Contribution to the International Workshop on Fast Neutron Physics, held September 5 - 7, 2002 at Dresden, Germany.

 

A critical assessment of neutron based humanitarian de-mining schemes

M. Drosg, Univ. Vienna, A-1090 Wien, Austria and

F. D. Brooks and M. S. Allie, Univ. of Cape Town, South Africa

 

Starting point: UN-requirements (before 1 Oct. 2001):

Reliability of clearance: > 99.6%

Depth < 20 cm of soil

Smallest mines: PMA-3: 0.035 kg explosive, 0.183 kg total, 104x40 mm2

PMA-2: 0.100 kg explosive, 0.135 kg total, 60x33 mm2

0. Introduction:

We have learnt that

- there is no single solution for de-mining

- the de-mining process consists typically of several steps, like

A. finding a mine

a. finding an anomaly (e.g. metal detector, ground probing radar)

b. reducing the false-alarm rate (verification process)

B. "removal" of the mine.

1. Essential properties of mine detectors:

1.1 Basic requirements (physics questions):

- very low missing-alarm rate (safety!); < 1/0.996 required (about <1/0.6 for one dog; so 5 dogs are required!)

a) high sensitivity,

b) good penetration of ground;

- low false-alarm rate (time!) (false alarm rate can be > e3 with metal detectors)

- fast response: < e2 s

1.2 Field requirements (technical questions):

- ease of operation (unsophisticated personnel)

- ease of transport (portable?)

- safety (radiation, high voltage, radioactivity, etc.)

- versatility (all types of mines, soil, terrain, climate)

- cheap to buy

- cheap to maintain (operating cost, repairs)

1.3 Discussion of basic properties (sensitivity vs. selectivity):

Many investigators appear to have not appreciated the difference between the requirements for de-mining and those for the detection of explosives in luggage. Whereas in luggage a unique identification of an anomaly is of outmost importance because false alarms must be very sparse the false alarm rate in de-mining "just" increases cost but does not completely disqualify the method (as is the case with metal detectors). However, with missing alarms it is just the other way round! Missing mines is deadly so only 4 missed mines in 1000 are acceptable according to UN requirements. The identification of mines in the ground is easier than that of explosives in luggage, insofar as mines occur in a limited number of shapes and masses. So, if ONE characteristic element of an explosive is detected in an anomaly in the ground and the strength of the signal is as expected an alarm will in most cases be correct. By using additional information (e.g. on the shape of this anomaly) the false alarm rate can be reduced to a very small amount (unless there are a lot of "dummy" mines in the ground of similar size and mass). So, a cursory identification of a buried mine suffices as long as the false alarm rate is tolerable.

1.3.1 Missing-alarm rate:

The best test of a design would be if the designer agrees to cross a minefield after he has cleared it by using his method. There are TWO essentials to be fulfilled simultaneously: a) the sensitivity must be so good that the smallest mines are detected b) the sensitivity must be so good that mines in a depth of 20 cm are detected. Usually there are two contributions to sensitivity: signal size and signal-to-background ratio. Besides the size of the signal must be within the operating range of the instrument. Any method that cannot provide the required sensitivity, at least to a good degree, can be dropped immediately and should not be considered further for humanitarian de-mining. It is naive to assume that it is possible to increase the sensitivity in the course of the development of the instrument by more than an order of magnitude. On the other hand, a device may still be valuable if there are situations in which the missing-alarm rate is too high. However, in such cases the instrument MUST indicate that it is not suited for such a situation.

1.3.2 False-alarm rate:

It is clear that a moderate false-alarm rate is NOT a problem. For de-mining it is NOT essential to identify the explosive, but only to find "all" the mines, even if false alarms occur. If the false alarm rate is too high it can be reduced by ANDing the signals of (two) independent mine indicators either simultaneously or sequentially.

