the dentist's office to the aircraft hangar, the use of x rays to
reveal the internal structure of objects is a time-honored practice.
However, during the past few decades, several industries have begun
to use thermal, or low-energy, neutron imaging as a complementary
technique to x-ray imaging for inspecting objects without taking
them apart. Now Lawrence Livermore researchers have demonstrated
the power of using high-energy neutrons as a nondestructive inspection
tool for evaluating the integrity of thick objects such as nuclear
warheads and their components.
Experiments conducted over
the past four years at Ohio University by a Lawrence Livermore team
have demonstrated high-energy neutron imaging's considerable promise
in probing the internal structure of thick objects composed of materials
that are essentially opaque to x rays. Indeed, the results have
proven more successful than computer models first indicated or than
Livermore physicists had expected.
The neutron imaging project
is funded through the Enhanced Surveillance Campaign, a key element
of the nation's Stockpile Stewardship Program, which is managed
by the National Nuclear Security Administration (NNSA) within the
Department of Energy. Nondestructive surveillance—the search for
anomalies from cracks to corrosion in aging stockpiled nuclear weapons
systems to assure their continuing safety and reliability—is much
more cost-effective than disassembling a warhead. Hence, the development
of improved nondestructive surveillance techniques is crucial to
the success of stockpile stewardship in the absence of nuclear testing
and to the nationŐs defense.
relies on a range of techniques, including x-ray imaging. X rays
are adequate for inspecting the condition of parts composed of what
scientists call high-Z (high-atomic-number) materials such as lead,
tungsten, and uranium. However, x-ray imaging is not always effective
in revealing voids, cracks, or other defects in so-called low-Z
(low-atomic-number) materials such as plastics, ceramics, lubricants,
and explosives when these materials are heavily shielded by thick,
high-Z parts. (See the box below.)
Neutron image of an object with defects taken at the Los Alamos
Neutron Science Center (LANSCE). (b) Computer simulations using
Lawrence Livermore's COG Monte Carlo radiation transport code.
The simulations show that neutron images taken at energy ranges
between 10 and 15 megaelectronvolts could reveal defects in
thickly shielded targets as well as LANSCE images, which were
taken at much higher energies.
forms of radiation are attenuated (weakened) by
a combination of slowing, scattering, and absorption
processes as they pass through materials. The variation
in attenuation through different parts of an object
forms the basis for radiation imaging. The most widely
used and commonly known form of radiation imaging is
the x-radiograph in which an object is exposed to x
rays and an image of the object (essentially a shadow)
is recorded on photographic film or with a solid-state
camera. Discovered more than 100 years ago, x rays today
have a wide range of industrial and medical applications.
in 1932, are electrically neutral particles similar
in mass to a proton and present in the
nuclei of all elements except hydrogen. Neutron imaging
(conceptually similar to x-ray imaging) is commonly
done today using neutrons that have an average energy
of about 0.025 electronvolts. These neutrons are generated
from fission neutrons produced in a nuclear reactor
or from the decay of a radioisotope and then passed
through thick layers of a hydrogen-rich material such
as polyethylene to reduce their energy to thermal levels.
applications using thermal neutrons exploit their strong
interaction with hydrogen. For example, thermal neutrons
can be used to inspect or detect explosives inside
brass shell casings and search for corrosion in the
aluminum skin of aircraft.
imaging (for example, in the 10- to 15-megaelectronvolt
range) is a relatively new technique
that offers unique advantages over conventional x-ray
and thermal neutron imaging, particularly for inspecting
light (low-Z, or low-atomic-number) elements that are
shielded by heavy (high-Z, or high-atomic-number)
elements. These advantages are due in part to their
greater penetrating power (that is, lower attenuation)
through high-Z materials and, compared to x rays, their
much stronger interaction (that is, higher attenuation)
physicist James Hall emphasizes that neutron imaging
yields different (and complementary) information to
that obtained with x rays. "The use of one does not
necessarily eliminate the need for the other," he says.
Hall notes that although the ultimate spatial resolution
attainable with high-energy neutron imaging—about
1 millimeter—is about 10 times less than the spatial
resolution of x-ray imaging done with the most penetrating
x-ray spectrum, it may be the only way that researchers
learn anything about the internal structure of some
extremely thick objects.
larger in size than the proposed Lawrence Livermore neutron
imaging system, the layout of the facility at the Ohio University
Accelerator Laboratory in Athens, Ohio, is similar in configuration.
