WO1990013900A1

WO1990013900A1 - Photoneutron method of detection of explosives in luggage

Info

Publication number: WO1990013900A1
Application number: PCT/US1990/002557
Authority: WO
Inventors: Kamil V. Ettinger; Joseph H. Brondo, Jr
Original assignee: Scientific Innovations, Inc.
Priority date: 1989-05-08
Filing date: 1990-05-08
Publication date: 1990-11-15
Also published as: AU6057090A; IL94327A0

Abstract

A method and apparatus are provided for scanning an object (12) for an element of interest and determining the concentration of the element of interest within the object (12). The apparatus includes a housing (10) for receiving the object (12) to be scanned. An accelerator (14) is incorporated within the housing (10) for producing a pulsed bremsstrahlung beam of high-energy photons. The photons are utilized to activate the nucleus of the element to be detected. The nucleus absorbs a photon and, subsequently, emits a photoneutron which is detected; and an output signal, representative of the flux density and energy of the photoneutrons, is processed and analyzed to determine the concentration of the element of interest within the object (12).

Description

PHOTONEUTRON METHOD OF DETECTION OF EXPLOSIVES IN LUGGAGE

This application is a continuation-in-part of application Serial No. 348,781, filed May 8, 1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention is directed to an apparatus for scanning an object for an element of interest, and especially for nitrogen in nitrogen-based explosives. More particularly, the invention is directed to an apparatus utilizing a photonuclear method. A pulsed bremsstrahlung beam of high-energy gamma photons is utilized to activate the nucleus of the element to be detected. The nucleus, thereby, absorbs a photon and, subsequently, emits a photoneutron that is detected and analyzed to provide a representation of the concentration of the element of interest within the object.

2. Description of the Prior Art Photonuclear methods have been utilized to locate materials, to direct radiation and to determine thickness.

A significant threat to human life and property exists when an explosive device is concealed in luggage or parcels brought into buildings, aircraft, etc. As a result, there is a need by both the public and private sector of the country for a reliable technique for the detection of such explosive devices. As the threat of terrorist activities through the world, especially in airports, has increased, the demand for an efficient and

STITUTE SHEET -_j_ practical device for scanning luggage to determine the presence of explosives has intensified.

It is well known that explosives may be detected by sensing the amount of nitrogen in the object being r- examined. One technique of detecting nitrogen is by photonuclear methods. U.S. Patent No. 2,726,338 relates to a photonuclear method for locating a particular material in a mass of associated material. The mass of material is irradiated with gamma rays of slightly higher _Q energy than the threshold energy associated with the particular material. The particular material emits photoneutrons which are then detected as an indication of the presence of the particular material. A predetermined element is incorporated into the particular material. It 5 is the incorporated element which reacts when irradiated. Beryllium and deuterium are set forth as the photoneutron source substance. The source of gamma rays disclosed is radioactive isotopes.

U.S. Patent No. 3,293,434 relates to the 0 detection of radiation in a fluid stream. A sample of the fluid stream is irradiated with gamma rays, and the neutrons produced are detected.

U.S. Patent No. 3,439,166 discloses a thickness measuring method which may include a gamma radiating source. The reflected gamma rays are then detected to indicate thickness.

U.S. Patent No. 4,497,768 discloses simultaneous photon and neutron interrogation to determine the presence of fissile and fertile nuclide material. The photon _Q interrogation is accomplished by pulsed high-energy gamma radiation as a result of bremsstrahlung in a heavy-metal target.

5

ET -_ SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method for scanning an object for an element of interest and for determining the concentration of said c element in the object. The apparatus includes a housing for receiving an object to be scanned. Incorporated within the housing is an accelerator for producing a pulsed bremsstrahlung beam of high energy gamma photons. The photons are utilized to activate the nucleus of the

•Lo element to be detected. The nucleus absorbs a photon and, subsequently, emits a photoneutron. The photoneutrons are slowed down and detected by a radiation detector or an assembly of radiation detectors adjacent to the sample. The slowed down photoneutrons are detected when the pulsed 5 bremsstrahlung beam is switched off. The moderator material captures a photoneutron and emits a gamma photon which is then detected. This method can be extended to include a detector capable of discriminating the neutron energy. o In a preferred embodiment, the method of the subject invention is utilized for quick screening of checked and unchecked luggage, at airports, for the presence of explosives. The method can also be used for screening briefcases, parcels and so forth at the entry 5 points to protected buildings, places and institutions.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of the apparatus for detecting the presence of an element of o interest, in accordance with the present invention.

