Detection of Nuclear Weapons and Materials: Science, Technologies, Observations

Jonathan Medalia
Specialist in Nuclear Weapons Policy

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 highenergy 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.


Date of Report: June 4, 2010
Number of Pages: 112
Order Number: R40154
Price: $29.95

Document available via e-mail as a pdf file or in paper form.
To order, e-mail Penny Hill Press or call us at 301-253-0881. Provide a Visa, MasterCard, American Express, or Discover card number, expiration date, and name on the card. Indicate whether you want e-mail or postal delivery. Phone orders are preferred and receive priority processing.