In 2007, the U.S. government set itself the goal of screening all aviation cargo loaded onto passenger planes and all maritime cargo entering the country for both explosives and nuclear materials. And to set up a system to do this within five years.

That’s a big ask given that the maritime traffic alone amounts to more than six million cargo containers per year.

And that’s before you get to the sheer technical difficulty of reliably spotting every kind of explosive along with all nuclear materials with a minimal percentage of false positives.

Today, Mark Goldberg at the Soreq Nuclear Research Center in Israel and a number of pals, mainly in Germany, outline plans for a single machine that they say could do the job.

The team propose using a particle accelerator to alternately smash ionised hydrogen molecules and deuterium ions into targets of carbon and boron respectively. The collisions produce beams of gamma rays of various energies as well as neutrons. These beams are then passed through the cargo.

By measuring the way the beams are absorbed, Goldberg and company say they can work out whether the cargo contains explosives or nuclear materials. And they say they can do it at the rate of 20 containers per hour.

That’s an ambitious goal that presents numerous challenges.

For example, the beam currents will provide relatively sparse data so the team will have to employ a technique called few-view tomography to fill in the gaps. It will also mean that each container will have to be zapped several times. That may not be entirely desirable for certain types of goods such as food and equipment with delicate electronics.

Just how beams of gamma rays and neutrons affect these kinds of goods is something that will have to be determined.

Then there is the question of false positives. One advantage of a machine like this is that it has several scanning modes is that if one reveals something suspicious, it can switch to another to look in more detail. That should build up a decent picture of the cargo’s contents and reduce false positives.

But some compromises will have to be accepted. For example the machine will be able to reliably distinguish uranium and plutonium from certain heavy metals such as lead and mercury but not from others such as tungsten or gold. Goldberg and co say this is a reasonable trade off since it would be in customs’ interests to detect these metals if they had been undeclared.

Finally, there is the issue of reliability and maintenance. This is not a simple device. Until now, particle accelerators have only been operated reliably in a few laboratories with highly trained staff. Making such a device reliable enough to operate in a real world environment may be the most difficult task of all.

Provided that the U.S. government is willing to pay, of course. The U.S. has 300 ports and 5,119 paved airports. Suppose it orders 1,000 of these machines at $5m each ( a highly conservative estimate), that comes to a total of $5 billion. And that’s before operating costs come into play.

One thing is for sure: even if the government decides that this is the machine for the task, Goldberg and co have yet to build one. Today’s paper just outlines the plan. That makes the 2012 deadline almost impossible to meet.

Ref:arxiv.org/abs/1001.3255: A Dual-Purpose Ion-Accelerator for Nuclear-Reaction-Based Explosives-and SNM-Detection in Massive Cargo