Tsahi Gozani

The role of neutron based inspection techniques in the post 9/11/01 era

Abstract Non-intrusive inspection of objects of all sizes, from luggage to shipping containers and from postal parcels to trucks is a vital component of any national security from aviation to the control of all land and sea ports of entries. The paramount importance of these inspections is more obvious now, in the post 9/11 era, as the spectrum of threats is wider and the probability of occurrence more real. The urgent need for reliable inspection underscores the key attributes they must possess: • High specificity • High sensitivity • Provide automatic decision The technologies being currently employed in the field, such as standard X-ray, X-ray based computed tomography, and trace detection (for luggage), and X-ray or γ-ray based radiography (for containers) are inherently deficient for lacking some or all of these attributes. The neutron based technologies, on the other hand, possess all three. They provide therefore accurate, rapid and automatic detection of a wide array of threats: explosives, chemical agents, nuclear materials and devices, other hazardous materials, drugs, etc. The nuclear based techniques achieve this feat through the production of characteristic elemental gamma rays by nuclear reactions, primarily (n,γ) with thermal neutrons and (n,n′γ) with fast neutrons. The principles and status of neutron based inspection techniques are reviewed below.

Novel applications of fast neutron interrogation methods

Abstract The development of non-intrusive inspection methods for contraband consisting primarily of carbon, nitrogen, oxygen, and hydrogen requires the use of fast neutrons. While most elements can be sufficiently well detected by the thermal neutron capture process, some important ones, e.g., carbon and in particular oxygen, cannot be detected by this process. Fortunately, fast neutrons, with energies above the threshold for inelastic scattering, stimulate relatively strong and specific gamma ray lines from these elements. The main lines are: 6.13 for O, 4.43 for C, and 5.11, 2.31 and 1.64 MeV for N. Accelerator-generated neutrons in the energy range of 7 to 15 MeV are being considered as interrogating radiations in a variety of non-intrusive inspection systems for contraband, from explosives to drugs and from coal to smuggled, dutiable goods. In some applications, mostly for inspection of small items such as luggage, the decision process involves a rudimentary imaging, akin to emission tomography, to obtain the localized concentration of various elements. This technique is called FNA — Fast Neutron Analysis. While this approach offers improvements over the TNA (Thermal Neutron Analysis), it is not applicable to large objects such as shipping containers and trucks. For these challenging applications, a collimated beam of neutrons is rastered along the height of the moving object. In addition, the neutrons are generated in very narrow nanosecond pulses. The point of their interaction inside the object is determined by the time of flight (TOF) method, that is measuring the time elapsed from the neutron generation to the time of detection of the stimulated gamma rays. This technique, called PFNA (Pulsed Fast Neutron Analysis), thus directly provides the elemental, and by inference, the chemical composition of the material at every volume element (voxel) of the object. The various neutron-based techniques are briefly described below.

GAMMA-RAY AND NEUTRON RADIOGRAPHY AS PART OF A PULSED FAST NEUTRON ANALYSIS INSPECTION SYSTEM

Abstract A gamma-ray and neutron radiography system has been developed to provide useful supplemental information for a Pulsed Fast Neutron Analysis (PFNA) cargo inspection system. PFNA uses a collimated beam of pulsed neutrons to interrogate cargoes using (n, γx) reactions. The PFNA source produces both gamma rays as well as neutrons. The transmission of both species through the cargo is measured with an array of plastic scintillators. Since the neutron and gamma-ray signals are easily separated by arrival time a separate image can be made for both species. The radiography measurement is taken simultaneously with the PFNA measurement turning PFNA into an emission and transmission imaging system, thus enhancing the PFNA radiography system.

Cargo inspection system based on pulsed fast neutron analysis

Abstract Pulsed Fast Neutron Analysis (PFNA) is a technique which uses a collimated pulsed beam of fast neutrons to excite the nuclei of common elements in bulk materials. Direct imaging of the elemental contents of the material is accomplished by using time-of-flight analysis to identify the position of the interactions and gamma-ray spectroscopy to identify the elemental gamma-rays. From the ratios and absolute measurements of elemental abundances the identification of the material can be deduced. The PFNA cargo inspection system uses a volume type negative ion source and a double drift bunching system to create an intense beam of nano-second bunched negative deuterium ions which, after acceleration to around 6 MeV, impinge on a deuterium gas target producing pulsed neutrons. A unique high speed data acquisition system digitizes and analyzes the time-energy data in real time. Experimental studies and computer simulations were extensively employed to characterize and optimize the design parameters of the system.

A TNA explosives-detection system in airline baggage

Abstract Existing technologies that are applied to explosives-detection in passenger baggage are briefly discussed. A system based on thermal-neutron analysis (TNA) is described. The actual performance of the system in the field on passenger bags is given. The application of the TNA in an integrated airport security system is discussed in view of the intense public debate on this issue.

