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Chapter 2. Advanced technologies: scanning cargo or analyzing a terrorist nuclear weapon with nuclear resonance fluorescence.

Two problems

Nuclear resonance fluorescence (NRF), described below, seeks to detect SNM in containers. At issue: Is NRF a useful approach for this task?

NRF may address a second problem. Discovery of a nuclear weapon in a cargo container would require an urgent effort to disable it and to gather forensic data. Both efforts would benefit from detailed information about the weapon's design. Several techniques can provide such information. Radiography or MT can show the shape and location of components; discovering that the weapon had a thermonuclear stage, for example, would show that it was manufactured by a nation and could have much higher explosive yield than a terrorist-made bomb. Interrogation using neutrons or high-energy gamma rays can provide information about SNM. NRF may be able to provide different types of data. Knowing what kind of chemical explosive the weapon contained, or combining information on location of electronics with information on their chemical composition, or knowing the mix of isotopes and impurities in SNM, would aid in dismantlement or might point to the source of the weapon. At issue: Is NRF a useful approach to determining the materials in a weapon?


When atoms of a given element are illuminated with photons above an energy threshold unique to that element, their electrons absorb the photons' energy and move to a higher energy level, a so-called "excited" state. The electrons then drop back to their normal state, emitting photons that are slightly less energetic than the inbound photons. For example, certain elements or minerals illuminated with ultraviolet light (which is more energetic than visible light but less so than gamma rays) give off visible light. (129) This emission of light is called fluorescence.

A different type of fluorescence provides more detailed information. Each isotope has a unique combination of numbers of protons and neutrons in its nucleus, so it vibrates at unique frequencies (the resonant frequencies). When the nucleus is struck by a photon at precisely that energy level--sometimes to within a few hundredths of an electron volt in a beam with photon energies spread over a range of 1 MeV or more--it will absorb the photon and move to an excited state. The nucleus then reverts to its initial state, giving off photons very slightly less energetic than those that it absorbed. This process is known as nuclear resonance fluorescence, or NRF. NRF produces a gamma-ray spectrum unique to each isotope (though different than the gammaray spectrum produced by radioactive decay). Identifying the spectrum of the emitted photons identifies the element and isotope. This is of particular importance in differentiating between fissile U-235, which can be made into a nuclear weapon, and non-fissile U-238, which cannot. Unlike the use of neutrons or high-energy photons to stimulate the emission of neutrons or photons in SNM, NRF causes fluorescence in almost all isotopes of elements with Z>2 (helium), so it can identify a wide range of materials, not just SNM and other radioactive isotopes. For example, if nuclear material is shielded by lead, the identification of the various lead isotopes and their ratios may provide information as to where the lead was mined. Technical experts consulted for this report were aware of no other technology that permits identification of a weapon's materials without opening the weapon. CBP could also use NRF to identify other contraband and to check customs manifests.

Absorption of photons creates an additional signature. X-rays are generated in a broad spectrum of energies without sharp peaks. If a beam of such photons is sent through a cargo container or other object, and the detector on the other side can record the energy of each transmitted photon, a hole or "notch" in the spectrum at a certain energy level means that some particular material has absorbed photons at that energy level through NRF and then subsequently re-emitted the absorbed photons at about the same energy level. Since NRF photons are emitted in all directions, only a small fraction of them reach the detector. The energy level of the notch indicates what material is present. For example, the notch for U-235 occurs at 1.73 MeV.

Technology description

To detect NRF, an accelerator generates a beam of x-rays, a photon detector records the radiation spectrum generated by the material being interrogated, and an algorithm matches NRF peaks against a library of such peaks. Photons resulting from NRF are differentiated from the incoming photon beam because the latter produces a broad continuum of photon energies while photons generated by NRF produce very narrow peaks. (130) Further, the photon beam travels in a forward direction, while the NRF signal is emitted in all directions, so a photon detector placed behind and to the side of the material being interrogated (relative to the direction of the photon beam) detects photons traveling backward from the beam direction, which are mainly NRF photons. Figure 18 illustrates this geometry.


