Fermilab Today Friday, March 28, 2008
Special Result of the Week

Toward a high-intensity muon source

Impact of a single pulse of 12 x 1012 protons at 14 GeV on a 1cm-wide jet of mercury flowing at 15m/s in a 10T solenoid field: a) at the moment of impact (T=0); b) 9ms after impact; c) 15ms after impact.

To solve the mysteries of nature, physicists operate accelerators that deliver beams of particles including electrons, positrons, protons, neutrons, photons, neutrinos and nuclei. Particle physicists would like to add a muon accelerator to their list. Results obtained with the Mercury Intense Target experiment show that this might be possible.

Muons are short-lived particles that are 200 times heavier than electrons. Physicists create muons by smashing a beam of protons into a solid or liquid target. The collisions of protons with atomic nuclei produce pions that decay within a fraction of a second to yield muons.

One of the key challenges for building a muon accelerator is the construction of an intense muon source. To be successful, scientists must build a source that consistently produces a large number of muons without destroying the target. They also must develop a strong magnet that channels these pions into a specific direction before they decay.

In November, physicists working on the MERIT experiment successfully tested a muon source concept developed by the DOE-supported Neutrino Factory and Muon Collider Collaboration. The NFMCC collaboration, which includes scientists from 41 institutions, proposed a target that consists of a free-liquid mercury jet flowing at 20m/s within the confines of a 20T solenoid field. The pions are produced by an intense proton beam that hits the jet with a beam spot size of a few millimeters.

The heat deposited in the mercury jet corresponds to the energy emitted by hundreds of 1kW heaters in a space that is 1cm in diameter and 30cm long. Can the jet survive this heat?

Using the Proton Synchrotron at CERN, the Mercury Intense Target collaboration demonstrated that the jet could, in principle, withstand a proton beam of up to 4MW power.

For the experiment, the MERIT scientists developed a pulsed solenoid capable of sustaining a 15T peak field for 1 second. The magnet surrounded the jet. They then exposed the mercury jet to proton beam pulses of 14 and 24 GeV with intensities up to 30 x 1012 protons per pulse. At 14 GeV, pulses with intensities up to 4 x 1012 protons did not disperse the mercury. At 24 GeV, the beam intensity reached a maximum energy of 115kJ in a single pulse, a new record for the CERN PS. This intense beam pulse temporarily disperses the mercury jet, but only after all protons hit the target. Flowing at 20 m/s, the jet replaces the dispersed mercury in 15 thousandths of a second. In principle, the target could receive a maximum of about 60 beam pulses a second at this intensity.

The results raise the prospect of using the NFMCC target concept in ultra-high intensity proton beam environments, bringing physicists a step closer to their dream of a muon accelerator.

The MERIT collaboration includes scientists from Brookhaven, CERN, Fermilab, Oak Ridge, Princeton University and Rutherford Appleton Laboratory and operates under NFMCC sponsorship with support from DOE.

Left to right, seated: Hugo Pereira, CERN; Harold Kirk, BNL. Left to right, standing: Chris Densham, RAL; HeeJin Park, BNL (obscured); Peter Loveridge, RAL; Roger Bennett, RAL; and Kirk McDonald, Princeton.Image courtesy of Robert B. Palmer.
Fermi National Accelerator - Office of Science / U.S. Department of Energy | Managed by Fermi Research Alliance, LLC.