Neutron Sources

Cliff Shull (right), co-winner of the 1994 Nobel Prize in physics for pioneering research using neutron scattering in the 1940s and 1950s at ORNL's Graphite Reactor, performs research with Ernest Wollan at the reactor.

Because isolated neutrons do not occur in nature, they must be extracted from atomic nuclei for use in scientific research. One source of neutrons is the nuclear fission process in a nuclear reactor. A second method of obtaining neutrons is spallation—bombarding nuclei of heavy atoms with energetic particles (usually protons) from a high-energy accelerator. When protons collide with target nuclei, 20 to 30 neutrons are knocked out, or "spalled," from each nucleus—thus the term "spallation."

Neutron sources have become invaluable tools for fundamental science, perfection of new industrial products, medical and biomedical applications, commercial power development, and defense purposes.

ORNL has a long history in using neutron sources for research. Because of the early neutron scattering research performed at ORNL's Graphite Reactor, a former ORNL researcher (Cliff Shull) received a 1994 Nobel Prize in physics. Today neutron scattering research is conducted at ORNL's High Flux Isotope Reactor (HFIR), which has the world's highest thermal neutron flux; about 290 users annually do neutron research at the HFIR, which is 30 years old.

Although the United States pioneered in developing neutron sources in the 1960s, the nation has now fallen behind Europe and Japan, where a new generation of research reactors and spallation neutron sources was built in the 1970s and 1980s. The justification and need for a new neutron source in the United States has been documented by numerous scientific and government assessments since the early 1970s. The development of new instruments and measurement methods has generally increased the demand for access to neutrons.

In a 1993 study by the Department of Energy's Basic Energy Sciences Advisory Committee (BESAC) Panel on Neutron Sources for America's Future, the scientific community overwhelmingly recommended constructing a new research reactor at ORNL, the Advanced Neutron Source (ANS), for which ORNL had recently completed a thorough conceptual design.

Construction requests for the ANS were included in the President's budget for fiscal year (FY) 1994 and FY 1995, but Congress did not appropriate funds for construction of the ANS because of concern over its high cost—almost $3 billion. In the FY 1996 budget process, DOE recommended termination of the ANS, and Congress appropriated funds to begin design of a new, lower-cost spallation neutron source. In early 1996, the BESAC panel assessed the need for neutron sources and recommended

National Spallation Neutron Source

The appropriations bill passed by Congress for FY 1996 provided ORNL with $8 million to begin design of a new spallation source. The National Spallation Neutron Source collaborative project was born as ORNL's response to DOE's charge to initiate conceptual design of a new spallation source. The project is organized to take full advantage of previous studies and assessments and of technical expertise and experience at industrial firms, universities, and other DOE laboratories.

The proposed neutron source consists of a high-energy particle accelerator that produces short bursts of protons at extremely high energies and power levels. These proton pulses bombard a heavy metal target and, through the spallation process, excite atomic nuclei, resulting in the emission of neutrons. The liberated neutrons are moderated (slowed down) to useful energies and then guided as beams into experimental areas for use in neutron science experiments.

The proposed facility will occupy an area about 0.8 by 1.6 kilometers (0.5 mile by 1 mile) on the ORNL Reservation. It will consist of a proton accelerator and storage ring, target stations for production of neutrons, and experimental halls and instrumentation for use of neutrons for research and product development. The conceptual design will take two years to complete. The total cost of the facility will be about $1 billion, and it should be in operation by the year 2005.

The NSNS will produce the highest-flux-pulsed-neutron beams in the world.

When a high-energy proton bombards a heavy atomic nucleus, causing it to become excited, 20 to 30 neutrons are expelled.

The NSNS will produce the highest-flux-pulsed-neutron beams in the world. It is estimated that the facility will attract 1000 to 2000 scientists and engineers each year from universities, industries, government laboratories, and foreign countries. With an expected operating budget of about $80 million per year, it is projected to provide more than 1200 permanent jobs and $40 million in new sales tax revenues.