DNDO is studying another approach to NRF using a beam of photons having a single energy level ("monoenergetic photons") near that needed to induce NRF in a particular isotope. At present, that method is technically difficult, costly, and requires large and delicate equipment, making it unwieldy for deployment in the field. However, according to DNDO, this method has been verified experimentally and the Stanford Linear Accelerator Center has demonstrated one type of accelerator technology "that might lead to a mono-energy photon source that could be compacted into a 20-foot cargo container." (131)

Passport Systems, Inc., is developing an NRF imaging system, Passport MAX (Material Advanced Inspection), under contract to DNDO to detect SNM in cargo containers. It uses a commercial electron accelerator with a beam that can be varied from 2 to 10 MeV, depending on the materials and containers being searched, to produce a photon beam with energies ranging from several hundred keV to the maximum energy of the electron beam. The beam is collimated; (132) as it scans a container, it excites nuclei in its path that emit photons. A germanium gamma-ray detector views the emitted photons scattered backwards from one region at a time, records their energies, and constructs a spectrum. The intersection of the collimated beam with the detector's view creates a voxel, and the spectrum shows the type and quantity of each isotope in that voxel. An algorithm constructs a three-dimensional image of the container's contents. Passport MAX would include other detector subsystems as well: EZ-3D, as described under AS&E CAARS; a radiography imager; and an NRF detector that detects notches in the transmitted photon spectrum. Passport Systems indicates that this approach could also be used to scan smaller items such as an aircraft cargo container (133) or a terrorist nuclear weapon. (134)

Because the NRF imaging component can examine only one region of interest at a time, the CONOPS envisions using the other components of Passport MAX to locate volumes of interest, and then using NRF to interrogate them. According to Passport Systems, "the complete system would scan a 40 ft container for SNM in an average of about 15 seconds. If there were indeed SNM in a container it may take longer (minutes) to identify the material as SNM. However, we anticipate that the actual number of containers with SNM would be very small." (135)

Potential benefits

For detecting a terrorist nuclear weapon or SNM: (1) The system would identify each isotope, and would alarm on threat substances, with no operator input required. (2) While GADRAS must account for all the spectral data, the algorithm required for the Passport system need only account for spectral peaks, making for a simpler algorithm. (3) The system is to identify most isotopes that CBP finds in contraband, eliminating some false alarms that occur with radiation portal monitors, such as from radioactive potassium. (4) An NRF-based system can scan a cargo container quickly for SNM and high-Z shielding material. The average scan rate for the EZ-3D mode is 15 seconds, but the system would automatically adjust the speed at which individual containers are scanned. If it detected little attenuation of the beam, it would scan faster. Conversely, if EZ-3D identified a region of interest, the system would reposition the container to analyze that region using NRF and photofission signatures and would likely increase scan time for that container. The impact of such delays on average throughput rate would depend on such factors as CONOPS and number of anomalies in containers being scanned. (136) (5) Passport Systems states that laboratory experiments and simulations predict that Passport MAX will be able to meet the scanning requirement that DNDO has set for CAARS, i.e., that it would have a 90% probability of detecting 100 cc of high-Z material, and a false alarm probability less than 3%, both with 95% confidence. (137)

For characterizing a nuclear weapon: (1) This system could determine the composition of a nuclear weapon. These data would be of value for disabling a weapon and for nuclear forensics. (2) An NRF-based system can identify the isotopic composition of uranium or plutonium and any impurities, which would be of value in nuclear forensics. (3) A potential difficulty with radiation detection is that large quantities of shielding in a container may block photons or neutrons as they enter and leave the container. The shielding problem would diminish if this system were used against an already-identified terrorist nuclear weapon because the weapon would be shielded only by its casing. (4) A bremsstrahlung source generates photons over a wide range of energies, and detectors are able to record a similarly wide range of emitted photons. As a result, the system could identify multiple materials quickly.

