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Physics for Safer Ports

New technology uses nuclear 'fingerprints' to scan cargo ships

The Port of Savannah is the fourth largest container port in the United States, importing hundreds of large metal boxes from cargo ships shown here. Credit: Georgia Department of Economic Development.
The Port of Savannah is the fourth largest container port in the United States, importing hundreds of large metal boxes from cargo ships shown here. Credit: Georgia Department of Economic Development.

While 700 million travelers
undergo TSA's intrusive scans and pat-downs each year, 11 million cargo
containers enter American ports with little screening at all. And the volume of
those containers, roughly equivalent to 590 Empire State Buildings of cargo,
could contain something even worse than box knives or exploding shoes, namely nuclear

Two teams of North
Carolina physicists are mapping the intricacies of the atomic nucleus, which
could provide better security at the ports. The scientists have identified new "fingerprints"
of nuclear materials, such as uranium and plutonium. The fingerprints would be
used in new cargo scanners to accurately and efficiently identify suspicious
materials. The physics might also be used to improve analysis of spent nuclear
fuel rods, which are a potential source of bomb-making materials.

The problem starts
at ports, where terrorists may try to smuggle an entire dirty bomb or even
smaller amounts of plutonium or uranium by hiding it within the mountains of
cargo that pass into the country each day. Cargo scanners using the new nuclear
fingerprints would be sensitive enough to spot an entire bomb or the smaller
parts to build one, according to Mohammad Ahmed, a nuclear physicist at Duke

Ahmed and his
colleagues are developing the fingerprints for the next-generation detectors
with HIGS, the High Intensity Gamma-Ray Source. It is the world's most intense
and tunable source of polarized gamma rays and is located on Duke's campus as
part of the Triangle Universities Nuclear Laboratory. HIGS produces gamma rays that
are guided to collide with target materials, causing a variety of nuclear

neutron fingerprints
In the reaction
Ahmed and his Duke colleagues study, the collision creates a spray of particles,
which fly into a group of detectors. The detectors count the number of neutrons
knocked from the atomic nuclei of the target material in either a parallel or
perpendicular direction, compared to the polarization plane of the gamma-ray
beam. Dividing the number of neutrons emitted parallel to the plane by the
number emitted perpendicular is distinct to each material, giving it a unique fingerprint.

Ahmed said these fingerprints
could eventually be used to distinguish special nuclear materials, like
weapons-grade uranium, from naturally occurring uranium or ordinary objects
such as clothing or granite countertops, distinctions that current port
scanners cannot make.

In a separate but related
project, nuclear physicists from three North Carolina universities are slamming
the HIGS beam into atomic nuclei and observing the energy pattern and
distribution of the gamma rays that fluoresce back out of the collision. Each
material has a distinct fluorescence pattern based on its nuclear structure,
according to physicist Calvin Howell, who leads the Duke group.

Howell and his
collaborators are studying the fluorescence patterns of potentially dangerous
nuclear materials and non-nuclear contraband such as explosives and drugs. They
are also identifying the patterns of steel and lead because terrorists can use the
metals to conceal and ship weapon-making materials.

The two
anti-terrorism projects were developed with the support of the Department of
Homeland Security's Domestic Nuclear Detection Office, or DNDO. The agency awarded
Ahmed, Duke physicist Henry Weller and their colleagues a $2 million grant, while Howell and his collaborators
received grants totaling $2 million. DNDO is funding both projects in response to
the SAFE (Security and Accountability For Every) Port Act of 2006, which
requires security agents to scan for nuclear materials in all of the containers
entering the United States through the nation's 22 busiest ports.

Five years after Congress
and the president approved the legislation, the equipment to satisfy this
mandate still doesn't exist. Meanwhile, the United States transfers about 20
percent of the world's freight across its borders and has more than 300
maritime ports for sea containers and an additional 300 access points, such as border
crossings, where dangerous materials might enter the country.

The Duke scientists
say their use of polarized gamma-ray beams could one day help satisfy the SAFE
policy, and they are building the fingerprint library to make it happen.

The HIGS data show,
for example, that a precisely tuned gamma beam at 6 MeV causes weapons-grade
uranium, U-235, to emit one neutron parallel to the polarization plane for each
neutron emitted perpendicular to the plane, giving the material a neutron
fingerprint of one.