1.3.3 Response time:

In nuclear methods based on counting sensitivity and response time are interconnected. To be competitive, a "verification" must be done within a few minutes, "detection" must be at least two orders of magnitude faster.

1.4 Field properties:

Field properties of a candidate for mine detection even if they are very important, should only be considered after the method has been found to fulfill the 3 basic requirements at least moderately well. It does not help the cause to solve the technical questions first just to have something to show.

2. Neutron physics methods:

2.0 How can neutron physics contribute:

It is essential that the de-mining process as described in the introduction is sped up.

1) There is no obvious way how neutron physics can be involved in step B)

2) Neutron physics can be involved in step A)

a) by completely taking over step A or

b) by reducing the false alarm rate of some other method that has established an anomaly in the ground (e.g. metal detector, ground-probing radar)

Obviously b) is easier to achieve.

2.1 Presently considered de-mining methods based on neutrons:

The following methods are considered:

(n,gamma) [1],

(n,x gamma) [2],

thermalization ("neutron backscattering") [3]

elastic backscattering (MNBRP) [4] and

neutron radiography (furrow method) [7], which is in a proposal stage.

Up to now only the elastic backscattering method has shown to work at a depth of more than 0.1 m (with a sample of 0.21 kg total mass). The other experiments are usually done with a 0.02 m soil cover and/or a total mass approaching 1 kg. In either case the improvement needed to achieve 0.2 m AND a total mass of only 0.135 kg means an improvement of the system by more than two orders of magnitude. (To a first order one can assume that the effort to detect smaller samples or penetrate deeper goes for either case at least with the square of such an improvement.)

2.2 Limiting experimental factors:

Table 1 compares pertinent properties (typical cross-sections, penetration (depth), max. count rate in the system (time constant of detector), signal-to-background ratio SNR) of various neutron based de-mining methods.

Table 1. De-mining methods based on neutrons

Physics

Typ. Xsect.

Sensitivity

Penetration

Time cst.[s]

SNR

Remarks

(n,gamma)

e-2 b

meager

marginal

e-6

good

Cf

(n,x gamma)

e-1 b

mediocre

mediocre

e-6

x

tritium

thermalization

e0 b

good

marginal a)

e-6

good b)

Cf

elast. backscatt.

e0 b

good

good

e-7

good c)

monoenerg.

radiography d)

e0 b

good

good

x

good

monoenerg.

 

a) a monoenergetic source is being considered

 

b) dry soil

 

c) with time discrimination, dry soil

 

d) in a proposal stage

2.2.1 Cross-sections:

The basic quantity that determines the speed of detection is the cross section. In a second step source intensity, detection efficiency, SNR etc. must be considered. However, these properties cannot make up for an unsuitable cross section. For this reason it is doubtful whether the first two methods considered in Table 1 will have the required sensitivity to measure deeper lying small mines. Maybe, (n,x gamma) at 8 MeV will have it. But this is not being tried, yet, to my knowledge. For the other three methods the relevant cross-section is about one order of magnitude bigger.

A) Thermalization using monoenergetic neutrons for good soil penetration is a candidate to be looked into.

B) The same is true for neutron radiography as proposed in the "furrow" method.

C) Only monoenergetic neutron backscattering with resonance penetration (MNBRP) has closely approached the UN requirements: in an unsophisticated pilot experiment a 0.2 kg sample with a sand cover of 0.14 m was detected [4]. MNBRP is based on two Austrian patent applications [5,6]. After some specific improvements of the system it can be expected that this method will FULLY fulfill the physics requirements of a mine verification device.

2.2.2 (Monoenergetic) neutron source:

For deeper penetration the resonance structure of the oxygen cross section must be taken advantage of. Therefore only accelerator based monoenergetic neutron sources are considered here.

2.2.2.1 Energy choices:

From what is known today, it appears that deeper-lying mines can only be "seen" at specific neutron energies depending mainly on the structure of the oxygen cross section [4]. This requires a "monoenergetic" neutron source.

