The Canadian Neutron Facility.
It is recognized worldwide that neutron beam experiments provide key information that is essential for the development of advanced materials. New neutron beam laboratories are being planned, constructed or commissioned in countries both large and small -- the United States, Germany, Australia, China, Taiwan and Switzerland, to name a few. Since 1990, there has been a sustained call for renewal of Canada's ability to exploit neutron scattering methods for advanced materials research. This call has come from NSERC studies on major facilities for materials research, the Canadian Association of Physicists, the Canadian Society for Chemistry, university leaders and researchers, industrial scientists and international experts. In 1998, the National Research Council of Canada (NRC) and Atomic Energy of Canada Limited (AECL) joined forces to advocate the construction of a national facility that would provide neutrons to support a wide spectrum of scientific and engineering research.
The proposed Canadian Neutron Facility (CNF) will be centred on a new medium-flux nuclear reactor as the source of neutrons for experiments performed both inside and outside the reactor core. Inside the reactor core, there will be specialized facilities that exploit the internal neutron flux for testing new fuel configurations and new reactor materials. This research will enable the responsible stewardship of installed CANDU power-generation stations, both domestic and foreign. Also, incore experiments will play an essential role in developing the next generation of CANDU power reactors, to meet the future energy needs of the world without adding to the global burden of greenhouse gases. The reactor core will also contain a coldneutron source. Beam tubes will carry thermal and cold neutrons to a suite of instruments located both inside and outside the reactor containment building. These instruments, operated by a team of scientists and technicians, will comprise a world-class neutron beam laboratory. The CNF neutron beam laboratory will be operated as a multidisciplinary user facility, available to researchers from universities, industries and government agencies from across Canada and around the world.
A Solid Foundation of Experience and Know-How
Operating an international, neutron-beam user facility is nothing new to Canada. The National Research Universal (NRU) reactor has been running at Chalk River Laboratories since 1957. The NRU reactor (together with its precursor, the NRX reactor) was the source of neutrons for the pioneering work of Dr. Bertram Brockhouse, who shared the 1994 Nobel prize in physics for his development of neutron scattering methods that continue to guide our understanding of solid-state and liquid-state matter. Today, at the NRU reactor, the NRC's Neutron Program for Materials Research (NPMR) operates a suite of five thermal neutron instruments as a user facility. About 100 user projects are completed each year. NPMR personnel collaborate in these projects with industry clients and visiting university researchers, often contributing to the education of graduate students. For research where the results are destined for open-literature publication, access to the neutron beam laboratory is granted free of charge, after the scient ific merit of the proposed research has been established by peer review. Industrial clients have the option to keep results proprietary by arranging for neutron beam access through a fee-for-service agreement. User projects span a wide range of topics, from the physics of frustrated magnets, to the kinetics of precipitation, to phase transitions of biomimetic membranes, to intergranular stresses in metals, and so on. In addition to collaborating with visiting researchers, the scientific personnel of the NPMR maintain individual research programs primarily in five subject areas: 1) materials engineering; 2) magnetism; 3) structure and dynamics of molecular systems; 4) biophysics; and 5) surfaces and interfaces. The thermal neutron beams available from the NRU reactor are well-suited to carry out high-impact experiments in these subject areas.
Why Neutron Experiments Are So Helpful
A neutron beam capability is an essential element of a nation's scientific infrastructure because of the unique way in which neutrons interact with matter. Thermal neutrons have wavelengths that match the distance scales found in condensed matter systems (ie. 0.05 to 2 nm). Their energies are in the range of crystal lattice vibrations, magnetic excitations and atomic diffusion (ie. 0.5 to 50 meV). When neutrons scatter from a material, changes in their energy and momentum are readily determined and can be related to the intermolecular dynamics and structures in the specimen. 'While much important information can be obtained by other techniques (X-rays, light scattering, NMR, msR and a host of microscopy methods), neutron scattering provides crucial knowledge that cannot normally be acquired even by a combination of these other probes.
Neutrons interact primarily with the nuclei of atoms, through a short-range interaction. This means that a neutron entering a material sees mostly empty space, and that neutrons penetrate deeply (many centimetres) into most materials. Whereas X-rays and electron beams probe the surface of a specimen, perhaps penetrating to a depth of a few microns, neutrons can survey the interior of a specimen. For example, neutron diffraction is applied to acquire three-dimensional maps of residual stress, non-destructively, in automotive engines, turbine discs, pipeline welds and other engineering components. It is also straightforward to carry out neutron diffraction measurements on specimens inside furnaces, cryostats and equipment to apply pressure, magnetic field or corrosive environments. Neutrons easily penetrate through the specimen environment to probe the specimen under the desired conditions.