The large orange vessel in the background is a Van de Graaff
accelerator. It is used to accelerate deuterium ions into a
cell containing deuterium gas to produce high-energy neutrons.
Complement X Rays
what is needed is a way to image shielded low-Z parts as a means
to complement standard x-ray imaging of nuclear warhead components
for stockpile surveillance. The answer seems to lie with high-energy
neutrons, which are able to easily penetrate high-Z materials to
interact with low-Z materials, yielding clear, detailed images that
are difficult to duplicate with x rays.
According to Lawrence Livermore
materials scientist Jim LeMay, deputy program leader for Enhanced
Surveillance, neutron imaging will be valuable to stockpile stewards
on a number of fronts. He notes that weapons are randomly selected
from the nation's nuclear stockpile for inspection. Neutron radiographs
could be used as a means to screen these weapons and select one
or more devices for complete disassembly and visual inspection.
Also, neutron radiography could serve as a valuable inspection tool
for identifying the warheads that actually need refurbishing as
well as a valuable quality control tool for inspecting refurbished
warheads before they are returned to the stockpile. Finally, neutron
imaging of a statistically significant number of units could serve
as a baseline assessment of the current state of a particular warhead.
Livermore physicist James
Hall, the neutron imaging project leader, notes that imaging systems
using thermal neutrons (average energy of about 0.025 electronvolt)
are well established as nondestructive inspection tools in research
and industry. However, these systems are generally limited to inspecting
objects only a few centimeters thick. In the early 1990s, scientists
at Lawrence Livermore and Los Alamos national laboratories speculated
that higher-energy neutrons could be used to image much thicker
objects such as nuclear warhead components.
began in 1994 at the Los Alamos Neutron Science Center (LANSCE),
a facility that produces neutron beams with energies of up to 600
megaelectronvolts (MeV), far greater than those used by industry.
The test object consisted of a 2.54-centimeter-thick lithium deuteride
(low-Z) disk that was sandwiched between two 5.08-centimeter-thick
uranium (high-Z) slabs. Small holes ranging from 4 to 12 millimeters
in diameter were drilled all or part way through the lithium deuteride
to simulate defects. A detector recorded images of the neutrons
transmitted through the object from the LANSCE source with a spatial
resolution of about 1 millimeter, revealing the presence of all
of the holes.
Livermore physicist James Hall assembles a test object called
a sandwich assembly for imaging at the Ohio University Accelerator
Laboratory. Behind Hall is a prototype multiaxis staging system
that secures and manipulates the test object. On its way to
the detector, the neutron beam passes through the test object
and immediately through a tapered polyethylene collimator set
into a 1.5-meter-thick concrete and steel wall.
Encouraged by the success
of these initial tests, Hall decided to model the LANSCE experiments
using Livermore's three-dimensional Monte Carlo radiation transport
computer code called COG. His computer simulations, however, focused
on a lower energy range (10 to 15 MeV) because neutrons with these
energies are known to penetrate high-Z materials effectively and
yet interact more strongly with low-Z materials than the much higher-energy
neutrons used at LANSCE. The COG simulations showed that neutron
imaging in the 10- to 15-MeV energy range should be capable of revealing
millimeter-size cracks, voids, and other defects in thick, shielded
targets similar to the one tested at LANSCE.
Hall was also drawn to two
other advantages of 10- to 15-MeV neutrons. The first is that neutrons
in this energy range are much less expensive to generate than higher-energy
neutrons such as those produced at LANSCE. Second, lower-energy
neutrons are easier to detect because they allow the use of plastic
scintillators, which are some 20 times more efficient than the conversion-type
detectors required for much higher-energy neutrons.
One disadvantage of the lower
energy range is the somewhat reduced penetrability of high-Z materials,
which means exposure times of a few hours and sometimes longer are
required for typical radiographs. However, says Hall, the greater
detection efficiency and lower overall imaging costs more than make
up for the longer exposure times.
Following the computer simulations,
Hall joined forces with colleagues Frank Dietrich, Clint Logan,
and Brian Rusnak to design and develop a full-scale neutron imaging
system for stockpile surveillance that would be capable of acquiring
both radiographic (single-view) and full tomographic (three-dimensional)
images. The system has to be relatively compact (about 15 meters
long), both as a prototype suitable for installation and use at
Livermore and in its fully developed form for eventual installation
at other NNSA weapons complex facilities.
