Radiation Detection Gets Direction

Scientists at Los Alamos recently learned something that they already knew: The ground at the Trinity site in south-central New Mexico is still radioactive. But not terribly so—eating a banana will deliver about as much ionizing radiation as 20 minutes at Trinity, where the first atomic bomb test was conducted. Further, the soil contains two kinds of radiation sources: direct decay products left over from the 1945 blast and elements native to the soil that were induced to radioactivity after capturing free neutrons released by the test. But while the radioactivity itself is not new, the way in which it was measured is.

Los Alamos engineer Jonathan Dowell has invented a suite of novel radiation detectors. Conventional radiation detectors operate on proximity—the closer the source, the stronger the signal—so pinpointing a source is a literal game of “hot-and-cold.” But Dowell’s detectors, which he named “lighthouse detectors” based on their sweeping and scanning field of view, are more sophisticated. They can pinpoint the direction of a radiation source without having to approach it and distinguish between sources when multiple sources are present, offering improvements to both safety and speed of material inventories, geological surveys, or radiological remediation. The survey at Trinity used a HAZMAT robot outfitted with lighthouse detectors and was a successful demonstration of how quickly large areas can be surveyed without sending in any people.

A self-described Ozark mountain hillbilly, Dowell was a teenaged ham radio operator. In 1989 he came to northern New Mexico for the skiing, stayed for the love of a lady, and in the meantime built an impressive engineering career at Los Alamos. In 2012, while surveying a contaminated glovebox, Dowell, being familiar with the directional nature of radio antennae, wished for a similarly directional radiation detector to pinpoint exactly which part of the glovebox was hottest. No such detector existed, so he invented one.

Similar devices block the radiation on all sides except one. But fully blocking gamma rays or fast neutrons requires a lot of bulky shielding material. What Dowell did differently, to keep his detectors small and agile, was to focus on attenuation, or reducing the signal, rather than trying to block it completely. Dowell likens it to a car with tinted windows—sunlight still enters through the rear and side windows but is attenuated, while the sunlight entering through the untinted windshield is not. Attenuated signals can be compared to unattenuated signals either through space—multiple detectors in different orientations to the source—or through time—one detector with a rotating field of view that intermittently points toward a source.

The gamma-ray lighthouse detectors consist of solid scintillator crystals surrounded on all but one side by attenuating tungsten plates. The fast-neutron lighthouse detectors consist of long narrow tubes filled with helium-3 gas and partially wrapped with an attenuating custom boron-carbide ceramic. At the back of each sits the electronics package, which is basically a small computer, including a power supply, signal processer, spectrometer, and web server, with both USB and Ethernet connectivity.

“The means by which we detect radiation is not new technology,” Dowell explains. “We advanced the engineering mainly through custom electronics. That’s how we got the detectors to be so versatile and portable.”

After proving the efficacy of his prototypes in 2012, Dowell and Los Alamos teamed up with several industrial partners to miniaturize and refine the physical designs. In 2015 the team demonstrated undersea capabilities of lighthouse detectors arrayed aboard a remotely operated submarine vehicle. Then, after further miniaturization—in three months the electronics package went from the size of a lunch box to half the size of a business card—the detectors were ready for mass production, enabling a myriad of remote field capabilities.

The safety benefit of lighthouse detectors for automated mapping of complex sites is twofold. First, the time and amount of exposure are minimized because the detectors can quickly zero-in on the location of a source. Second, dangerous tasks—like entering a site after an event, which can include physical instability, cumbersome maneuvering in radiation suits, and eventual fatigue—can be exchanged for less dangerous tasks, like sitting in a control booth a safe and comfortable distance away, controlling a HAZMAT robot carrying lighthouse detectors.

In keeping with the noble job of their namesake, lighthouse detectors cast their gaze into the darkness, helping to keep people out of harm’s way.