I n any assessment of potential terrorist attacks, the nuclear threat takes center stage. Although weapons-grade nuclear materials are heavily guarded, a plausible scenario involves terrorists detonating a simple radiological dispersion device (RDD) capable of broadcasting nonfissile but highly radioactive particles over a densely populated area. In most cases, a motor vehicle would have to transport the device and its payload-commonly known as a "dirty bomb"-to the target destination. As a final defense against such a weapon, select traffic choke points in the US have large portal monitoring systems to help detect illicit isotopes. The Distributed Sensor Network project at Los Alamos National Laboratory, in cooperation with the University of New Mexico, is developing a network of radiation detectors that, coupled with other sensors that collect supportive data, is suitable for RDD interdiction in either urban or rural environments. Compared to a portal monitor, a DSN is much less visible, uses less power per detector, is hand carried and thus more rapidly deployable, and simplifies coverage of multiple transport avenues. Also, to function effectively, portal monitoring systems typically require slow or halted traffic, whereas our DSN can be tailored for any moderate traffic speed. Expanding on earlier work suggesting that bigger is not necessarily better in radiation detectors, 1,2 our project seeks to provide a flexible, discreet radiation detection solution that enhances not just national security but also global nonproliferation. Our model DSN consists of arrays of 75-mm sodium iodide (NaI) scintillators directly connected to PDA-sized platforms that provide in situ processing of raw gamma counts. In situ processing eliminates a single point of failure and can potentially weed out faulty measurements. We chose PDAs because they have the processing capability to cope with more complex algorithmic requirements in the future, and a PDA is usually smaller than the radiation detection equipment to which it attaches. Also, PDAs with a general-purpose OS-in this case, Linux-can use familiar and well-tested software tools to manipulate and communicate data. On each side of a typical two-lane road, 6 to 7 meters wide, we deploy an independently operating array of detectors. The detectors are several meters apart and well away from the roadway, so that the two arrays are approximately 10 meters from each other. In the forward and rear positions and interspersed among the radiation detectors are simpler nodes, such as Crossbow's MICA2 mote, that use accelerometers, magnetometers, and similar sensors to directly detect and track vehicles through the DSN operational space. To compensate for the smaller detectors' reduced efficiency and source interaction time, the system combines gamma counts across the detection array and coordinates this data with the radioactive source's motion. This coherent signal addition method uses an integration window that follows the source as it moves past each detector in the array. During algorithm development, 3 we discovered that when this window length matches detector spacing and expected time lag, increasing the number of detectors also increases the signal-tonoise ratio (SNR) along a √n curve. In the absence of traffic, the system collects background radiation measurements, compiles these as a mean and standard deviation to produce a baseline threshold, and updates its statistical noise model. When a vehicle approaches the DSN, the forward sensors cue the detection apparatus. These forward units typically detect either seismic vibrations (using an accelerometer) or variations in the local magnetic field (using a magnetometer) and broadcast a time-stamped report when the source exceeds threshold values. This chatter rapidly propagates to the first radiation detector, which informs its neighbors of an oncoming target. The motes use a separate radio frequency from the PDAs, but one or more PDAs can listen in via an attached mote gateway. Using the mote time stamp, the first radiation detector records gamma counts while the vehicle is within a designated interaction range. For example, the detector can begin counting when the vehicle is 10 meters from its closest approach and cease when it has receded 10 meters. At this moment, the next detector begins
In our own previous work, the Distributed Sensor Network project at LANL looked at the feasibility of a small network of sensor nodes using PDA-sized platforms for processing raw gamma counts REF .
Radiation detection with distributed sensor networks
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Computer Science