The resulting design features
three primary components: an accelerator-driven neutron source generating
an intense beam of 10-MeV neutrons, a remotely controlled staging
system to support and manipulate objects being imaged, and a detector
system with relatively high efficiency (about 20 to 25 percent)
that can resolve defects of about 1 millimeter in diameter. To expedite
the system's development and minimize technical risks, the team
decided to use commercially available components and proven neutron
imaging techniques wherever possible.
Making of a Neutron Imaging System
design of Livermore's neutron imaging system consists
of a high-energy neutron source, a multiaxis staging platform
to hold and manipulate an object, and an efficient imaging
detector. The development of these components has proceeded
in parallel over the past several years.
Neutrons can be
produced using accelerators, radionuclides, or nuclear
reactors. To achieve a high-energy neutron flux sufficient
to image thick objects of interest within reasonable imaging
times (a few hours), an accelerator-driven source appears
to be the most practical option for stockpile surveillance
based on a commercially available design, will be built
to Livermore specifications. The unit will focus a narrow
(1.25-millimeter-diameter), pulsed (75-hertz), 300-microampere
beam of deuterium ions into a 4-centimeter-long cell containing
deuterium gas. (Deuterium is an isotope of hydrogen containing
one proton and one neutron in its nucleus.) The collision
of the deuterium ions with deuterium gas in the cell will
produce an intense, forward-directed beam of neutrons
with an energy of about 10 megaelectronvolts.
requirements of a high deuterium-ion current and small
beam diameter preclude the use of typical thin-walled
("windowed") deuterium gas cell designs. At an average
power of about 170 kilowatts per square centimeter,
the incident deuterium ion beam would generate far too
much heat for any window material to withstand.
As a result,
Lawrence Livermore researchers have teamed with nuclear
engineering professor Richard Lanza
at the Massachusetts Institute of Technology (MIT) to
develop a "windowless" deuterium gas cell that can be
efficiently coupled to a high-current, pulsed, deuterium
accelerator. One design under consideration features
a high-pressure (3-atmosphere) gas cell mounted at the
exit port of a vacuum system. The cell's several pumping
stages are isolated from each other by a series of rotating
with small holes synchronized to the pulse frequency
of the accelerator.
In this way, the holes in the rotating disks line up about
75 times a second to allow the ion beam to penetrate the
cell without letting substantial amounts of deuterium
gas leak out.
to the rotating aperture design is also being pursued
by the Lawrence Livermore–MIT team. This approach, developed
at Brookhaven National Laboratory, uses an intense plasma
discharge to effectively plug the opening of the gas cell
by rapidly heating and ionizing any deuterium leaking
out. Similar "plasma windows" are being developed for
use in electron-beam welding applications.
The object under
inspection will be secured to a staging system that was
originally designed at DOE's Y-12 Plant in Tennessee for
x-ray imaging. The unit goes up and down and back and
forth and rotates a full 360 degrees to permit both radiographic
and tomographic imaging. Calculations and tests conducted
at the Ohio University Accelerator Laboratory by Livermore
researchers indicate that placing the staging system halfway
between the source and the image plane of the detector
will minimize the neutron scattering that can fog the
Detector Has Nevada Heritage
The design of
the imaging detector will be based on technology originally
developed by Lawrence Livermore's Nuclear Test Program
for use at DOE's Nevada Test Site. The full-scale detector
will consist of a 60-centimeter-diameter transparent
plastic scintillator viewed indirectly by a camera with
a high-resolution (2,048- by 2,048-pixel) charge-coupled
device (CCD) imaging chip.
A thin turning
mirror made of aluminized glass will be used to reflect
the brief flashes of light generated by neutrons interacting
in the scintillator into the CCD camera, which will
itself be located in a shielded enclosure well out
of the neutron beampath. The camera will be fit with
a fast (f/1.00 or better) lens to enhance its sensitivity
and cooled with liquid nitrogen gas to —120 degrees
Celsius to minimize thermal electronic noise.
Lawrence Livermore design for a high-energy neutron imaging
system consists of a powerful neutron source, a multiaxis
staging platform to hold and manipulate an object, and
an efficient imaging detector.
Nine step wedges fabricated from lead, Lucite, mock high explosive,
aluminum, beryllium, graphite, brass, polyethylene,
and stainless steel were imaged. Each step wedge has 10 steps
ranging in thickness from 1.27 centimeters to 12.7 centimeters.
(b) The nine wedges were imaged as a single unit. (c) The radiographs
clearly differentiated the various materials and steps.