In the proposed NSNS, protons will be accelerated to high energies, formed into pulses, and directed onto a mercury target. Each pulse of protons will produce a burst of neutrons that will be moderated and guided into an experimental area.

The NSNS project was organized as a collaborative design project involving ORNL, Argonne National Laboratory (ANL), Brookhaven National Laboratory (BNL), Los Alamos National Laboratory (LANL), and Lawrence Berkeley National Laboratory (LBNL). Other laboratories, U.S. industrial firms, and universities will be involved as the project matures. This collaborative approach was taken to access the best technical expertise available. In addition, memoranda of understanding have been signed with the European Spallation Source—a design effort similar to NSNS—and several European laboratories, to access research results and technology developments that could further leverage the NSNS design effort. BNL, LANL, and LBNL are responsible for the accelerator system design, which will be coordinated by an accelerator design group at ORNL. ORNL is responsible for target design, experimental systems, project management, and conventional construction. ANL and ORNL are responsible for experimental systems design.

The reference design recently chosen for the new spallation source is consistent with recommendations of the 1996 BESAC panel that assessed needs for a new short-pulse spallation source for DOE. The approach is to design a 5-megawatt (MW) spallation source that can be built in stages. The first stage, which must be built for about $1 billion, will be a 1-MW source that can be upgraded to higher powers in the future, as funds allow. The 1-MW source will consist of a linac accelerator that injects negatively charged hydrogen (H­) ions into an accumulator ring, where ~1-microsecond pulses of 1-billion-electron-volt protons are produced and directed to a liquid mercury target from which neutrons are generated. The target-moderator and experimental systems provide for cold and thermal neutrons servicing 18 beam lines. Several staged upgrades are possible leading to higher powers of 4 to 5 MW.

Because of the large user community expected at NSNS, ORNL, the University of Tennessee (UT), and the state of Tennessee have initiated plans for a Joint Institute for Neutron Science (JINS). This joint venture between ORNL and UT would provide meeting facilities, offices, laboratories, a communication center, and housing for scientists and engineers from universities, industries, and the international community. It would also be a focus for expanding neutron science R&D with UT, regional universities, and industrial collaborators. Funds have been included in the state of Tennessee's FY 1996 budget to begin design of JINS.

This new spallation neutron source would be the most powerful facility in the world for neutron scattering and would restore a much needed capability to the U.S. neutron science community.—Bill Appleton, ORNL Associate Director for Advanced Materials, Physical, and Neutron Sciences.

High Flux Isotope Reactor Upgrades

The HFIR is a multipurpose research reactor, with missions in four nationally important areas: isotope production, neutron scattering, neutron activation analysis, and irradiation testing of materials.

Overview of the High Flux Isotope Reactor with proposed additions of a Thermal Guide Hall and Cold Guide Hall for providing additional beams of thermal neutrons and new beams of cold neutrons for research.

It was originally designed for production of the transplutonium isotopes—elements that do not occur naturally on earth and that are beyond plutonium in the periodic table of elements. One of these is californium-252, which has been used to treat more than 450 patients with advanced cervical cancers, improving the 5-year survival rate from 12% with older treatments to 54%. The HFIR remains the sole source of these isotopes in the western world and the best source of many other radioisotopes, including important medical isotopes. The HFIR's annual sales of californium-252 total about $2 million, and its radioisotopes are sold to about 800 customers a year.

Over time, neutron scattering has grown astonishingly in scientific and economic importance—providing, for example, most of our knowledge about the molecular and magnetic structure and behavior of new, high-temperature superconductors. Although the proposed package of upgrades would enhance our capabilities in all four areas mentioned above, the biggest impact would be on neutron scattering.

One part of the upgrades, a return to the original 100-MW design power level (instead of the 85 MW that has been used since 1989), will benefit all the applications discussed, because the neutron production rate, which is proportional to reactor power, will be increased by nearly 20%. Likewise, modernizing instrumentation and electrical equipment, as well as replacing certain other components of the reactor system, will reduce maintenance downtime and will increase operating time, hence more neutrons, available for all users.