Obvious optimum average neutron energies are 2.35 and 6.5 MeV [4]. Both oxygen resonances have a suitable width. (There are additional relative optima around 7 and 8 MeV).

2.2.2.2 Timing provision:

Neutron events can be timed either by using bunched beam or the associated particle technique. The latter method can be used in special situations only because it requires associated particles of high enough energy (see e.g. Table 3). Besides it cannot be applied to very strong sources. Timing can provide information on the depth of a mine, and the SNR can be tremendously improved by applying time discrimination for background reduction.

2.2.2.3 Source strength:

Basically the response time is inversely proportional to the source strength. So take the strongest source you can get. However, in general, the weight, the amount of shielding, the amount of electric power the target power and the cost increase with increased strength. Besides, it does not make sense to overload the detection system with counts as a consequence of too strong a source. Without an eye on the field requirements no final decision can be made on the type of source and its strength. So first choice will depend on the actual design. However, two factors contributing to the source strength should not be overlooked - thickness of neutron target and - target material. Obviously, to a first order, the monoenergetic neutron yield is proportional to the target thickness that determines the energy resolution. To take best advantage of the resonance the energy width of the source should match its half-width. If the target is not isotopically pure a decreased neutron output will result. So the target construction deserves some attention, both with regard to neutron output as well as durability and easy handling.

2.2.2.4 Count rate:

If there are no other limitations to the source strength, the maximum count rate of the detecting system will limit it. By some extent one can increase the maximum total count rate by increasing the number of counting channels. The ultimate limitation is the pulse-pair resolution, which depends on the intrinsic time constant of the detector and the pulse shaping time constants in the main amplifier. Usually gamma detectors have longer intrinsic time constants. If energy resolution is an issue, longer pulse shaping time constants are needed, too. So, generally higher count rates can be handled with fast (organic) neutron detectors than with gamma detectors.

2.2.2.5 Signal-to-background ratio:

The background has usually two contributions: instrumental (detector intrinsic), and physics-related ones.

2.2.2.5.1 Detector connected background:

Although it will strongly depend on the individual situation there are some general rules that can be applied:

- signals at the upper edge of the pulse height spectrum will have little background (14N(n,gamma), MNBRP)

- resonant, i.e. energy-selective detectors will give low background (e.g. thermal neutrons)

- special signal suppression (like neutron-gamma discrimination) can be applied.

2.2.2.5.2 Physical background:

Generally, signals that do not originate from the volume of interest (anomaly) can be considered background.

- Source background: Shielding of the detector (or the source) is required. Further reduction of such background is possible by using direction sensitive detectors or radiation discriminating circuits (e.g. n-gamma discrimination).

- Soil background: This type of background can be reduced by applying time-discrimination (see 2.2.2.2).

- Intrinsic background: If also the soil contains the element that is the indicator for the presence of the mine the true signal gets veiled.

As a result of an excessive intrinsic background there can either be a false alarm, or, if the threshold for the discrimination between signal and noise is set wrongly, NO alarm. The latter must be avoided by all means (in particular MNBRP and the thermal neutron detection are exposed to this problem).

3. Realization of a neutron based mine detector:

Up to now only MNBRP has shown the promise to have enough sensitivity to fulfill the UN requirements for both size AND depth of anti-personal mines.

3.1 Description of MNBRP:

3.1.1 Basic principle:

The intention of this method is to obtain a shadow picture of the mine by means of backscattering of monoenergetic neutrons [4]. If hydrogen (protonium) is present in the ground there will be fewer fast neutrons returned from this place. This is a unique property of protonium. The shadow is generated by a deficit of backscattered neutrons of the highest energy. Therefore pulse-height-discrimination can be used for a good SNR. False alarms can be expected from objects that have about the same shape, hydrogen concentration and hydrogen mass as the mines. It is not likely that many such objects are buried so that a rather low false alarm rate can be expected. Missed alarms are unavoidable, if the hydrogen concentration in the ground near the mine is similar to that in the mine. Therefore a provision is necessary (and feasible) not to use this instrument under such conditions.