Figure 2 is a side view schematic illustration of the apparatus for detecting tne presence of an element of interest, in accordance with the present invention.

5

SUBSTITUTE SHEET ^ Figure 3 graphically illustrates bremsstrahlung spectra normalized for a target for different energies of an electron beam.

Figure 4a graphically illustrates the energy of _t- the bremsstrahlung beam versus time, in accordance with the present invention.

Figure 4b graphically illustrates when the detector is on or off relative to the pulsed bremsstrah¬ lung beam, in accordance with the present invention. 0 Figure 4c graphically illustrates the thermal neutron fluence relative to the pulsed bremsstrahlung beam, in accordance with the present invention.

Figure 5 graphically illustrates the stepwise form of a photoneutron yield curve. 5 Figure 6 graphically illustrates the photo cross section for deuterium.

Figure 7 is a schematic illustration of the apparatus for detecting the presence of an element of interest utilizing an array of photomultipliers, in 0 accordance with the present invention.

Figure 8 is a schematic illustration of the apparatus for detecting the presence of an element of interest wherein the background signals are subtracted from a combined signal from the element of interest and the background, in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The subject method is essentially an activation technique based on a photonuclear effect. The photo¬ 0 nuclear effect is the absorption of a photon by the nucleus of the element of interest and subsequent emission of a nucleon. It is necessary that the energy of the photons exceeds the binding energy of the nucleon.

5 In general, the photoactivation technique is rarely used because the radioactivity induced in irradiated materials is predominantly of the beta+ (positron) type, all emitting 0.511 MeV quanta on annihilation. Therefore, to distinguish one emitter from another, it is necessary to resolve the decay curves into components, which is not always possible. Counting of the decay of positron emitters is not a practical technique to be used in the screening of luggage because of the time involved. Instead, in the method described herein, the photoneutrons are detected.

All the nuclei, other than hydrogen, exhibit photoneutron production; and excluding the lightest nuclei, the cross sections for reactions caused by photons have similar shape, albeit the position of threshold differs. The photonuclear absorption cross section is comparatively small and has a maximum value of only a few militants per nucleon. If a photonuclear interaction takes place, the nucleus can be excited in a number of different ways, depending on the energy of the photon.

At low energies (10 - 25 MeV), the dominant feature of the photoabsorption cross sections is the giant resonance which occurs in all elements. Light nuclei have a resonance peak at an energy around 22 MeV, which value decreases to about 13 MeV for heavy nuclei. In the giant resonance region, the most probable result of photonuclear absorption is the emission of a single neutron; but other processes must also be considered (such as an emission of a gamma ray) and particularly, for light nuclei, the emission of protons. The photoneutron cross section can be found in, e.g. in B.L. Berman, "Atlas of Photoneutron Cross Sections Obtained with Monoenergetic Photons," Atom.

UTE SHEET Nucl. Data 15.319, 1975 or E.G. Fuller, et al. ,

"Photonuclear Reaction Data 1973," NBS, Washington, 1973.

The energy of a photoneutron is equal to the excess energy of the original photon in relation to the value of the threshold of the (n, gamma) reaction. The threshold for photoproduction in N-14 is 10.55 MeV, which lies below the thresholds for the most common components of luggage (carbon and oxygen) . With photons of energy just sufficient to produce neutrons in nitrogen, there is, however, a number of light and heavy nuclei which will also exhibit a nuclear photoeffect. Light hydrogen can¬ not generate photoneutrons, but heavy hydrogen, i.e. deuterium, has a very low threshold of 2.23 MeV. Fortunately, a very low natural abundance of deuterium (0.0148%) removes the possible danger of swamping the detectors with photoneutrons originating in the deuterium content of baggage. Lithium and beryllium are rather unlikely components of luggage (the former appearing in some lubricants, battery cells and in medicines alone) . Boron (in the form of borax and boric acid) may find its way to luggage among cosmetics and cleaning agents; but the usual amounts are too small to cause concern.

