|Fig. 3: A visual diagram of the basic idea behind Muon Tomography.  (Courtesy of the U.S. Department of Energy)|
The world is dawning a new era in emerging nuclear capable nations and entities. While the threat of nuclear weapons and war across seas is a highly political matter, the Deprtment of Homeland Security is making efforts in facing the problem of eliminating nuclear threat on american soil. In 2005, the Domestic Nuclear Detection Office (DNDO) and the National Science Foundation (NSF) funded several universities and national laboratories around the United States to begin innovating new methods of nuclear detection with a multi million dollar dispersement through their Academic Research Initiative (ARI). 
The prevention of smuggled nuclear materials into the United States is of the utmost concern. This leaves cargo checkpoints and border crossings with the tremendous responsibility of having to ensure the credibility of any cargo entering the US. While detection techniques are already being employed, they are not without problems; missing detections of passive radiation from shielded nuclear material, short detection distance, and the use of a radiation source which in the instrumentation that distorts readings. 
One of the latest innovation in nuclear detection being developed is Muon Tomography (MT). MT is a techinique in development geared towards detecting nuclear material in vehicles, cargo freight and general mobile containers crossing security checkpoints.
MT utilizes incoming galatic muons, which are highly energetic, and their observed flight deflection from cargo collisions to determine the threat of nuclear materials. As muons pass through matter they are deflected according to the density and the atomic number, Z, of the matter it collides with. The collision of muons with high density, high-Z matter provides a characteristic deflection that is highly discernable to that of non-nuclear materials. Current MT methodology uses this fact by arranging a fully encompassing tunnel of gas filled tubes (drift tubes) that will enclose a vehicle under inspection. When a muon passes through the tube, an electron-drift effect sends a signal to a data acquisition system which records a timestamp and location of the muon. A muon's trajectory can then be tracked and the degree of deflection can be found to determine if nuclear material is present. 
As mentioned before, MT uses naturally occuring galatic muons as the main source for detecting any nuclear materials. Past methods and detectors have been using an actual radioactive material which involves a certain health risk and danger to those operating the device, or even the cargo in the container being analyzed. [2,3] MT eliminates this concern. In addition, data resolution is greatly improved and the event of false-positives is greatly reduced. Also, the detecion tunnel of a MT detector would only have to take up a little area a typical traffic lane would. This is ever important in creating an efficient and realistic process at border crossings and security checkpoints. But by far, the most appealing aspect of the MT detector is that it can detect nuclear materials regardless of any shielding used. [2,3] This is due to the fact that muons are high energy and highly penetrating particles (mouns travel near the speed of light and can penetrate approximately 2m of lead). 
The Department of Homeland Security is making progress towards improving the efficiency and success of detecting nucear material on American soil. MT technology is among one of the many interesting innovations that are making their way into the developing nuclear related technologies of today.
© Donish Khan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 C. Burress, "UC Berkeley Team to Study Nuclear Detection For Homeland Security," San Francisco Chronicle, 4 Sep 07.
 J. Medalia, "Detection of Nuclear Weapons and Materials: Science, Technologies, Observations," CRS Report for Congress R40154, 4 Jun 10.
 B. Fishbine, "Muon Radiography: Detecting Nuclear Contraband," Los Alamos Research Quarterly, Spring 2003, p. 13.