3.2 Neutron source properties: 3.2.1 Primary neutron energy:

To get best penetration of the ground either the oxygen resonance at 2.35 MeV or at 6.50 MeV must be used. As was done in the pilot experiment [4] we concentrate on 2.35 MeV. The alternate resonance at 6.50 MeV has the advantage that neutron sources with much higher efficiency are available (see 3.2.3). However, such neutrons are more difficult to produce, need more shielding and are faster by a factor of about 1.7, reducing the background rejecting capability by means of time slicing. In addition, neutron and gamma ray background could be prohibitive. So all this and a reduced hydrogen cross section speak against the choice of the 6.5 MeV resonance.

3.2.2 Neutron energy width:

Although an energy width of about 0.1 MeV (the half-width of the resonance) yields the optimum effect, an energy resolution of at most 20 % (<0.26 MeV) can be tolerated for MNBRP to get a higher source intensity for the same beam current. When using even thicker targets neutrons of a different primary energy will illuminate the "shadow". For deuteron beams fully stopped in a deuterium target this means maximum neutron energies of 3.06 MeV (or 3.97 MeV allowing for an effective energy resolution of 20 %).

3.2.3 Neutron source reaction:

To make the system acceptable for as many countries as possible, radioactivity in the neutron source should be avoided, if possible. Table 2 compares the specific neutron output for the production of monoenergetic neutrons at 0 with an energy spread of 0.10 MeV for use in connection with the oxygen resonance of 2.35 MeV.

Table 2. Properties of the best sources of 2.35 MeV neutrons with a 0.10 MeV energy spread

Reaction type

specific yield

beam energy

Radioactivity

Gas target

 

[n/(sr*pC)]

[MeV]

 

 

t-1H

340.

4.901

yes

yes

p-3H

154.8

3.182

yes

yes

7Li-1H

109.4

13.97a)

no

yes

p-7Li

25.6

4.076

no

no

p-11B

7.81

5.211

no

no

 

a) By using triply charged ions the machine energy is correspondingly smaller.

Disregarding sources that involve radioactivity (which are, unfortunately, the best) 7Li-1H and p-7Li are the best choice.

3.2.3.1 1H(7Li,n)7Be:

The intrinsic properties of 7Li-1H are great. Especially the kinematic collimation of the neutron beam into a forward cone of about 15 which makes shielding unnecessary and the very good yield are outstanding. However, the use of a windowless target adds weight and cost so that the neutron background from the target structure, which is highly energetic and not collimated, is bothersome. Besides, using, for practical reasons, a solid hydrogen target instead of a gas target reduces the yield by about an order of magnitude. In addition, the energy of the neutrons depends strongly on the emission angle, so that, at a given acceptance angle, the acceptable beam energy width of the accelerator is reduced because of the "monoenergetic" neutron beam requirement.

3.2.3.2 7Li(p,n)7Be:

Everything considered p-7Li is the favorite for the production of "2.35" MeV neutrons. The contamination with lower energy neutrons stemming from the excitation of 7Be is irrelevant as shown in the pilot experiment, the gamma emission from the source might or might not require neutron-gamma discrimination.

3.2.3.3 2H(d,n)3He:

Recently, hand held neutron generators based on the d-D reaction have been developed. The use of such sources would be very attractive. Unfortunately these sources cannot produce 2.35 MeV neutrons at 0, but only at back angles. Besides the beam is not bunched. Table 3 summarizes relevant properties of the d-D reaction.

Table 3. Neutron production by the 2H(d,n)3He a) reaction for fully stopped beam, unless shown otherwise.

av. En

d beam energy

emission angle

specific yield

3He energy

[MeV]

[MeV]

[deg]

[n/(sr*pC)]

[MeV]

2.35

0.10

113.6

0.04

1.03

2.35

0.15

110.9

0.11

1.08

2.79

0.10

0.