The interaction between neutron and nucleus varies widely and unsystematically from one atom in the periodic table to the next, and from one isotope of a given atom to the next. It is often the case that neighbouring atoms in the periodic table have easily distinguishable scattering properties, and that light atoms are as easy to see with neutrons as are heavy atoms. With probes like X-rays, which are sensitive to the number of electrons in an atom it can, for example, be difficult to obtain a clear picture of the crystal structures in lithium battery materials without the complementary information provided by neutron diffraction. The light atom, lithium, scatters neutrons in a strong and distinctive manner, compared to the surrounding atoms, such as manganese or oxygen. Neutron scattering is very sensitive to the presence of hydrogen and readily distinguishes its heavier isotope, deuterium, from the light isotope. This distinctive scattering characteristic allows researchers to highlight specific groups in complex molecules by selective deuteration and leads to major advances in our understanding of structures that occur, for example when viruses attack membranes, or when polymer layers interdiffuse.
Finally, neutrons are neutral particles but possess a magnetic moment, which interacts with magnetic ions in materials. Much of our detailed understanding of magnetic excitations, phase transitions and critical phenomena is based on neutron scattering experiments. Neutron scattering is well suited to investigate hard magnet materials and nano-scale magnetic multilayers. Progress towards understanding the mechanisms of high-temperature superconductivity also depends on the experimental data that can most effectively be obtained by neutron scattering measurements.
CNF Expands Canada's Neutron Scattering Capability
The neutron beam laboratory at the CNF will support a wider range of scientific topics and a much larger user community than has been possible with the NRU-based program. The cold neutron source in the core of the CNF reactor will boost the flux of lower-energy, longer-wavelength neutrons to more than ten times the level that can be achieved at the NRU reactor, which does not include a cold source. With so many more "cold neutrons", it will be practical to apply neutron beam methods to investigate materials with larger length scales and lower-energy processes than has been possible with the thermal neutrons of the NRU. Of particular interest are soft materials such as polymers, lipids, peptides, gels, colloids and perhaps cell organelles. These are the hydrogen-rich materials of the 2lst century, where knowledge obtained by neutron scattering will underpin the development of organic devices, intelligent drug-delivery systems and nano-machines.
The CNF will be operated as an international user facility. Over the 40-year lifetime of the facility, more than 20,000 research projects will be completed, involving between 600 and 1000 users each year. Canadian researchers will steer this powerful national facility to meet our own strategic scientific and technical needs. Canadian companies will benefit from access to a domestic resource that provides unique information to solve their particular problems and increase their competitiveness in a global market. The CNF will also function as an element in an international network of neutron laboratories and other big-science facilities, leveraging access by Canadian scientists to specialized instruments abroad, and preparing Canadian researchers to make optimal use of foreign facilities, when required.
CNF Nuts and Bolts
The NRU reactor has a large, distributed core that operates at 120 MW (thermal energy output). The CNF will have a compact core, and operate at 40 MW. The smaller reactor with a modern control system will cost less to operate, use less fuel and produce only one third of the radioactive waste produced by the NRU.
The CNF's cold neutron source will be a half-litre container of liquid hydrogen, at a temperature of 20 K. When neutrons enter this container, repeated collisions with the cold hydrogen atoms will shift the neutron energy spectrum close to thermal equilibrium with the moderator liquid. Neutrons from the cold source will travel through "optically flat" coated guides over flight paths of the order 10 to 30 m. Such guides minimize the loss of neutron flux, through the phenomenon of total external reflection, analogous to the way light is transported with minimal loss through fibre optics via total internal reflection. Separate neutron guides will diverge to deliver cold neutrons to a suite of neutron beam instruments, which will be located in an experimental "guide hall" that is outside the reactor containment building. Thermal neutrons emanating from core regions outside the cold source will mostly be directed through beam tubes to instruments located inside the containment building. However, it is also planne d to install thermal-neutron guides to deliver thermal neutrons to one or two specialized instruments located in the guide hall.