University Test Bed
The team chose the Ohio University
Accelerator Laboratory (OUAL) in Athens, Ohio, to evaluate the performance
of a prototype imaging detector beginning in 1997. Although the
accelerator facility at OUAL is much larger than that proposed in
the Livermore design, its layout and configuration are similar.
In addition, the OUAL staff has extensive experience in the production
of accelerator-driven, high-energy neutron beams.
For the Lawrence Livermore
experiments at OUAL, a 10-MeV neutron beam is generated by focusing
deuterium ions into a cylindrical 1-centimeter-diameter by 8-centimeter-long
deuterium gas cell attached to the end of the beam line. The gas
cell is capped with thin entrance and exit windows and maintained
at a pressure of about 3 atmospheres to limit the spread in energy
of the resulting neutrons. The typical deuterium ion beam current
arriving at the gas cell is on the order of 10 microamperes, which
corresponds to about 60 trillion ions per second. In comparison,
Lawrence Livermore's proposed design will feature a 300-microampere
accelerator with a 4-centimeter-long deuterium gas cell.
The result is a neutron
beam flux only 15 times less intense than the intensity called for
in the full-scale system. As a result, images take about 15 times
longer to complete at OUAL than they will at Livermore. Nevertheless,
the flux is sufficient to evaluate the performance of prototype
detectors and for Lawrence Livermore researchers to gain valuable
experience in neutron imaging. In many ways, says Hall, the Ohio
University accelerator lab has been a "perfect test facility."
The experiments conducted
thus far at OUAL have focused primarily on radiographic imaging
of step wedges made of different materials and slab or sandwich
assemblies, most with holes or other features machined into them
to test the system's resolving power. The sandwich assemblies are
typically composed of blocks of low-Z materials, such as polyethylene,
that are shielded by various thicknesses of high-Z materials, such
as lead or depleted uranium (D-38). Tomographic images of several
cylindrical test objects composed of nested shells of high- and
low-Z materials, with machined features, have also been obtained.
The test objects are mounted
on a multiaxis staging system, which is located on the beam axis
about 2 meters downstream from the neutron source and about 2 meters
in front of the prototype imaging detector. The detector is housed
in a shielded area behind a 1.5-meter-thick concrete and steel wall
with a tapered polyethylene collimator to help minimize background
A lead cylinder with a 10.16-centimeter outside diameter, a
5.08-centimeter inside diameter, and a polyethylene core was
imaged. (b) The polyethylene core was split into two half-cylinders.
One served as a blank, and the other had a series of holes that
were 10-, 8-, 6-, 4-, and 2-millimeter-diameter by 1.27-centimeter-deep
machined into its outer surface. (c) The resulting tomographic
reconstructions clearly showed the core's structure, including
the slight gap between the two halves.
Steps, and Cylinders
One of the first experiments
conducted at OUAL involved imaging a 12.7-centimeter-thick lead
and polyethylene sandwich (with features machined into the polyethylene)
and a set of 9 step wedges fabricated from lead, Lucite, mock high
explosive, aluminum, beryllium, graphite, brass, polyethylene, and
stainless steel. Each step wedge had 10 steps ranging in thickness
from 1.27 centimeters to 12.7 centimeters. The nine wedges were
grouped together and radiographed as a single unit (looking up the
steps from thick to thin) in a series of two 1-hour exposures. The
radiographs clearly differentiated the different materials and step
Another series of experiments
involved imaging a 7.62-centimeter-thick D-38 and lithium deuteride
sandwich (similar in design to the lead and polyethylene assembly
previously described) and tomographic imaging of a lead cylinder
with a 10.16-centimeter outside diameter, a 5.08-centimeter inside
diameter, and a polyethylene core (see figure above).
The polyethylene core was
split into two half-cylinders. One served as a blank and the other
had a series of holes machined into its outer (curved) surface that
were 10, 8, 6, 4, or 2 millimeters in diameter by 1.27 millimeters
deep. A series of sixty-four 10-minute exposures was taken of the
cylinder at angles evenly distributed over 180 degrees. Resulting
tomographic reconstructions clearly showed the core's structure.
Although not well resolved, the narrow (less than 0.25-millimeter-wide)
gap between the two halves of the polyethylene core was also visible
in the reconstructed images.
Additional experiments at
OUAL have focused on imaging objects made of other materials with
a variety of machined features. One object consisted of a 10.16-centimeter
by 5.08-centimeter by 2.54-centimeter-thick slab of ceramic set
atop a polyethylene slab of similar size and shielded by 2.54 centimeters
of D-38. The ceramic piece featured two sets of 4- and 2-millimeter-diameter
holes machined to depths of 4, 2, and 1 millimeters (the smallest
hole corresponded to a defect with a volume of about 3 cubic millimeters).
