|Compact accelerator neutron generators
|David L. Chichester and James D. Simpson
For industrial applications that require neutrons, users have
three primary sources from which to choose: nuclear reactors, radioisotopes,
and accelerator-based neutron sources. Nuclear reactors are clearly
the largest and most prolific sources of neutrons. However, their
size, complexity, and cost have limited their industrial applications
for purposes other than electricity generation.
In contrast, radioisotope neutron sources are used in a myriad
of industrial applications, including thickness gauging and petroleum
exploration. Although radioisotope sources are ideal for fixed
installations that run continuously, they are not well suited for
applications that require pulsed neutrons, and they may create
safety issues and logistical complications as well. Moreover, concerns
have increased recently about the physical security of radioisotope
sources because of their possible use in dirty bombs.
Particle accelerators are the third source of neutrons for industry.
These systems vary in size and diversity, and they include large
installations such as the Spallation Neutron Source under construction
at Oak Ridge National Laboratory, and smaller photoneutron sources
such as that at the Gaerttner Linear Accelerator Laboratory at
Rensselaer Polytechnic Institute. Among the various light-ion accelerators,
compact devices designed as hermetic, sealed tubes that use deuterium–deuterium
(D–D) and deuterium – tritium (D–T) reactions
have found the most widespread use in industry.
These accelerators generate neutrons of ~2.5 and ~14.1 MeV, respectively.
Thousands of such small, relatively inexpensive systems have been
built over the past five decades, and the number and variety of
their applications are growing steadily (Figure 1).
||Figure 1. A
case containing a newspaper, ammonium nitrate, steel plates,
and urea (a) is examined with a pulsed neutron generator using
associated particle imaging, which produces a two-dimensional
density plot (b) and a yellow spectrum characteristic of ammonium
nitrate (c) (J. Kocher, Dynamics Technology Inc./P. Hurley
and J. Tinsley, U.S. Dept. of Energy, Special Technology Laboratory)
The basic design of a modern compact accelerator neutron generator
(Figure 2) does not vary much from those of other particle accelerators.
It consists of a source to generate positively charged ions; one
or more structures to accelerate the ions (usually up to ~110 kV);
a metal hydride target loaded with either deuterium, tritium, or
a mixture of the two; and a gas-control reservoir, also made of
a metal hydride material. The most common ion source used in neutron
generators is a cold-cathode, or Penning ion source, which is a
derivative of the Penning trap used in Penning ion gauges. This
simple ion source consists of a hollow cylindrical anode (usually
biased 1–2 kV) with cathode plates at each end of the anode
(usually at ground potential). An external magnet is arranged to
generate a coaxial field of several hundred gauss within the ion
When deuterium and/or tritium gas is introduced into the anode
at a pressure of a few millitorr, the electric field between the
anode and cathodes ionizes the gas. Electron confinement is established
in this plasma because of the orientation of the electric and magnetic
fields, which forces the electrons to oscillate back and forth
between the cathode plates in helical trajectories. Although some
low-energy electrons are lost and strike the anode, which creates
more secondary electrons, most remain trapped and ionize more gas
molecules to sustain the plasma. The ions are not similarly trapped,
and when they strike the cathodes, they also release secondary
electrons, which enter the plasma and help sustain it. Ions, however,
can escape the chamber into the acceleration section of the tube
through a hole at the center of one of the cathodes, called the
Other types of ion sources are also used in industrial applications,
including hotcathode sources, magnetrons, and radiofrequency ion
sources. However, the simple design and durability of the Penning
ion source have made it the most commonly used in industrial neutron
Construction methods used in building sealed neutron tubes include
joining techniques such as welding, metal brazing, ceramic-to-metal
brazing, and glass-to-metal seals. Materials used in accelerator
neutron systems include glass, ceramics, copper, iron, different
alloys of stainless steel, and Kovar. Most compact accelerator
neutron tubes are loaded with 1 to 2 Ci of tritium; for comparison,
a typical tritium exit sign used in an airplane or hotel might
contain as much as 10 to 20 Ci of the isotope.
Compact accelerator neutron generators are made by several companies
(Table 2) and used for many industrial purposes (Table 1). Although
the initial sealed-tube devices addressed the military’s need
for a long-lived neutron source, the technology made the transition
to peaceful applications soon after World War II. The first commercial
products were developed for petroleum exploration.
Since the development of these first systems, applications have
focused on detecting and quantifying the presence of different
elements in a variety of materials, with the goal of improving
process optimization and control. Usually, neutron-generator-based
systems can provide this information much more quickly than traditional
laboratory techniques. Because fast neutrons have a large effective
range of penetration in most materials—several tens of centimeters—neutron
analysis of bulk materials has significant advantages over certain
laboratory techniques. This is particularly true where sample collection
and preparation are a problem, as when samples are difficult to
obtain or are not representative. For some uses, such as nuclear-waste
assaying or explosives detection, the noncontact, nondestructive,
and remote-measurement capability that neutron analysis techniques
allow are an additional advantage.
