In the OSTI Collections: Neutron Sources for Studying Matter
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Two sources and things explored with their use
Research Organizations and Facilities
Reports available through OSTI's SciTech Connect
Understanding what makes any particular material strong or weak, reactive or inert, conductive or insulating, ductile or brittle, transparent or opaque, or possessed of any other qualities and behaviors is largely a matter of knowing how its atoms and the chemical bonds between them are arranged. This arrangement determines how the material affects anything it interacts with, including the beams of photons that make up light. If different features of objects are made of different materials, they will be distinguishable as the materials differently affect the light’s color, intensity, or polarization.
Light has limitations in what it can reveal about materials, since some materials’ differing atomic arrangements don’t have very different effects on photons. However, even those materials may have significantly different effects on other particles besides photons, so features that don’t look different when seen with photons can be easily distinguished when observed with other radiations.
Beams of neutrons have been put to work to show us things about materials that photons can’t. Whereas a beam of photons will interact electromagnetically with anything that has an electric charge, particularly the electrons that constitute the outermost parts of atoms, neutrons are primarily affected, not by the electromagnetic force, but by what’s known as the strong force, by which the neutrons interact with the nucleons (protons and other neutrons) in atoms’ nuclei. Since electric charges are likely to appear throughout an atom, while nuclei occupy very small fractions of the volume of each atom (roughly 10-15, the ratio of a cubic millimeter to a cubic hectometer), photons tend to interact more with atoms near a material’s surface without reaching its interior, while neutrons are likely to travel well into the bulk of a material before interacting with anything. Also, while photon beams are affected more by atoms that have more electrons, neutron beams’ behavior is less simply described, as the neutrons can be affected more by smaller nuclei than by some larger nuclei. Thus neutrons often show the locations of small, few-electron atoms like hydrogen more clearly than photons do.
While neutrons are mainly affected by the strong force, each neutron is a magnet and so is affected electromagnetically by other magnetic particles. Observing the results of neutrons’ electromagnetic interactions can thus reveal atom-sized (or smaller) details of a material’s internal magnetic fields.
Every source of light consists of electric charges or magnetic particles that, when appropriately disturbed, generate photons. Neutron beams are ordinarily made with more difficulty, by removing preexisting neutrons from nuclei using any of several nuclear reactions. In many cases, the difficulty is worth undertaking to discover things that only neutrons can show. Some small-scale devices produce low-intensity neutron beams, but a few larger-scale neutron sources have been developed to produce more intense beams for experiments that require them. Some experiments conducted with the larger sources’ beams, as well as the sources’ technical features, are described in several reports available through OSTI’s SciTech Connect.
Two sources and things explored with their use
Two of the largest neutron sources are at Oak Ridge National Laboratory (ORNL): the High Flux Isotope Reactor and the Spallation Neutron Source. Each exemplifies a different method of producing neutrons from nuclei; each method is also used at other neutron sources.
The High Flux Isotope Reactor works like any nuclear reactor that involves a chain of fission reactions, a process described in detail in a previous article[OSTI]. Briefly, the fuel nuclei are composed of nucleons held together by the strong force. While neutrons have no net electric charge, the net charge of protons is positive, so the cohesion of each nucleus is opposed by the mutual electric repulsion among its positive charges. When a nucleus of the reactor fuel absorbs a sufficiently energetic neutron from its surroundings, it becomes less stable, enabling the positive charges’ repulsion to split the nucleus apart into two smaller nuclei. The smaller nuclei have too little attraction among their protons and neutrons to hold onto the most energetic neutrons, so these neutrons fly out of the nuclei. Some of them may be absorbed by other fissile fuel nuclei to continue the reaction chain, but others may miss the other nuclei entirely and escape from the reactor. The High Flux Isotope Reactor (HFIR) is designed to allow many such neutrons to escape, where they can be diverted to “illuminate” materials and reveal details about them that a flux of photons couldn’t.
For some purposes it’s useful to have neutrons arrive in pulses instead of a steady flux. To produce neutron pulses, one can use electromagnetic forces to accelerate pulses of charged particles into a heavy metal target; the Spallation Neutron Source (SNS) accelerates protons into a target of mercury. The protons smash nuclei of the mercury atoms into fragments (spall) that include tens of energetic neutrons. The neutrons generally have more energy than one wants for the observations to be made with them, so they are slowed down by being passed through substances that can absorb much of their energy before they reach the material that they’re meant to illuminate.
