a hermetically sealed germanium gamma-ray detector is coupled
to a small, low-power cooler that is available commercially.
The unit weighs 4.5 kilograms and can operate for 7 to 8 hours
on two rechargeable lithium-ion batteries.
UNTIL recently, the U.S. relied
on large oceans and friendly bordering countries to provide security
against a terrorist attack. It was believed that an attack would
most likely arrive in the form of missiles launched from land,
terrorist threat now lies much closer to home. Experts believe
that a possible method of weapon delivery will be a suitcase concealing
contraband radioisotopes hidden in a car or plane’s luggage
compartment. Or a seemingly harmless shipment of medical or industrial
radioisotopes could mask potent radioisotopes destined for a dirty
bomb—an ordinary explosive laced with radioactive material.
To counter such threats, security agencies are looking for a new
generation of portable radiation detection devices that will allow
military, law-enforcement, public-health, and medical personnel
to easily and quickly identify radioactive materials and distinguish
that detect x and gamma rays have been available for a few decades.
However, precision energy-resolution detection devices,
which unambiguously identify radioisotopes, are large and power-intensive
and require a consumable liquid cryogen, all of which make them
difficult to use in the field. A Lawrence Livermore team has developed
a portable, handheld germanium radiation detector called CryoFree/25
that can duplicate the energy resolution and efficiency of a laboratory
gamma-ray spectrometer. Physicist John Becker and engineers Norman
Madden, Lorenzo Fabris, and Chris Cork are collaborating on the
project. Begun in 1998, the project is sponsored by the Department
of Energy’s Office of Nonproliferation Research and Engineering,
which is part of the Office of Defense Nuclear Nonproliferation.
team’s goal,” says Madden, “was to create
the smallest possible handheld, mechanically cooled gamma-ray spectrometer
with the largest amount of germanium. The more germanium, the higher
the detection efficiency.” As part of Livermore’s Measurements
Science Team, originally formed at Lawrence Berkeley National Laboratory
and now part of the Physics and Applied Technologies Directorate,
the team had previously developed technology for a radiation detector
that is onboard a NASA spacecraft. Becker thought it would be worthwhile
to explore terrestrial applications for the radiation detector.
gamma-ray photons illuminate the CryoFree/25 detector, their
energy is converted to electrical signals that can be measured
and recorded. Every radioisotope has a unique signature. Shown
are the combined characteristic fingerprints of americium-241,
cobalt-60, and cesium-137 radioisotopes.
Powerful and Lightweight
under the name Cryo3 but now dubbed CryoFree/25, this technology
makes a quantum leap forward in portable
The detector system features a handheld, gamma-ray spectrometer
and book-size auxiliary equipment, such as a portable computer,
power supply, and conditioning units. The gamma-ray spectrometer
nearly replicates the precision energy resolution found in the
larger, less-portable laboratory units used for unambiguous radioisotope
unit weighs less than 4.5 kilograms and can run for 7 to 8 hours
on two rechargeable lithium-ion batteries.
CryoFree/25 operates on only 16 watts of
dc power and can operate continuously for more than 6 months before
the unit’s cooling mechanism needs a short recycling period. “The
beauty of CryoFree/25 is that the device can deliver the level
of gamma-ray energy resolution associated with laboratory germanium
spectrometers in a portable, lightweight, germanium detector without
liquid nitrogen and with long field life,” says Becker.
has long been the detector material of choice for precision gamma-ray
spectroscopy. Compared with other semiconductor materials used in detectors,
such as silicon or cadmium telluride, germanium provides better detection efficiency,
line-shape characteristics, and precision energy resolution, which are needed
to produce the detailed x- or gamma-ray spectra for identifying radioactive materials.
The germanium crystals must be cooled to approximately 90 kelvins (about –300°F)
to operate. Liquid nitrogen has been the cryogen of choice, but more than 10
liters per week of liquid nitrogen are required to cool an average laboratory-size
detector. This cooling requirement makes standard detectors awkward to transport,
store, and handle in the field. Access to liquid nitrogen is a requirement for
Becker’s team overcame the size and access problems by joining the germanium
crystal to a commercially available mechanical device commonly used to cool low-noise
cell-phone antennas. The device, originally designed for the aerospace industry,
requires only 12 watts to cool the germanium. “Our innovation is coupling
a germanium radiation detector with a small, low-power cryogenic cooling mechanism,” explains
Madden. “This combination offers extremely high-resolution gamma-radiation
analysis in a portable package.”
As gamma-ray photons
interact with the germanium in CryoFree/25, their energy is converted into charges
that can be measured and recorded. Every radioisotope
has a unique gamma-ray signature, which can be inferred by measuring the magnitude
of the charge created in the detector and processed by the detector’s electronics.
Unlike many other detectors, CryoFree/25 can identify all of the gamma rays originating
from the known radioisotopes.
|The germanium detector
is hermetically encapsulated within a thin layer of aluminum
that contains a nitrogen blanket. An external metal “getter” and
a miniature ion pump are used to maintain the utility vacuum.
Adaptable for Multiple Applications
“CryoFree/25 is a forensics science tool for gamma-ray detection,” says
Madden. “The resolution of the CryoFree/25 is so precise that it provides
a unique fingerprint of the sample. For example, the detector can reveal not
what radioisotope is present but also the history of the radioisotope—whether,
for instance, it came from a spent fuel rod or another source and how long ago
the chemical alteration to the radioisotope occurred. The device might even reveal
approximately where the alteration occurred.”
In addition to finding
a cooling mechanism for the portable unit, the Livermore team added signal-processing
electronics to minimize electronic noise that would
otherwise obscure the detector’s energy resolution. A readout that shows
detector pulse height with sharp, defined peaks accurately depicts the types
and amounts of radiation present.
The Livermore team
is working with the Coast Guard to adapt the unit for use
on shipping vessels. “The Coast Guard needs the least obtrusive, most rugged
and lightweight unit with proven reliability,” says Madden. CyroFree/25’s
small size makes it particularly attractive for applications where small spaces
may obscure field use of other detectors, such as at border crossings, airport
terminals, and cargo ports.
The group is applying
the same detector technology used in CryoFree/25 for space applications. Last
year, an expanded team from Lawrence Livermore and the Space
Science Laboratory at the University of California at Berkeley, delivered a flight
detector to Johns Hopkins Applied Physics Laboratory for research on a NASA spacecraft
designed to retrieve radioisotope data on the geochemistry of Mercury. CryoFree/25
technology may also prove valuable for the Department of Energy’s work
in monitoring the nation’s nuclear weapons stockpile and protecting the
weapons and nuclear energy facilities from terrorists.
The team is hopeful
that government agencies will take advantage of the portable
unit’s technology, which can be adapted to many applications. In emergencies,
where time and power outages may limit the ability of laboratories to process
radiation samples, CryoFree/25 provides the unambiguous radioisotope identification
that is required to protect vital resources.
Key Words: counterterrorism, CryoFree/25, portable germanium radiation
For further information contact John Becker (925) 422-9676
(email@example.com) or Norm
Madden (925) 423-1934
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