0.08

0.52

2.87

0.15

0.

0.23

0.46

2.94

0.20

0.

0.48

0.42

3.31

0.50

0.

5.22

0.26

3.97b)

1.200

0.

53.7

0.06

6.50c)

3.344

0.

107.

0.000d)

6.50b)

3.855e)

0.

632.

0.003d)

 

a) standard solid targets give about a factor of 10 lower yield, for special solid targets a factor of 3

 

b) with a mean energy resolution of 20%

 

c) with a total energy width of dE=230 keV

 

d) 3He not at 180 but at 0 in the lab

 

e) energy loss in target 1.39 MeV

Because a low energy d-D source is lightweight, cheap and convenient it is prudent at this stage not to disregard this source. Although it is not likely that a hand-held MNBRP-device is feasible, a bunched d-D source with sufficient intensity might qualify for a verification device despite the low yield. The negligible gamma emission from this source is another advantage.

3.2.4 Neutron source effectiveness:

Figs. 1 to 3 explain the effectiveness of the p-7Li source and the d-D source in dependence of the (max.) neutron energy as based on computer simulations of MNBRP with time discrimination using a 135 g mine (0.13 g/cm2 hydrogen) and 33.8 g/cm2 soil cover (22 cm).

Fig. 1. Simulated energy dependence of the shadow effect using time discrimination for background subtraction.

 

Fig. 2. Energy dependence of the effectiveness of neutrons from p-7Li (dashed) and d-D (full) with a (max.) energy resolution of 20%. As a reference the effectiveness of neutrons of exactly 2.350 MeV was taken to be 1.0.

Fig. 3. Energy dependence of the effective specific neutron yield from p-7Li (full) and d-D (dashed) for isotopically pure targets and a (max.) energy resolution of 20%. (Based on Fig. 2.)

3.3 Geometry and detectors:

Many reasons demand a well-shielded neutron source; besides some kind of collimation is required, too. To save (precious) space heavy (metal) shielding is advisable. Materials with good shielding properties above about 1 MeV to 3 MeV must be selected. By making the neutron flight path from target-to-detector noticeably smaller than the path from the target to the mine (and back) time discrimination can be used for background suppression so that shielding becomes less important. Besides a neutron detector that is direction sensitive could be developed facilitating shielding and collimation.

3.3.1 Geometry:

3.3.1.1 Source geometry:

Let us, rather arbitrarily, assume that such a shielding/collimator can be restricted to a volume of 3 to 4 liters (a conical shape with typical dimensions of 15 to 20 cm). Towards the ground the shielding may be less thick for obvious reasons so that we arrive at a somewhat optimistic target-to-surface distance of 6 cm. To provide the necessary local discrimination an outer collimator diameter of 1 cm at a distance of 5 cm from the center of the target appears to be an upper limit resulting in a source solid angle of 0.031 sr.

3.3.1.2 Detector geometry:

Let us further make the plane of the detector face 15 cm from the surface, i.e. it cuts the axis 9 cm back from the target position. Then a circular detector ring with outside radius of 25.5 cm and inner radius of 20.5 cm is at about 150 with regard to an assumed back-scattering center at a depth of 25 cm. For radiation from this depth such a detector has an accepting solid angle of 0.293 sr (getting bigger with less soil cover).

3.3.2 Detector system:

The detector of a final device must be chosen according to cost, ruggedness and reliability. At this stage these important factors cannot be considered here. Only those detector properties contributing to a success are discussed.