Naturally occurring
uranium, U-238, emits three parallel neutrons for every one emitted
perpendicular to the polarization plane of the beam, giving it a neutron
fingerprint of three.

Beryllium, which can
also be a neutron source in nuclear weapons, has a neutron fingerprint of 10. The
team is now measuring the neutron fingerprints of plutonium and other fissile
materials, Ahmed said.

Howell and his
collaborators, meanwhile, work at lower energies on HIGS, about 3 MeV. (Surgeons,
for comparison, use a "Gamma Knife" at roughly 1 MeV to treat brain
tumors.) Their team has already identified the fluorescence patterns of several
special nuclear materials and lead.

Both teams will
report their results at a meeting with DNDO officials on Thursday, April 28 in
Washington D.C. and will store their results in a nuclear identification database.

Ahmed and Howell
said that engineers at one private security company and scientists at U.S.
national laboratories have already begun using the database to design new port
security scanners.

The new detectors will search cargo for
the fingerprints using an electron accelerator, possibly coupled to lasers that
produce a finely tuned gamma-ray beam, said Craig Wuest of the Global Security
Principal Directorate at Lawrence Livermore National Laboratory (LLNL).

The design sounds complex, but in some ways it resembles medical
scanning equipment and appears promising to pursue, he said.

Howell's "nuclear resonance fluorescence" approach
is interesting because it uses a beam with lower-energy gamma rays and reduces
the potential irradiation and contamination of cargo while providing "sufficient
detection sensitivity," Wuest, who was not involved in the research, added.

One of Wuest's colleagues
at LLNL, nuclear physicist Dennis McNabb,is more intrigued with Ahmed's and Weller's technique. Scientists
are only just beginning to measure the fingerprints and background signatures
from this neutron-scattering process, and because "the research is in
progress, how to best use the data is still an open question," McNabb said.

He also explained that cargo scanners using the data from
both teams could be ready for use at ports in about 10 years.

Still, some
scientists question whether the emerging science and technology can mature fast
enough to meet the real-world threats of terrorists and dirty bombs. For
instance, Thomas Cochran, a physicist and senior scientist at the Natural
Resources Defense Council, voiced "serious doubts" and said the
government should focus instead on eliminating inventories of highly enriched
uranium, improving port security, boosting intelligence efforts and training
first responders.

Other experts disagree and are urging the government to
accelerate research on new science and technologies that could significantly reduce
the threat of nuclear weapons smuggling, which seems likely to persist into the
next decade. McNabb, a proponent, said, "it takes time to develop new technologies"
and suggests that the research may accelerate development in other areas of
nuclear security.

HIGS physicists
The new information from HIGS could improve analysis of spent
nuclear fuel rods, which are an environmental issue as well as a potential
source of bomb materials, according to Duke physicist Anton Tonchev.

He works on the nuclear resonance fluorescence project
with Howell and said the technique provides a nondestructive way to measure the
quantities of plutonium and other nuclear materials that remain after the rods
are removed from a nuclear reactor.

Currently, the spent fuel rods must be opened and tested
to assess what materials remain in them. The process is expensive, but critical
for the International Atomic Energy Agency to accurately calculate the amount
of leftover fissile and nuclear materials. McNabb and Tonchev said that a new
technique to distinguish the leftover U-235, U-238 and plutonium in the spent rods
without opening them could substantially lower the costs to manage and account
for nuclear waste to prevent nuclear proliferation by terrorists.

Regardless of how fast engineers turn the fingerprint data
and new approaches into workable scanning and nuclear fuel devices, the Duke scientists said there is
immediate value in compiling a robust database of both the neutron and nuclear
resonance fluorescence fingerprints. Government officials at the DNDO concur
and cite HIGS as the only facility with the ability to produce such a database,
according to Ahmed.

Because of the
demand, the physicists have recruited graduate and undergraduate students from
Duke, University of North Carolina, North Carolina Agricultural and Technical
State University, North Carolina Central University, James Madison University and
George Washington University to help with the effort. They especially encourage
students from historically black colleges and universities to participate, hoping
the effort will help broaden the diversity of nuclear physicists working to
identify new ways to curb the threat of future terror attacks.