Bevatron

From Wikipedia, the free encyclopedia

Edwin McMillan and Edward Lofgren on the shielding of the Bevatron. The shielding was only added later, after initial operations.

The Bevatron was a particle accelerator — specifically, a weak-focusing proton synchrotron — at Lawrence Berkeley National Laboratory which began operating in 1954. The antiproton was discovered there in 1955, resulting in the 1959 Nobel Prize in physics for Emilio Segrè and Owen Chamberlain.^[1] It accelerated protons into a fixed target, and was named for its ability to impart energies of billions of eV. (This is the reason why it was named Bevatron: Billions of eV Synchrotron.) It received a new lease on life in the 1971^[2], when it was joined to the SuperHILAC linear accelerator as an injector for heavy ions^[3]. The combination was known as the Bevalac. It could accelerate any nuclei in the periodic table to relativistic energies. It was finally decommissioned in 1993. The building that houses the Bevatron is still present but has been slated for demolition beginning in June 2008 and ending in 2011.

The Bevatron was largely designed to be energetic enough to create antiprotons, and thus test the hypothesis that every particle has a corresponding anti-particle^[4]. At the time it was built, there was no known way to confine a particle beam to a narrow aperture, so the beam space was about four square feet in cross section. In order to create antiprotons (mass 938 MeV) in collisions with nucleons in a stationary target while conserving both energy and momentum, a proton beam energy of approximately 6.2 GeV is required. The combination of beam aperture and energy required a huge, 10,000 ton iron magnet. The next generation of accelerators used "strong focusing", and required much smaller apertures, and thus much cheaper magnets. The AGS (Alternating Gradient Synchrotron) at Brookhaven was the first next-generation machine, with an aperture roughly an order of magnitude less in both transverse directions, and reaching 30 GeV proton energy, yet with a less massive magnet ring.

In the years following the antiproton discovery, much pioneering work was done here using beams of protons extracted from the accelerator proper, to hit targets and generate secondary beams of elementary particles, not only protons but also neutrons, pions, "strange particles", and many others. These could in turn be passed for further study through various targets and specialized detectors, notably the liquid hydrogen bubble chamber, for which Luis Alvarez received the Nobel Prize in 1968.