3.3.2.1 Desirable detector properties: An ideal detector is highly efficient, is insensitive to background neutrons (e.g. direction sensitive), is fast (allows high count rates), is position sensitive (for providing a picture), and has a favorable pulse-height characteristics.

a) A relatively low neutron energy allows for a thinner detecting medium reducing cost and weight.

b) Heavy collimation can provide the direction sensitivity. The development of a detector material that is direction sensitive is a (promising) alternative.

c) If a very strong neutron source is used, count rate limitations might be important. Using several detectors in parallel will help somewhat. More important is the use of fast detectors (e.g. organic scintillators). MNBRP does not require good pulse-height resolution so that a fast pulse-height discriminator may be used. This is definitely true if the gamma background is small so that no neutron-gamma discrimination is necessary. Otherwise neutron-gamma-discrimination might limit the count rate capability.

d) Areal resolution could be introduced which is needed for picturing a potential mine by making the detector ring (3.3.1.2) from several (independent) directional detectors.

e) The use of deuterated scintillators could improve the sensitivity of the instrument as the pulse height distribution of such detectors peaks at highest pulse heights (see e.g. [8]). Thus the signal-to-background ratio is enhanced because the information is contained in the higher energy portion of the pulse height spectrum. However, the cost of such scintillators is considerable higher.

3.3.2.3 Instrumentation:

Although it is premature to go into details it might be worth mentioning that the requirements are minimal (especially, if no neutron-gamma ray discrimination is necessary). The pulse-height channel only needs a (fast) discriminator, the time channel just 4 bits. Differential linearity of the ADC is no issue so that even ultra-fast flash converters may be used for that. It is clear that the final instrument will contain a micro controller that will coordinate all necessary safeguard operations. To avoid missing alarms (from wet soil) the reflectivity of the soil must be taken into account. Besides, additional parameters like source-to-ground distance and source intensity (e.g. via the beam current) could be of interest.

A fully automatic operation is envisioned doing the data reduction online in parallel to the measurement because the amount of data to be handled ultimately is small. Thus also the required measuring time would be determined by the instrument itself.

3.4 Feasibility:

It very much looks as if a hand-held mine-detecting device that fulfills the UN requirements is not within the range of neutron-based methods. So, there remain 3 possible applications of neutron based methods in humanitarian de-mining

- a vehicle-based mine-detection device

- a portable mine verification device

- a transportable mine verification device

Obviously, a method that cannot be applied for verification will not succeed in mine detection. So we concentrate here on verification.

3.4.1 Results of the pilot experiment at NAC using MNBRP:

In an experiment at the National Accelerator Centre (NAC) in South Africa four measurements with a dummy mine of 0.21 kg (0.3 g/cm2 hydrogen) were performed [4]. Two of them where at an average neutron energy of 2.37 MeV, i.e. right on the resonance. Combining these two measurements gives a result with 3.5 sigmas, i.e. only 5 out of 10000 measurements can be expected to be "wrong". Half of those would be a false alarm, the other half a missed alarm. This is more than an order of magnitude better than the UN requirement (four out of thousand). Table 4 summarizes some experimental parameters and results.

Table 4. Data summary of the pilot experiment at 2.37 MeV.

FG Run #

38+40

Ep nom.

4.14 MeV

soil cover

11 cm

collected charge

2.60 mC

time window width

6 ns

counts: FG/BG

185/253

effect/at bias

36.8% at ch. 18

sigmas

3.5

total time

20467 s

3.4.2 Optimizing the pilot experiment:

Table 5 compares the parameters of the pilot experiment with those expected for an ideal dedicated device (as sketched in 3.3) measuring the mine-out vs. mine-in ratio with an accuracy of two sigmas.

Table 5. Parameters of the pilot experiment and of (ideal) dedicated devices (as sketched in 3.3) for an uncertainty of 2 sigmas.

 

#38 pilot exp.

p-7Li source

d-D sourcea)

Beam:

 

 

 

particle type

p

p

d

particle energy [MeV]

4.115

4.076

0.1

current [microA]

0.25

25.

25.

burst length [ns]

2.

2..4

2..4

burst freq. [MHz]

2.

<~10

<~10

Source:

 

 

 

av. n energy [MeV]

2.376

2.350

2.788

n energy width [MeV]

0.127

0.100

0.128

target power [W]

1.03

102.

