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 US.
Their solution is Cryo3, a 10-pound, battery-powered detector that promises to bring advanced 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,’ said Lorenzo Fabris of Berkeley Lab’s Engineering Division. ‘This offers extremely high-resolution radiation analysis in a portable package.’
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 reportedly 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 overcome 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 is said to utilise 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 fibres 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. Fabris turned to a specially designed small, low-power preamplifier that minimises electronic noise without sapping battery power.
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. The modified mechanical coolers have almost twice the heat lift for the same input power when compared to the conventional mechanical cooler.