Both of ORNL’s large neutron sources are “user facilities”[DoE] at which researchers from ORNL and elsewhere can apply for time to bring their own materials and observe how neutron beams affect them. Numerous examples of the kinds of experiments done, described in reports like “2010 Neutron Review: ORNL Neutron Sciences Progress Report”[SciTech Connect], show the variety of things that can be learned from having materials interact with neutron beams. The following is a small sample of the research described in that report:
Neutron-beam experiments with a wide variety of iron-based superconductors have shown how different superconductors’ atoms and their magnetic fields are arranged, leading to an understanding of which kinds of iron-based materials would and would not superconduct, among other things. (Pp. 16-18)
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Figure 1. Crystal structures in the iron-based superconductor LaFeAsO above the superconducting temperature TS (a) and below the normally conducting temperature TN (b). The structure below TN is antiferromagnetic, so the minimum-sized repeating unit of the atom arrangement in structure (b) is double the size of that in structure (a). The study by Li et al. suggests that these two structures coexist dynamically at temperatures TS > T > TN. (“2010 Neutron Review: ORNL Neutron Sciences Progress Report”[SciTech Connect], p. 18.) |
Exhaust-gas recirculation coolers, intended to make auto engines more efficient by reducing the formation of nitrogen oxides in the cylinders, can become clogged with deposits of hydrocarbon particles. Since the nuclei of the coolers’ metal atoms interact less with the neutrons than the hydrocarbons’ numerous hydrogen nuclei do, neutron-based images of the coolers show clearly where and how the particle deposits form. The details of this process provide clues for figuring out ways to prevent it. (P. 20)
The way atoms are arranged in a material may change as the material undergoes stress—either weakening the material if its atoms are moved further apart, or strengthening it if the atoms are pushed closer together. The various processes that turn materials into useful products are intended to enhance their strength and reduce or eliminate their weaknesses for the job required of them. Neutron beams have been used to map atomic rearrangements in products like automobile components and the welds of pressurized-water nuclear plants that join their coolant pipes to the plants’ reactor vessels, in order to see how current manufacturing techniques can be improved to make the products stronger. (Pp. 24-25)
When a virus infects the cells of its host, the cells’ functioning is taken over by the virus so that the cells replicate numerous copies of the virus, which may then be released from those cells to infect other cells or other hosts. Viral interactions with neutron beams have shown, though, that a virus that infects different species can replicate itself with somewhat different structures in each species. The development of antiviral compounds that stop the viral replication may be informed by an understanding of how viral structures change with infection. The insect-borne Sindbis virus consists of a single-stranded RNA core inside a protein shell (a capsid) surrounded by a lipid membrane that is itself surrounded by a second protein shell. Neutron beams have shown that, compared to Sindbis virus grown in insect cells, Sindbis grown in mammal cells has significantly more cholesterol in its lipid bilayer, a more extended outer protein coat, and a different distribution of RNA in its core. (Pp. 36-37)
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Figure 2. The degree of neutron scattering at different radii of the Sindbis virus particles is different for viruses grown in mammal cells and in insect cells, indicating a difference in the virus’ composition. (“2010 Neutron Review: ORNL Neutron Sciences Progress Report”[SciTech Connect], p. 37.) |
Neutrons, like photons, can also do more than reveal features of the material they interact with; in some cases, they can cause lasting changes in those materials. As a first step in experiments to prepare samples of element 117 (ununseptium), an element not found in nature, neutrons from the HFIR were captured by less-massive nuclei to form atoms of the berkelium isotope 249 [Wikipedia], most of which will undergo beta decay with a half-life of 330 days. Berkelium-249 atoms were taken to the Joint Institute for Nuclear Research in Dubna, Russia, where those that did not undergo radioactive decay into other elements first were bombarded with neutron-rich nuclei of calcium (Ca-48). Berkelium and calcium nuclei fused together would form atoms of element 117. (P. 19)
Other reports describe single investigations involving either or both of ORNL’s large neutron sources. Some of the most recent studies deal with material properties of nuclear fuel that are relevant for the design and operation of nuclear power plants. Nuclear fuel, through its controlled chain reactions, radiates subatomic particles that generate heat which (a) can be partially converted into electric power and (b) affects the atomic arrangement and thermal conductivity of the fuel itself. An April 2014 report[SciTech Connect] from Idaho National Laboratory’s Center for Materials Science of Nuclear Fuels deals with the use of neutrons from both the SNS and the HFIR to determine what these effects are in uranium dioxide fuel. How effectively uranium dioxide can conduct heat, in the form of random vibrations in its atoms’ lattice structure, depends on what ways that lattice structure is able to vibrate. This can be determined by observing how neutrons are scattered from uranium dioxide atoms’ nuclei. The experiments described in the report were intended to determine how the fuel’s thermal conductivity depends on the fuel’s temperature and on defects in its atomic arrangement brought on by radiation; information gained from the measurements revealed some deficiencies in existing theoretical simulations of uranium dioxide’s behavior.
