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Muon detector takes 30-60s and costs $1 million (estimated)

Muons in search for hidden pyramid chambers
PhysOrg.com - Evergreen,VA,USA
... pyramid. The team will be looking for any surplus of muons striking a portion of its detector array compared to the background flux. ...

False positives in nuclear detection

As of March, there are some 10,000 radiation detectors that can false-detect on patients who undergo medical treatment. Can't this be fixed with more frequency selective detectors?

Medical tests can trigger radiation detector's alarm
Consumer Reports (subscription) - USA
... 11, 2001. The number of people undergoing nuclear-medicine procedures has also been growing, increasing the likelihood of tripping a detector. ...

Gamma ray imaging using the Compton effect at LLBL

A week ago, (11/29) I attended a fascinating talk by Dr. Kai Vetter, a researcher at Lawrence Livermore National Lab. He is pioneering a new technique to detect not only the energy of gamma ray photons, but also determine their direction of arrival upon the detector which conventional gamma ray detectors are incapable of. His talk was titled "High-Sensitivity Gamma Ray Imaging for Homeland Security Applications."

Gamma rays are interesting since they are emitted by radioactive materials and therefore can be used to detect the presence of dirty-bombs, improvised nuclear devices, or stolen nuclear weapons. The amount of gamma rays emitted from radioactive materials like highly enriched uranium (HEU) can also be tiny, and these gamma rays are also emitted in small quantities from the environment and from outer space (background). Therefore it is desirable to find better ways of detecting where the gamma rays come from in order to distinguish them from the background and pinpoint their source.

The technique Vetter described is very cool and also very hard but explain and I'm not sure I understand it completely. But I'll try. The gamma rays are photons range in energy from 150keV to 3MeV. The technique involves tracking the position of the photon as it scatters within the detector (Compton scattering) until it is finally absorbed within the detector material (Photoelectric effect). Hence Vetter's technique is called "Compton imaging" and the detector is known as a Compton camera. The distance between scatters is of the order of millimeters and the time measured in picoseconds. Such high-speed tracking of photon scattering has only become feasible due to advances in digital signal processing over the last five years. The photon's location is measured by a rows of detectors where each detector produces different signal signatures based on the particular position in which that the scatter took place. Using this location information, they can  "triangulate" the photon's angle of arrival and generate an image (picture) of where the gamma rays come from.

In comparison, an older technique that tries the to accomplish the same thing is "collimation" in which shields are used to reflect/absorb gamma rays that come from within a small predetermined direction (2-3 degrees). The drawback with collimation is that all the other photons are lost. Compton imaging removes this drawback making the detector more efficient.

Ultimately, Vetter and colleagues are shooting for detection of nuclear materials at 100 meters distance. Since this is still in research, this technique costs several hundred thousand dollars.

How to detect an atomic bomb (4)

Just found what is probably the most direct hit so far on the topic of detecting shielded nuclear weapons--once again by the Fetter crew called "Detecting Nuclear Warheads." The thinking here could probably be generalized to any sort of nuclear weapon or improvised nuclear device.

The paper looks at the tradeoffs of detection distance/time. An intriguing observation it makes is that signals below the background can be detected given enough time. The idea is that the background counts are subject to the central limit theorem (many sources) and so their fluctuations from the mean rate grow proportional to the square root of time. Whereas a single source, however weak, contributes a constant number of counts per second growing steadily over time. After enough time the weak source can be unambiguously detected if the background is well-known. The numbers in this paper are very conservative, and they also discuss how these assumptions are likely to be relaxed in practice.

This and other related references can be found at http://www.puaf.umd.edu/faculty/papers/fetter/publications.htm

Deployments of passive sensors for nuclear and conventional explosives

As part of the US National Nuclear Security Administration annouced that as part of their "Megaports Initiative," the Belgian seaport of Antwerp will be getting radiation sensors.

Separately, Implant Sciences annouced that they recieved an order for their passive detection system to be deployed at Pakistani airports. The handheld system can detect conventional explosives at a distance without the swabbing required by most other systems deployed throughout the world.

How to detect an atomic bomb (3)

Found a few cold-war era papers on detection of nuclear weapons. The first is "Gamma-Ray Measurements of a Soviet Cruise-Missile Warhead" in which an American team of nuclear scientists were asked to board a Soviet cruiser ship and detect the nuclear weapons onboard. They used high-res gamma spectroscopy to detect the weapon at a distance of 4 meters. The second is paper describes the efforts of Soviet scientists to use of neutrons to detect nukes at a distance of tens of meters from a helicopter, called "The Use of Helicopter-Borne Neutron Detectors to Detect Nuclear Warheads in the USSR-US Black Sea Experiment." The paper suggests that neutron emissions from Plutonium-based nukes can be useful for detection. Both these papers are referenced by the NTI writeup on nuclear detection, "A Tutorial on Nuclear Weapons and Nuclear-Explosive Materials."

