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Airport Baggage Screening Using Nuclear Techniques

Tsahi Gozani Science Applications International Corporation

The development of a nuclear technique for on-line detection of explosives in luggage sounds like a very non-technological subject. After all, it is a politi­cal and anti-terrorist type of activity. Nevertheless, it calls for the use of the best technology available since, until recently, terrorists were a few steps ahead of us.

For some time there have been techniques that claim to detect explosives, specifically "plastic" explosives. Such explosives are non-metallic and com­posed of the benign elements C, H, N, and O. These techniques, such as manual search of baggage, have been used in the past with some success. Others, like vapor detection and x-ray, do not work for explosives made of military plastic explosives.

X-rays do not provide elemental analysis except when x-ray fluorescence is used. They only indicate the density of the object. Vapor detection depends on the vapor pressure. The vapor pressure from the C4 and Russian-Czech plastic explosive (Semtex) is, unfortunately, very low. A few more orders of magni­tudes of sensitivity are needed to detect plastic explosives using vapor pressure. In addition, sealing this type of explosive in a suitcase with some plastic wrap­ping would make it extremely difficult to detect. However, there is room for these two techniques, vapor pressure and x-rays, in looking for other contraband and guns.

Why nuclear techniques? The nuclear techniques use neutral particles that penetrate the object to be searched. The fundamental idea is to create a specific nuclear reaction that will signal the presence of the explosive.

The nature of an explosive detection system based on nuclear techniques is shown in Fig. 1. It's very simple in concept. A beam of neutrons or gamma Irays penetrate the object. Each of the elements in this object has a certain prob­

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How Hidden Objects Are Detected In Luggage

Neutron Source

Gamma

Detector

Alarm

Hidden Ob jeets

Luggage

Computer

Figure 1: Thermal Neutron Activation Principle

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ability of interaction (scientists call this the cross section). These elements can give off characteristic gamma rays when the neutrons interact with them. Such gamma rays are called "capture" gammas if they are produced by thermal neutrons. These characteristic gamma rays are high in energy, so they can eas­ily leave the suitcase without attenuation and be detected by an array of detec­tors. The information from the detector goes into a computer which looks for certain signature patterns that are incorporated in the decision on whether an ex­

plosive is present.

An explosive is defined by its nuclear and other physical signatures. The nu­clear screening technique does not look at the chemistry of the object. Instead, it probes for the element, or the isotopic composition. The general characteris­tics of high-powered military and commercial explosives are that they have high densities of nitrogen and oxygen, compared to ordinary plastic, and a relatively low concentration of carbon and hydrogen.

Ratios of elements such as carbon to oxygen, which represent the specific molecular structure of the material, can be very specific to the explosives. On the other hand, though explosives are dense, many other materials, including metals and some plastics, are also dense. Hence, the density by itself is not a good signature of explosives. Shape is not much help at all for identification, because all military explosives are pliable. They can be made into any shape and form. Therefore, shape cannot be used as a signature.

What nuclear reactions are available for this screedng for explosives? As a nuclear phy"icist I can tell you there are a host of them. You could spend a lot of time, probably more than a lifetime, considering all the possibilities. Table 1 lists some of these. You do not need to learn this table by heart. The point is: on the one hand, there are a large number of nuclear interactions that can be used to see some of these elements; on the other hand, if the criteria of prac­ticality, simplicity, reliability, and specificity are applied, the number of reac­tions is reduced to a very short list.

Concentrating for a moment on nitrogen, the most promising reaction is reac­tion #12. This is the neutron capture gamma ray in the isotope nitrogen 14. The advantage this has is that the capture gamma ray has an energy of 10.8 MeV. This is a very-high-ellergy gamma ray, essentially the highest in nature.

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~ Isotope Probe ~ Reaction COmments

1 It! Ilt.h It! (Ilt.h' ..,)~ Low specificity

2 11 (and llB)

n (slowing down spect.)

thermalization absorption

Low specificity

3 ~ ..,(>2.2 MeV) ~(-y, n) It! Low specificity, very low sensitivity

4 ~ n(14 MeV) ~(n,2n)1t! Low specificity, very low sensitivity

5 l7c nth l7c (nth,"') l:t; medium specificity, low sensitivity

6 l7c n(14 MeV) l7c(n, n' ..,)l7c* medium specificity, low sensitivity

7 17c n(14 MeV) l7c(n, p)lll* medium specificity, low sens i tivi ty

8 l7c n(14 MeV) l7c(n,a)&Se* medium specificity, low sensitivity, high background