A preliminary report prepared for the Spring 2014 conference of the Materials Research Society describes “Neutron Scattering of CeNi at the SNS-ORNL”.[SciTech Connect] The flight times and directions of neutrons scattered from cerium-nickel alloy under various conditions depend on, among other things, the behavior of particular electrons in the cerium atoms that determine the cerium’s magnetic properties. The preliminary report is very brief and provides very little detail about the neutron scattering experiments or the reasons for undertaking them, but their relevance for the behavior of nuclear fuels, such as plutonium, is indicated by the statement that “The manifestations of electron-correlation in Pu [plutonium] and Ce [cerium] have interesting parallels”.
The present large neutron sources, while already useful, have the potential to become even more useful if improved in various ways. Several reports deal with various advancements in neutron-source technology.
Safe operation of the High Flux Isotope Reactor has been supported for over 50 years by computer models of its thermal hydraulic characteristics. While these models were sufficiently accurate, even more accurate models are now possible with modern computational tools. Work described in the University of Tennessee, Knoxville dissertation “Thermal Hydraulic Characteristics of Fuel Defects in Plate Type Nuclear Research Reactors”[SciTech Connect] describes such an update, occasioned by a proposed conversion of the HFIR’s nuclear fuel from highly-enriched uranium (93% of the uranium being the fissile isotope U-235) to a low-enriched uranium (19.75% of which is U-235). The low-enriched fuel is to be a monolithic foil that is denser than the packed-powder cermet and filler containing highly-enriched uranium, so that the new fuel elements can be arranged in the same overall configuration as the old ones and still produce the same neutron flux. The dissertation work included the mathematical modeling of the fuel plates’ interiors, the turbulent flow of coolant around them, and variable fluid properties, and demonstrated through computations and experimental observations that COMSOL modeling software[Wikipedia], designed to simulate physical processes that involve multiple simultaneous phenomena[Wikipedia], can accurately represent the reactor’s behavior and provide useful information about how defects in its fuel plates would affect its performance.
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Figure 3. Top: a cutaway view of the core of the High Flux Isotope Reactor, with an inset photo of the fuel elements. Bottom: an axial view of the radial cross-section of an Inner Fuel Element Fuel Plate. (From “Thermal Hydraulic Characteristics of Fuel Defects in Plate Type Nuclear Research Reactors”[SciTech Connect], pp. 3-4.) |
Such a mathematical analysis of a process like this, which involves multiple physical phenomena, is feasible when the phenomena, and their relative importances for the process, are understood well enough to describe mathematically. When one is still at the stage of finding out what contributes to a process, one has to do real experiments instead of simulated ones; indeed, experiment is the ultimate source of information that informs the development of mathematical models in the first place. This is the situation for the work described in a report entitled “Volume and Surface-Enhanced Volume Negative Ion Sources”[SciTech Connect], a chapter in the proceedings[CERN] of a 2012 CERN Accelerator School on ion sources that describes experiments done to develop proton sources for the Spallation Neutron Source. The chapter title refers to “Negative Ion Sources” because the protons are most easily produced by first generating negative hydrogen ions (specifically, hydrogen atoms each made of a proton bound to two electrons instead of one), accelerating the ions, stripping their electrons off in transit, and further accelerating the remaining protons. While the ion sources described are of the same basic types used in some particle accelerators and magnetic fusion experiments, achieving the Spallation Neutron Source’s neutron-beam output required a combination of large ion current and high percentage of “on” time (high duty cycle[WhatIs.com]) that would mutually conflict using only previous ion-source technology. The chapter’s description of experiments that revealed more about how different ion production methods worked and showed how to overcome the conflict serves as a short history of how these types of ion source were developed.
