations is that although the presence of a hydrogen-rich anomaly in many
cases is quite clear, images of mines with various diameters and shapes all turn into a circular spot on the detector. This can be explained by the large distance a thermalized neutron can drift in the soil before emerging at the
surface. The smaller mines produce a spot with a slightly
smaller diameter, but the mine depth and the standoff distance influence the shape of the image. However, it is
still possible to distinguish roots, sticks and long narrow
shapes from actual mines.
The main advantage of using timing is the large increase in the mine/no-mine signal ratio. Since the distance of the operator to the vehicle can be large a high
neutron flux is not a problem. This is favorable for getting good statistics and quick measurements, so that the
vehicle can move with a reasonable speed.
With this system it might be possible to reduce the
amount of false positives which are generated by metal
detectors. When the metal detector indicates the presence
of a small piece of metal, the neutron imaging device
can add information on the presence of hydrogen-rich
materials, possibly explosives, at the same spot. If these
are not found, it is more than likely that no land mine
is present. Neutron backscattering imaging can provide
valuable additional information and may be combined
with other imaging techniques of land mines like GPR,
IR, EMI, etc.
A prototype of a device as described above, in which
imaging and timing are applied, is currently under development. The 2D position-sensitive neutron detection
system will contain 16 position-sensitive helium-3 tubes
with a length of 50 cm and a diameter of 2.5 cm. The total
detection area of this imaging device is therefore 40x50
cm. The position resolution of this device will be in the
order of 25x10 millimeter. A neutron reflector (glass container filled with heavy water) is placed above the source
to increase the flux in the direction of the soil.
REFERENCES
1. Brooks F. D., Buffler A., and Allie M. S., “Detection
of plastic landmines by neutron backscattering,” in 6th
International Conference on Applications of Neutron
Science, June 99, Crete, Greece.
2. Datema C. P., Bom V. R., and van Eijk C. W. E., Electronic
Journal of the Society for Counter-Ordance Technology,
2002.
3. Datema C. P. et al., “Landmine detection with the neutron
backscattering method,” in IEEE Trans. Nucl. Sci., Vol. 48:4
(2001), pp. 1087-1091.
4. Craig R.A., Peurrung A.J., Stromswold D.C., Proceedings
of UXO/Countermine Forum, Anaheim (CA), May 2000.
5. G. Nebbia G., Electronic Journal of the Society for
Counter-Ordnance Technology, 2002.
6. Csikai J., Király B., Dóczi R., Proceedings of the 2nd RCM
on Nuclear Demining Techniques, St. Petersburg, Sept. 2001.
7. Datema C. P., Bom V. R., and van Eijk C. W. E., Nucl. Inst.
& Meth., A 488, 441-450 (2002).
8. Datema C. P., Bom V. R., and van Eijk C. W. E.,
“Experimental results and Monte Carlo simulations
of a landmine localization device using the Neutron
Backscattering Method," in Nucl. Inst. & Meth. A. (2002), in
print.
9. Datema C. P. et al., “Optimization of the Delft University
Neutron Backscattering LAndmine Detector (DUNBLAD),”
in Proceedings of the 7th International Conference on
Applications of Nuclear Techniques, June 2001, Crete,
Greece.
10. Datema C. P., van der Schoor L. A., Bom V. R., and van
Eijk C. W. E., “A Portable Landmine Detector Based on
the Combination of Electromagnetic Induction and Neutron
Backscattering,” in Proceedings of the IEEE Nuclear
Science Symposium and Medical Imaging Conference, San
Diego, 3-10 November 2001.
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