9 ~ ..,(>19 MeV) l7cz (-y, N) lie medium specificity, low sensitivity, unacceptable very high dose rate, complex

10 l:t: nth l:t: (Ilt.h' ..,) l:t: medh... specificity, very low sensitivity

11 l:t: ..,(>5 MeV) l:t; (-Y, n) l7c medium specificity, very low sensitivity, probably unacceptable high dose rate

12 l~ Ilt.h l~(nth,..,)l~ specific, basis for TNA to date most practical

able I: Assessment of accessible nuclear reactions for neutrons or high-energy photon interrogation.

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li2... Isotope Probe ~ Reaction Comments

13a 1\.1 "1(>11 MeV) 1\.1("1, n) 1~ specific, low sensitivity, very high dose rate, high induced activity, neut. background, complex, unacceptable

13b 1\.1 "1(>30 MeV) 1\.1("1, 2n) 1~ specific, low sensitivity, very high dose rate, high induced activity, high neut. background, complex, unacceptable

14 1\.1 n(14 MeV) l\.1(n, n' "1)1\.1 specific, high background

15 1\.1 n(14 MeV) l\.1(n,Q"1) lJs specific, high background

16 1\.1 n(14 MeV) l\.1(n,p-y)lt specific, high background

17 1\.1 n(14 MeV) l\.1(n, d"1)lt specific, high background

18 1\.1 n(14 MeV) l\.1(n,2n)1~ specific, high background low sensitivity

19 lib n(14 MeV) l'b(n,n'"1)l'b specific

20 lib n(14 MeV) l'b(n, p) lIN specific, low sensitivity

21 lib n(14 MeV) l'b(n,Q)lt specific, low E signatures

22 1'b "1(>16 MeV) l'b("1, n) lb specific, low sensitivity, very high dose rate, high neut. background, complex, unacceptable

Table I cont.: Assessment of accessible nuclear reactions for neutrons or high-energy photon interrogation.

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Except for this vital positive fact, nitrogen detection is all trouble. Nitrogen has a very low cross section for this reaction (approximately 10 mb). The effi­ciency of detection of such a high gamma energy is low. However, because of this specific high energy, it is possible to distinguish nitrogen in the presence of many other perturbing materials. This is the basis for the technique called thermal-neutron analysis (TNA), that is, detecting gamma rays from thermal neutron capture in nitrogen.

Why not use one of the other reactions? There are enough reactions here to keep a lot of physicists busy. The problem here is that one must deal with real life. The limitations of the real application must be considered. It is not possi­ble to give the suitcase a megarad of radiation in order to find an explosive. For example, with reaction #13a there is no question that it is possible to detect nitrogen. In the process, the material or the suitcase may be destroyed or, at the very least, any film that is present will be completely fogged. One of the sen­sible criteria that the Federal Aviation Administration (FAA) has is that the sys­tem should not fog film nor damage magnetic material. There are still physicists playing games with other neutron-gamma reactions. One possibility is a photonuclear reaction at 30-60 MeV (reaction #13b). A 60-MeV electron accelerator for that purpose would be a delightful machine to build and do re­search with. However, such an arrangement would never be practical. Propos­ing possibilities of that kind is one sure way of losing the confidence of the po­litical decision-making echelon.

While there are other possible reactions, only the one I have highlighted in Table 1, using thermal neutrons on N14, does not require an accelerator. Ther­mal neutrons can be generated from any type of neutron source, whether it is a radioisotope or a small accelerator.

One of the most important differences between the accelerator needed for this machine and one having a 52-mile circumference (which you are familiar with) is the size. For application in an airport, we have to think small. The ac­celerator must also be reliable and cheap. This approach may run counter to what we have learned in our youth until a few years ago.

How do we go about taking the thermal neutron method and making it practi­cal? We start with a Californium source or with a yet non-existent, reliable, small accelerator. By the way, unrealiable small sources do exist. However, we

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are seeking a small, reliable accelerator. A' 'not so small" accelerator is ac­ceptable, as long as it is reliable and does not take up half of the airport space.

The process starts with fast neutrons. They have to be thermalized in a very efficient way. Neutron economy is very important here. Neutrons are expen­sive, and a neutron that is not used is even more expensive, since costly shield­ing must be provided to absorb it, and that also takes space.

