Originally published by APS Division of Physics of Beams at: https://www.aps.org/units/dpb/newsletters/upload/fall18.pdf.

SNS Spotlight: The world’s most powerful accelerator-based neutron source

Today, thanks in part to its superconducting radio-frequency (RF) linear accelerator, the Spallation Neutron Source remains the world’s most powerful pulsed neutron beam source, operating at or near its design power of 1.4 MW. The SNS accelerator complex, stretching nearly the length of three football fields, consists of a hydrogen ion source, a 1 GeV linac and a proton accumulator ring.

The proton beam is initially accelerated through a normal conducting copper drift-tube linac and a coupled-cavity linac. Once the particle beam reaches approximately 0.4c, the superconducting section of the accelerator takes over, accelerating the beam to 0.88c. Superconducting cavities permit more rapid ion acceleration per meter than a room-temperature copper linac and provide operational flexibility.

The SNS complex can deliver up to a 1.4 MW proton beam—the highest beam power ever delivered during a neutron production run cycle—in pulses 60 times per second to a mercury target. The mercury not only provides a ready supply of available neutrons, it also circulates to help dissipate the sudden bursts of energy that are produced by the proton pulses impacting the target. Each proton hitting the nucleus of a mercury molecule in the target “spalls” off 20 to 30 high-energy neutrons, a portion of which are directed to advanced beamline instruments. Cryogenic moderators are located next to the target to lower the neutrons’ energy for specific types of experiments.

Oak Ridge scientists and engineers have significantly extended the life expectancy of SNS targets by studying the performance of earlier targets and making modifications, including injecting small bubbles of helium gas into the target vessel’s liquid mercury jet flow—an improvement that reduces mechanical strain and cavitation damage caused by the proton pulses.

Neutron scattering provides essential details about atomic-, meso- and nanoscale structures, forces and activities that in many cases simply cannot be obtained using any other technique. Neutrons have ideal energies for observing atoms in motion, and they are non-destructive, deeply penetrating and uniquely sensitive to magnetism and lighter elements such as hydrogen. SNS, with its 19 advanced beamline instruments, enables a wide range of science under ambient conditions, as well as in extreme and complex environments.

SNS and ORNL’s other world-class neutron source, the continuous beam High Flux Isotope Reactor, together in FY 2018 provided 102,883 hours of beamtime for research, hosted 1,205 unique visiting users who conducted 1,188 user experiments that resulted in 646 published papers (457 instrument publications by users and189 other publications by ORNL’s neutron science staff ).


Looking to the Future

With Department of Energy approval, ORNL will move ahead with two major upgrades at SNS: implementing a Proton Power Upgrade (PPU) and building a Second Target Station (STS). The PPU will double the available proton beam power to 2.8 MW by adding more superconducting cryomodules and new RF sources in the klystron gallery including 28 klystrons, three high-voltage converter modulators and associated support equipment. Some existing RF equipment will be upgraded to accommodate the increased beam loading.

PPU will enable faster discovery and the study of smaller and more complex samples. It will also allow the SNS linac to deliver a portion of its additional proton power to the STS instrument hall—at 15 pulses per second—to power up to 22 new world-leading instruments. STS will enable breakthroughs in materials research including biological, polymer, quantum, complex and engineered materials. STS will deliver capabilities far beyond those of current U.S. sources, producing more cold (lower energy) neutrons, with a factor of four increase in range of wavelengths and a 40 times increase in pulsed brightness.

With a new suite of instruments boasting the latest advances in high-resolution optics, instrument design and neutron spin manipulation, STS will deliver instrument-specific performance gains that are 100 to 1,000 times better than existing neutron instruments. Together, these improvements will offer unprecedented neutron science capabilities vital to the US economy and research community, speed up the pace of discovery with faster data collection, and provide more opportunities to visiting scientists to complete their materials research at ORNL. 

In recognition of the 75th anniversary of Oak Ridge National Laboratory’s founding as a world leader in innovative research and technology, this article looks back at some of the more notable events involving neutrons and accelerators at the lab.

