Development of compact neutron sources

. Many small research reactors, neutron sources built in the 1950s and 1960s are being shut down. The activities that these supported have therefore ended, which motivates their replacements with new facilities. Compact accelerator-based neutron sources under development are able to take up many of those activities. This paper paints the background for these developments, summarizes the replacement technologies, relates recent work that supports them, and alludes to some of the new activities that they enable.


Small research reactors
During the 1950s and 1960s, numerous laboratories and universities built and operated several dozens of research and test reactors (distinguished from power reactors). That was the optimistic era of "Atoms for Peace" programs around the world. The reactors were of many different types, operating at total thermal powers between 1 and ~10 MW and providing steady thermal neutron fluxes between 10 12 and 10 13 n/cm 2 -sec. The table below presents a representative list. Most have offered materials irradiation (neutron damage) and activation analysis facilities, beams for neutron cross section measurements, and capabilities for medical and industrial isotope production. They also provided for reactor physics tests and included arrays of instruments for slow-neutron scattering research. In addition, they supported nuclear data and nuclear astrophysics measurements and still more applications such as the following: materials science neutron scattering studies of atomic structures and motions, neutron-induced radiation damage in power reactor materials, cross sections for design and production of medical isotope production systems, and cross sections for neutron-modulated nuclear reactions relevant to astrophysical isotope production. Around most of these reactors grew programs of nuclear science education, small-source technology (especially moderator development relevant to both small and large sources), reactor physics tests, neutron physics and materials science instrumentation development, neutron detector improvements, radiation applications, and training. Many of these became full-fledged university Nuclear Engineering departments, which furnish undergraduate and graduate education in broad programs in radiation physics and power reactor development.
About 15 neutron summer schools established in those times, as well as new ones coming into being (see Table of Neutron Summer Schools, below), train students in the use of the local facilities and for entering research facilities at the major (~ 10 15 -10 17 n/cm 2 -sec) neutron sources based on fission (notably, for many years starting in the 1970s and still so, ILL) and spallation reactions (notably, SNS at ORNL).

What's being lost
A large fraction of the original complement of small neutron sources has now been or will soon be shut down. Some have been dismantled on the basis of fear of reactorrelated incidents, none of which ever involved any laboratory-or university-based reactors. And some midscale reactors have been shut down to be replaced. Capabilities are being lost as the small reactors are shutting down, especially for device prototyping, techniques development, and education and practice in the field, as well as opportunities for hands-on student projects and laboratory experiments.
Also being lost as some of the larger facilities are closing are medical isotope production facilities (especially for Mo 99 production), a significant fraction of materialsscience neutron-scattering capabilities, and facilities for neutron-induced transmutation doping of high-purity silicon to produce precisely controlled volumetric n-type P 31 -doped material for use, for example, in photovoltaic cells (the reaction is Si 30 + n -> Si 31 -> P 31 + e + ). Also diminished are the concentrations of expertise of engineers and scientists residing there and the consequent loss of bases for education and training of young scientists.

New sources
All these losses motivate development of new neutron facilities, probably compact, small, accelerator-driven neutron sources, CANS, and based on low-energy (typically less than 100 MeV particle energies) neutronproducing charged-particle reactions. Simply put, it's not that CANS are much better than reactors, it is that they are different, with a broad range of applications that are now being elaborated. Slowing down in room-temperature moderators, these produce Maxwellian neutron spectra with temperatures ~ 300 K, mean energies 0.025 eV. As sources of neutron beams, accelerator-driven sources compensate for losing some of the reactor-based installations. Furthermore, the accelerator-based facilities are more than neutron sources; they are also sources of charged-particle beams useful for other purposes. The topics covered in the 2018 UCANS VII meeting were Accelerator and beam optics, Neutron detection, Nuclear astrophysics and nuclear data, Target radiation damage and heat removal, CANS projects and innovative instrumentation, Medical applications, Material characterization, Computer simulations and data analysis software, Optical devices, Other applications of CANS, and Education. See Carpenter, et al. 2018 [2]. The 2019 meeting in Paris, UCANS VIII, is the first UCANS meeting recorded in a published proceedings. Consult www.ucans.org for current information.

A conceptual CANS installation
The figure is a schematic representation of a compact accelerator-based neutron source. Such systems are either purchased from existing suppliers (smaller systems) or designed and built in-house (larger ones) to local specifications.

Applications
Existing CANS support a wide variety of applications (Anderson, et al. 2017 [3]), for example, in the developing field of characterizing cultural artifacts (Kardjilov and Festa 2017 [4]). Several groups are developing and deploying portable neutron radiography technology for nondestructive infrastructure (roads and bridges) inspection and contraband nuclear materials (cargo) detection. Fissile materials give themselves away by emitting fission-generated neutrons that return with a time and energy signature detectably distinct from those scattered from common nonfissionable materials. CANS cargo inspection facilities are probably inexpensive enough to deploy permanently at ports where they are needed and to justify mobile inspection CANS stationed to move around when called for.
CANS are well-suited for simulations of single-event upsets (SEUs) caused by fast-neutron scattering in semiconductor devices in the high atmosphere. For these purposes, it would be easier in CANS to tailor neutron spectra than in the large facilities. Also, some of materials-science investigations that researchers carry out with higher resolution and at higher rates of throughput at the major sources rely on preliminary tests at a small source. Furthermore, a long-foreseen and hoped-for application of CANS in boron capture neutron therapy, BNCT, awaits development of boron-containing cancertumor-seeking pharmaceuticals. And there is pressing need for alternatives to reactor-produced medical isotopes, notably Mo 99 , but other isotopes also, where hopes lie with accelerator-based production. Table 4

An example
The 2-MW Ford Nuclear Reactor (criticality 1957) at the University of Michigan's Phoenix Memorial Laboratory is an example of a response to the shutting down of an Atoms for Peace reactor. There, the Nuclear Engineering and Radiological Sciences Department has installed a d,d (2.2 MeV neutrons) and a d,t (14 MeV neutrons) neutron generator in its new Neutron Sciences Laboratory and an array of several low-energy charged-particle accelerators (p, d, , …) in its Ion Beam Laboratory. These serve both teaching and research purposes involving students and faculty (see [5] for more details).
Throughout, these applications follow from the intrinsic advantages of CANS over reactors and the large, multiuser neutron facilities: low acquisition and maintenance costs, flexible scheduling, lighter safety concerns, easier access, local management, and more.