We propose raising HFIR's power level and adding a cold
neutron source and five neutron guides.

For neutron scattering, there are two really major enhancements. The first is the installation of a small cold neutron source—small because there's no room for a large one—in one of the four existing beam tubes. The source will be an aluminum chamber holding about a half liter of liquid hydrogen, at a temperature of 20 K (about ­420°F): at this temperature, the hydrogen molecules move fairly slowly and neutrons that enter the chamber are slowed down by collisions with those molecules. When a beam of these very slow neutrons is directed onto a sample, even very small, weak, or subtle features of atomic structure and motion can influence the speed or direction of the neutrons (as a very slowly moving golf ball is more susceptible to small irregularities or tussocks on the putting green than is a fast moving one). Thus, the more complex materials, and the weaker, more subtle forces in them, are best explored using these cold neutrons.

The best cold neutron beams in the world are those at the Institut Laue-Langevin (ILL) in France. However, because of the higher power at HFIR (100 MW vs 59 MW) and the use of new design concepts developed for the ANS, our cold beams will be brighter. Although space limitations dictate that we will have far fewer beams and instruments, the ones we do have will be the best in the world.

The second change will be to put five neutron guides at an existing beam port (called HB 2) to bring neutrons from close to the reactor out into a big hall that will have space for nine experiments. Neutron guides work exactly like fiber optic guides—they are rectangular conduits, typically, 5 × 10 centimeters (2 × 4 inches), whose inside surfaces may be coated with multiple layers of nickel that will reflect any neutrons that strike the surface at a glancing angle, if they are not traveling too fast. Thus, the guides can bring neutrons from close to the reactor, in a series of ricochets, to an instrument more than 30 meters (100 feet) away, with little loss. This capability is important for two reasons: first, there isn't enough room close to the reactor for nine big instruments; second, near the reactor is considerable background radiation, including gammas and fast neutrons, that tend to swamp small signals that scientists look for. Because each guide reflects only rather slow neutrons, this undesirable background radiation escapes through guide walls, and it is absorbed by shielding before reaching the experimental sample and the detector at the far end.

The HFIR already has the world's most
intense beams of thermal neutrons.

The HFIR already has the world's most intense beams of thermal neutrons—those that have slowed down to the equivalent of room temperature or so. This part of the upgrade will make them brighter and purer still, as well as increasing the number of users that can be accommodated by increasing the number of beams and instruments. Office and laboratory space will also be provided for outside and ORNL researchers using the new beams.

Another important research use for the HFIR is
neutron activation analysis.

The blue glow of the HFIR fuel core submerged in cooling water.

Another important research use for the HFIR is neutron activation analysis. This very sensitive, quick technique determines elemental composition, including trace elements, in a sample. The sample is inserted in the reactor for a predetermined, usually short, time. The intense neutron bombardment will make many of the elements radioactive. The elements can be identified by detecting and measuring energies of characteristic gamma rays from the sample. A recent, important example is the analysis of samples from the floodplain of East Fork Poplar Creek in Oak Ridge for mercury, cadmium, chromium, and other environmental pollutants. Another, rather dramatic, application was the analysis of samples of hair and nails from the body of President Zachary Taylor: Almost 150 years after his death, neutron activation analysis at the HFIR showed conclusively that his arsenic levels were not elevated above normal, proving that, contrary to some historical theories, Taylor did not succumb to arsenic poisoning. The upgrade proposal includes more, and larger, facilities for accessing the reactor with samples for neutron activation and measurements on the irradiated samples.

The development of new fusion and fission reactor systems depends on the development and testing of materials that can retain, for example, their strength, ductility, and shape, even under intense radiation. Because of its high flux and versatility, the HFIR is the preferred reactor for conducting these materials tests. The upgrade will provide improved capabilities for both handling and dismantling the test capsules.

Because neutrons are now so important to so many fields of science and engineering, the upgrade program is carefully planned to minimize the time that the HFIR will be out of service and to complete the work before the HFBR at Brookhaven shuts down for a planned upgrade of its own. The goal is to ensure that the United States will have use of one or the other of the two sources at all times.Colin West, manager of the Neutron Sciences Program.