SUBSTITUTE SHEET TABLE 1

RADIONUCLIDE PRODUCED IN (n, gamma) REACTION WITH COMMON NUCLEI

Specific

Target Threshold Product Gamma Constant Product *

Nuclide MeV Nuclide {(R.h)} (Ci.m) Half Life

C-12 18.72 C-ll 0.59 20.34 min

N-14 10.55 N-13 0.59 9.96 min

0-16 15.67 0-15 0.59 123.00 sec

Al-27 13.03 Al-26m 0.59 6.23 sec

Fe-54 # 13.62 Fe-53 0.65 8.51 min

Cu-65 @ 9.91 Cu-64 0.38 12.80 hr

Zn-70 9.29 Zn-69m 0.27 13.80 hr

Zn-70 9.29 Zn-69 beta only 57.0 min

Se-82 9.18 Se-81m beta almost only 18.6 min

Ag-107 9.39 Ag-106m 1.35 8.5 d

Ag-107 9.39 Ag-106 0.46 23.96 min

In-115 9.03 In-114m 0.14 50.0 d

In-115 9.03 In-114 0.004 72.0 sec

Sb-121 9.28 Sb-120m 0.33 15.89 min

Sb-121 9.28 Sb-120 1.43 5.8 d

1-127 9.15 1-126 0.25 12.8 d

Pr-141 9.37 Pr-140 0.33 3.39 min

Ta-181 7.64 Ta-180 0.04 8.15 min

W-182 7.99 W-181 0.07 140.0 d

Au-197 8.07 Au-196m 0.14 9.17 hr

Au-197 8.07 Au-196 0.28 6.18 d

Pb-204 8.38 Pb-203m 0.33 6.1 sec

Pb-204 8.38 Pb-203 0.18 52.1 hr

Where two entries are made for the same nuclide, one of them refers to the metastable state. The following nuclides undergo photodisintegration (in parentheses are threshold energies in MeV) D-2 (2.23), Li-6 (5.66), Be-9 (1.67) and B-10 (8.43).

# Fe-57 with a natural isotopic abundance of 2.2% has a photoneutron threshold of 7.6 MeV.

@ Natural isotopic abundance of 31%. Iron, aluminum and chlorine (in PVC) , which are commonly found in luggage, have their thresholds above that of nitrogen. The heavier elements (including copper, zinc, tin, silver, platinum and gold) have their photo- thresholds below 10.55 MeV and will be detected by the photonuclear method. In order to separate the contri¬ bution of neutrons originating from nitrogen from that of other elements, it is necessary to perform the measurements of neutron yield at a number of energies (a few above the nitrogen photoneutron threshold and a few below) and having done that, decompose the yield curve into components.

The decomposition of the summary yield curve is facilitated by the knowledge of cross sections for photoproduction from individual elements, which permits construction of the yield curves for these elements. However, it is necessary to know the excitation spectrum of bremsstrahlung for a particular accelerator for use in the luggage checking machine. ^Tne scanning apparatus of the present invention

(as shown in Figure 1 for the embodiment where luggage is scanned) includes a housing 10 for receiving an object 12 to be scanned. The housing may include a means 13 for transporting the object 12 into the housing 10. The illustrative embodiment in Figure 1 shows a conveyor belt to transport the object 12 into the housing 10; however, any other suitable means may be utilized. An accelerator 14 provides a bremsstrahlung beam of high-energy gamma photons directed toward the object 12. The nucleus of the element of interest with the object 12 absorbs a photon and, subsequently, emits a photoneutron. The moderator 15, adjacent to the object 12, captures the emitted

SUBSTITUTESHEET photoneutron and emits gamma photons. Means 16 detects the gamma photons and provides an output which is proc¬ essed and analyzed by means 17 to indicate the concen¬ tration of the element of interest within the object 12. The energy response of the moderator, together with the gamma detector system, depends on its geometry and can be used to provide information on the photoneutron energy.

The apparatus shown in Figure 1 is shown in a side view in Figure 1. The bremsstrahlung beam 20 interrogates the object 22 (where the element of interest emits photoneutrons which follow a random path 23 in moderator 25) and, subsequently, emits gamma photons which are detected by means 26.

Since monochromatic (or quasi-monochromatic) high-energy photon sources are very large and very costly, the bremsstrahlung beam obtained when electrons hit a target is used as photon source in almost all photo- activation studies. The energy spectrum of the photons in a bremsstrahlung beam, from a thin target, is well known and is shown in Figure 3, (L.I. Schiff, "Phys. Rev.", 53.252 (1951)).