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Figure 4. Schematic of the Spallation Neutron Source’s hydrogen ion source and the initial stage of its proton accelerator, the Low Energy Beam Transport. (From “Volume and Surface-Enhanced Volume Negative Ion Sources”[SciTech Connect], p. 273 in the proceedings of the CERN Accelerator School on Ion Sources held May 29-June 8, 2012 in Senec, Slovakia.) |
It’s obviously efficient to have all the protons in a beam reach their target. There’s also another reason to keep from losing protons from the beam along the way: protons that deviate from the intended beam path can run into components of the accelerator that drives and directs the beam, possibly damaging the components and/or making them radioactive. Several mechanisms can cause a beam of charged particles to spread and form a halo around the intended path large enough to intercept the surrounding accelerator components. A report[SciTech Connect] from the Republic of Korea’s Institute for Basic Science, published last year in Physical Review Special Topics – Accelerators and Beams, describes evidence of a new halo-forming mechanism in the SNS’s linear accelerator (linac) and how it was mitigated.
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Figure 5. Graph of the measured vertical beam distribution of the nominal Medium Energy Beam Transport optics (top) and the round beam optics (bottom) in which the halo is substantially reduced. Different colors indicate different densities of protons. Each graph’s horizontal axis represents proton positions, while the vertical axis shows the rate at which a proton’s position changes for every unit of distance the proton travels through the accelerator (expressed in milliradians, or millimeters per meter). (From “Evidence of a halo formation mechanism in the Spallation Neutron Source linac”[SciTech Connect] , p. 5.) |
Reducing losses from a particle beam, and optimizing how an accelerator affects the beam’s shape and motion, requires knowing what condition the accelerator puts the beam into. The ORNL report “Noninterceptive method to measure longitudinal Twiss parameters of a beam in a hadron linear accelerator using beam position monitors”[SciTech Connect] describes a new way to use standard beam-position monitors to parameterize the distribution of protons along a proton beam’s length, as well as a demonstration of the new method’s applicability in the SNS’s linac. The report also compares the results of the new method and a different measurement technique, and discusses the new method’s limitations and advantages.
A neutron source’s usefulness may also be enhanced by providing greater control over the neutron beams it ultimately produces as well as increased information about the neutrons’ state after they interact with objects exposed to the beams. A particular type of control and information can be provided by the same device, as a brief summary[SciTech Connect] of work done at Indiana University shows. This report documents the development of a “spin filter” device, consisting of a solenoid that surrounds a cell full of helium-3 atoms. The device is unequally transparent to neutrons whose “spins”, or directions of rotation, are oriented differently. Because of the unequal transparency, the spins of most neutrons that get through such a device will be clockwise around a direction that depends on how the solenoid’s magnetic field is oriented. Materials containing nuclei that interact differently with neutrons of different spin will affect beams of differently-spinning neutrons accordingly. How the neutron spins are oriented after the neutrons have interacted with the material can be determined by how many of the neutrons pass through another spin filter of the same type when its magnetic field is also oriented in various ways. The Indiana work was done in collaboration with spin filter programs at the Spallation Neutron Source, the NIST Center for Neutron Research at the National Institute of Standards and Technology (NIST), and the Jülich Centre for Neutron Science at the Forschungszentrum Jülich GmbH (Jülich Research Centre).
Neutron source technology didn’t begin and end with the large facilities at Oak Ridge National Laboratory. The facilities of the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory, for example, include a similarly-used neutron source that uses an 800,000,000-electronvolt beam of protons to produce neutrons by spallation from a tungsten target. (Oak Ridge’s SNS produces neutrons from its mercury target with more energetic protons accelerated to 1,000,000,000 electronvolts each.) But one of the newest additions to the Los Alamos neutron source uses neutron beams to study, not other materials, but the nature of the neutron itself.