Fruits of Neutron Research

Neutrons are electrically neutral, but their impact on research has been positive. Because these neutral particles interact weakly with most materials, they penetrate to great depths in a target, unlike charged particles, light, or X rays, which are stopped near the surface. This property is especially important for controlled experiments such as observing lubricant flow in an operating engine or biological molecules immersed in water. Those few neutrons that do scatter from atomic nuclei inside a material can provide unique information about its atoms, including their arrangement, motions, and vibrational energies.

Neutrons are used to guide the creation of new materials and new products such as Kevlar bulletproof vests and complex synthetic oils. In complex materials such as polymers and plastics that contain many atoms bound into very large molecules, a technique called molecular substitution provides unique opportunities for molecular engineering. Because neutron scattering probabilities vary substantially between isotopes such as light and heavy hydrogen (deuterium), one isotope can be selectively substituted for another within a molecule, and this tagged molecule can be observed. Such information helps researchers understand and determine the processes that produce the best plastic for a particular use.

Small-angle neutron scattering patterns showing the first observation of a surprising complexity in the response of a soap solution to shear flow (motion that results from spreading soap solution by rubbing hands together). As shown here, the molecular structures can become disentangled and aligned enough to flow freely.

Neutrons also behave like tiny magnets. This property has made them indispensable in the development of magnetic materials such as those used in credit cards, computer storage disks, videotapes, and electric motors. In fact, virtually all knowledge about magnetic media has come from neutron science.

The metal alloy INVAR does not expand at around room temperature. This property makes it ideal for uses requiring dimensional stability (e.g., microwave guides, lasers, metal seals, radar cavities, pressure gauges, and precision machine tools). Neutron scattering has shown that magnetic interactions of atoms in INVAR may counterbalance thermal expansion effects. This understanding opens up possibilities for new applications.

New high-temperature superconducting materials, such as yttrium-barium-copper oxides (YBa2Cu3O7-x), which conduct electricity without resistance when chilled by inexpensive liquid nitrogen, may be the heart of tomorrow's energy-saving technologies. Neutron scattering has been an essential technique for understanding how the best superconducting materials are built, how they work, and how to make wires and magnets from them.

Traditional polymer processing to make new plastics creates undesirable by-products and contamination. Recently, using small-angle neutron scattering, scientists discovered new ways to synthesize polymers in environmentally safe carbon dioxide. This discovery could lead to new low-cost polymer processing techniques.

The commercial and technological success of fluids we use daily—:such as lubricants, paints, and shampoos—depend largely on the ordering of their molecules under flow. For example, paint should stick generously to the brush dipped in the can but flow smoothly when brushed on the wall. This convenient property shift is caused by molecular reordering. Neutron scattering investigation of such fluids helps us understand how such reordering occurs.

Recently, researchers from Harvard Medical School and Forsyth Dental Center have jointly used neutron scattering to study interactions between collagen molecules and mineral deposits that make up the structure of bones. Neutron scattering investigations show that in healthy bones, collagen molecules form a regular nearby periodic array with an average spacing of about 1.5 nanometers, and minerals deposit in spaces along this array every 87 nanometers. Diseases like osteoporosis cause loss of mineral deposits, so adjacent spaces are no longer locked together, weakening the bones and causing them to shrink. This highly specific diagnostic tool makes it possible to devise and test remedies for demineralizing diseases.

In manufacturing processes, such as when two metal pipes are welded together or a metal part is heated, residual stresses may be trapped deep inside the metals. These stresses can cause parts to warp or break when put into use. Because neutrons can penetrate to great depths in these metal parts and "measure" variations in distances between planes of atoms compared with the normal structure, it is possible to actually map residual stresses caused by manufacturing. Such neutron residual stress measurements have been used to guide changes in manufacturing processes to produce improved automobile gears and brake rotors, welds in fuel tanks for the space shuttle, and boiler pipes in high-pressure steam generators. Penetrating neutrons could help U.S. industry penetrate new markets.

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