2.5

strength [e8 n/s]

0.53

41.2

0.16

yield [e6 n/sr/s]

8.2

640.

1.99

solid angle [msr]

1.36

31.

31.

Detector:

 

 

 

solid angle [msr]

12.4

293.

293.

slow count rate [cts/s]

<0.11

<6100.

<19.

fast ct.rate [e3 cts/s]

0.0031

167.

0.52

FG measuring time [s]

5289

0.13

41.

FG+BG meas. time [s]

10055

0.25

78.

 

a) gas target

3.4.2.1 Using a 7Li(p,n) source:

From column 2 of Table 5 we see that the increase of the source solid angle by a factor of 23 and the detector solid angle by 24 would result in a measuring time of 19 s under the same source conditions as in the experiment (current=250nA, energy width=0.127 MeV). With an average beam intensity of 25 microA (and an energy width of 0.1 MeV) this time is further reduced to 0.25 s. This time could be further decreased by a factor of four using the optimum neutron energy spectrum with a 20% energy resolution (see Fig. 3) and even further, using an even higher beam current, if such a target power can be handled. (Of course, both provisions mean a higher neutron source strength requiring additional physical shielding).

With such a fast response not only verification but also searching for mines might be possible. Considering the worst case situation (20 cm depth and 135 g mass) instead of the experimental values (11 cm soil cover, 210 g mass) increases the measuring time (assuming a quadratic dependence) by about a factor of 8 to 2 s. So, for verification within 3 minutes a factor of one hundred can be spared for non-ideal solid angles, e.g. a less stringent geometrical arrangement (and detection efficiency), with some additional margin if the neutron source strength is increased by increasing the target width or the current.

3.4.2.2 Using a 2H(d,n) source:

According to column 3 of Table 5 d-D is much less efficient by orders of magnitude as a source of (nominal 2.79 MeV) neutrons. Using the usual solid targets would decrease the yield further, by about a factor of ten (or at least three with a special target construction). Raising both the current and the voltage (to about 0.5 MV) would be necessary to compete with p-7Li.

3.4.2.3 Source comparisons:

Using the d-D source (at acceptable current levels) would require matching very closely the ideal conditions given in Table 5, i.e. there would not be enough freedom for an optimum design of the instrument. However, for tests in the laboratory such a source could still be acceptable.

3.5 The instrument:

3.5.1 The accelerator:

A newly patented RFI Linac Structure yields small and light weight structures, even for the relatively low rf frequency of 200 MHz. [9] An innovative pulsing technique, developed for the Superconducting Super Collider, could provide the required sub-harmonic structure to the beam.

Specifications:

 

Proton beam energy: 4.6 to 4.8 MeV

 

Beam structure: macro rep. rate, pulse length: 1 kHz, 50 micro-s

 

Min. pulse distance: 100ns (count down from 200 MHz by 20)

 

Bunches: length, charge, number/s: < 1ns, 50 pC, 500000

 

rf power: <5 kW

 

Electr. power: <12 kW

 

Dimension: hanging: 2.5 m high (ion source through the entire linac structure)

 

Weight: 200 kg

Such an accelerator can easily provide 25 microA of average beam, and even more without adding much in cost, weight or size. The weight given includes only the accelerating structure (ion source through the total linac structure). An estimate of the additional weight (Turbomolecular Pumps, rf power system, power supplies, cooling system and primary electrical power system) gives about 300 kg, so that it can be expected that the total weight of the system will not exceed 600 kg by much if at all. Obviously, this weight could be reduced (at quite some cost) because it has not been optimized. The accelerator portion would hang vertically from a boom with the primary power source and other systems in the bed of a vehicle.

3.5.2 The neutron source:

Optimum proton energy and target thickness is discussed in 3.2 and 3.4.2. Some thought must be given to the removal of the target power which, however, should not really be a problem (just 0.1 kW).