Let n(E, E dE be the number of photons with energies between E and E + αE per unit target thickness per second in a bremsstrahlung beam where E₀ is the maximum photon energy. The energy flux in the irradiated sample, at a unitary distance from the target, is then

Uu(vE^χ-_nΛ) = I Eβ nm(E__,, E____n, )f(E)dE (1)

SUBSTITUTESHEET 2 where f(E) is a correction factor, which accounts for the distortion of the bremsstrahlung spectrum by the effects of the photon absorption in the machine target, in the walls of the accelerator chamber and in the sample. _r- Let us define the number of equivalent quanta Q by

Q = U(E₀)/E_Q ...(2)

0 (i.e. Q is the number of quanta with energy E_Q) that have the same energy content as the bremsstrahlung beam. Thus the cross section per equivalent quantum σ , is defined as

5

^Gq⁽V = ) { {σσ((EE))nn((EE,, E_Q)f(E)dE}/Q (3)

^■ ('

The bremsstrahlung activation yield measured in o arbitrary monitor response units can be written in a form

The monitor measures only some part of the energy of the beam and thus R(E_Q) gives the sensitivity of the monitor for a bremsstrahlung beam with the upper 0 energy limit of E.

While f(E) and R(E) are quantities specific for the laboratory set up, n(E, E_Q) can be tabulated for a

5 bremsstrahlung spectrum. The relationship between the number of photons per MeV per incident electron per g/cm of a target characterized by the atomic number Z and atomic mass A can be expressed as

n(E, E₀) = (N_Av/A)σ(E, E_Q) = ...(5a)

= 32(N_Av/A)(Z²r^α/E) (Z, E, E_Q) = ...(5b)

= 111.84 (Z7AE)Φ(Z, E, E_π)

where re is the classical electron radius

N is Avogadros number α is the fine structure constant Table of values of function Φ (Z, E, E_Q)dE as well

as table of values of (E, E_Q) can be found in

A.S. Penfold and J.E. Leiss "Analysis of Photo Cross Sections", Physics Research Laboratory, University of Illinois (1958). There are very severe restrictions on the type of neutron detector which could be employed in the photoactivation technique. The photoneutrons, which are to be detected, are fast depending upon the difference between the energy of the photon and the threshold energy. Direct detection of the fast photoneutrons does not seem possible in the luggage inspection equipment because in limited space, it is impossible to screen the detectors

SUBSTITUTESHEET from the bremsstrahlung flash. While there exists a number of neutron detectors which are almost entirely insensitive to gamma quanta of energies up to few MeV, the recoil counters, detectors based on reaction of He-3, B-10 and Li-6 and fission detectors (with Th, U or Np) will all respond directly to a bremsstrahlung photon of energies below the nitrogen threshold. Needless to say, generation of photoneutrons, within the detectors themselves, will render the detection system useless. Furthermore, detection of fast neutrons is characterized by a low sensitivity owing to interaction cross sections being small in comparison with slow and thermal neutrons. What is needed is to avoid interference in the detection process from an intense bremsstrahlung flash and, also, to increase the detection sensitivity by slowing down the neutrons and thus making use of relatively very large interaction cross sections with thermal neutrons. Both these aims can be achieved by an introduction of a moderator and application of the pulsing technique. The role of the moderator is to slow down neutrons by collisions with light nuclei. Any hydrogenous material can serve as moderator and so can materials containing deuterium or carbon. In the application of the photoneutron technique to the detection of nitrogen in luggage, the presence of deuterium should be avoided or minimized in both the moderator and detector.

The physical basis of the detectors of neutrons in a photoactivation system, particularly intended for use in a luggage checking machinery, but not necessarily limited to it is an application of a pulsed mode of excitation. The machine generating bremsstrahlung operate in a pulsed mode. During the bremsstrahlung pulse, fast

SUBSTITUTESHEET neutrons are being generated inside the baggage. These neutrons are undergoing moderation, i.e. slowing down, in a system comprising the piece of luggage itself and the bulk of moderator placed in the immediate vicinity of the luggage, e.g. lining the walls of the irradiation cavity in which the luggage is placed for the inspection.