The most advanced industrial applications of compact accelerator
neutron generators are in the petroleum industry, which has relied
on the devices for nearly 50 years. These probes penetrate far
more deeply into rock formations than most other techniques, and
thus, they allow analysts to “see” farther than with
other downhole probes. This capability is especially useful when
analyzing cased holes, where neutron-based techniques allow the
probe to measure rock properties behind steel casings.
|Figure 2. Schematic
design of a sealed-tube neutron generator with a Penning
ion source. (Ian Worpole)
Early probe systems used gas-filled thermal- neutron detectors
to measure the thermal- neutron intensity decay over time between
the neutron pulses emitted by the generator. Taking advantage of
the large thermal-neutron absorption cross section of chlorine
(33 barns for chlorine versus 0.33 barn for hydrogen) and subsequent
improvements in the implementation of the technology, analysts
began using measurements of thermal-neutron decay to distinguish
between hydrocarbons and saline water in underground formations.
Tools for these measurements typically operate at 1,000 pulses/s,
with each 100-µs pulse followed by a 900-µs
pause before the next pulse. These respites allow enough time for
the fast neutrons from each pulse to become fully thermalized and
absorbed. Photon detectors are most commonly used in these tools;
they measure gamma-ray intensity versus time, which is proportional
in intensity to the number of surviving thermal neutrons.
An important and growing market for neutron generators is in analyzing
bulk materials. Taking advantage of recent improvements in neutron-generator
performance that have extended typical operating lifetimes from a few
hundred hours to several thousand hours, companies have built commercial
systems for the real-time analysis of materials such as cement and
coal moving on conveyor belts. Newer neutron-generatorbased systems
typically run both fast- and thermal-neutron activation analyses to
measure the elemental content of the major constituents in the bulk
material, and use stoichiometric relationships to convert the elemental
information to chemical assays.
In the cement industry, this information enables the optimal blending
of raw materials before processing and the verification of chemical
uniformity of the final product. In the coal industry, on-line
measurements have found particular use in reporting the thermal
energy and sulfur content of coal and for determining the fraction
of the coal that is not hydrocarbon and will remain as ash after
combustion. Although neutrongenerator systems do not necessarily
provide analytical advantages over radioisotope- based systems,
they have found particular use among customers who are sensitive
to radiation safety and security issues related to the use of chemical
|Figure 3. When
a 122-mm projectile filled with high explosives is examined
with a pulsed neutron generator, a gamma-ray spectrum taken
during a neutron pulse (blue) reveals characteristic carbon,
nitrogen, and oxygen peaks, while a gamma-ray spectrum taken
between pulses (Data courtesy of P. Womble, Applied Physics
Institute, Western Kentucky University)
R&D programs at several national laboratories, universities, and
private companies are investigating the development of neutron- generator-based
systems for detecting high explosives, chemical weapons, and nuclear
materials in a variety of objects (Figure 3). The goals of these projects
include developing sensor systems for border security, airline-cargo
inspection, and first response in the investigation of unknown packages.
The advantage of the technique over conventional approaches to
dealing with unexploded ordnance, for example, lies in the ability
of the system operator to perform measurements without physically
touching the object being analyzed—a particularly useful
feature when dealing with old, potentially unstable high explosives.
The use of neutron generators for this application also has other
benefits over chemical sources. They allow pulsed measurements,
and they simplify safety procedures for deployment and storage
because the neutron source can be turned off.
Another application of neutron generators is in the measurement of body
composition. Techniques similar to those described above allow the
measurement of the body’s carbon and oxygen content with neutron
inelastic scattering, and this data is used to assess the total amount
and the distribution of fat in the body. This information is useful
for evaluating the health of individuals with respect to obesity, aging,
cardiovascular disease, and the amount of energy stored in body fat,
as well as for assessing the nutritional effectiveness of different
Advances in compact sealed-tube neutron generators are focused toward
the development of smaller, lighter, less expensive systems with longer
lifetimes and higher outputs. Reductions in size and weight are mostly
driven by the growing demand in the homeland-security market for field-portable
applications, and all markets demand price, reliability, and performance
improvements. Also important are ongoing advances in neutron-generator
support systems, such as updating analog control systems to digital
controls that incorporate advanced diagnostic routines that can be
remotely accessed for wireless operation, servicing, and upgrading.
Similarly, improvements in generator designs and power supplies have
lowered the energy consumption of these systems to allow longer operation
when working off batteries.
Particularly noteworthy advances have come in devising neutron-generator
systems suitable for associated particle imaging. These generators
incorporate charged-particle detectors near the target, which can
record the helium atoms produced during the D–D and D–T
fusion reactions. Using this information, outgoing neutrons can
be identified by their emission angles and times, and used with
external gamma-ray detectors and gating circuitry to acquire three-dimensional
elemental information within objects (Figure 1). In the past, systems
capable of generating high-resolution images were large and bulky,
but newer systems are now emerging that will allow the deployment
of this technique for field applications.
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David L. Chichester is
manager of R&D and James
D. Simpson is manager of sales and marketing at Thermo
Electron Corp. in Colorado Springs, CO.