Status, schedule, and funding

From 2004 to 2008, DHS awarded Passport Systems several contracts totaling $8.4 million to build a proof-of-concept (PoC) scanner that is fully integrated and functional. In 2005, the contract was transferred from the DHS Homeland Security Advanced Research Projects Agency to DNDO for management. According to Feinstein,
   The NRF PoC system demonstration and evaluation completed on August
   4-6, 2008. The primary purpose of this PoC test is to demonstrate
   full functionality and automation. This requires all critical
   components to operate as specified in an integrated architecture
   similar to an operational scanner. The Passport PoC subscale system
   successfully demonstrated its ability to automatically select
   high-Z [regions of interest] with EZ-3D and auto-identify the
   isotopic content of [these regions] with NRF. This was accomplished
   with a variety of cargo and with a mixed set of high-Z material and
   contraband. Other NRF applications are still being explored
   including cargo manifest-checking and forensics. (138)

Most of the major hardware components that the system uses--accelerator, detector, computer, and display--are commercially available. Passport Systems estimates that its system will be available for commercial delivery by mid-2010 at a unit cost of $5 million to $10 million depending on system configuration. Feinstein states, however, that that system "will not have completed the DHS phased-milestones of development, testing, evaluation and cost-benefit analysis" by that time. He further states that DNDO is developing enabling technologies, such as improved accelerators and detectors, "that, if successful, could significantly reduce the overall size and cost [of the NRF system] (by more than a factor of two) as well as improve its performance and speed," though such technologies would not likely be ready for use in a commercial system by mid-2010. (139)

In September 2008, DNDO awarded Passport Systems a contract worth up to $9.3 million to build a full-scale prototype unit for an Advanced Technology Demonstration (ATD) of the NRF system under DNDO's Shielded Nuclear Alarm Resolution program. (140) This contract runs for 2// years. The ATD scanner is intended to demonstrate performance on cargo containers and is the continuation of the previously demonstrated proof-of-concept (PoC) installation built by Passport at the University of California at Santa Barbara (UCSB), also under DNDO contract.

Many organizations have been conducting research into NRF to address national and homeland security issues since 2004. Lawrence Livermore National Laboratory (LLNL), Pacific Northwest National Laboratory (PNNL), and Passport Systems have collaborated to characterize the NRF response of U-235 and of Pu-239. PNNL has developed a computer model of NRF and has used simulations from it to reproduce results from laboratory measurements; (141) Los Alamos National Laboratory is developing another such model. (142) Duke University, Idaho National Laboratory, Idaho State University, LLNL, PNNL, Passport Systems, Purdue University, and University of California at Berkeley are conducting basic research on NRF. Passport Systems built its proof-of-concept prototype at University of California at Santa Barbara in order to use the accelerator in that university's free electron laser facility, and PNNL and Duke University conducted NRF measurements on U-238 and Pb-208 using the High Intensity Gamma Source at Duke as a source of nearly-single-energy photons.

Risks and concerns

Scientific risks and concerns

(1) A key scientific task is to conduct more experiments to identify the energy levels at which materials of interest undergo NRF. In particular, it would be useful to measure NRF spectra of isotopes of uranium and plutonium other than U-235 and Pu-239, and to measure spectra of materials used in nuclear weapons of other nations (e.g., for alloys) for purposes of nuclear forensics. (2) Another task is to develop the algorithms to identify materials quickly based on their NRF energies. (3) The Passport MAX geometry is designed to improve the signal-to-noise ratio by focusing on photons scattered backwards as a result of NRF, thus avoiding most of the photons generated by the interrogation beam, which travel in a forward direction. However, beam photons may have an energy range of nearly 10 MeV, while the energy range of photons that produce NRF in a particular isotope may be only a few hundredths of an electron volt. As a result, only one out of a few hundred million photons may have an energy level that produces NRF in that isotope, making it hard to detect NRF-generated photons. On the other hand, if the NRF system is tuned correctly, very few photons of the same energy as the NRF photons scatter backwards to the detector, facilitating detection by increasing signal-to-noise ratio. (4) While NRF has routinely been used to detect gram samples, the limitations of the detectable mass of various isotopes has apparently not been quantified, so it is not clear that NRF can be used to detect microgram (or smaller) quantities of all isotopes, possibly limiting the applicability of NRF to nuclear forensics.