During the process of the slowing down, part of the photoneutrons will escape from the system. Those which will be slowed down to the region of thermal energies will undergo capture by nuclei of the moderator. A neutron capture by the nucleus of hydrogen is accom¬ panied by an emission of a gamma photon of energy of 2.23 MeV. The process of slowing down is not identical for all neutrons. Because of the stochastical nature of the slowing down process, the thermalization will occupy an interval of time following the injection of the brems¬ strahlung pulse. Both theory and experiment (e.g. K.V. Ettinger, Nuclear Activation in Vivo in "Analytical Techniques in Nuclear Medicine", ed. R. , Cesareo, Publ. Elsevier, Amsterdam 1987; U.J. Miola, "Medical Neutron Physics", Ph.D. Thesis, University of Aberdeen, Scotland UK, [1981]) lead to the conclusion that the first thermalized neutrons will appear 1 - 2 microseconds after the appearance of bremsstrahlung. This pulse will then pass through broad maximum and gradually disappear. The exact timing depends upon the shape and volume of the moderator. IF blocks of moderator are covering the sides of the luggage of common dimensions and are about 20 cm thick, the build up of thermal fluence will have its peak after about 20 - 40 microseconds following the bremsstrah¬ lung pulse. It will take about 200 - 400 microseconds for the thermal flux to disappear. For smaller volumes of moderator, the build up will be faster; and the thermal flux will also disappear faster. The times given here are indicative. The slow down, build up and decay times of the thermal flux depend also on the composition of the moderator, namely on the amount of hydrogen present. The actual values for a specified moderating system can be computed by Monte Carlo numerical modelling or, more practically, can be determined experimentally. As a consequence of the use of a pulsed excitation system, the detection of a slowed down photoneutron takes place after the beam has been switched off and, thus, its interfering effect has been eliminated or reduced.

The actual dimensions of the moderator may be optimized for the detection of nitrogen. When exciting the photonuclear process with gamma quanta slightly above the threshold of nitrogen-14, photoneutrons produced by elements of lower threshold, e.g. from copper, will have much higher energies than from nitrogen. A proper choice of the volume and thickness of the moderator will result statistically in larger escape of more energetic neutrons from the moderator and thus in lesser chance of detection.

This approach can be extended further, if thermal neutron detectors were available which could operate in the presence of bremsstrahlung and were much less sensitive to neutrons of higher energies, then with an excitation energy just above the photoneutron threshold of a sought element, almost only neutrons from that element will be detected, providing that there is no excessive moderation in the system that will produce thermalized neutrons from other elements. Further discrimination in this case can be provided by the time selection in counting; neutrons from the sought element

SUBSTITUTE SHEET 2 would require a much shorter thermalization time than those from elements with lower thresholds.

The actual detection of a photoneutron may take place in one of the following ways: 1. The moderator itself is made of a hydro¬ genous scintillator, plastic or liquid. Scintillators produced by Nuclear Enterprises, Ltd, UK, type NE 102A, NE 104, NE 104B, NE 110 and others of the same or different manufacturer 0 could be used. These scintillators are made of plastic and are characterized by a large hydrogen content as moderator and by a fast decay time of scintillation measured in nanoseconds or tens of nanoseconds. These scintillators can be cast or machined in any desirable shape and are durable and rigid.

The use of liquid scintillators, poured into suitable vessels, is also possible. In this case, it is necessary to keep the solution o oxygen free by careful deoxygenation and, perhaps, regular bubbling of scrubber gas through the scintillator. A suitable scintillator is NE 224 produced by Nuclear Enterprises, Ltd. or similar types. A gamma ray, from the capture of a thermalized neutron in the scintillator, can be detected by a photomultiplier or an array of photomultipliers, as shown in Figure 7, optically coupled to the scintillators in such o a way as to collect the maximum emitted light.

The spectrum of such events will show no photopeak because of the lack of heavier elements in the scintillator.

SUBSTITUTESHEET 2. The efficiency of the detection of captured gamma rays can be improved by adding atoms of heavier elements to the scintillator wherein the atoms have a photoneutron threshold above that 5 of nitrogen. Aluminum, phosphorus, sodium and calcium are examples of such elements, which when added to the scintillator as a part of the non-quenching compounds, will increase the stopping power of the scintillator for gamma 0 rays and provide some fairly modest amount of photoabsorption.

3. Inorganic scintillators, which do not contain elements with a photoneutron threshold below that of nitrogen-14, can be coupled to a 5 moderator block, which does not need, in that case, to be in itself scintillating.

Jn cases 1 and 2, the signals from photo- multipliers may be used to operate a coincidence circuit in order to reduce the effect of o background and their amplitudes added before providing an amplitude selection prior to counting. The coincidences should take place between the signals from photomultipliers looking at neighboring volumes of the 5 scintillator only, to avoid losing capture events.

