Long before September 11, engineers at Lawrence Berkeley National Laboratory,
in collaboration with researchers at Lawrence Livermore and Los Alamos
National Laboratories, have been working to outsmart terrorists attempting
to smuggle radioactive material into the country.
Their solution is Cryo3, a 10-pound, battery-powered detector that promises
to bring state-of-the-art radiation spectrometry anywhere radioactive
materials are found.
"The innovation is coupling a germanium radiation detector with
a small, low-power cryogenic cooling mechanism originally designed for
the aerospace industry," says Lorenzo Fabris of Berkeley Lab's Engineering
Division. "This offers extremely high-resolution radiation analysis
in a portable package."
The need for a hand-held radiation detector was first born from a necessity
to monitor nuclear weapon stockpiles to ensure nations adhered to treaty
obligations. An even more pressing need surfaced after the dissolution
of the Soviet Union, when national security experts worried the former
superpower's nuclear arsenal could spark a black market in fissile materials.
In the wrong hands, these isotopes could be used to build both nuclear
bombs and conventional bombs laden with radioactive material a so-called
dirty bomb. And rather than being delivered via intercontinental missiles,
contraband isotopes can be hidden in backpacks and car trunks, meaning
airports, border checkpoints, and shipping terminals provide the last
best chance to thwart smuggling.
To complicate matters, any tool used to screen for isotopes in busy terminals
must detect not only the presence of radiation, but also the type. A terrorist
could mask radioactive material destined for a dirty bomb in a seemingly
benign package of medical isotopes, and therefore sneak past a Geiger
counter.
That's where the Cryo3 comes in. At the heart of the unit is a high purity
germanium crystal. Energetic photons, X and gamma rays, interact in the
germanium crystal to create a corresponding charge. When further processed,
this charge depicts both the quantity and type of radioactive isotope
present. Although germanium offers higher radiation resolution than other
semiconductor detectors, such as silicon and cadmium telluride, it must
be deeply cooled, traditionally with liquid nitrogen. And although liquid
nitrogen is very common in the laboratory, it is awkward to transport,
store, and handle in the field.
To sidestep this limitation, Berkeley Lab engineers coupled the germanium
crystal to an off-the-shelf mechanical cooling device currently used to
cool low-noise cell phone antennae. The device, which utilizes the Sterling
cycle to reach low temperatures, only requires 15 watts to cool the germanium
to 87 degrees Kelvin. When the cryogenic mechanical cooler is vacuum sealed
to a germanium detector, the result is a lightweight, highly sensitive
radiation detector that operates up to six hours on two rechargeable camcorder
batteries.
The mechanical cooler requires 16 hours to cool the detector from room
temperature to operating temperature, but because the batteries are hot
swappable, a fresh supply guarantees unlimited operational time.
In the field, the solid-state detector performs much like its lab-based
cousins. Incident photons are absorbed by the germanium and converted
into electrical signals at a resolution of 3.5 keV at an incident energy
of 662 keV.
To keep the system portable and low power without sacrificing resolution,
Fabris and colleagues made additional refinements. Borrowing from lessons
learned in satellite-based germanium detector applications, they protected
the delicate crystal in a hermetically sealed, nitrogen-filled capsule.
The encapsulated germanium detector is suspended with Kevlar fibers in
a close-fitting utility vacuum chamber.
Another obstacle was electronic noise, a byproduct of all electrical
systems that is particularly troublesome in radiation detectors because
it degrades the electronic readout's depiction of the absorbed radiation.
In short, electronic noise softens the readout's sharp spikes into rounded
hills, meaning valuable data is lost. Fabris turned to a specially designed
small, low-power preamplifier that minimizes electronic noise without
sapping battery power a critical component, given that conventional
preamplifiers are too power-hungry to be used in a battery-powered device.
So far, Fabris and colleagues have developed detectors of modest size,
or so-called 25 percent efficient detectors. In the future, they hope
to increase the detector size and therefore the efficiency to 50 and even
100 percent by using modified mechanical coolers that only cool to 105
degrees Kelvin, a temperature still within germanium's operating parameters.
The modified mechanical coolers have almost twice the heat lift for the
same input power when compared to the conventional mechanical cooler.
Ultimately, Fabris foresees a time when next-generation iterations of
Cryo3 safeguard the nation with lab-quality, portable radiation detection
and characterization.
"Whatever you can detect with a germanium crystal, you can detect
with the portable system," says Fabris. "Ideally, we would be
able to place one at any customs port."
|