The CNF neutron beam laboratory is intended to start up with 11 instruments. Five existing thermal neutron instruments from NRU will be upgraded and transferred to the CNF. These will maintain Canada's capabilities in non-destructive stress-scanning, materials testing, powder diffraction and inelastic scattering with thermal neutrons. One existing instrument will be rebuilt into a novel facility called "Diffractometer for Extreme Conditions", to be located outside the reactor containment building. In this facility, it will be possible to handle specimens that are too hazardous for experiments inside the reactor containment building, such as a running gas turbine, a highly radioactive specimen, a pressure vessel or even an explosive.
Six new instruments will be constructed to explore the new realms of knowledge opened up by the existence of a cold neutron source. The start-up suite of cold neutron instruments was decided at a workshop of the Canadian Institute of Neutron Scattering (CINS), considering criteria such as: that neutrons provide clear advantages over other methods; that a reactor-based neutron source provides an advantage over a spallation source; that the instrument would serve a strong demand (present or potential) in Canada; and that a "champion" could be identified within the CINS community.
The cold-neutron instruments to be included in the initial CNF project are:
1. A 30 m small-angle neutron scattering (SANS) machine, the workhorse for microstructural studies on polymers, bio-materials, colloids, gels, magnetic domains, precipitates in a matrix and fractal networks (for length scales from 1 nm to 500 nm);
2. A horizontal-surface neutron reflectometer, to characterize interface thickness and roughness in polymer films, oxide layers, electronic devices, biomimetic membranes, even on liquid surfaces (for length scales from 0.5 nm to 500 nm);
3. A diffractometer specially adapted for structural analysis of biological materials (e.g. aligned membranes in physiologically relevant conditions of pH, temperature and hydration, with inserted peptides or proteins);
4. A high-resolution spectrometer, to explore low frequency excitations, as might occur in diffusion of guest molecules in a clathrate or of water in cement as it cures;
5. A high-bandwidth spectrometer, to characterize broad excitations in superconductors and quantum spin systems;
6. A neutron instrument prototyping station.
The CNF is designed for expansion over its 40-year life expectancy, with space to add more neutron guides and up to 12 additional instruments to meet the emerging needs of science in the twenty-first century and to exploit technological breakthroughs as they occur. There is already a request from a private company (N-ray Services, Inc.) for space at the CNF to install a dedicated instrument for neutron radiography. This company will invest in a facility to enable it to capture a large share of the world market for radiographic and tomographic imaging of industrial objects, such as aircraft turbine blades and explosives.
CNF Schedule and Cost
The CNF has been "approved in principle", by a cabinet committee, but funds have not yet been allocated. Once the project starts, it will take about six years to complete the reactor and commission a subset of the start-up neutron instruments. The remaining neutron beam instruments will be commissioned over the two years after neutrons become available. The total cost of the reactor plus the neutron beam laboratory plus the facilities for carrying out the CANDU R&D associated with the CNF is estimated at $388 million in 1998 dollars, ($466 million, when escalated over the time it will take to complete the project). Of this total, the neutron beam laboratory will cost $90 million in 1998 dollars.
To operate the CNF-based neutron beam laboratory as a competitive resource for 600 to 1,000 users each year will require an on-site staff of about 50 persons, including scientists, post-doctoral fellows, technicians and administrators. With reasonable support to cover the costs of materials, supplies, services and equipment upgrades, the operating cost will be about $9 million per year, of which 10% to 15% is expected to be recovered as revenue from industrial clients.
John Root is a senior research officer and program leader of the NRC's Neutron Program for Materials Research at the Chalk River Laboratories in Chalk River, ON.
(1.) Root, J.H., P. Wanjara, S. Yue, R.A.L. Drew, A. Oddy, F. McDill, F. Marsiglio and R. Fong, "Neutron Diffraction for Industry: Optimized Processing, Failure Analysis and Regulations", Physica B 241-243 (1998) 1181-1188.
(2.) Root, J.H., T.M. Holden, D.C. Tennant and D. Leggett, "Non-invasive Measurements of Temperature by Neuton Diffraction" in Temperature its Measurement and Control in Science and Industry, J.F. Schooley, Ed., AIP 6, 1992, pp. 1231-1236.
(3.) Tun, Z., J.J. Noel and D.W. Shoesmith, "Electrochemical Modification of the Passive Oxide Layer on a Ti Film Observed by In Situ Neutron Reflectometry", J. Electrachemical Soc. 146: 988-994, 1999.