National security experts believe terrorists continue to be interested in such devices for terror plots. Now researchers have proposed a new technique remotely to detect the radioactive materials in dirty bombs or other sources. It is the increased ion density that the researchers aim to detect with their new method. They calculate that a low-power laser aimed near the radioactive material could free electrons from the oxygen ions.
In 2004 British national Dhiren Barot was arrested for
conspiring to commit a public nuisance by the use of radioactive
materials, among other charges. Authorities claimed that Barot had
researched the production of “dirty bombs,” and planned to detonate them
in New York City, Washington D.C., and other cities. A dirty bomb
combines conventional explosives with radioactive material.
Although Barot did not build the bombs, national security experts
believe terrorists continue to be interested in such devices for terror
plots. AIP reports
that now researchers from the University of Maryland have proposed a
new technique remotely to detect the radioactive materials in dirty
bombs or other sources. They describe the method in a paper in the
journal Physics of Plasmas, from AIP Publishing.
While the explosion of a dirty bomb would likely cause more damage
than the radioactive substances it spreads, the bombs could create fear
and panic, contaminate property, and require potentially costly cleanup,
according to the U.S. Nuclear Regulatory Commission.
Radioactive materials are routinely used at hospitals for diagnosing
and treating diseases, at construction sites for inspecting welding
seams, and in research facilities. Cobalt-60, for example, is used to
sterilize medical equipment, produce radiation for cancer treatment, and
preserve food, among many other applications. In 2013 thieves in Mexico
stole a shipment of cobalt-60 pellets used in hospital radiotherapy
machines, although the shipment was later recovered intact.
Cobalt-60 and many other radioactive elements emit highly energetic
gamma rays when they decay. The gamma rays strip electrons from the
molecules in the surrounding air, and the resulting free electrons lose
energy and readily attach to oxygen molecules to create elevated levels
of negatively charged oxygen ions around the radioactive materials.
It is the increased ion density that the University of Maryland
researchers aim to detect with their new method. They calculate that a
low-power laser aimed near the radioactive material could free electrons
from the oxygen ions. A second, high-power laser could energize the
electrons and start a cascading breakdown of the air. When the breakdown
process reaches a certain critical point, the high-power laser light is
reflected back. The more radioactive material in the vicinity, the more
quickly the critical point is reached.
“We calculate we could easily detect ten
milligrams [of cobalt-60] with a laser aimed within half a meter from an
unshielded source, which is a fraction of what might go into a dirty
bomb” said Joshua Isaacs, first author on the paper and a graduate
student working with University of Maryland physics and engineering
professors Phillip Sprangle and Howard Milchberg. Lead could shield
radioactive substances, but most ordinary materials like walls or glass
do not stop gamma rays.
UMD notes that the lasers themselves could
be located up to a few hundred meters away from the radioactive source,
Isaacs said, as long as line-of-sight was maintained and the air was not
too turbulent or polluted with aerosols. He estimated that the entire
device, when built, could be transported by truck through city streets
or past shipping containers in ports. It could also help police or
security officials detect radiation without being too close to a
potentially dangerous gamma ray emitter.
The proposed remote radiation detection method is not the first, but
it has advantages over other approaches. For example, terahertz
radiation has also been proposed as a way to breakdown air in the
vicinity of radioactive materials, but producing terahertz radiation
requires complicated and costly equipment. Another proposed method would
use a high-power infrared laser to both strip electrons and break down
the air, but the method requires the detector be located in the opposite
direction of the laser, which would make it impractical to create a
single, mobile device.
So far the researchers at the University of Maryland have analyzed
the feasibility of the new approach and experiments are underway to test
it in the lab.
Isaacs said it would be difficult to estimate when a detection device
based on the new method might be commercialized, but he didn’t foresee a
specific manufacturing challenge that would stand in its way.
“We specifically chose well developed technology for each component of the proposed system,” he said.