In order to avoid an interference from the small amount of deuterium present in the scintillator, an improvement can be made by synthesizing it from hydrogen o depleted of deuterium. Depleted water and other compounds are available from certain chemical reagent suppliers, e.g. from Aldrich Inc.

5

SUBSTITUTESHEET 2 Even if the photomultiplier tubes are not directly exposed to the primary bremsstrahlung flux, they will receive a considerable amount of scattered flux. In consequence, some darkening of the glass envelopes will be observed, which will change the characteristics of the tubes and eventually render them useless. To avoid this situation, the envelopes should be made of radiation resistant material, e.g. from silica glass of suitable grade. The use of silica, is also preferred on another 0 ground; the components of silica, silicon and oxygen have both photoneutron thresholds above that of nitrogen-14.

In order to avoid the fatigue of photocathodes in the PM tubes and to restore them quickly to normal operation following the cessation of bremsstrahlung pulse, the tubes should be of a gated type; or if gated tubes of an adequate type are not available, a gating circuit should be used. An example of such a circuit is a scheme in which pulses of suitable amplitude and polarity are applied to the number of dynodes, so as to reduce the gain o of the tube. In addition, it is desirable to gate the linear amplifying chain to avoid overload, which may create a dead time in the detection process.

The beam pulse of the bremsstrahlung source should last no more than 2 - 3 microseconds, preferably 5 about 1 to 3 microseconds. The rise and fall times are not critical, but a sharp cut off is necessary. There is a need to maintain flatness of the pulse top. The repetition frequency is not critical as long as the period between two successive pulses is not less than 40 - 50 o microseconds; otherwise, the counting losses will ensue.

The pulse generator controlling the operation of equipment should also provide the gating pulses for the photo¬ multipliers and for the counting gate. Figure 4a illustrates the timing of the pulsed bremsstrahlung beam. The pulse being approximately one microsecond and the rest period being about 10 to about 50 microseconds. The detector is turned on when the bremsstrahlung beam is not on, as illustrated in Figure 4b. The detected response, therefore, measures the thermal neutron fluence slightly before its peak value through the remainder of the area of the fluence. The thermal neutron fluence present prior to turning on the detector, the shaded area of Figure 4c, is, therefore, not observed.

The photoactivation method of inspecting the luggage for the presence of explosives is a particular case of a more general technique of detection. The required photon fluxes and the energies of the electron beam depend upon the particular application. The following considerations refer specifically to the detection of nitrogen in luggage.

Figure 5 is the stepwise form of a photoneutron yield curve as a function of electron energy for a simple system containing deuterium nitrogen and elements of higher threshold.

Figure 6 is a photo cross-section for deuterium. In order to provide a positive identification of the presence of a suspicious causing quantity of nitrogen inside the piece of luggage, it is necessary to consider the numbers of photoneutrons which will originate from the "threatening" quantity of nitrogen.

The numbers of atoms of nitrogen in the minimum detectable quantity of explosive stipulated by FAA are:

SUBSTITUTE SHEET C4 3.35 x 10²⁴ Ammonium nitrate 3.40 x 10 24

PETN 1.72 x 10²⁴

TNT 0.81 x 10²⁴ Dynamite 40% 0.70 x 10 24

24 Black powder 0.25 x 10

Composition varies

With certain simplifying assumptions concerning the detailed shape of neutron yield curves near the threshold (cf. e.g. V.P. Kovalev, "Secondary Radiation

From Electron Accelerators", Atomizdat, Moscow 1979) and, also, assuming the shape of the bremsstrahlung spectrum corresponds to the thin target theory, the minimum number of photoneutrons, which must be detected emerging from a single piece of luggage in order to conclude that it contains the minimum FAA quantity of nitrogen, is about 400 at each spot energy. This corresponds to a 5% statistical uncertainty, in the absence of background. The analysis of the yield curves can be performed either in a continuous way or by selection of a number of "spot" energies. The choice of the spot frequencies is governed by the maximum detectability of nitrogen. In order to understand the yield curve of neutrons emerging from luggage, it is necessary to estimate the background generated by deuterium and by Fe-57 (2.2% isotopic abundance, 7.6 MeV threshold), together with the possible presence of boron, lithium

SUBSTITUTESHEET 2 and beryllium (all below 8.6 MeV). The next step is determination of heavy and medium heavy elements about 10.5 MeV, i.e. just below the threshold for nitrogen. Nitrogen is determined above its threshold of 10.55 MeV. 5 All together, four or five spot energies are preferred.