The ceramic was carefully cracked along its centerline and then
reassembled so that the fracture was barely visible to the naked
eye. The polyethylene piece featured the same set of 4- and 2-millimeter-diameter
holes but no crack.
neutron radiograph of a fractured ceramic and polyethylene test
object shielded by 2.54 centimeters of depleted uranium shows
the crack separating the two ceramic halves as well as a series
of 4-millimeter-diameter (top) and 2-millimeter-diameter (bottom)
holes machined into the ceramic. (A narrow slot was cut in the
top of the ceramic to a depth of 2.54 centimeters to facilitate
cracking the piece along its centerline.)
The object was imaged in
a series of forty-eight 30-minute exposures. The final processed
image and associated lineouts clearly showed the crack in the ceramic
slab and all of the machined features, including the smallest 2-millimeter-diameter,
Hall says the contact gap
between the two ceramic pieces was probably less than 0.01 centimeter
wide, far less than the designed resolution of the imaging system.
Yet, the gap can still be resolved. "We're very pleased we can see
this kind of detail through more than 2 centimeters of uranium,
even though we can't really quantify the gap," he says, adding,
"we're seeing more than we ever expected."
number of images have been taken of (a) the British Test Object
(BTO), which consists of (b) a set of six nested cylindrical
shells made of graphite, polyethylene, aluminum, and tungsten.
A series of exposures was reconstructed into mock tomographic
images, which show the BTO viewed (c) from the top and (d) through
of images have also been taken of the British Test Object (BTO),
on loan from the Atomic Weapons Establishment in Aldermaston, United
Kingdom. The BTO consists of a set of six nested cylinders of graphite,
polyethylene, aluminum, tungsten, polyethylene, and tungsten (respectively)
with a solid polyethylene core. Twelve 30-minute exposures were
taken of one side of the assembly and then reconstructed into a
mock tomographic image. The reconstruction clearly shows the gross
structure of the object as well as the detailed joint structure
in the outer shells.
the experimental success enjoyed thus far, much work remains to
be done to meet the goal of having a full-scale neutron imaging
system in operation at Livermore by late 2003 or early 2004. Vendors
need to be selected to build the accelerator, the detector's optics
system, and the multiaxis staging system. Meanwhile, plans are under
way to modify an existing Lawrence Livermore laboratory to house
the system's performance is validated at Livermore, it will be transferred
to other DOE facilities such as the Pantex Plant in Texas or the
Y-12 Plant in Tennessee by late 2005 or early 2006. The continuing
success of the Ohio University experiments makes it likely that
neutron imaging will be serving the nation's stockpile stewardship
needs within a few short years.
Brookhaven National Laboratory, COG Monte Carlo radiation transport
code, deuterium, Enhanced Surveillance Campaign, lithium deuteride,
Los Alamos Neutron Science Center (LANSCE), Massachusetts Institute
of Technology (MIT), neutron radiography and tomography, Nevada
Test Site, Ohio University Accelerator Laboratory (OUAL), Pantex
Plant, scintillator, stockpile stewardship, x-ray imaging, x-ray
radiography, Y-12 Plant.
information contact James Hall (925) 422-4468 (firstname.lastname@example.org).
received his B.S. in physics and mathematics from the University
of Southern Colorado in 1974 and his M.S. and Ph.D. in physics
from Kansas State University in 1977 and 1981, respectively.
He joined Lawrence Livermore in 1987 as a physicist charged
with the design and execution of a variety of nuclear device
diagnostic experiments, primarily neutron and gamma-ray measurements,
for the underground nuclear test program at the Nevada Test
Site. With the end of underground testing in 1992, Hall refocused
his efforts on the development of detailed computer simulations
of inertial confinement fusion diagnostics, flash x-ray systems,
and a variety of nonintrusive inspection systems proposed for
use in stockpile stewardship, cargo and luggage inspection,
and nuclear counterterrorism schemes. As an outgrowth of this
work, in 1994 he was selected to serve as the DOE representative
and chief science advisor to the 8th Joint Compliance and Inspection
Commission meetings associated with the Strategic Arms Reduction
Treaty. Hall is currently a principal investigator for the development
of high-energy neutron imaging techniques in support of nuclear
stockpile stewardship applications.