  • 1943: The Secret City
    World War II is raging as some of the world’s brightest minds help establish a top-secret research and development facility in eastern Tennessee, which will later be named Oak Ridge National Laboratory.
  • 1944: Putting neutrons to work
    From its origins in the Manhattan Project, including the X-10 Graphite Reactor, ORNL helps pioneer nuclear engineering and energy technologies. Ernest Wollan, Lyle Borst and others begin using neutrons produced by the reactor to demonstrate neutron diffraction and make the first observations of Bragg reflections.
  • 1948: Building accelerators to produce neutrons
    Tasked with developing an accelerator program to, initially, produce neutrons for research, Oak Ridge scientists begin acquiring or building Van de Graaff accelerators, the only known source of neutrons with precise energies. They later acquire a Cockcroft-Walton unit, an early particle accelerator named for its inventors, to test radiation effects at lower energies.
  • 1950: First proof of neutron decay
    Arthur Snell and Frances Pleasonton measure the half-life of neutrons and prove they decay into a proton, an electron and an electron antineutrino.
  • 1951: Discovery of crystal magnetic structure
    Clifford Shull, Wilbur Strauser, and Ernest Wollan use neutrons to reveal the magnetic structure of a crystal, manganese oxide, providing the first direct evidence of antiferromagnetism predicted by Louis Néel.
  • 1963: Oak Ridge Isochronous Cyclotron
    Among the first new-generation cyclotrons, ORIC begins operations to exploit the azimuthally varying field principle to achieve significantly higher energies for a wide range of particles.
  • 1965: High Flux Isotope Reactor
    ORNL’s High Flux Isotope Reactor (HFIR) achieves criticality and begins producing super-heavy elements, such as californium, while researchers apply its neutron beams to materials studies.
  • 1969: Oak Ridge Electron Linear Accelerator
    ORELA begins full operation to provide neutron cross-section data that indicates the likelihood of interaction between an incident neutron and a target nucleus. It features beam energies up to 180 MeV, a neutron production rate up to 1014 n/sec and 50 kW of beam power.
  • 1975: Radiopharmaceutical heart-imaging agent
    Using isotopes produced at HFIR, ORNL researchers demonstrate a radioactive imaging agent that detects how much of a patient’s heart muscle is still alive after a heart attack.
  • 1979: 25 MV Tandem Electrostatic Accelerator
    The tandem electrostatic accelerator is a high-voltage generator inside a 100-foot-tall, 33-foot-diameter pressure vessel. It features a folded configuration with both low- and high-energy acceleration tubes contained in the same column structure.
  • 1980: Holifield Heavy Ion Research Facility
    Consisting of the new 25 MV tandem accelerator and the modified Oak Ridge Isochronous Cyclotron, this facility uses one of the devices, or both in a coupled mode, to provide a wide range of energetic beams for experiments.
  • 1982: Award for studying magnetism and superconductivity
    Herbert A. Mook Jr. wins the Department of Energy’s Outstanding Scientific Accomplishment in Solid State Physics award for using neutron diffraction to demonstrate the coexistence of magnetism and superconductivity in rare-earth rhodium borides.
  • 1994: Nobel Prize for neutron research
    Clifford Shull shares the Nobel Prize in Physics with Bertram Brockhouse. Shull wins for research he conducted while at ORNL with the late Ernest Wollan, which enabled neutron scattering techniques that led to improved materials and technologies.
  • 1996: Holifield Radioactive Ion Beam Facility
    Originally the Holifield Heavy Ion Research Facility, HRIBF opens after ORNL reconfigures its 25 MV tandem electrostatic accelerator and its isochronous cyclotron to produce high-energy radioactive ion beams for research into nuclear structures and astrophysics. HRIBF will continue operating until 2012.
  • 1999: Spallation Neutron Source construction begins
    The $1.4 billion SNS is the first U.S. science facility of its scale to be constructed in more than a decade. Attending is special guest and Nobel Laureate Clifford Shull.
  • 2006: Spallation Neutron Source begins operations
    SNS begins operations, and by 2009 increases its beam power to 1.0 MW, or eight times that of the world’s leading pulsed spallation source. This increase in power, when combined with advanced instrument technology developed at SNS, gives researchers a 50- to 100-fold net improvement in measured neutron beam peak intensity.
  • 2007: Oak Ridge Electron Linear Accelerator
    The American Nuclear Society recognizes the 38-year-old ORELA as a Nuclear Historic Landmark. Research at this linac has led to more than 500 published papers. The accelerator will continue operating until 2015.
  • 2010: HFIR helps discover “tennessine” (element 117)
    Berkelium is a radioactive element needed to produce element 117, and it can only be made at HFIR. After 250 days of irradiation, 22 mg of berkelium-249 are sent to the Joint Institute for Nuclear Research in Dubna, Russia, which produces the first six atoms of element 117—later named “tennessine” in honor the contributions of ORNL, Vanderbilt University and the University of Tennessee.
  • 2015: HFIR turns 50 and achieves landmark status
    Highlights of HFIR’s half-century of research include the invention of neutron polarization analysis in 1969, the implementation of small-angle neutron scattering in 1978 and investigating hightemperature superconductivity in the early 1990s.
  • 2016: SNS uses plasma cleaning on superconducting cavities
    SNS staff develop in-situ plasma processing to remove contaminants, which limit operational performance, from the surface of superconducting accelerator cavities. This reduces cavity cleaning time from up to eight months to just a few weeks without removing the cavities.
  • 2016: New state of water discovered
    Neutron scattering and computational modeling reveal a unique and unexpected quantum tunneling behavior of water molecules under extreme confinement.
  • 2017: Laser stripping process targets beam loss
    Significant beam loss in the injection of the accelerator caused by the ion-stripping material interacting with the circulatingring proton beam prompts SNS accelerator scientists to develop the first laser-assisted hydrogen ion-stripping process to aid ring injection and reduce beam loss.
  • 2017: Neutrons peer into a running engine
    Researchers use neutrons to investigate the performance of a new aluminum-cerium alloy in a gasoline-powered engine—while the engine is running.
  • 2017: First nanoscale look at a living cell membrane
    Scientists use neutrons to make the first-ever direct nanoscale examination of a living cell membrane. Researchers identify tiny groupings of lipid molecules, called lipid rafts, that are key to a cell’s ability to function, resolving a longstanding debate.
  • 2018: Complete 6D characterization of accelerator beam
    SNS scientists produce the world’s first six-dimensional measurement of an entire accelerator beam. To avoid monopolizing the SNS accelerator during long periods of data acquisition, researchers conduct the measurements at the ORNL Beam Test Facility, a functional copy of the SNS linac injector.
  • 2018: Neutrons probe a metal-organic framework
    Scientists at Oak Ridge use neutrons to show how an MOF exhibits a selective, fully reversible and repeatable capability to remove nitrogen dioxide gas from the atmosphere.
  • 2018: SNS sets record for neutron production beam power
    After years of innovations in mercury target design, linac improvements, and other advancements, SNS completes a full production cycle at 1.4 MW, the highest beam power ever delivered for a full cycle.
  • 2018: Plasma processing enables 1 GeV beam energy
    The plasma cleaning process developed at SNS in 2016 helps bring linac beam energy to the design goal of 1 GeV.