Engineering risks and concerns

(1) More work is needed to develop the detection algorithms. (2) The accelerator for this system, called a Rhodotron, (143) is built to order, and it may take the manufacturer a year to build one; (144) unless the manufacturer can build them much faster, it would be difficult to procure Passport MAX units in quantity. As of February 2010, Passport Systems was building a prototype of a high duty cycle accelerator (145) that could be produced more rapidly. In addition, since the accelerator footprint would be much smaller than that of the Rhodotron, Passport MAX would become smaller, which Passport Systems argues would make it or other NRF-based scanners much more attractive for mobile operations. (3) Once the tasks noted under "scientific risks and concerns" are completed, an engineering task would be to integrate hardware and software into an operational system, requiring many tradeoffs between cost, performance, and schedule. This may be particularly complicated for Passport MAX given the many separate detector units it uses, as shown in Figure 18. A demonstration of the full-scale ATD scanner under the Shielded Nuclear Alarm Resolution program is scheduled for 2010-2011; if successful, it would significantly reduce this risk.

Cost and schedule risks and concerns

The projected unit cost, $5 million to $10 million, is at a level that might preclude ordering large numbers of units. Regarding schedule, as of 2008 it was difficult to predict when an early-stage development program would become commercially available given the work that remained to be done and the possibility of unanticipated problems.

As of March 2010, Passport MAX had progressed from early-stage development to Advanced Technology Demonstration (ATD), with construction of the ATD unit scheduled to begin in 2010. As it has progressed, the amount of work remaining and the range of potential problems have decreased, so risk to schedule has decreased as well. Also as of March 2010, DNDO continued to fund enabling technologies, such as low-cost/high-resolution detectors and high duty cycle accelerators, that may, if successful, significantly reduce the cost and size of the commercial system and increase its NRF performance speed. As a result, Passport Systems stated that if the MAX ATD scanner demonstration is successful in 2010, the company could deliver a commercial version of that system in 2011. (146)

Operational risks and concerns

The system would need to be large enough to hold a tractor-trailer truck. The Rhodotron used in the Passport MAX generates a considerable amount of radiation that requires containment. To meet a radiation safety requirement of no standoff zone, the system is completely enclosed, and a preliminary estimate by Passport is that it would be 90 to 100 ft long; 20 to 30 ft wide at its widest point, where the detection equipment is located; and several stories high at that point. These dimensions are dictated by the requirement to scan a container that is 40 ft long, 9 ft wide and 14.5 ft high. A system for smaller objects would be correspondingly smaller. Passport states that the enclosure will reduce the radiation dose outside the system to levels low enough to not require an exclusion zone. On the other hand, the system is quite large using existing commercial off-the-shelf technologies, which could be a problem in ports where space is at a premium.

Potential gains by increased funding

Passport Systems, a small company, does not have some key equipment of its own; for example, it is using an accelerator at the University of California at Santa Barbara as the photon source for its prototype. Added funds, Passport says, would enable it to buy needed equipment, advance supporting technologies, and hire more staff for engineering, algorithm development, and manufacturing. The prototype Passport MAX uses a considerable amount of expensive off-the-shelf hardware; with added funds, Passport says it could design less costly components specifically for use in its system.

Potential synergisms and related applications

(1) NRF could help identify illicit cargo in addition to SNM, such as explosives or chemical weapons. (2) NRF could help verify the manifest (list of contents) of a cargo container. (3) NRF might be used in nuclear forensics to identify rapidly the materials present in radioactive debris from the detonation of a terrorist nuclear weapon. (4) In nuclear nonproliferation applications, it could be used to analyze the isotopic composition of spent nuclear fuel. (5) New detector material could improve the sensitivity and resolution of the system, reducing the amount of electrical energy needed to run it and the amount of shielding needed, thereby reducing acquisition and operational costs.
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Title Annotation:Detection of Nuclear Weapons and Materials: Science, Technologies, Observations
Author:Medalia, Jonathan
Publication:Congressional Research Service (CRS) Reports and Issue Briefs
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2010
Previous Article:Chapter 2. Advanced technologies: muon tomography.
Next Article:Chapter 2. Advanced technologies: detecting SNM at a distance.

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