3.5.3 Detectors and instrumentation:

Most topics of interest are discussed in 3.3. A fully automatic operation can be achieved with rather little logistic, giving a shadow picture of the mine and e.g. an alert if the soil condition does not allow a reliable answer.

4. Further laboratory tests:

The following tests should be performed before a field test will make sense. They could be done in any fast neutron laboratory that can provide bunched proton beams of energies up to 5 MeV (and currents of 1 microA) OR bunched low energy deuteron beams of high intensity. A provision to take data two-dimensionally would be helpful (at the beginning).

Measurement program:

- measurement of a 135 g device (unfused PMA-2 and/or PMA-3) at 20 cm depth

with a Li-target with 0.1 MeV width and with a 10% and 20% energy width

- measurements of elastic backscattering from different soil samples in the size of the smallest mine (PMA-2)

- measurements without neutron-gamma discrimination (p-7Li source!)

- one dimensional measurement (time spectra) using a fast trigger to reject lower pulse-height pulses

- considering deuterated scintillators

- development of a high aperture source-detector geometry and determination of the required source strength

(beam current) and radiation protection measures

- development of an "imaging" detector system (collimators, direction sensitive detectors)

5. Conclusion:

As shown in a previous pilot experiment MNBRP has the potential to fulfill the UN requirements for de-mining. (Up to now, no other neutron based method has shown that.) The strength of MNBRP lies in its overall sensitivity, its background insensitivity (only high pulse height signals are involved), its high count rate capability (the fast signals of organic scintillators may be used) and its simplicity (a shadow picture of the mine is envisioned). Drawbacks are the total weight (about 600kg) requiring some kind of vehicle and the limitation of its use, partly because of its bulkiness, partly because there are soil conditions that will make MNBRP fail. However, the latter is not a real obstacle, because the instrument itself will recognize such conditions so that nobody should be endangered.

Although this method is intended for mine verification its high sensitivity calls for its use as a mine detection device by scanning the ground.

Obviously more laboratory tests are needed to prove experimentally above claims and to develop the best geometry for the instrument.

References:

[1] M. Cinausero, "The EXPLODET Project. Progress Report 2000", report DFPD 01/NP/10, March 2001.

[2] P.C. Womble, F. J. Schultz, and G. Vourvopoulos, Nucl. Instr. Meth. B 99, 757 (1995)

[3] F.D. Brooks, A. Buffler, and M.S. Allie, "Detection of plastic landmines by neutron back-scattering", IAEA Research Contract No. 10987, report IAEA/PS/RC-799, Dec. 1999.

[4] M. Drosg, "Use of back-scattered monoenergetic fast neutrons for humanitarian de-mining.", Final Report to IAEA, Vienna, Dec. 2001 www.ap.univie.ac.at/users/drosg/mnbrp/drosgfi1.htm

[5] M. Drosg, 'Procedure for the detection of hidden hydrogen-containing objects by means of fast neutrons', Austrian Patent Application A1038/2000,G01N, June 15, 2000.

[6] M. Drosg, 'Procedure for the analysis of oxygen-containing materials or the detection of light elements in such materials, resp., by means of fast neutrons', Austrian Patent Appl. A1039/2000,G01N, Vienna, June 15, 2000.

[7] F.D. Brooks, in IAEA/PS/RC-799-2, Report of the Second RCM on Application of Nuclear Techniques to APL Identification, St Petersburg, September 2001 (CD available from IAEA, Vienna)

[8] F.D. Brooks, A. Buffler, M.S. Allie, K. Bharuth-Ram, M.R. Nchodu, and B.R.S. Simpson, Nucl. Instr. Meth. A 410,319 (1998)

[9] D. A. Swenson, Linac Systems, USA, priv. communication


© Manfred Drosg, Institut für Experimentalphysik der Universität Wien
Letzte Änderung am 12.09.2002 um 14:15 von MD