Congress has been interested in detecting nuclear weapons and the special materials needed to make them for many years, especially since 9/11. However, they are generated in different ways. Atomic number and density

Atomic number, abbreviated "Z," is the number of protons in an atom's nucleus. Thus a way to detect an object, such as a bomb, in a container is to beam in x-rays or gamma rays to create a radiograph (an opacity map) like a medical x-ray. Each point on the spectrum shows the number of photons emitted (vertical axis) at each energy level (horizontal axis). Detectors, algorithms, and CONOPS are the eyes and ears, brains, and hands of nuclear detection: effective detection requires all three. For example, shielding a bomb with lead to attenuate gamma rays would create a large, opaque image on a radiograph. This section discusses nine U.S. technologies selected to include different (1) agencies sponsoring projects (the Defense Threat Reduction Agency (DTRA), an agency of the Department of Defense (DOD); the Domestic Nuclear Detection Office (DNDO), an agency of the Department of Homeland Security (DHS); and the National Nuclear Security Administration (NNSA), an agency of the Department of Energy (DOE)), (2) organizations performing the work (national laboratories, industry, universities, and collaborations between them), (3) types of technology (materials, algorithms, simulation, systems), (4) physical principles (muon tomography, radiography, stimulated emission of radiation, nuclear resonance fluorescence), (5) distances between the detector and the object being scanned (near and far), and (6) levels of maturity (in use for many years but constantly updated, near deployment, and anticipated to be available for deployment in several years). There are two main types of scintillators. Many variables affect detector performance. Operational analysis. In an effort to overcome these limitations, DNDO is conducting R&D on advanced radiography systems under the Cargo Advanced Automated Radiography System (CAARS) program. A different physical principle enables a radiography system to search for materials with high atomic number (Z). (The third CAARS system utilizes a different principle.) It produced its first image in May 2009. Based on tests conducted in early 2010 with its prototype scanner, DSIC estimated in April 2010 that its system would take less than 1 minute to clear most containers with 95% confidence; that it could automatically detect a cube of unshielded SNM 5 cm on a side in a container in that time; and that it could automatically detect that cube inside shielding (e.g., a larger cube) in less time. (5) Because muons are highly penetrating, they can be used to detect high-Z material even when shielded. Such materials are not high Z. NRF may be able to provide different types of data. At issue: Is NRF a useful approach to determining the materials in a weapon? 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. This section considers the Photonuclear Inspection and Threat Assessment System (PITAS), a project funded by DTRA and conducted by Idaho National Laboratory (INL). Chapter 3. Other technologies, such as improved detector material and improved algorithms, also have the potential to improve detection capability. It is difficult to predict the schedule of new detection technologies . It is easier, less costly, and potentially more effective to accelerate a program in R&D than in production. Technology development is dynamic. Concept of operations " (CONOPS) is crucial to the effectiveness of detection systems. Current equipment to detect and identify SNM makes use of two main signatures of this material, opacity for radiography to detect SNM, and gamma-ray emissions for spectroscopy to detect and identify SNM. Congress may wish to address gaps and synergisms in this portfolio. Observations on Technical Progress and Terrorism

Ongoing improvement in U.S. detection capabilities produces uncertainties for terrorists that seem likely to increase over time, adding another layer of deterrence beyond that of the capabilities themselves. However, that beam will also cause other materials to fission, including natural uranium, so emission of a burst of neutrons and gamma rays resulting from interrogation by a high-energy gamma ray beam is a possible, but not a definitive, indicator by itself of the presence of SNM. Detecting Signatures of a Nuclear Weapon or SNM

			Overview: How are signatures gathered, processed, and used? Denser, higher-Z material within a solid object absorbs photons of lower energy and scatters photons of higher energy. Evasion of Detection Technologies

In order to understand the capabilities of detection systems, it is important to know their weaknesses as well as their strengths.
Detection of nuclear weapons and special nuclear material (SNM, plutonium, and certain types of uranium) is crucial to thwarting nuclear proliferation and terrorism and to securing weapons and materials worldwide. Congress has funded a portfolio of detection R&D and acquisition programs, and has mandated inspection at foreign ports of all U.S.-bound cargo containers using two types of detection equipment.
Nuclear weapons contain SNM, which produces suspect signatures that can be detected. It emits radiation, notably gamma rays (high-energy photons) and neutrons. SNM is dense, so it produces a bright image on a radiograph (a picture like a medical x-ray) when x-rays or gamma rays are beamed through a container in which it is hidden. Using lead or other shielding to attenuate gamma rays would make that image larger. Nuclear weapons produce detectable signatures, such as radiation or a noticeable image on a radiograph. Other detection techniques are also available.
Nine technologies illustrate the detection portfolio: (1) A new scintillator material to improve detector performance and lower cost. This project was terminated in January 2010. (2) GADRAS, an application using multiple algorithms to determine the materials in a container by analyzing gamma-ray spectra. If materials are the "eyes and ears" of detectors, algorithms are the "brains." (3) A project to simulate large numbers of experiments to improve detection system performance. (4, 5) Two Cargo Advanced Automated Radiography Systems (CAARS) to detect high-density material based on the principle that it becomes less transparent to photons of higher energy, unlike other material. (6) A third CAARS to detect material with high atomic number (Z, number of protons in an atom's nucleus) based on the principle that Z affects how material scatters photons. This project was terminated in March 2009. (7) A system to generate a 3-D image of the contents of a container based on the principle that Z and density strongly affect the degree to which muons (a subatomic particle) scatter. (8) Nuclear resonance fluorescence imaging to identify materials based on the spectrum of gamma rays a nucleus emits when struck by photons of a specific energy. (9) The Photonuclear Inspection and Threat Assessment System to detect SNM up to 1 km away, unlike other systems that operate at very close range. It would beam high-energy photons at distant targets to stimulate fission in SNM, producing characteristic signatures that may be detected. These technologies are selected not because they are necessarily the "best" in their categories, but rather to show a variety of approaches, in differing stages of maturity, performed by different types of organizations, relying on different physical principles, and covering building blocks (materials, algorithms, models) as well as systems, so as to convey many points on the spectrum of detection technology development.
This analysis leads to several observations for Congress. It is difficult to predict the schedule or capabilities of new detection technologies. It is easier and less costly to accelerate a program in R&D than in production. "Concept of operations" is crucial to detection system effectiveness. Congress may wish to address gaps and synergisms in the technology portfolio. Congress need not depend solely on procedures developed by executive agencies to test detection technologies, but may specify tests an agency is to conduct. Ongoing improvement in detection capabilities produces uncertainties for terrorists that will increase over time, adding deterrence beyond that of the capabilities themselves.
This report will be updated occasionally.