As shown in Figure 8, means 86 detects the gamma photons emitted from the moderator 85, which is processed and analyzed by means 87, to provide first output signals representative of the concentration of the element of 0 interest, including background signals from other elements; and means 88 detects the background gamma photons emitted from the moderator 85, which is processed and analyzed by means 89, to provide second output signals representative of the concentration and energy of the 5 photoneutrons emitted by the other elements. Means 90 subtracts the background emission from the first output signal to determine the concentration and energy of the element of interest.

The optimization of the choice of the numbers o and energies of spot frequencies must involve the experimental measurements of the photoneutron yield in the energy range reaching up to about 13 MeV, just below the threshold of aluminum.

With the number of atoms of nitrogen given by the requirements of FAA and the relevant cross sections for the photoproduction of neutrons of an order of 1 mb

(near the threshold), the macroscopic cross section is in the range of 1 x 10 -4 to 1 x 10-3 per piece of luggage.

The detection efficiency is estimated to be in the region

-4 -5 0 of 1 x 10 or even 1 x 10 . to produce a detected number of 1000 photoneutrons, one needs a photon yield of about 1 x 10 12 quanta per piece of luggage. The actual

5 required number of photoneutrons may be higher because the identification of nitrogen is done in the presence of background produced by other elements. These consider- ations lead to an estimate of 3 x 10 12 photons per energy step (spot energy) per piece of luggage.

Similar estimations can be performed for other applications of the photoactivation system.

The photoactivation explosive detection system can be used in a mode which permits measurement of the photoneutrons emerging from a part of the luggage, albeit the delineation between adjacent parts cannot be sharp because of neutron scattering inside the luggage and in the process of moderation. This process can be achieved by limiting the size of the beam spot, "illuminating" the piece of luggage and mechanically moving the piece or by an electronic displacement of the beam. When operating in this mode, it is highly desirable that the photoactivation system is used in conjunction with the radiographic x-ray imaging system or equivalent technique in which the signal i dependent upon the atomic mass or atomic number of the detected material. The technique employed may depend upon transmission or scattering of x-rays. The technique of imaging may include 3-dimensional imaging. Techniques are already employed alone to detect metallic objects in luggage, such as guns and knives. Techniques of this type are capable of distinguishing typical organic material from medium and heavy atomic mass elements like copper, zinc or gold.

Such a technique allows identification of heavier nuclides, when present, whose photoproduction spectra are overlapping that of nitrogen to a significant extent. The heavier nuclides having much larger photo- 2 neutron cross sections would swamp the relatively weak photoproduction signal for nitrogen (on a gram per gram basis) . The information from the apparatus determining atomic mass or atomic number is used to decide for each of 5 the pixels into which the area of the projection of the investigated piece of luggage is divided, whether any photoneutron flux arising from the projected volume comes from (1) low atomic number nuclides with low thresholds, (2) nitrogen or (3) medium or high atomic number nuclides. o Iron and aluminum, which are common in cargo and luggage, will not cause interference. The pixel by pixel evalu¬ ation will be done automatically following the process of scanning the luggage. The evaluation may be done by an operator or by computer. This eliminates false alarms 5 from materials with high automatic mass. The use of the system containing both a photonuclear apparatus and an apparatus to detect the medium or high atomic number nuclides increases contour resolution while determining the atomic mass value. o While illustrative embodiments of the subject invention have been described and illustrated, it is obvious that various changes and modifications can be made, therein, without departing from the spirit of the present invention, which should be limited only by the 5 scope of the appended claims.

0

5

SUBSTITUTESHEET

Claims

WHAT IS CLAIMED IS :

1. An apparatus for scanning an object to determine the concentration of an element of interest in the object comprising: a housing for receiving the object to be scanned; means for generating a pulsed bremsstrahlung beam of high energy gamma photons wherein the photons have energy equal to or greater than the threshold energy for photoproduction in the element of interest; means for directing said bremsstrahlung beam towards said object; means for detecting the photoneutrons emitted by the element of interest contained in said object and for producing output signals representative of the flux density and energy of said photoneutrons; and means.for processing and analyzing said output signals for determining the concentration of said element of interest in said object. 2. The apparatus of Claim 1 wherein said pulsed bremsstrahlung beam is generated when accelerated electrons hit a target.

