Radiation detection portals

Some of you might have seen the CBP (Customs and Border Patrol) announcement today re: the deployment of radiation detection portals at borders. The idea is to interdict trafficking of nuclear materials (among others) across US borders. If these are effective, border patrol have the ability to intercept nuclear weapons as they are brought across the borders. This would obviously be a good thing.

How might such a detection system work? I'm going to discuss this in very general terms because I have some misgivings on revealing potentially sensitive information. Consider a uranium bomb with, say, 12 kg of weapons-grade uranium and tungsten "tamper" that acts as a radiation shield. In a sense, this is a conservative weapons model (derived from Fetter, et al) - it is more likely that a terrorist group would use a gun-type bomb which would require about 50kg or more of highly-enriched uranium.

Such a bomb would emit neutrons and gamma rays, but the number of emissions observable at a detector may be smaller than the background rate of neutrons/gamma rays coming from cosmic rays, natural radioactivity, etc. So this presents an interesting problem of resolving signal from noise.

How can you make this detection problem easier? One obvious way is to move the detectors closer to the sources. Another is to increase the exposure time. To explain the latter point, consider a source that generates 20 neutrons/sec at the detector. The neutron background is 50/second with a standard deviation of 7/second (assuming a Poisson process with standard deviation equal to half the mean). Now if you see counts per second of 70, 75, 68, 75, 70..., you might notice a trend of 2-sigma events and conclude that there is a neutron source in your field emitting about 20 neutrons/sec. Well, the same goes for 1-sigma events, over a larger number of intervals, since the probabilities are multiplicative: a string of counts such as 59, 61, 64, 60, 59, 63, 56, 58, 60,... for instance, might lead you to conclude that what you're seeing is a smaller but still definite number of counts (perhaps 8-9 neutrons/second) above the background. So, given longer exposure times, it is possible to definitively detect weaker sources of radiation.

The truck or vehicle pulls up to or passes through the portal (a few meters wide) at pedestrian speeds (say 5 mph). This provides proximity and exposure time, aiding detection. Even so, this is a tricky problem, as noted earlier. Further, maximizing detection time is at odds with the goal of increasing throughput by reducing delays.

The above remarks primarily apply to passive detection, which consists of passively measuring gamma/neutron counts and registering counts that exceed a specified threshold. There is also active detection which involves actively probing the contents of a truck or car using gamma rays or x-rays and using the results to infer the presence of nuclear materials (this is conceptually similar to taking an x-ray image). This works quite a bit better, but obviously, since this is an invasive procedure that could affect any humans within the vehicle, this technique is not as popular as passive detection. However, this technique might be feasible at border checkpoints, where it might be feasible to require the passengers to step out of the vehicle for the duration of the inspection.