Imagine that you have a cavity. On one side of it you have a very efficient moderator with a radioactive source like Californium 252, or a small accelerator with a deuterium beam on a deuterium target (DD) or some other combination. Each of these has its own advantages or disadvantages. Most of our current sys­tems use Californium. The source is moderated and is surrounded by neutron cavity reflectors. The system gives rise to a cloud of low-energy or thermal neutrons. This is why one uses thermal neutrons. Thermal neutrons are the only ones you can effectively reflect back and accumulate to form a cloud. It is not possible to accumulate fast neutrons.

As the suitcase moves through the cavity, an array of detectors around the cavity looks for nitrogen gamma rays. The detectors are also capable of yield­ing information on the gamma ray spatial distribution.

We image (like in an emission tomography case) the baggage to get spatial information from the nitrogen signal. Otherwise, it would not be possible to distinguish between benign material and an explosive. Most of the materials that we carry onto an airplane contain some nitrogen. For example, there is ni­trogen in nylon and wool.

How do you distinguish a bomb, which may have a few hundred grams of nitrogen, from a suitcase which may have a couple kilograms of nitrogen in benign material? One very important feature is that the explosive is usually a contiguous body. This does not define the shape of the body, it only says it is contiguous. If you cut the explosive into small pieces and scatter them around, it will no longer explode.

For this reason, some sort of imaging is essential. It is very crude imaging when compared to imaging used by physicians. The TNA images are made from signals with rather poor signal-to-noise ratio. As you know if you are fa­

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miliar with counting statistics and dealing with an effective cross section of 10 millibams, you just do not get that many counts.

If the passenger must leave the suitcase for an hour, he could miss his plane. For that reason, the measurements must all be done quickly. A decision has to be made within six seconds as to whether the suitcase is suspect or not suspect.

Figures 2 and 2a illustrate the imaging process. The image cuts the suitcase into four slices. Figure 2 shows a clean suitcase containing only clothing. Fig­ure 2a shows the situation when there is an explosive in the lowest comer (the bright left comer in the first image from the left). The explosive is a 2-inch­thick C4 bar. This is one of the most powerful explosives available. Notice there are clear artifacts. Some of the distortion is part of our software, and some of it is just the nature of the beast, because of the statistical fluctuations. Nevertheless, the explosive is definitely visible in the lower part of the suitcase. The software looks at the concentration of Intense regions and then makes a judgment as to whether or not there is an explosive.

We used a lot of sodium iodide detectors which are not the fastest available, but they have good resolution. This is very important because it is necessary to do some nuclear spectroscopy, since the nitrogen signal is about four to six or­ders of magnitude less than the background. Fortunately, most of the back­ground is at low energy; yet the background can distort the signal through either spectroscopic contamination because of limited energy resolution, or by pile-up. Lines from elements like iron may contaminate the signal if the resolu­tion is not good. Pile-up is a process by which two random pulses appear within the time resolution of the detector system. Pile-up happens when the count rate is very high.

Since no material gives a signal which is energetic enough, except nitrogen, one way to get pile-up is from a 2.2-MeV hydrogen line. The hydrogen line is a very strong signal. If it piles on top of another pulse with energy, say of 8.6 (which happens to be a chlorine line), the combined signal can fall exactly at 10.8 MeV. We must eliminate pile-up, which we have done by developing our own special electlonics. It represents almost an order-of-magnitude improve­ment over conventional electronics.

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Figure 2: Nitrogen 3-D image of suitcase without explosive.

Figure 2a: Same as above, but with explosive.

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The optimIzation of the system starts from the neutron cavity design and moderator design, goes through the choice of detectors and electronics, and on into the computer and software.

Suppose one has done everything one can and then tries to make a separation between a bomb and non-bomb case. There are still some problems. When the program looks at one feature of the explosive detection, it finds two distribu­tions: one of suitcases alone, and another with a simulated explosive added to them. Figure 3 illustrates these distributions. Notice that there is still some overlapping. Moving the threshold affects the tradeoff between the non­detection probability and the false alarm or the false positive.

Obviously, it is easy to trade off. I can decrease the false alarm by raising the threshold and reducing the detection probability. For 100 percent detection probability, where all bombs of a given size will be detected, there is a 30 per­cent false alarm rate. For a 70 percent detection probability using one feature only, there is a zero percent false alarm rate.