3. The apparatus of Claim 1 wherein said means for generating the pulsed bremsstrahlung beam includes means for pulsing said bremsstrahlung beam for about 1 to about 3 microseconds with rest periods of about 10 to about 50 microseconds.

4. The apparatus of Claim 1 wherein the energy of the gamma photons is greater than or equal to 10.55 MeV, and the element of interest of nitrogen-14.

5. The apparatus of Claim 1 wherein said means for detecting the photoneutrons further comprises a moderator. 2 6. The apparatus of Claim 5 wherein said moderator comprises a substance which does not affect the process of moderation.

7. The apparatus of Claim 5 wherein said 5 moderator is a plastic hydrogenous scintillator or a liquid hydrogenous scintillator.

8. The apparatus of Claim 5 wherein said means for detecting the photoneutrons comprises a moderator and a photomultiplier tube. o 9- The apparatus of Claim 8 wherein the envelopes of the photomultiplier tube are made of silica.

10. The apparatus of Claim 8 wherein the photomultiplier tube is an array of photomultiplier tubes.

11. The apparatus of Claim 1 further com- prising: second means for detecting the photoneutrons emitted by other elements in the object during photo¬ production and producing second output signals representa¬ tive of the concentration and energy of the photoneutrons o emitted by the other elements; means for processing and analyzing said second output signals for determining the background emission of other elements and means for subtracting the background emission 5 from said output signal to determine the concentration and energy of the element of interest.

12. A method of scanning an object for an element of interest in the object, comprising: placing an object to be scanned in a housing; 0 generating a pulsed bremsstrahlung beam of high energy gamma photons wherein the photons have energy equal to or greater than the threshold energy for photoproduc¬ tion in the element of interest;

5 2 directing said bremsstrahlung beam toward said object; detecting the photoneutrons emitted by the element of interest contained in said object when the bremsstrahlung beam is off and producing an output signal representative of the flux density and energy of said photoneutrons; and processing and analyzing said output signals for . determining the concentration of said element in said 0 object.

13. The method of Claim 12 wherein said bremsstrahlung beam is generated when accelerated electrons hit a target.

14. The method of Claim 12 wherein said element of interest is nitrogen-14.

15. The method of Claim 12 which further includes detecting the photoneutrons emitted by other elements in the object and producing secondary output o signals representative of the concentration and energy of the photoneutrons emitted by the other elements during photoproduction; processing and analyzing said secondary output signals for determining the background emission of other elements; and subtracting the background signal from said output signal to determine the concentration and energy of the element of interest.

16. A system for scanning an object to determine the concentration of an element of interest in the object comprising:

SUBSTITUTE SHEET A.) an apparatus to determine the combined photoneutron flux from the element of interest and heavier nuclides comprising: a housing for receiving the object to be 5 scanned; means for generating a pulsed bremsstrahlung beam of high-energy gamma photons; wherein the photons have energy equal to or greater than the threshold energy for photoproduction in the element of interest; 0 means for directing said bremsstrahlung beam towards said object; and means for detecting the photoneutrons emitted by the element of interest and heavier nuclides contained in said object and for producing first output signals representative of the flux density and energy of said photoneutrons;

B.) an apparatus to determine the presence of heavier nuclides within the object including means from producing second output signals representative of said o heavier nuclides; and

C.) means to subtract said second output signals from said first output signals.

17. The apparatus of Claim 16 wherein said pulsed bremsstrahlung beam is generated when accelerated 5 electrons hit a target.

18. The apparatus of Claim 16 or 17 wherein said apparatus to determine the presence of heavier nuclides is an x-ray imaging system.

19. A method of scanning an object for an o element of interest in the object, comprising: placing an object to be scanned in a housing; generating a pulsed bremsstrahlung beam of high-energy gamma photons wherein the photons have energy

5 equal to or greater than the threshold energy for photo¬ production in the element of interest; directing said bremsstrahlung beam toward said object; detecting the photoneutrons emitted by the element of interest and heavier nuclides contained in said object when the bremsstrahlung beam is off and producing a first output signal representative of the flux density and energy of said photoneutrons; 0 determining the presence of heavier nuclides within the object and producing a second output signal representative of said heavier nuclides; and subtracting said second output signal from said first output signal. 5 20. The method of Claim 19 wherein said bremsstrahlung beam is generated when accelerated electrons hit a target.

21. The method of Claim 19 or 20 wherein the presence of heavier nuclides is determined by an x-ray o imaging system.

5

SUBSTITUTE SHEET