In 1956, the father of the atomic bomb, J. Robert Oppenheimer, suggested to Congress a reliable means of detecting nuclear weapons within a suitcase destined to be detonated in an American city. It was a screwdriver. Prying open and inspecting each and every case or container capable of concealing a nuclear weapon is obviously an impossible task, which was precisely Oppenheimer’s point; that nuclear weapons are hard to detect. They remain so today.

Fortunately, science and technology have improved since Oppenheimer’s day, but could authorities find a nuclear weapon smuggled into an American city today? Maybe, though existing technologies are only marginally better than Oppenheimer’s screwdriver. 

What Makes Nuclear Weapons Hard to Find?

To find a nuclear weapon, radiation detectors are typically used to measure radiation emissions. The problem is that these detectors are rarely able to measure long enough, close enough, and without shielding in the way. This is especially true in city environments. Let’s look at a scenario in which a nuclear weapon is in a vehicle driving down a street. Scanning vehicles with detectors positioned on the side of roads or on overpasses only gives a couple seconds of measurement per vehicle and the cargo surrounding the weapon as well the vehicle itself provides some shielding.

A majority of the current technologies deployed today are for finding radiological sources by gamma ray detection. They primarily consist of detectors the size of cell phones attached to a police officer’s belt. Unfortunately, these are not so effective for finding a nuclear weapon, unless you were standing next to one. Why? Compared to a radiological source that may be used in a terrorist attack, nuclear weapons emit only a small number of gamma rays that can be overwhelmed by background radiation. The world consists of a sea of background radiation that includes mostly gamma rays and some neutrons. Detectors are not so immune and register this background as potential signals that can make finding true signals, those from a nuclear weapon, difficult to spot. 

Another issue with these gamma-ray detectors is naturally occurring radioactive materials, or NORM. NORM is everywhere, such as gamma-rays emitting from radionuclides in your banana for breakfast or in your cat’s kitty litter. In larger quantities, NORM can trigger false alarms that might, for example, have police wasting their time and resources chasing down something no more dangerous than a truckload of bananas.

Is There Any Hope of Finding a Nuclear Weapon in Our Cities?

Maybe, and neutron detectors can help. Searching for nuclear weapons by neutron detection has two main advantages. First, background neutron radiation is relatively low. This makes the problem of distinguishing true signals from background signals easier. Second, there exists little significant neutron emitting NORM. This means no wild goose chases for truckloads of kitty litter. However, there remains a major issue using neutron detectors for finding nuclear weapons – they can only find some of them. While weapons-grade plutonium emits a hardy amount of neutrons that can make it detectable at distances up to 25 yards away, weapons grade uranium emits about 50,000 times less neutrons, making it much harder to detect.

This also assumes that the terrorist did not elaborately shield the nuclear weapon. If this shielding were of appropriate materials and thickness, it could absorb a majority of the radiation emitted from the weapon. Without radiation from the weapon, any radiation detector is blind to the weapon. To deal with this problem, “active” detector systems are at the forefront of nuclear detection research. The concept of active detector systems is similar to the use of x-ray machines at the airport, where an external radiation source is used to peer into the inner contents of luggage. Currently however, the size, expense, and the fact that significant amounts of external radiation can be harmful to living beings has limited the concept’s development primarily to shipping ports. 

So, is science and technology at the point of reliably finding a nuclear weapon in our cities? Only partly – for some weapons and some adversaries. For those cases there is hope.

Realizing the Hope

Much like technology solutions to other difficult problems, the solutions for this problem are only partial and complicated by the nuances of the science. However, the technology solutions still have value. The next step is to construct a network of neutron detectors and actually use them. Only that can show how well they really work and how local governments can operate them. Lessons can be learned, from how to interdict a potential threat to how different government agencies at various levels can cooperate. The construction of a detector network would also provide information for future improvements to the detectors and how they may be optimally deployed and utilized.

At the same time the United States must continue to invest in other methods of interdicting a nuclear weapon. This would close the gaps and develop other barriers to nuclear weapons proliferation. These include activities like intelligence gathering and police work, and reducing the number of weapons and quantities of weapons grade nuclear materials, There will not be a single comprehensive technology or policy but rather a patchwork of technologies and policies that may provide a workable solution. Together, these investments should steadily make America’s cities harder to attack.

Edward Cazalas is a former Stanton Nuclear Postdoctoral Fellow at RAND Corporation with research interests in the technical and policy related aspects of the detection and interdiction of nuclear and radiological materials in the context of homeland security and countering nuclear terrorism. Edward holds a Ph.D. in Nuclear Engineering from the Pennsylvania State University, where he aided the development of radiation detection technologies and scientific concepts at the intersection of nuclear physics and material science for and micro- and nano-sensors. He also holds an M.S. in Nuclear Engineering from Oregon State University, where he studied the dosimetry of nuclear and radiological exposure events and processes.

Image: National Nuclear Security Administration via Wikimedia