Studying the neutron itself
Photons, as far as we’ve determined to date, are relatively featureless entities that have no component parts. Neutrons, on the other hand, are assemblages of quarks held together by the same strong force that dominates neutrons’ interactions with each other and with similar particles. A neutron is thus a system of more basic particles—a system whose characteristics, in fact, are still imprecisely known. Increasing the precision of this knowledge is one purpose of LANSCE’s new Ultracold Neutron Source.
The fact sheet “Nuclear Physics: The Ultracold Neutron Source”[SciTech Connect] from Los Alamos describes this purpose and some details of the ultracold source itself. The source is “ultracold” in that the neutrons, although they start out at high speeds when the tungsten target is spalled, are slowed down to the very low, few meter-per-second speeds characteristic of nucleons cooled to millikelvin temperatures (i.e., about a thousandth of a kelvin above absolute zero). While these neutrons’ interactions with other matter are also of interest and are in fact being studied, their low energy lets the neutrons be contained and transported with relative ease, and their properties be precisely measured. Some of these properties relate to a type of interaction that neutrons are subject to besides the strong force and the electromagnetic force, namely the “weak” force, which is closely related to the electromagnetic force, whereby quarks, leptons, and things made of them interact with the particles known as Ws and Zs. (Particles called gluons interact with quarks and with each other by means of the “strong” force; photons interact with quarks, charged leptons, and W particles by the electromagnetic force. And all particles interact gravitationally.)
Because all known physical processes apparently amount in the end to strong, “electroweak”, and gravitational interactions of material particles, then the more detailed our understanding is of the laws that govern these interactions, the more capable we become of accurately analyzing and predicting the courses of physical processes, at least the most mathematically tractable ones. Neutrons that aren’t bound to other nucleons, for instance in an atomic nucleus, have a half-life of about 880 seconds: in that time, on average, half of any collection of free neutrons will decay by a weak interaction into a proton, an electron, and an electron antineutrino. More precisely, one of the neutron’s d quarks transforms into a u quark as it emits a negatively-charged W particle; the W particle in turn decays into an electron and an electron antineutrino.
The law governing weak interactions determines the probability that a free neutron will undergo this type of decay in any given time t after the neutron is formed. The same law also dictates the probability that the resulting proton will recoil from the W particle in a given direction relative to the spin orientation of the original neutron or some other direction; likewise, the probabilities that the momenta of the electron and electron neutrino will have particular directions are expressed by the weak interaction law. Thus measuring the momenta of the end-product particles in many neutron decays provides a great deal of information about the law that governs all weak interactions, including processes that don’t involve neutron decay. A more detailed presentation of what this information consists of, and how observations of decaying ultracold neutrons produce it, is represented by the slide set entitled “The Science Program at the Los Alamos Ultracold Neutron Source”[SciTech Connect]. It indicates how the mathematical expression of the weak interaction law is related to observable features of neutron decay, and describes in some detail the experiments planned and already done for observing those features.
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Figure 6. Correlations between the momenta of neutron decay products (
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Wikipedia
WhatIs.com
Research Organizations and Facilities
- Oak Ridge National Laboratory (ORNL)
- High Flux Isotope Reactor
- Spallation Neutron Source
- Joint Institute for Nuclear Research
- Idaho National Laboratory
- Materials Research Society
- University of Tennessee, Knoxville
- CERN
- Institute for Basic Science
- Indiana University
- National Institute of Standards and Technology (NIST): NIST Center for Neutron Research
- Forschungszentrum Jülich GmbH (Jülich Research Centre): Jülich Centre for Neutron Science
- Los Alamos National Laboratory: Los Alamos Neutron Science Center (LANSCE): Ultracold Neutron Source
Reports Available through OSTI’s SciTech Connect |
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- “In the OSTI Collections: Fission Theory”
- “User Facilities”, DoEOffice of Science
- CERN Accelerator School, Ion Sources (held May 29-June 8, 2012 in Senec, Slovakia)
- Physical Review Special Topics – Accelerators and Beams
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