Although this is far better than what x-ray or other devices are doing today, it is unacceptable. To get around the problem, more features are added. A multi­dimensional feature space is created.

In the final system,there is a conveyor belt going through the system's three modules: an entrance module, an interrogation module, and an exit module. As the suitcase moves, various properties are measured, such as the size and weight.

In the interrogation module, the suitcase is interrogated using neutrons either from an accelerator or a Californium source. The moderator and detector ar­rangements are somewhat different for the two types of sources.

By the time the suitcase leaves the exit module, the decision has been made whether it is a suspect or non-suspect bag. If it is a non-suspect bag, it con­tinues. If it is a suspect bag, it is diverted and the operator must do something with it. Procedures are being developed on what to do at that point. For ex­ample, the suitcase can be rerun in a different orientation to try to clarify the situation, with some measurable loss in the detection probability.

We have obtained results running for almost a year in San Francisco Interna­tional Airport and Los Angeles International Airport with two different

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N PosslOle ThresholdU 10 M B E 8 R

0 6 F

C 4 A S E 2 S

0 -15 -10 5 10 15

Region of False Alarm Region of Non-Detection

- WITH EXPLOSIVE -t- SUITCASE ONLY

Figure 3: Decision Analysis, one feature.

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machines; a DD accelerator and the Californium-based machines. These demonstration studies covered different seasons of the year and different des­tinations. What the FAA wanted to see was the probability of explosive detec­tion. We usually set the probability of detection at a certain level (say 95 per­cent) and looked to see where the false-alarm rate was. The trade-off curve (or TNA performance curve) based on the above tests is shown in Fig. 4. With cargo, we were getting even better results.

By incorporating a special kind of x-ray device and doing mathematical im­age correlation, we could reduce the false-alarm rate by approximately one-half.

The TNA prototype system, based on the DD accelerator, is shown in Fig. 5. The accelerator (KAMAN 711) is based on a 25-year-old technology. Basi­cally, the length of the system is determined, in this case, by the accelerator power supply and the cooling unit. This system has a 200-kV power supply, us­ing very old technology, which could be greatly improved today. The ac­celerator sealed tube head, that contains the ion source and DD target, has a lim­ited life of, at best, a few hundred hours.

With today's technology, this unit could be smaller and much more reliable. Unfortunately, there is no commercial accelerator currently available to do this job. Nevertheless, even within the lifetime limits of the seal tube, the TNA sys­tem worked very well. The source requirements are quite modest. About a bil­lion neutrons per second are needed from a DD reaction. DD reaction is used rather than DT because it is much more efficient to thermalize DD as opposed to DT. If we could (inexpensively) get neutrons below 1 MeV, we would be very happy. This would require the p-lithium reaction near its threshold and, unfor­tunately, means a costlier and larger accelerator.

Because of our results and the disaster over Lockerby, Scotland, the FAA ac­celerated production of six units. The first six units will all be Califomium­based systems. Californium is very reliable, always produces neutrons, and is also very compact. A picture of the compact cf-based TNA system is shown in Fig. 6. The system is as big as a gOOd-size x-ray system, about 13 ft long, 7 112 ft at its widest point, and 6 ft high, and can screen 10 bags per minute. The first system, which is much smaller than the prototype, was supplied to TWA at JFK in New York in the summer of 1989. We have made it compact because space is very expensive at an airport.

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P 100% r 0 b 95% a b i 90% I j

85%t Y

80% 0 f

75% D e

70%t e c 65%t i 0 60%

+ MORE

curve for system at minimum explosive threat.

Increased detectability for 1/3 more explosive.

n 0% 2% 4% 6% 8% 10% 12% 14% 16% 18 % 20%

Probability of False Alarm

Performance curve for SAle TNA-EDS. The tradeoff curve shows the operating points the system may be set at;

for example, at 95% probability of detecting the minimum explosive threat, it will be false

positive 2% of the time.

Figure 4: Performance of Explosive Detection.

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EXPLOSIVE DETECTION SYSTEM STRAIGHT THROUGH CONFIGURATION (TOP COVER REMOVED)

Cooling unn for Sealed tube neutron source the (O,Oj target, placed at center of the ion source and the system above the cavity power supply

l'igure 5: Explosive Detection System.

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'4

•

Figure 6: Airport baggage screening using nuclear techniques.

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The second system will be installed at the Miami International Airport, and the third system at Gatwick Airport near London in early 1990. The final des­tination of the other three machines is still being investigated.