Digital signal processing for high rate gamma-ray spectroscopy

Abstract A fully digital system for acquisition of NaI gamma-ray spectra was developed and tested. A fast digitizer is used to digitize unprocessed photomultiplier anode pulses to 8-bit resolution at 5 ns intervals. Digital filtering and a pulse searching algorithm are used to identify pulses, detect and correct for pileup and determine pileup corrected pulse areas. A pulse height distribution is generated, corresponding to a conventional multichannel analyzer spectrum. Pileup is detected with excellent sensitivity compared to conventional pileup detection methods. Since pileup events are corrected rather than rejected, all events are retained, with no loss of counting statistics. No electronics are used between the photomultiplier and the digitizer, eliminating the usual amplification, filtering, baseline restoration, etc.

Application of pulsed fast neutrons analysis to cargo inspection

Abstract Pulsed Fast Neutron Analysis (PFNA) is a technique which uses a collimated pulsed beam of fast neutrons to excite the nuclei of common elements in bulk materials. Direct imaging of the elemental contents of the material is accomplished by using time-of-flight analysis to identify the position of the interactions and gamma-ray spectroscopy to identify the elemental gamma rays. From the ratios and absolute measurements of elemental abundances the identification of the material can be deduced. The PFNA Cargo Inspection System uses a volume type negative ion source and a double drift bunching system to create an intense beam of nano-second bunched negative deuterium ions which, after acceleration to around 6 MeV, impinge on a deuterium gas target producing pulsed neutrons. A unique high speed data acquisition system digitizes and analyzes the time-energy data in real time. Experimental studies and computer simulations were extensively employed to characterize and optimize the design parameters of the system. The system described is scheduled for full scale laboratory testing in the fall of 1994 and for field testing at a Government Testbed in Tacoma, WA in 1995.

Time of flight fast neutron radiography

Abstract Neutron radiography with fast or thermal neutrons is a standard technique for non-destructive testing (NDT). Here we report results for fast neutron radiography both as an adjunct to pulsed fast neutron analysis (PFNA) and as a stand-alone method for NDT. PFNA is a new technique for utilizing a collimated pulsed neutron beam to interrogate items and determine their elemental composition. By determining the time of flight for gamma-rays produced by (n,n′ gamma X) reactions, a three dimensional image can be produced. Neutron radiography data taken with the same beam provides an important constraint for image reconstruction, and in particular is important in inferring the amount of hydrogen within the interrogated item. As a stand-alone device, the radiography measurement can be used to image items as large as cargo containers as long as their density is not too high. The use of a pulsed beam gives the further advantage of a time of flight measurement on the transmitted neutrons. By gating the radiography signal on the time of flight appropriate to the energy of the primary neutrons, most build-up from scattered neutrons can be eliminated. The pulsed beam also greatly improves the signal to background and extends the range of the neutron radiography. Simulation results will be presented which display the advantage of this constraint in particular for statistically limited data. Experimental results will be presented which show some of the limitations likely in a PFNA system utilizing neutron radiography data. Experimental and simulation results will demonstrate possible uses for this type of radiographic data in identifying contraband substances such as drugs.

Thermal neutron analysis (TNA) explosive detection based on electronic neutron generators

Abstract Thermal neutron analysis explosive detection systems have been developed and demonstrated for inspection of checked airline baggage and for detection of buried land mines. Thermal neutrons from a moderated neutron source impinge on the inspected object, and the resulting capture gamma ray signatures provide detection information. Isotopic neutron sources, e.g. 252 Cf, are compact, economical and reliable, but they are subject to the licensing requirements, safety concerns and public perception problems associated with radioactive material. These are mitigated by use of an electronic neutron generator — an ion accelerator with a target producing neutrons by a nuclear reaction such as D(d, n) 3 He or 9 Be(d, n) 10 B. With suitable moderator designs based on neutron transport codes, operational explosive detection systems can be built and would provide effective alternatives to radioactive neutron sources. Calculations as well as laboratory and field experience with three generator types will be presented.

Nuclear-based techniques for explosive detection

Abstract The status of bulk explosive detection techniques based on nuclear interrogation methods is reviewed. The desirable characteristics of an operational system capable of meeting the requirements for civil aviation security are compared with what can be expected from nuclear-based techniques. The comparison is limited to those techniques that utilize penetrating neutron and photon probes on the target elements of explosives most likely to be encountered in the airport scenario. The physical properties of a relevant group of explosives are surveyed for unique characteristics that could provide detectable signatures to nuclear-based techniques. A survey of the accessible reactions and their relative detection sensitivities are tabulated for a selection of practical interrogating probes. Some results obtained with systems currently under development are reported.