In addition, there is a report from Los Alamos National Lab called "Nondestructive Assay of Special Nuclear Material for Uranium Fuel-Fabrication Facilities," similar to the report by the British National Physical Laboratory that I blogged earlier. Except, this report provides more quantitative models/estimates of things like self-shielding and the times required to take a measurement using gamma ray spectroscopy. One thing they note here is that unlike Plutonium as discussed in the Soviet report above, passive neutron detection of Uranium is likely to be less effective due to the small number of neutrons emitted.

How to detect an atomic bomb (2)

Buried somewhere in the Nuclear Threat Initiative website is a readable summary called Detecting Nuclear Materials and Nuclear Weapons (see first 10 paragraphs or so). In particular, they discuss the tradeoffs of

1. active versus passive scanning
2. detecting highly enriched uranium (HEU) versus plutonium (Pu) and dirty bombs
3. Nuclear Emergency Search Teams (NEST)

What they say here is consistent with what I have found in other references.

Due to its low level of radioactivity, passive HEU detection appears to be significantly harder than detecting Pu or dirty bombs. The following references discuss specifically how HEU might be detected:

1. Uranium Attributes and Measurements by Lawrence Livermore National Lab
2. Emission and Absorption of Radiation in Science and Global Security.

Active neutron scanning of shipping containers

To detect the presence of concealed nuclear or chemical material/weapons in cargo shipping containers, US national labs and defense contractors are investigating an approach where the container is sent through a scanner that floods the container with high energy neutrons (~14MeV). The energetic neutrons are penetrate almost anything that gets in their way and they will excite shielded contraband material (Uranium, Plutonium, etc.) into emitting detectable high-energy gamma rays (several MeV). Unlike the spontaneous radioactivity, these induced gamma rays are not usually present in the background and can be identified readily by the scanner.

Several articles have appeared in the press about this, but here is a technical feasibility study by Lawrence-Livermore National Labs that answers just about any question you could possibly ask "Detection of special nuclear material in cargo containers using neutron interrogation" (August, 2003). The study draws two interesting conclusions:

1. The radiation produced by the scanning is a potential concern, but it will drop to acceptable levels within minutes.
2. The scans will take about a minute, and they envision only suspect containers will be diverted through the scan after first being screened by passive techniques and traditional inspections.

An article describing the personal story of the dedicated physicists behind this effort appeared in the Daily Californian, Innovative Detection Methods May Help Tighten U.S. Security. The effort began in 2001 immediately after 9/11, and it is in laboratory testing as of mid-2004. They optimistically expect to achieve widespread deployment at ports by 2007.

Books on WMD & terrorism

The following books to help in understanding the nature of nuclear, biological, and chemical threats. In particular, they give you a good sense for how what threats are most likely to materialize based on the type of perpretrator (terrorist, nation-state, individual, etc.). These books also expose what the US government is not doing, but could be doing, to protect us against these threats.

1. Nuclear Terrorism is short and reads very easily, sparing lots of detail to give you the big picture. It is written by the founder of the Kennedy School of Government.
2. The Four Faces of Nuclear Terrorism by the Monterey Institute of International Studies is the most recent, most comprehensive, and has an extensive bibliography. It covers policy recommendations and also includes technical details, such as the amount of U or Pu  needed for a improvised nuclear device (IND). The full text is available online as a pdf, so you don't have to buy it unless you prefer the book format.
3. America's Achilles' Heel, written back in 1998 by another Kennedy School professor (Falkenrath) who was subsequently tapped by Tom Ridge for the newly formed Department of Homeland Security, covers a wide variety of possibilities spanning nuclear, biological, and chemical threats.

Another reference is a report on WMD by the Office of Technology Assessment.

U.S. Congress, Office of Technology Assessment. 1993. Proliferation of Weapons of Mass Destruction: Assessing the Risks. OTA-ISC-559.

Muon detection

Earlier this year, researchers at Los Alamos National Lab discovered a new passive detection technique for heavy elements that works better the more the shielding... their idea involves cosmic muons. Read about it in a BBC science article or the original article in Nature cited below.

These highly energetic particles (GeV range) shower the earth at the rate of 10,000 per sq. meter per minute and mostly pass right through anything they encounter. However, they will scatter just slightly and the scattering angle is highly dependent on the atomic mass. Although it is not specific to nuclear materials, this technique can therefore differentiate between ordinary materials in say a container or truck and heavier materials like Uranium/Plutonium and shielding such as lead . So a property of this approach is that greater or denser the shielding, the greater the angle of scattering. This forms a passive substitute for (active) X-ray scanning that can even be used on live cargo, such as sheep, to detect a concealed cargo.

The original article appearing in Nature (below), which have to have online access to or purchase, shows the shape of shielded matter can reconstructed exhibiting the detection capability and providing formulas on how the scattering works.

Nature 422, 277 (20 March 2003); doi:10.1038/422277a, "Surveillance: Radiographic imaging with cosmic-ray muons"