Negatively charged hydrogen ions are produced by an ion source. Each ion consists of a proton orbited by two electrons. The ions are injected into a linear accelerator, which accelerates them to very high energies. The ions are passed through a foil, which strips off each ion's two electrons, converting it to a proton. The protons pass into a ring where they accumulate in bunches. Each bunch of protons is released from the ring as a pulse. The high-energy proton pulses strike a heavy-metal target, which is a container of liquid mercury. Corresponding pulses of neutrons freed by the spallation process are slowed down in a moderator and guided through beam lines to areas containing highly specialized instruments for conducting experiments. Once there, neutrons of different energies are used in a wide variety of experiments.
The SNS front-end system includes an ion source, beam formation and control hardware, and low-energy beam transport and acceleration systems. The ion source produces negative hydrogen (H-) ions—hydrogen with an additional electron attached—that are formed into a pulsed beam and accelerated to an energy of 2.5 million electron volts (MeV). This beam is delivered to a large linear accelerator (linac).
The linear accelerator, or linac, accelerates the H- beam from 2.5 to 1000 MeV, or 1 GeV. The linac is a superposition of normal conducting and superconducting radio-frequency cavities that accelerate the beam and a magnetic lattice that provides focusing and steering. Three different types of accelerators are used. The first two, the drift-tube linac and the coupled-cavity linac, are made of copper, operate at room temperature, and accelerate the beam to about 200 MeV. The remainder of the acceleration is accomplished by superconducting niobium cavities. These cavities are cooled with liquid helium to an operating temperature of 2 K. Diagnostic elements provide information about the beam current, shape, and timing, as well as other information necessary to ensure that the beam is suitable for injection into the accumulator ring and to allow the high-power beam to be controlled safely.
Proton Accumulator Ring
The accumulator ring structure bunches and intensifies the ion beam for delivery onto the mercury target to produce the pulsed neutron beams. The intense H- beam from the linac must be sharpened more than 1000 times to produce the extremely short, sharp bunch of neutrons needed for optimal neutron-scattering research. To accomplish this goal, the H- pulse from the linac is wrapped into the ring through a stripper foil that strips the electrons from the negatively charged hydrogen ions to produce the protons (H+) that circulate in the ring. Approximately 1200 turns are accumulated, and then all these protons are kicked out at once, producing a pulse less than 1 millionth of a second (10-6seconds) in duration that is delivered to the target. In this way, short, intense proton pulses are produced, stored, and extracted at a rate of 60 times a second to bombard the target.
Inside the target vessel, when a high-energy proton hits the nucleus of a mercury atom, 20 to 30 neutrons are "spalled" or thrown off. Those neutrons are guided out of the target vessel into beam guides that lead directly to instrument stations. The neutrons coming out of the target must be turned into low-energy neutrons suitable for research—that is, they must be moderated to room temperature or colder. The neutrons emerging from the target are slowed down by passing them through cells filled with water (to produce room-temperature neutrons) or through containers of liquid hydrogen at a temperature of 20 K (to produce cold neutrons). These moderators are located above and below the target.
SNS instrument layout.
The pulsed, moderated neutrons are guided through a beam tube to specially designed instrument stations. Each instrument at SNS is unique and is designed to for certain types of experiments with specific types of samples. Researchers bring samples of materials (anywhere from centimeters in width to meters) to put within the path of the neutron beam. A variety of "sample environments" are available with each instrument, meaning that researchers can select from different environmental conditions, such as high temperature or high pressure, under which their experiment can be conducted.
Oak Ridge National Laboratory is managed by UT-Battelle for the Department of Energy