New Beams Take Flight at RHIC

This guest post is written by Brookhaven physicist Thomas Roser, Chair of the Collider-Accelerator Department. Roser, who earned his Ph.D. from the Swiss Federal Institute of Technology, worked at the University of Michigan before joining Brookhaven in 1991.

Thomas Roser.jpg

Thomas Roser

The chain of accelerators that leads into two of Brookhaven's major research facilities - the Relativistic Heavy Ion Collider (RHIC) and the NASA Space Radiation Laboratory (NSRL) - will soon have a new starting point.

A new ion generator, called the Electron Beam Ion Source (EBIS), will produce and accelerate beams with greater versatility than the current system, allowing studies with new kinds of ions previously unavailable to researchers. EBIS recently received formal approval to start operations from the U.S. Department of Energy.

This new pre-injector system uses EBIS and two small accelerators as a replacement for the Tandem Van de Graaff accelerators, which have been running successfully for 40 years, but with certain limitations. For example, because the Tandems must start with negatively charged ions -- atoms with one extra electron -- they can only produce beams from about half the elements in the periodic table. EBIS can start with positive ions or even neutral atoms, allowing the creation of ion beams from almost any element.

One of the newly available ion beams is a beam of uranium ions. Collisions of uranium ions are interesting for the RHIC program -- where physicists are recreating conditions of the early universe to learn more about the forces that hold matter together -- because of their football shape and large number of protons and neutrons. Head-on collisions of two tiny uranium ions positioned like colliding spiraling footballs would produce a speck of matter with even greater energy density than the spherical gold ion collisions currently creating a hot soup of quarks and gluons at RHIC. This higher energy density will allow scientists to study the evolution of this early-universe substance over a wider range of conditions than has been previously accessible at RHIC.

Head-on collisions with the uranium ions oriented vertically, like footballs prepared for a kick-off, are also of great interest: They will permit scientists at RHIC to separate the effects of extremely hot matter formed when two spherical gold ions overlap only partially on impact (forming a similar football-shaped interaction region) from effects of the ultra-strong magnetic fields produced in such non-head-on collisions.

As with the gold ion collisions at RHIC, which have used less gold than is found in a single wedding ring over RHIC's 10 years of operations, the amount of uranium used will be extremely small and not pose any radiation risk to either Brookhaven Lab staff or the public. Also, EBIS will use the dominant naturally occurring form of uranium, U238, which cannot "split" and sustain a nuclear chain reaction like U235.


A cut-away of the EBIS pre-injector. The central "trap" region, where electrons are stripped off atoms to create positively charged ions, is 1.5 meters long.

At NSRL, scientists study the effects of space radiation to help find ways to protect astronauts. Researchers here are interested in testing the effects of ions of noble gases like helium, neon, and argon, because these are common components of galactic cosmic rays and could pose a significant risk to astronauts on long-term space missions.

Another major advantage is that EBIS can feed beams of different ions to each facility at practically the same time and also quickly switch between different ion species. This feature will allow the exposure of samples at NSRL to a variety of different ions, therefore more realistically simulating exposure to deep space radiation.


Brookhaven's NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and access the risks of space radiation to astronauts.

EBIS works by trapping atoms or ions in an electrically charged chamber inside a 1.5-meter-long cylindrical superconducting magnet. The voltage holds the charged ions in the chamber while an electron beam generated at one end passes through, systematically stripping electrons off the trapped atoms. For helium gas, there are only two electrons to be stripped off to create a beam of helium ions, where each has a 2+ charge. For gold -- the heaviest ion collided at RHIC to date -- the ions start off with one electron already stripped off, but they are held in the trap until 32 electrons have been removed. At this point the ions are released from the trap forming a beam that then moves from EBIS through two small linear accelerators that bring the beam up to the energy needed for injection into the next accelerator -- the Booster and then NSRL or the Alternating Gradient Synchrotron and then RHIC.

The high charge achieved in EBIS along with its modern accelerator technologies makes the new pre-injector system very compact and easy to operate and maintain. EBIS will begin providing helium ions to researchers at NSRL this fall. The first uranium collisions at RHIC may take place as early as late winter or spring 2011. Meanwhile, and even after EBIS starts operating, the Tandems will continue to run to provide beams for a community of outside scientists who use that facility for dedicated research on electronic components for industrial and space applications.

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