Celebrating 75 Years: A History of Oak Ridge National Laboratory

1 The original “research proposal” Ernest Wollan wrote in 1944 requesting funding for neutron experiments at the ORNL X-10 pile. 2 Ernest Wollan’s 1944 hand-drawn graph of the first observation of Bragg reflections using neutron diffraction at Oak Ridge. 3 Conceptual image of laser electron stripping. Left to right: incoming hydrogen particle with two electrons (red); first electron is stripped in a magnetic field; excitation (purple beam) of the remaining electron by a laser (center); remaining electron is stripped by a second magnetic field; resulting proton particle (yellow). Image credit: ORNL/Jill Hemman 4 Behind the work station, an SNS cryomodule undergoes in-situ plasma processing. Inset shows a 6-cell cavity with monitored plasma inside each cell. Image credit: Genevieve Martin/ORNL. 5 Conceptual image of 6D beam measurement in a particle accelerator, showing that the beam’s structural complexity increases when measured in progressively higher dimensions. Image credit: ORNL/Jill Hemman 6 Planned upgrades to the Spallation Neutron Source include doubling the power through the Proton Power Upgrade and adding a new-generation neutron source, the Second Target Station. 7 Spallation Neutron Source linac accelerator’s ring-to-target beam transport tunnel. 8 Cryomodules at the Spallation Neutron Source linac. 9 The 1.4 MW Spallation Neutron Source at Oak Ridge National Laboratory is the world’s most powerful pulsed-beam neutron source.

Edited by Paul Boisvert, ORNL Communications

The Research Accelerator Division at Oak Ridge National Laboratory welcomes inquiries from industry, academia and government agencies to collaborate with ORNL in accelerator science and the development of new accelerator technologies.

For more information, contact:

Fulvia Pilat
Research Accelerator
Division Director

Sarah Cousineau
Group Leader
Beam Science & Technology